Hydraulic Lime, Practical Advantages

Hydraulic Lime, Practical Advantages

Natural Hydraulic Lime (NHL) is generally associated with building conservation or as a green, eco-building material. That’s hardly surprising, hydraulic lime goes back as far as the Greeks, Egyptions and Romans. It’s use diminished in Britain in the 19th century as people adopted modern Ordinary Portland Cement (OPC).

However, we’re learning more about these traditional materials now, than was ever known when they were popular. The inherent natural properties of hydraulic lime provide the same practical benefits of modern OPC but with none of the disadvantages. Naturally, NHL is perfectly suited for new build and can play an important role in the future of modern construction.

Lime-Putty

Fast Setting

Hydraulic lime is fast setting, it can be used on-site just as efficiently as modern cement.

Hydraulic

It sets by hydrolysis, this means it will also set underwater.
Excellent for marine construction or masonry exposed to severe weather conditions.

No Shrinkage

Shrinkage cracks in mortar and render are virtually eliminated due to its hydraulic setting characteristics.

Breathable

It is breathable (vapour permeable), absorbing and evaporating moisture from surrounding masonry.
This also helps to protect the masonry, there is less risk of salt and frost damage.

Flexible

It has a low modulus of elasticity.
This means it is extremely flexible and allows for movement and thermal expansion.

Adaptable

Available in NHL 2, NHL 3.5 and NHL 5 grades.
This allows design of mortars suitable for masonry of varied density and degrees of weather exposure.

Workable

It is supplied in powdered form, it is pretty much the same as mixing and working with a modern cement.
Mixed to the right consistency, hydraulic lime mortar, render or plaster are beautiful materials to work with.

Sympathetic

It is softer than modern cement, a softer mortar will not wear away the surrounding masonry over time. Bricks and stones are also reclaimed more easily if dismantled.

Aesthetic

Hydraulic lime mortars are indistinguishable from non-hydraulic limes.
They carry the same chamring, traditional appeareance.

Lime for Contemporary Construction

The benefits of using lime to create a vapour open construction are well known for historic solid walled buildings, however its use in modern construction is not so well documented.

The most common argument for using a lime mortar in modern buildings is for its green credentials. There is an immediate reduction in carbon at the production stage because lime is burnt at lower temperatures than cements. Lime mortars also set through carbonation, which is the reabsorption of CO2 and this reabsorption further reduces the embodied energy of the mortar.

When compared to cement based mortars, lime can offer a significant reduction in carbon, however the benefits do not stop there.

causing the bricks to cement

Modern buildings are designed to prevent water ingress through the use of a cavity. Masonry is porous and will draw water and moisture into the wall from internal and external elements, the cavity serves to drain this away and prevent it from being built up.

However, a cavity alone cannot protect a building entirely from water related issues. It has been estimated that up to 75% of building failures are due to water, with the most common causes of failure being water penetration and interstitial condensation.

Water related issue do not always result in failure, they can also impact the thermal performance of the structure and the occupier’s health.

Water can affect a building in 3 key areas –

Outer Envelope – penetration from external conditions (rain etc.)

The Middle – interstitial condensation; when warm, moist air reaches its dew point and turns into water.

Inner Envelope – surface condensation and internal air quality.

Across Europe it has been estimated that anywhere from 10% to 50% of buildings suffer with damp and moisture related issues.

External Conditions

Water should be kept out of buildings, this much is obvious. The conventional method for doing this in modern construction is to use renders that are waterproof or contain waterproof additions and do not allow for water to reach the porous masonry: this is known as a capillary block or a closed render.

This makes perfect sense in theory, unfortunately in practice it is not always the case.

A render that is closed can lead to an increase in water run-off. This increased displacement of water can be injected into the wall (or frame) through junctions and detailing, which in turn can result in movement and failure.

A closed render will rely on chemical additions or treatments, such as silicone to prevent water ingress. As the render is subject to weathering and minor movement which takes place over the life of a building, the capillary block can and will break down. This break down will then allow water to enter the render, with little means of escape, which can lead to failure if it eventually reaches the porous masonry (or frame).

As water becomes trapped within a render it can be susceptible to freezing. When water freezes it has an expansion rate of 9%, meaning for every freeze cycle the crack is further widened, allowing more water to enter every rain cycle.

By using a vapour permeable render, water is absorbed more evenly across the entire surface. This in turn reduces the concentration of water in isolated locations, especially areas that may be more vulnerable.

This water and moisture is then allowed to evaporate back through the render and is not held captive.

Whilst this is a viable solution for external elements, there is always a risk of water/moisture penetration being too high and being able to transfer itself internally. This risk is heightened if the cavity is not wide enough or if it has been filled with insulation. However, this issue can be overcome by applying a vapour open mineral silicate paint that contains a hydrophobe, to repel liquid, which in turn with further increase the lifespan of the render coating. Detailing is still critical and has to be carried out sufficiently, however as a mineral paint chemically bonds to the render, it is not as susceptible to weathering, unlike modern closed systems and as a mineral paint is vapour open, it will ensure that any moisture contained within the render can escape.

The Middle: Moisture, Interstitial Condensation and Thermal Performance

Interstitial condensation is when warm, moist air penetrates a material and reaches its dew point, condensing on a surface.

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Interstitial condensation is a complex issue and relies on several factors, but in essence it is pressure and temperature differences that pull warm humid air through materials until they hit a point where it is cold enough to turn to water. The point where the air hits a sufficient colder temperature is known as the dew point.

Interstitial condensation can lead to issues such as mould and rot, but it can also have a detrimental impact on the thermal performance of a structure.

Most standard insulation materials do not mix well with water. Water molecules will fill the voids within the insulation and create a cold bridge. This in turn significantly reduces the thermal performance of the building, leading to an increase in heating bills and carbon emissions.

A building cannot be sealed to combat this issue, as that will simply shift the issue to another area. We generate far too much moisture within structures to seal them off.

Temperatures and relative humidity play a significant role in this subject.  Relative humidity is defined as the amount of water air can hold, for example 50% relative humidity means that the air contains 50% of the water it can hold. When the relative humidity is 100%, dew point is reached and moisture will condense from the air. So the higher the relative humidity, the quicker air can condense. Relative humidity will decrease as the temperature increases, as we have no control over external conditions this has to be managed internally. If the heat within a room is increased, the relative humidity will decrease, in turn lowering the available moisture within a room.

However, a breathable render can help combat this issue, by offering water and moisture a means of escape. A breathable render will be hygroscopic and porous, allowing moisture and water to enter. It has the ability to store the moisture without damage, until conditions allow for it to be released. By holding moisture within the render/plaster it will prevent it from being able to reach the insulation and have a detrimental impact. The moisture will then be released back into a room when the relative humidity is low enough to accommodate it, acting as a form of natural ventilation. It is also vital that if the finish is to be decorated a suitable, vapour permeable paint is used.

Inner Envelope: Surface Condensation and Internal Air Quality

We already know the problems that can be caused if moisture is able to permeate through materials and settle within insulation, but what if the moisture is able to sit on surfaces within the internal environment?  When warm air hits a cold surface it is able to condense and form water droplets as condensation, this can be commonly seen on windows.

Air holds moisture and the temperature of the air determines the amount of moisture it can hold, with warmer air holding more than cold air. Much like with interstitial condensation, a balance between temperature and relative humidity needs to be achieved.
If condensation is allowed to dwell on a surface, it can result in mould. Mould is a fungal growth and whilst mould itself is not toxic, certain moulds can produce toxins that can be harmful to the occupant, triggering conditions such as allergic reactions, asthma and other auto-immune diseases.

Mould requires 4 elements to grow:

  • Mould spores
  • Food source
  • Sufficient temperature
  • Moisture

Mould spores are microscopic and can be found everywhere, they are commonly contained within household dust. Unfortunately, no matter how thorough you clean, dust will likely always be present.

Mould requires a food source, with the majority of moulds feeding on any substances that contain organic matter. The majority of building materials, from plaster to paint, contain organic matter and will always be present within a building.

For mould to grow, it requires a sufficient temperature, unfortunately most moulds are amazingly persistent and can grow in a variety of conditions. Mould will generally grow in temperatures that humans are comfortable within. Moulds can still grow in temperatures close to freezing, however they will thrive in warmer temperatures.

One area we can control is the level of moisture available within a building. A typical 3-bedroom house, with 4 occupants, will create 112 pints of moisture a week from breathing, cooking, showering and boiling the kettle. As we strive to create buildings with lower emissions and improved insulation through sealing our buildings, we in turn reduce the means of escape for this moisture.

Most moulds depend upon high levels of moisture to grow, typically they require a relative humidity of 70% and above. For comfort, most humans will prefer humidity levels below 70%, with the optimum levels being between 40% to 60%. Dust mites will thrive at 70% plus humidity and it is also worth noting that VOC’s are also more harmful outside of 40% to 60% humidity.

By sealing buildings and improving efficiency we are also increasing the capacity of water that the internal air can hold, if this is able to condense we are then increasing the ability for mould to grow. Ventilation is critical in maintaining stable conditions, by removing stale and warmer air, however highly insulated building usually rely on equipment, such as MVHR’s, to enable this process. Whilst MVHR’s work extremely well, we become reliant on them. A breathable surface, such as plaster or render has the ability to absorb and release moisture, meaning if humidity levels become too high the surfaces can act as a natural form of ventilation. It is also worth noting that the alkalinity of lime can help prevent mould growth by killing spores that may dwell on the surface.

Cornish Lime produce and supply a range of breathable mortars, render systems and paints designed for contemporary and new build construction 

Cornerstone Newbuild Mortar Range

The Cornerstone Newbuild Mortar range offers a variety breathable mortars for both internal and external use in modern construction

Materials Overview:

  • A blend of specially selected sands, St Astier NHL, specialist cements and working aids
  • The Cornerstone Newbuild range offers a system that can be applied to render carriers (timber frame), block, brick and stone as a render.
  • It can also be used as a building and pointing mortar
  • Vapour open, allowing moisture to escape the surface
  • Highly flexible, can accommodate movement
  • Highly workable, with all of the benefits of lime
  • Can accommodate heat and regulates humidity
  • Reduced risk of shrinkage and cracking
  • Batch blended for consistency
  • Colour is derived from the sand, so no pigment additions.
  • No chemical additions (such as silicone)
  • With up to half the embodied carbon of conventional cement based mortars.
  • 1 tonne of Cornerstone New build mortar can absorb 30kg of CO2 – equivalent to the amount of CO2 absorbed by a young tree in 3 years.

Beeck Mineral Silicate Paints

Beeck Mineral Silicate Paints offer a range of paints for both internal and external use. Beeck paints work through a chemical bond between the paint and the surface. This chemical bond results in a vapour open paint, that is not subject to weathering like conventional paints.

Externally Beeck paints contain a hydrophobe, which repels liquid from the surface, whilst offering moisture a means of escape from the surface.

Internally, Beeck paints form the same chemical bond to the surface and offer the same unsurpassed durability.

Materials Overview:

  • Beeck paints are fully breathable, ensuring that no moisture will be held captive within the fabric.
  • Due to their breathability they help regulate the internal climate by absorbing moisture and releasing it, creating a healthy internal climate.
  • Beeck external paints contain a hydrophobe, meaning water is repelled from the coating and not allowed to enter the fabric, whilst still allowing moisture to escape.
  • High alkalinity results in a mould resistant coating.
  • Free from solvents and chemicals.
  • As it is non film forming it will not flake or peel.
  • As it contains a low acrylic content and no chemical additions it reduces dust build up on surfaces, keeping the internal environment clean.
  • Suitable in critical applications such as hospitals and schools as it fulfils the strictest air hygiene requirements.
  • Due to their chemical bond they offer superb durability and longevity.
  • Easily recoatable and available in over 250 colours.

 

Lime Mortars for Pointing

Pointing is the action of filling the gap between masonry units (joints) with a mortar to protect the masonry from water ingress and associated decay.

Before any work begins it is important to observe how the wall was originally built and take note of this. If you observe traditional solid masonry construction, you will very often note the inclusion of small stones between larger stones. These pinning stones (also referred to as gallets or snecks) may be there to offer support or “chock” or act as large pieces of aggregate, reducing the amount of mortar used within the joint. This reduction in mortar mass aids curing and to some degree the cost. In order to maintain the visual integrity, it is important that these pinning stones are placed back within the wall as close as possible to that of the original.

Lime-pointing-enhanced

Joint Preparation

In most repointing cases the least popular aspect of the process, although vital that it’s done as thoroughly as possible is the removal of the existing mortar joints which, in the case of original lime mortars, is relatively straight forward; however, more frequently this involves the removal of hard cement mortars and in order to mitigate further damage to the original masonry this should be removed as carefully as practical leaving as square a profile as can be practically achieved to the back of the joint.  Exactly how it’s achieved will be very subjective, as will the tools used to achieve it. There may well be occasions where mechanical cutting out is deemed appropriate and when and where this method is suitable we would advise due diligence to mitigate unnecessary damage to the masonry units.

When removing existing pointing it’s important to keep any damage to the masonry to an absolute minimum, and themost obvious element to manage that is “common sense”. Remove by cutting out in a controlled manner where the existing is mortar is cut out to the open face of the joint, don’t direct energy from the chisel directly into the mortar.  This reduces the energy and stress to the masonry unit.  Tools should be appropriate to the joint size and chisels should be as sharp a chisel as possible, while it may be counter intuitive you don’t necessarily require big heavy hammers, and rarely would we advocate the use of mechanical type “breakers” or similar.

The joints need to be raked out to a suitable depth, with the general rule being that it should be at least one and a half times the width of the joint, in the case of wide joints common sense should prevail.  Insufficient depth will result in nothing more than a token gesture with an increased risk of the mortar becoming loose or simply falling out within a relatively short time.  Conversely too much mortar, can lead to masonry becoming looser while benefiting the vendor.

Once the joint has been cut out it should then be squared off at the back and further prepared by a thorough brushing out, with no loose material present to compromise the bond of the fresh mortar once it is placed.

Dampening the Wall

While this should be important when working with any mortar it’s far more so with lime, to ensure that once the mortar is placed that it’s not allowed to dry out too quickly. To best manage the process, before any mortar is applied the background of the joints will need to be dampened down.  As different masonry will have different characteristics the advice we offer here is generic; however, existing lime mortars tend to be highly absorbent and assuming the work is being carried out on typical masonry with an average amount of residual moisture (approx. 20% in northern Europe), spray the wall using appropriate methods that will place sufficient moisture into the background to prevent desiccation of the fresh mortar after placing.  NEVER place your mortar into a situation where there is liquid water present.

Lime Mortar

A suitable mortar should be selected for specific application.

The table below offers a simple overview of suitable mortars; however, we do advise that you contact us for more specific advice.

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The mortar for pointing should be workable without being too wet, as a rule the stiffer the mortar the cleaner the work can be executed. If the mortar is too wet, it can be difficult to apply and will readily stain the masonry; also, the wetter a mortar is the more prone it will be to shrinkage cracking.

Tools

There are a wide variety of tools that can be suitable for pointing and for most applications it will come down to what is suitable to the joint size and that of personal preference and what feels comfortable etc.  Using too large a tool will result in mortar being smeared over the stone work or prove difficult to apply in tighter joints.

A (Plasterer’s) small tool, also known as a leaf & square & trowel & square, can be very useful for where you have smaller joints.

In our opinion the churn brush is the most important tool when it comes to pointing with its main role to beat the mortar back after application; this will be discussed in more detail shortly.

Filling the Joints

When filling joints, it is important that the mortar is thoroughly pressed home applying sufficient pressure to the tool forcing mortar into the whole of the joint.   When placing the mortar, avoid filling large voids with just mortar, instead think about packing them out with appropriately sized stones that act as aggregate thus reducing the mortar mass which will in turn aid the curing process; regardless of the type of lime used. While there are many ways that pointing can be finished we only advise that of “Fully Flush Pointing” or “Pointing to the Weathered Edge,” where the mortar is finished in such a way that allows water to run down over the face of the stone.  We never advocate or recognise recessed pointing where a ledge is created which in turn can allow easier access for water into the masonry fabric from wind driven rain.

Finishing the Joints

The best tool and method for finishing is to use the Churn Brush to strike the mortar so as to compact it within the joint in a tamping action, throwing the brush square onto the mortar, in such a manner that you compact the mortar closing up any initial shrinkage that may have occurred, thus improving the contact between mortar and masonry.  This action serves several purposes, firstly it improves the contact between the stone and mortar, it also helps clean any material that may be on the edge of the stone and finally it leaves a textured surface. A textured surface increases the surface area of the mortar and allows for greater evaporation from the mortar joint, as it opens the surface. This tamping operation can and should be further repeated at an appropriate time (12 -48 hours) while the mortar is in a suitable state to receive such treatment.

Buckland+shit+repointing

The action of finishing the mortar is extremely important for the obvious technical reasons but probably more so because of the visual impact of pointing works; this is after all what you will be displaying for many years to come.

As for the right time to carry out this task it should be done when the mortar has reached a state where it has started to set to the point that it’s “leather dry” e.g. when your thumbnail can just indent the mortar; too soon and you’ll simply smear the mortar over the masonry and too late you’ll just wear the brush out quicker.  It can be as soon as a couple of hours or as much as 48 hours, or longer during colder weather, especially with lower strength limes.

The exact time mortar will take to reach this point will vary considerably on weather conditions at the time (temperature especially), with the mortar taking longer during colder conditions and greatly accelerated during the summer.   Therefore with the explanation offered above the best advice we can provide here, is that you should wait for what we hope will be relatively apparent, but please don’t hesitate to contact us for further advice.

Aftercare

While dampening the wall is a vital process for pointing, as it helps cure the mortar there are further measures that need to be considered once the mortar has been placed.  Weather in the UK is dynamic and while there are several factors to manage the two main concerns to watch out for especially will be frost and drying out.  Most importantly, if a lime mortar is allowed to dry out too quickly (rapid moisture loss) the result will likely be a mortar that is chemically deficient and vulnerable to accelerated weathering, and one of the most common causes of mortar failures.  Lime mortars demand baby sitting in their infancy with appropriate measures adopted for good curing and appropriate to the prevailing conditions at the time they are placed.

The mortar should then be protected; this is usually achieved with Hessian sheeting. Hessian has a dual purpose, firstly it protects the mortar from the elements and secondly it helps keep the mortar damp to aid its cure. For further more specific information please refer to our guidance sheet titled “Curing Lime Based Renders & Mortars

This article is to provide general guidance and an overview of pointing, we strongly advise that you contact us for more specific information for individual applications.

COVERAGE FOR OUR LIMES

Coverage for our Non Hydraulic Limes

For Plastering/Rendering

As a Backing Coat

1 tonne of lime mortar/render will plaster/render 48m sq @10mm. A 25kg tub will cover 1.2m sq @ 10mm

Applications-For-Lime-Mortar

As a Finishing Coat

1 tonne of lime mortar/render will plaster/render 120m sq @3mm. A 25kg tub will cover 2.5 – 3.5m sq @ 3mm

For Pointing

1 tonne of lime mortar/plaster will repoint 100 -150m sq of stonework.

A 25kg tub will repoint 4m sq in standard joints.

Laying Bricks

1 tonne will lay approximately 900 bricks.

 

Coverage for our Natural Hydraulic Limes

For Plastering/Rendering

A 25kg bag will cover 8m sq @ 10mm

As a Finish Plaster

A 25kg bag will cover 26.5m sq @ 3mm

For Pointing

A 25kg bag will point 16m sq of brickwork.

A 25kg bag will repoint 10m sq of rubble.

lime-mortar

Laying Bricks

25kg will lay approximately 125 with 10mm joints.

Coverage for our Limewash

Limewash is much thinner than modern paints, as such a new lime plaster render will require at least four coats.

1 ltr of limewash will cover approximately 3-6sqm, a 15 litre tub will cover approximately 75sqm with one coat

 

Lime, Hydraulic or Non-Hydraulic

Hydraulic lime sets by hydrolysis whereas non-hydraulic lime sets by carbonation. The hydraulic lime can set underwater, that’s because hydrolysis is a reaction caused by water. The non-hydraulic lime needs air to carbonate and thus set.

In physical terms, the two materials are very different to work with. Hydraulic lime is available as a bagged powder whereas the non-hydraulic lime is a putty, hence the name fat lime putty. This makes working with hydraulic lime much easier for most builders as it’s practically the same as working with cement.

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Non-hydraulic lime is softer and sets much more slowly. The carbonation process is very slow and the material remains soft and flexible. This of course, can be extremely advantageous if that’s what is required. The hydraulic lime is available in degrees of strength, is faster setting and more durable.

Practical Considerations

With these general differences in mind, let’s consider the practical considerations.

Project

If it’s a new build then there’s an incredibly strong case for hydraulic lime. If you’re working to existing with maintenance or repair work, it’s always best to replace like with like. Technically there’s no reason that hydraulic and non-hydraulic can’t co-exist but it’s better conservation practice to keep it original.

Experience

What skills have you got on-site? Bricklayers or masons experienced in conservation work may be highly adept with either material. Less experienced workers are generally more comfortable working with hydraulic lime.

Budget

Non-hydraulic lime is considerably cheaper than hydraulic lime and could give you a good saving on large jobs. At the same time, hydraulic lime could save you a fortune on labour and lessons learned throughout the course of the project.

Timescale

If you’re on a tight schedule then hydraulic lime may be more appropriately suited. Hydraulic limes are faster setting, it is also generally accepted that non-hydraulic limes can require up to 40% more labour.

Limes & Cement in Comparison

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Lime Mortar Not Setting? A Dash of Cemen

Please don’t do this. If you’re using a lime mortar this will compromise the integrity of the mortar. Adding cement to make it set faster will undermine the benefits of a lime mortar and may negate the very purpose it was specified.

traditional-london-laying-3

Lime mortars set more slowly than a cement mortar by nature but a suitably designed lime mortar mix will set of it’s own accord. Free of cement the mortar will be flexible, breathable and more sympathetic to surrounding masonry.

It’s understandable that this happens in the midst of modern building and tough schedules. When selecting your lime mortar, consider the practical considerations. If you do need a lime mortar and time is an issue you might consider the faster setting hydraulic limes.

Hydrated Lime Mortar

When people refer to lime mortar they’re generally referring to mortar made with hydraulic lime or fat lime puttysand and no cement. However, for most modern bricklayers and builders, the only exposure they’ve had to lime products is hydrated lime.

St_Nich_IWP_067_lc

Hydrated Lime is readily available at the local builder’s merchant and can be used to increase the workability of cement mortar. It is used more widely in schools and training centres where it is mixed with sand (no cement) to make a mortar that doesn’t set, ideal for teaching brickwork.

Naturally, many people new to lime mortar assume that hydrated lime is the key ingredient, they know by experience that they’ll need cement to make it set. Unfortunately this couldn’t be further from the truth and is pretty much contrary to the use of lime mortars altogether.

True lime mortars contain hydraulic lime or fat lime putty (non-hydraulic lime). These two types of lime do indeed set with no need of cement, albeit more slowly than cement. The real goal with lime mortar is to eliminate the use of cement which is unnecessarily strong, rigid and is not breathable.

harehills church after

Natural lime mortars are flexible, allowing for movement in the building and thus preventing cracks in the masonry. Furthermore, they are breathable (vapour permeable), drawing the vapour present in the masonry back out into the air. This breathability naturally draws and damp and also helps to prevent freeze thaw action.

Technical Advice Note for Bag Rub Finish

Bag rubbing is a technique where the pointing of masonry is conducted in a manner closer to rendering, that does not (have to) extinguish the ‘character’ of the wall.

Preparation

The joints to the masonry are prepared the same as for pointing, cleaned out to an average depth of 25mm to form a key. This depth will vary according to the nature of the stonework, but should not be less than 20 mm.

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The joints are then thoroughly cleaned and washed out by use of a hose (fine spray nozzle only) or suitable pump up sprayer. This is preferably done the day before application, and again immediately before application with a fine mist spray. A single coat (following any preparation required for deeper pockets) of lime mortar is then applied fully into the joints.

The mortar should be mixed using a blend of 50/50 standard coarse and fine sand, mixed at the ratio of 2:5 hydraulic / non hydraulic lime to sand.

Bag Rubbing

Once the mortar has cured sufficiently it should then be rubbed up using hessian sacking or some other type of suitable material in order to press it home. Then after the initial “set” has taken place it is rubbed over again. The rubbing action should be carried out in such a manner as to press it well back into the recessed joint closing up any initial shrinkage that has taken place. At this point it is often scoured up with a sponge or suitable medium to achieve the desired finish.

Material consumption will be in the region of approximately 50 square metres per tonne of mortar (for guidance only). This will obviously vary depending on how and what the wall has been built from and it’s overall condition.

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Protection

Upon Completion the work should be covered over with hessian or other suitable material in order to offer protection from the elements, this should be kept damp in warm weather. The finished coating should be further dampened down by a fine mist spray as necessary, to allow the ‘render coating’ to cure naturally. Rapid drying, either by wind or sun will often result in a failure and adequate protection is therefore a must.

Bag rubbing is a very simple and cost effective technique. It can be subsequently painted (not essential) with either a lime wash or a lime paint. The overall appearance of the stonework needn’t be lost, resulting in a surface that has been unified and tidied up at a fraction of the cost of pointing or plastering. This makes a bag rub finish an extremely cost effective wall finish that was very often meant to be covered in any case. This particular approach does not require a high level of skill to execute but care and consideration will determine the outcome.

 

HOW TO APPLY LIMEWASH

Traditionally LIMEWASH was the principal finish applied externally and internally to historic buildings, quite often applied directly to the masonry or brickwork and more commonly to pre-applied lime coatings (i.e. harling, plaster, render etc;). Although often thought of as a decorative coating, the limewash was first of all a protective layer to the lime coatings and masonry substrate. On new lime renders and plasters it unifies and protects the surface particularly while strength is developing within the new plaster.

As with all lime coatings, limewash is a breathable coating allowing evaporation of moisture and water vapour. Limewash is also a repairing material, being used to fill small shrinkage cracks on the lime coverings. Limewash can also be used in conjunction with various aggregates to make shelter coats for friable masonry and will act as a sacrificial protective coat.

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In its simplest form, limewash is lime putty diluted by water to turn it into a milky consistency.

Additives for Limewash

When used externally limewash requires regular renewal, usually every two years.  The addition of a binder such as casein, tallow or linseed oil will improve its durability.

Casein is an animal deriative and can be added to a limewash to act as a water repellent and to prevent dusting.

Tallow is the refined fat of a cow, pig or sheep and serves a similar function but is more commonly found.

Both casein and tallow should be incorporated in the lime slaking process.

Linseed oil is a traditional vegetable-based additive which is used as a binder and weatherproofer.  Its water shedding properties are considerable, generally a one per cent solution is all that is required i.e. 50ml to 5 litres of wash.  It is worth remembering that although a greater quantity of additive will increase the protection against the weather, it will also inhibit moisture movement within the wall.

Using Limewash Internally

Limewash is ideal for internal use as it is unaffectd by rising damp which continually breaks down most other paints.  However, it is best not to use additives such as casein, tallow or linseed oil as this will reduce absorption and hinder the evaporation of moisture within the wall.

Plain Limewash

Plain un-coloured limewash will take on the colour of the lime used; this can range from pure white through to gray or buff coloured.

Coloured Limewash

Generally earth PIGMENTS were used to colour the limewashes, most commonly ochre’s, but also siennas and umbers, which produced a range of yellows, reds and oranges. Broadly speaking these produced pastel shades, although deeper colours are not uncommon. Coal dust, ash, blood and ground stone dust have all been found as additives in historic limewashes to achieve the desired colour

Application of Limewash – Preparation
Surfaces to be limewashed must be clean, free from grease and they must be porous. Previously limewashed surfaces must be well brushed down and any loose limewash scrapped off. Any mould should be treated with fungicide and thoroughly washed off with clean water. Do not use fungicides, which contain silicon.

Damping Down

Limewash should never be applied to a dry surface, as this will cause rapid drying out of the limewash and result in dusting. SPRAY about 3 sq/m of the surface to be limewashed with water until the surface is damp but not running with water. Do not try to damp down the whole wall or ceiling at one time, as most of the area will be dry before it can be limewashed. Dry joints must be avoided as these will result in the limewash gaining a patchwork appearance.

Application

Limewash is best applied by using a flat brush or masonry paintbrush. Stir the limewash well before and during application, apply working the wash well into the surface. The limewash should be applied in several thin coats. Avoid runs or drips running down the face of the work.

The limewash will appear transparent when first applied so care must be taken not to build up the limewash too quickly as this will craze on drying. Each coat should be allowed to dry before the next coat is applied. We recommend 24 hours between coats. It is very important to rewet the previous coat before applying the next coat. At least 4 coats will be needed to cover new work. Each coat will need to be burnished into the surface with a dry brush as it starts to ‘gel’. This will give a surface free from brush strokes and leave a unified finish.

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As earth pigments are a natural product slight variations in colour do occur. We highly recommend when ordering coloured limewash order the whole amount required plus 15%, this should then be mixed together in a large container and will avoid variation in colour over the job.

Common Mistakes and Solutions

Limewash dry but powdery: Dried too fast, spray with water and re coat with limewash. Limewash not absorbed: Unsuitable non-porous surface remove and use alternative product. Limewash patchy Insufficiently mixed, mix following coat thoroughly. Limewash dries too quickly: Remove flaky limewash and damp down the background

A practical guide to rendering and plastering with lime

Lime renders were traditionally applied to give protection to walls built of poor quality rubble stone or porous brick or to walls in exposed locations facing driving winds. They help by acting like a sponge, absorbing rainfall then allowing it to evaporate rather than soak into the wall. Most cottages and houses built of rubble stone would have been rendered originally and they tend to suffer from penetrating damp if the lime render is removed or replaced with a cement rich render.

There is a very wide range of types of lime rendering. Rubble walls of many vernacular buildings were often treated with just a single coat of render, amounting to not much more than a very full, flush pointing. Such a render is thicker in the hollows and very thin over the stone faces. There was no attempt to create a flat surface so the undulations of the wall and even some of the stones themselves were not concealed. For a smarter finish or on more prestigious buildings the aim would be for a more uniform render achieved by applying a scratch coat to fill the hollows and take up some of the unevenness followed by one or two more coats which were worked to a flatter surface. Sometimes joint lines were ruled into the damp top coat to create the illusion of ashlar stone, but a common finish for many houses and cottages was a rough-cast where the final coat consisted of a mortar slurry containing coarse grit applied by throwing from a special trowel. For interiors a fairly smooth surface could be obtained using a coarse render mix, but for top quality internal plastering the final coat would be richer in lime and polished up to a smooth, close finish.

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Renders and plasters can be applied to a variety of backgrounds including earth (which should nearly always be rendered), stone and brick. Plaster is also applied to wooden laths for ceilings and internal partitions.

By carefully selecting appropriate aggregates it is possible to match existing renders and successfully repair failed patches without the need for complete re-rendering. Hollow or detached plaster can sometimes be consolidated and saved and further advice should be sought before replacing it, especially if it is very old.

Preparing the wall
For stone and brick any hollow or decayed render should be hacked off and any loose pointing should be raked out and replaced prior to rendering. Brush the wall to remove loose material. Do not rake out pointing to provide a key. Do not use plastering bead on corners as this will give a modern appearance. Do not use chicken wire or metal lath to form a key as it can cause stress in the render due to differential thermal movements and can lead to large-scale failure, especially when it rusts.

For plastering onto existing wooden laths check that they are firmly fixed and free of lumps of old plaster. New laths should ideally be riven oak or chestnut. Sawn laths are inferior as they are smoother and weaker than those split along the natural grain of the wood. Laths should be fixed so that the distance between them is approximately 8-10mm. This allows the right amount of space for the plaster to be pushed between the laths and flop over to form a key. Do not apply preservative treatments to either old or new laths as they can introduce harmful salts into the plasterwork. Metal lath is sometimes used internally instead of timber laths as it is quicker to fix and cheaper, but it is harder to plaster onto as it is slippery and the sharp edges may cut into and weaken the plaster key. Plenty of hair in the mix is essential.

For masonry, thoroughly wet the wall with clean water using a hose-pipe or . The more porous the background the more water will be required. Allow the water to soak in a bit then spray again, and repeat until the surface layers of the wall are thoroughly damp. When the renderFor stone and brick any hollow or decayed render should be hacked off and any loose pointing should be raked out and replaced prior to rendering. Brush the wall to remove loose material. Do not rake out pointing to provide a key. Do not use plastering bead on corners as this will give a modern appearance. Do not use chicken wire or metal lath to form a key as it can cause stress in the render due to differential thermal movements and can lead to large-scale failure, especially when it rusts.

Mixing plaster and render
A basic lime render or plaster can be made using the same ingredients and mixed in the same way as a pointing mortar. A coarse sand can still give a smooth finish suitable for most vernacular buildings although the mix should be slightly richer in lime than a standard pointing mix, say one part of lime to two-and-a-half or three parts of sand. Do not be tempted to use a soft, fine sand – you will just end up with lot of cracks. The mix will need to be slightly wetter than for pointing but it should be slightly stiffer than its modern cement or gypsum counterpart. Once you start plastering you will soon discover whether your mix is the right consistency: too thick and it will be virtually impossible to apply it smoothly and get it to stick to the wall; too thin and it will go on beautifully then slump, sag or drop off.

Applying-Lime-Plaster

Whether you mix your own or buy ready mixed lime render or plaster it is a good idea to ensure that the lime putty used is at least three months old. This will ensure that the lime is thoroughly slaked. If the lime is younger than this any unslaked particles in the mix may slake some time after plastering causing a small eruption or ‘lime blow’. Many practitioners advocate the use of six month old putty for plastering, but few suppliers stock it.

When plastering onto lath it is essential to add hair to the mix to help in forming a key of plaster between the laths. Whilst it is not essential to use a haired mix for other backgrounds it can help to reduce shrinkage. Hair should be added to the mix just before use. Do not add hair to coarse stuff that is to be stored for more than about six weeks as the hair may rot if left in damp lime mortar for a long time. The hair should be gradually teased into the mix so that it is well distributed and does not form clumps (often referred to as ‘dead mice’). As a rule add 4 Kg of hair per cubic metre of mortar for work on laths. For the second coat halve the quantity of hair. To check whether there is sufficient hair in the mix, scoop a dollop of mix into a gauging trowel, tap the underside of the trowel smartly against a hard edge so that the blob flattens and the surplus mortar falls off the edge of the trowel. There should be a fringe of hair at 1-2mm intervals around the edge of the trowel.

Applying Render
There are many different ways to apply render depending on the type of finish required, the type of lime used and the preference and experience of the person applying it. The following should give you an idea of some of the common procedures but is by no means the final word.

Rendering or plastering is not something that can be easily taught without a practical demonstration, but if you have already done some plastering or can get someone to show you how, there are several guidelines that will help you to use lime render successfully.

Lime renders shrink as the water in them evaporates. This can be minimised by using a well graded aggregate, by ensuring that the wall is well wetted before you start and by applying the render in thin coats of no more than half an inch. It also helps if the mix is as dry as possible but obviously it has to be wet enough to be workable and if you observe the other points you can get away with a slightly wetter mix which is easier to use.

If there are deep hollows in the surface of the wall dub them out first using lime mortar and small bits of stone or tile, and allow this to firm up before applying the first coat of render. There is no need to try to create an absolutely smooth flat surface as on most old buildings lime render and plaster looks best if it follows the contours of the wall.

Lime renders must be applied using as much pressure as possible to force the mortar into the surface crevices or between the laths to form a close contact between mortar and backing. For masonry walls, whilst it is possible to apply render using either a gauging trowel or a plasterers trowel the best result is achieved by throwing the mortar on from a trowel. This technique ensures the best bond between the mortar and the wall, expels any air in the mix and ensures that the mortar is well compacted. If you use a float or gauging trowel it is very difficult to apply the render with equal pressure all over the wall: it will tend to be under more pressure over the high spots and under less pressure in hollows and therefore more likely to drop off. Throwing render sounds difficult but it is surprisingly easy particularly for the scratch coat or dubbing-out coat, and involves less physical effort than using a trowel. This is particularly important if you are not used to plastering on a regular basis. It doesn’t matter if the first coat goes on rather unevenly as you can remove any excess mortar by running the edge of a trowel over the surface to cut off the rough bits. Just remember to protect windows, rainwater goods and any other areas that you do not want covered with lime mortar, including yourself, particularly your eyes. If you prefer to trowel the mix on you may find it easier to apply the mortar using a gauging trowel rather than a plasterers trowel as it is better for getting into the hollows and maintaining an even pressure over the entire wall.

Once you have applied the scratch coat and got a fairly flat surface, subsequent coats can be applied successfully using a trowel, although it is still easier to throw it on.

Tyrolean rendering machines which splatter mortar onto the wall do not achieve the necessary level of compaction of the mortar as it hits the wall and are not suitable for the application of traditional lime renders.

For plastering onto wooden laths a plasterers trowel is suitable, but you must apply the plaster with enough pressure to force the mix between the laths so that it can flop over behind the laths and form the key.

Use a clean tarpaulin or sheet of polythene to protect the floor or ground along the foot of the wall. You will then be able to scoop up and re-use any mortar that doesn’t stick to the wall first time. If you simply cannot get the mortar to stick try re-wetting the wall or experiment with a slightly wetter mix.

As you apply the render do not try to smooth the surface by stroking with a steel trowel or float. Working the surface of the wet mortar with a steel tool will draw the lime to the surface creating a lime-rich layer over a weak, lime-depleted layer which can lead to premature failure of the render. Simply apply with one stroke, pushing hard or throw it on. If there are high spots or ridges hold the edge of the trowel or float against the wall and draw it across the surface. This will cut off the rough bits and leave a good open texture.

As the mortar starts to firm up it may develop cracks, although cracking will be minimised if the points in the first paragraph are observed, and hydraulic lime tends to suffer less from shrinkage and cracking. Cracks in the base coats can be left as they will not compromise the strength of the plaster and will be covered by subsequent coats. However, you must make sure that the cracks are due to shrinkage and not because the coat is peeling away from the backing; push the coat gently to check that it is firm against the backing.

Create a key for the second coat by scoring the surface in a pattern of diamonds using a lath scratcher. Do not use the edge of a trowel as this will create too fine a groove to provide a good key. Some practitioners advocate throwing on the scratch coat and leaving it rough to provide the key for the next coat, but this depends on being able to throw the mortar on reasonably evenly in the first place or it will be almost impossible to render over it if there are huge humps and hollows. It is vital to take time and trouble over creating a good key as the adhesion of subsequent coats depends on it. Do not be tempted to miss areas in awkward places or at junctions with architraving or ceilings. Do not scour or float a scratch coat on laths. as this may damage the nibs.

There are two schools of thought regarding when to apply the second coat. Some practitioners advocate applying the second and subsequent coats whilst the previous coat is still ‘green’ or ‘leather hard’, that is after it has firmed up sufficiently to resist indentation with a thumb but is still soft enough to scratch with a finger nail and is still damp.This varies according to drying conditions. For internal plaster it may be anything up to a week or so between coats. This method requires less dampening of the surface before application of the second coat and may achieve a better bond between coats. However, there is a risk that there might be further shrinkage in the base coat after the second coat has been applied, and it will take a lot longer for the undercoats to fully carbonate and achieve full strength.

The alternative approach is to allow the base coat to dry out slowly and start to carbonate. The work should be protected from drying out too quickly by covering with damp hessian for at least one week, and often for two to three weeks according to conditions. By this stage carbonation will have started (but not be very far advanced) and there should be no further shrinkage in the base coat. The base coat needs to be thoroughly dampened down before applying the next coat. This method is more dependent on a good mechanical key between the coats, and requires a longer period for completion of the work than the first method.

In the second coat cracks should be closed up by scouring the surface with a wood float using a circular movement and pushing hard to consolidate the coat. This will also enforce the bond between the coats and remove the high spots creating a flatter surface ready for the next coat. You may need to do this several times until the mortar is firm and no further cracks develop. The importance of this scouring and consolidation process for the success of lime rendering cannot be over emphasised. It must be done thoroughly no matter how wrist-breaking it may be.

Timing is also important. For non-hydraulic lime the mortar must be firm enough that the scouring will not just re-work the mortar, but not so firm that the cracks cannot be closed up. This can be anything from several hours to several days depending on conditions.

Always ensure that the previous coat is damp before applying the next one. If applying a fairly fine top coat, keying of the previous coat is best carried out using a comb scratcher or a devil float which creates a finer key than a lath scratcher. If the keying is too coarse it may well result in cracking of the top coat along the lines of the key because the render will be comparatively much thicker in these places.

The final appearance depends on the type of mortar used for the top coat and the tools used to finish it. Scouring with a wood float will result in a fairly open texture suitable for the majority of external renders on vernacular buildings. For internal plastering a combination of wood and sponge floats and a plasterers trowel can be used to create a smooth polished finish. A traditional type of external render finish for simple cottages and farmhouses is known as roughcast. A slurry of mortar containing some quite coarse particles of gravel is thrown onto the top coat of render from a dashing trowel. This creates a rough texture with a large surface area which helps in allowing the wall to breathe, but requires some expertise to avoid a porridgey mess.

The work must be protected to prevent rapid drying. Both hydraulic and non-hydraulic limes must be kept slightly damp but at the same time air must be allowed to circulate. If using polythene or tarpaulin it should be fixed so that there is a gap between it and the wall. Remember that water will drain down through the render under gravity so the upper parts of a wall will start to dry out faster than the lower parts and you may need to spray these areas more frequently. Frost can be a particular hazard to a young render as it can cause damage weeks after the render was applied, especially if preceded by heavy rainfall. It is a fallacy to believe that by using hydraulic lime or a pozzolanic additive you can “beat the frost”. It is only the initial setting which takes place by hydraulic reaction and the mortar still requires a long period of time to carbonate and strengthen. If hard frost is forecast within a couple of months of application then ideally the render should be protected by hanging polythene, bubble wrap or hessian over it, although on most buildings this may be impossible. A render which survives its first winter unscathed is more likely to wear well subsequently.

Repairing Render
If patches of render have fallen off cut back the edges to sound plaster. The best tool for this is a craft knife. You can cut back using a bolster chisel but you risk loosening adjacent plaster. Ensure that the edges of the patch are cut square to provide a key for the new mortar, and eliminate feather edging.

Where render is cracked or hollow it may be possible to consolidate and save it and further advice should be sought before hacking it off, especially if it is believed to be very old. However if it is beyond repair or not worth saving hack it off, cutting back round the edges to sound plaster.

Rake out any loose pointing in the wall behind and repoint roughly. Brush out loose dust and apply the render in thin coats following the guidance above and taking care to ensure that it is well pushed in beneath the undercut edges of existing plaster. Apply the final coat with a neat butt joint between the old and new and with a slight bulge in the middle of the patch so that the final consolidation will create a flat patch, flush with the surrounding render. Do not feather the edge of the patch over the surrounding plaster. Push back and consolidate the patch with a wood float. For small patches it is best to use miniature wooden floats which can be made by fixing a small handle to a short piece of batten and rounding off the edges with sandpaper.