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This is your very first post. Click the Edit link to modify or delete it, or start a new post. If you like, use this post to tell readers why you started this blog and what you plan to do with it.
Hydraulic lime is available in three different grades; NHL 2, NHL 3.5 and NHL 5. The NHL stands for Natural Hydraulic Lime. The number relates to compressive strength in N/mm² (Newtons per square millimetre).
These grades are traditionally associated with the terms feebly hydraulic, moderately hydraulic and eminently hyrdraulic. These terms refer to each grade’s degree of hydraulicity, that is their ability to set under water without exposure to air.
The beauty of NHL is that each grade is suitable for different kinds of masonry and weather exposure. This means you can select a mortar or render that is sympathetic to the masonry but with a maximum durability, the perfect mortar for the job.
Soft, Permeable Masonry
NHL 2 is softer and slow setting, suitable for internal applications or where conservation is a primary concern with soft or deteriorating stones and bricks
NHL 2, Typical Uses:
Hydraulic lime plaster, chalky stones, soft, crumbling, badly worn, flakey or scaling sandstone, limestone or bricks. Note: NHL 3.5 will still be a better choice for internal walls, etc
Medium Density Masonry
NHL 3.5 is for general building, suitable for moderately permeable masonry materials. Basically, if you don’t need to use NHL 2 or NHL 5 then use this.
NHL 3.5, Typical Uses:
Bricks, facings, commons, blockwork, sandstone, limestone, terracotta, internal & external building work, cavity & solid wall construction, bedding, pointing, re-pointing
Dense, Impermeable Masonry
NHL 5 is stronger and faster setting, more suitable for dense, durable or impermeable materials with severe exposure to weather or water.
NHL 5, Typical Uses:
Granite, engineering brick, basalt, flint, paving, roofing, chimneys, parapets, cornice, balustrades, copings, plinths, bridges, harbours, marinas, canals, dams, sea-fronts
Things to consider
Remember when choosing lime, strongest is not always best, mortars should be expendible.
It is important that the mortar is softer than the masonry. A softer hydraulic lime mortar will absorb and evaporate the moisture from the surrounding masonry, thus allowing it to breathe. It will also accomodate movement and thermal expansion and will not damage the masonry by wearing it away over time.
This doesn’t mean that softest is always best either though.
It is also important that the mortar is strong enough to last. If the mortar is too soft then it won’t hold up to the elements or the surrounding masonry. You need to consider the weather conditions and degree of exposure too. The mortar should be as soft as it needs to be, but no softer.
So in a nutshell, don’t just choose the hardest or the softest grade of hydraulic lime, select the most suitable one. Our specialist team can help you make this choice, just make an enquiry We have tons of practical experience and we’re here to help.
Non-hydraulic lime or as it’s also known, fat lime putty is, as the name suggests, a white, putty like material with a fairly solid consistency. It is relatively simple to work with but there’s two fundamental things you’ll need to know before you get started. That is knocking up & cherishing.
As fat lime putty is not a powdered lime, it can’t be mixed with sand and water in the conventional way you’d make mortar, render or plaster. It needs to be knocked up. Knocking up is the process of kneading, beating, ramming and chopping the materials together with a wooden stick or batten.
If you’re thinking about making dough right now then you’re thinking along the right lines. The great thing about non-hydraulic lime mortar, render and plaster is that it can be knocked up time and again. This gives it a pretty much indefinite shelf life.
Once knocked up, moisture in the material will evaporate, it will harden after a while. You’ll be able to knock up again to re-introduce workability, only the smallest amount of water is added. Just remember that the lime or any mortar should be kept sealed, air-tight in a container when not in use. Non-hydraulic lime sets by carbonation, this happens when exposed to the air, it can’t be knocked up again once set.
Cherishing is the process of moisture control undertaken during the curing process of mortar, render or plaster. Fat or high calcium limes set by carbonation which make them particularly prone to shrinkage. Controlling moisture is critical within the first few days of mortar placement.
As a first step, before the placement of any mortar, render or plaster, it is essential that the masonry substrate is wet (but not saturated). When placed, slow drying or wet curing will help prevent shrinkage cracks developing and will also help improve the pore structure.
When fat or high calcium limes dry, carbonation will occur at the margin of wet and dry material present. The wet curing effectively slows and stabilises this process. As the mortar dies out it should be sprayed periodically with a fine mist sprayer at intervals determined by the weather conditions on site but generally anywhere from once a day to every 4 hours.
Remember that the work should be screened from the sun and wind to prevent rapid drying or from rain and frost to prevent damage during the drying cycle. This can be achieved using screening materials such as mortar fleece, hessian, plastic sheets or tarpaulins.
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.
Hydraulic lime is fast setting, it can be used on-site just as efficiently as modern cement.
It sets by hydrolysis, this means it will also set underwater.
Excellent for marine construction or masonry exposed to severe weather conditions.
Shrinkage cracks in mortar and render are virtually eliminated due to its hydraulic setting characteristics.
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.
It has a low modulus of elasticity.
This means it is extremely flexible and allows for movement and thermal expansion.
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.
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.
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.
Hydraulic lime mortars are indistinguishable from non-hydraulic limes.
They carry the same chamring, traditional appeareance.
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.
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.
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.
Interstitial condensation is when warm, moist air penetrates a material and reaches its dew point, condensing on a surface.
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.
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 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
The Cornerstone Newbuild Mortar range offers a variety breathable mortars for both internal and external use in modern construction
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 Non Hydraulic Limes
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
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
1 tonne of lime mortar/plaster will repoint 100 -150m sq of stonework.
A 25kg tub will repoint 4m sq in standard joints.
1 tonne will lay approximately 900 bricks.
Coverage for our Natural Hydraulic Limes
A 25kg bag will cover 8m sq @ 10mm
As a Finish Plaster
A 25kg bag will cover 26.5m sq @ 3mm
A 25kg bag will point 16m sq of brickwork.
A 25kg bag will repoint 10m sq of rubble.
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
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.
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.
With these general differences in mind, let’s consider the practical considerations.
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.
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.
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.
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.
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.
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.
When people refer to lime mortar they’re generally referring to mortar made with hydraulic lime or fat lime putty, sand and no cement. However, for most modern bricklayers and builders, the only exposure they’ve had to lime products is hydrated lime.
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.
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.
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.
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.
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.
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.
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.