Internal Lime Plastering

Tim Ratcliffe

The promotion of modern gypsum-based plasters has led to the almost complete demise of lime plastering, and of many of the traditional skills associated with the craft. This has been exacerbated by the plastering trade being divided into flat and decorative work, with new ‘fibrous plasterwork’ being made in workshops. Many youngsters entering the trade are now just taught the basic skills to enable them to stick up plasterboard and skim plaster onto it. We are told that it is all down to ‘supply and demand’; if this is the case, those of us involved in work on old buildings need to be more demanding.

There is a real need for skilled plasterers who can plaster with lime, and also turn their hands to repairing and reinstating dado and cornice mouldings in situ. The current training system works against anyone gaining this set of skills.

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An article like this cannot resolve this skills shortage, nor attempt to even describe the range of skills that a traditional plasterer should have. There are, however, some general principles which anyone involved with lime plastering should be aware of. Sadly there are too many cases of lime plasters failing because the people who have specified the work or the people carrying out the work don’t have adequate knowledge or experience.

THE RIGHT MATERIALS?

Most people using lime in old buildings have a vague understanding of the benefits of a ‘breathing’ mortar or plaster, but if they perceive that lime is too difficult to use they may decide not to bother with it. We need to make it clear that the revival of the use of lime is not some ‘airy fairy’ idea dreamt up by a bunch of idealists. On the contrary, it is driven by the realisation that buildings are suffering because they have been coated with inappropriate materials, and the people living in them may be less healthy as a result. There is compelling evidence that modern gypsum plasters encourage condensation and consequent mould growth if used on walls that are supposed to ‘breathe’. We are beginning to see a revival in the use of lime plasters and we need to encourage a revival of the skills required to use them.

Having bemoaned the lack of proper training for anyone wanting to learn traditional plastering skills, it has to be said that lime plastering is not rocket science. With a basic understanding and a willingness to learn, most plasterers can pick up the skills required to produce a reasonable job in a few days. This is not the same as the skill required to repair plaster in a fine quality country house, or the experience required to match a range of historic finishes, but these things come with time, and we can hope that as more plasterers learn to use lime they will be inspired to develop their skills and understanding further.

There are two characteristics that differentiate lime plasters from modern plastering materials. The first is that they set slowly by absorbing carbon dioxide from the air, in the presence of moisture. The second is that they will shrink as they dry.

Although hydraulic limes, which set more quickly than white/fat limes, are occasionally used for plastering in damp conditions, they are less flexible and breathable than the latter, and their use internally should generally be avoided. All the evidence on old buildings and in written documentation indicates that for centuries, if not millennia, plasterers have chosen to use white/fat limes for internal plastering.

There is some debate about whether we really need to use traditionally slaked lime putty, or if bags of dry hydrated white lime from the builders’ merchant are just as good. Although chemically they are the same (both are calcium hydroxide), a traditionally slaked lime, which has been matured for three months, will have broken down into much smaller particles and started to form crystal chains. This gives it better adhesive qualities, helping it to grip the wall more tenaciously. The difference seems to be in the maturing process; so if recently hydrated lime from a fresh bag is left to soak in water for three months, it should be as good as a traditionally slaked and matured lime. However, in practice most of us find it is easier to buy the matured stuff from a specialist supplier.

APPLICATION

Lime plaster in most buildings from the second half of the 17th century onwards was applied in three coats, which enabled a flat finish to be achieved. In agricultural buildings two coats of plaster are common, or even one single coat may be found where an undulating surface was acceptable. Similarly, pre-18th century buildings often have undulating plaster finishes, and this usually indicates that fewer than three coats were used.

Before embarking on any plastering project it is worth assessing the number of coats used originally and/or the quality of finish required. On the basis that three-coat work is the most common in historic buildings, it is best to understand how to apply this and then reduce to two or one coats where appropriate.

SCRATCH COAT

The first coat is known as the ‘scratch coat’, because the surface is scratched with lines to give a key for the next coat. The mix used is usually one part lime putty to two and a half parts of coarse, sharp, well-graded sand. If the grading of the sand includes more or less of a particular grain size the amount of lime may need to be varied slightly. An experienced plasterer will be able to tell instinctively whether another half part of lime or sand needs to be added. Another way to tell is to take a sample of the dried sand and measure the volume of water required to fill all the voids between the grains; the amount used is equal to the amount of lime required.

Hair can be added to the mix to give it tensile strength. Although this isn’t absolutely necessary when plastering onto stone or brick, its benefit in the long term is that if the building moves or any patches of plaster detach from the substrate, the hair will help bridge over any gaps. Old plaster can sound hollow in places when tapped, but is usually still sound if it contains hair. When plastering onto laths, the addition of hair becomes a necessity, because plaster does not adhere well to timber once it has dried; it relies on interlocking fingers formed as the wet plaster squeezes though the laths and slumps over, so the tensile strength imparted by the fibres is vital.

It is important, before applying new lime plaster, to ensure that it isn’t going to be sucked dry by the background it is applied to, as this will cause it to shrink and potentially fail. The suction can be reduced and controlled by wetting the substrate before applying the plaster. If the wall is very dry and porous it may need to be sprayed with a hose pipe a couple of times on the day before, and then once again on the day of application, but if it is less porous and the environment is relatively humid, spraying with a hand-held spray on the day of application may suffice. There needs to be enough moisture in the wall for it still to be damp to the touch after an hour, but no longer glistening with droplets of water.

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The scratch coat should be no thicker than 15mm (5/8 inch). Any deep recesses or holes should be ‘dubbed out’ beforehand, using a stiffer (drier) mix, and allowed to dry to avoid deep pockets in the scratch coat. If plastering onto laths it is important to apply the plaster diagonally to the line of the laths, joining up each time with the previous area laid, to achieve a consistent key between the laths.

While still wet, the surface should be scratched with a three pronged lath scratcher or a single pointed lath (which is slower but gives a better job). The scratching should be in straight lines, diagonally to the laths or the line of the wall, in both directions, to create a diamond or lattice pattern. The quality of the scratching affects the keying of the next coat, so it should be done carefully to achieve an even pattern, and, on laths, particular care should be taken not to cut through to the lathing.

The scratch coat should then be left to dry and shrink before attempting to apply the next coat. In most circumstances it will need two weeks to dry out, but can take up to four weeks in some cases. Shrinkage cracks are likely to appear as it dries, but this is not a problem. The important thing is to avoid it drying too rapidly, which can cause it to fail. Exposed areas of plaster (adjacent to open windows, for example) may need to be covered with hessian or polythene, and the use of dehumidifiers should be avoided.

Once the pad of a thumb can no longer indent the scratch coat it is ready to take the next coat. At this stage the surface should still be slightly damp to the touch and will just need brushing down to remove any loose grains and then lightly dampened with clean water, using a hand-held spray. If it has been left too long and has dried out completely, more water will be required.

FLOATING COAT

The second coat is known as the ‘floating’ or ‘straightening’ coat, and is used to bring the surface to a level plane. The mix is usually slightly less rich than that of the base coat, typically one part of lime putty to three parts of coarse, sharp, well-graded sand, and normally without any hair. Again, it should not exceed 15mm (5/8 inch) in thickness. A level surface is achieved using long ‘floating rules’ or ‘straight edges’, passed over the wet surface to remove undulations.

In the best quality work, wooden blocks (known as dots) are temporarily applied and plumbed and levelled; lines of plaster (known as screeds) then join between the dots, and are levelled using a floating rule; finally the spaces between are filled using trowels and levelled with a floating rule with its ends bearing on the screeds. This method was used in finer quality Georgian and Victorian buildings.

Once it has begun to stiffen up, the floating coat needs to be consolidated by ‘rubbing up’ the surface using a wooden float to counteract shrinkage. This is likely to be required once or twice on the day of application, and may be necessary on the following day as well. The timing depends on the speed of drying. Sprinkling the surface with water, using a brush, assists the circular rubbing action if it has dried out too much.

Once the surface has been compacted, a ‘devil float’ (wooden float with nails or screws driven through the corners to project about 2mm) is rubbed over the surface to form a key for the finish coat. It should then be left for about a week or so before it is ready for the final coat.

FINISHING COAT

The final coat is known as the ‘setting’ or ‘finishing’ coat. It is usually thinner than the other two coats and uses a finer sharp sand. The mix can vary depending on the hardness and the type of finish required; the richest mix being three parts of lime to one of fine sharp sand, and the leanest mix being one part of lime to three parts of sand. More sand will give a harder finish and is more suitable for open textured float finishes; more lime will give a softer surface but allows it to be polished smoother. For standard work, a mix of one to one is suitable.

The thickness of this coat can vary between 2mm (1/16 inch) and 5mm (3/16 inch). In order to achieve an even finish on the surface, it needs to dry out consistently, so is applied in two or three very thin layers. The additional time and labour required for this is well worthwhile because it controls the drying of the surface and allows the plasterer to achieve the finish required.

Before applying the setting coat the floating coat needs to be lightly dampened with water to control the suction. Each layer is skimmed on as thinly as possible, working in alternate directions each time, and is laid over the previous one as soon as it has had a chance to ‘steady-up’ (usually in about half an hour). When the work is firm enough it should then be scoured to compact and consolidate the surface.

A straight-grain float (that is to say, one with the grain running the length of the float) can be used for the scouring process, but if a very flat surface is required, a cross-grain float is better. The ‘cross-grain’ prevents the edges wearing down and ensures that any projections are shaved off as the float passes over the surface. Cross-grain floats are only used to rub or scour over a surface and any specification that refers to using them to apply lime plaster is clearly wrong.

Depending on what is required, the surface can then be worked over using either a trowel, to achieve a fine closed finish, or a combination of wooden and sponge floats, to create an open textured finish. Some water is likely to be required in this process, splashed on with a brush.

The rubbing and scouring process required to achieve a suitable finish can present a problem when patching up to old plaster. The first important point to note is that the exposed edge of the old plaster is likely to suck more moisture out of the new plaster than the wall itself, so more water will be required to control this suction at the edges. Once the area has been patched, particular care will be required when rubbing over the setting coat, to avoid forming an indent at the junction with the old plaster.

n some cases, where an absolutely blemish free surface is required, the setting coat mix can be gauged with Plaster of Paris (a form of gypsum) to minimise the need for scouring. However, the decision to add another material needs to be taken carefully, and the visual compatibility of the repair needs to be balanced with its technical compatibility. This brings us back to the need for traditionally skilled plasterers who understand the materials they are working with. Without these skills we are lost.

Lime Mortar for use in Today’s Restoration and New Build Projects

Why use Natural Lime Mortar

The use of natural lime mortars and renders has increased considerably in recent years compared to just a couple of decades ago. These simple, basic materials are fundamentally important to the long term survival of historic buildings, yet there are many who still regard their use with suspicion. Natural Lime Mortars are the only viable way to repair and restore much of the great architectural heritage of this country.

Invariably, our Historic Buildings, Monuments and Structures will have been constructed with the Natural Lime ‘Products of the Era’. Attempts to repair these national treasures with modern ‘Cement’ instead of a compatible Natural Lime Mortar, will frequently produce a worse result than no repair at all. These ‘Bodged Repairs’ then have to be reworked first to undo the damage inflicted by the alien chemical nature of modern cements before they can be repaired using the Natural Lime Mortar and Render that should have been used in the first place.

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If you are involved in a repair or restoration project at the early planning stage you would be well advised to contact ‘Cornish Lime’ for advice on the appropriate Mortars and Renders for the repairs being undertaken. You may also like to attend one of our regular Lime Training sessions, a one day practical and theory course covering such topics as pointing, rendering, bag rubbing and plastering.

We have an extensive portfolio of prestigious projects where our expertise, advice and Natural Lime Mortar has formed the basis for extensive repairs and restoration including The National Trust and English Heritage. You may wish to view our information on Lime Putty and Coloured Mortars.

The Use of Natural Lime Mortar

Selecting the right Lime Mortar for your particular application can appear very daunting. That is where ‘Cornish Lime’ become not only a supplier, but a source of knowledge gained over decades of successful restoration projects using the same Natural Lime Mortars that they themselves manufacture today.

In addition to the freely given advice and comprehensive literature packs available, Cornish Lime also run regular, informative training sessions on the use of Lime Mortars, Renders and Plasters including the versatility of their own Mature Natural Lime Putty.

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One example of our Training is the course entitled: “Introduction To And Working With Lime” which consists of the following content:

  • Selecting the right materials
  • Compatibility and interaction of Lime Mortars, Renders and Plasters with other materials
  • Surface preparation and suitable or necessary base coats etc
  • Mixing the Lime Mortar, Render or Plaster
  • Useable life span of Limes after mixing
  • Curing times and protection during the curing stages
  • Adequate Q & A session time for you to explore your specific thirst for knowledge
  • Comprehensive handouts to keep for your own reference
  • Building with Lime Practical ‘Hands On’ session using the appropriate materials
  • Pointing, Rendering and Plastering with Lime
  • Sands and aggregates, the role they perform and their suitability
  • Decoration

Please Contact Us to find about more about our lime training and for availability on the next course.

 

COB RENDERING

Introduction to Cob Rendering

Cob walls should always have a well maintained roof/capping and where possible the plinth should be exposed and repaired re-pointed as necessary using suitable lime mortar. Or as the old saying goes the wall should have a good hat and boots to keep the moisture out.

A cob wall in good condition in a sheltered situation does not need render coats for protection. A render finish even lime can never look as good as cob.

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Cob, chalk and earth walls are by nature of a soft friable composition.

Rendering and repairs should be carried out with lime putty mortar – MEDIUM STUFF or SINGLETON BIRCH NHL2 mortar 2.5:1.

External lime coatings should be applied in relatively thin coats, therefore any defects in the form of hollows must be corrected before the application of the render coats by either rebuilding with cob material (normally available in the immediate area) or with cob blocks available from us.

Defects should not be corrected through coats of “dubbing out” (Thick applications of render/mortar). Areas of varying thickness are prone to shrinkage, carbonation and curing problems. Careful background preparation plays a vital part in the weather resistance capability of the wall.

Materials to be used in background repairs, should, wherever possible, be matched to the existing fabric. In doing so, the repair will be compatible and produce a similar performance pattern. Where previous remedial work has taken place with unsuitable materials, (i.e. dense cement mortars or bricks), these should be removed if possible without causing more damage and repaired with matching lime mortars or chalk/chalk cob.

Cob Wall Preparation
The background must be free from dirt, grease and vegetation. These elements should be removed before repairs are underway. The removal of biological growths should be thoroughly carried out, as any remaining lichen, algae, etc; will grow back and attack the bonding between lime finishes and the background.

Cob and earth walls have a friable surface which needs to be prepared before render or plaster can be applied. The wall surface should be wet down the day before application. On the day of application it should be dampened (but not wet). A harling coat is then applied to the wall by casting vigorously from a HARLING TROWEL (or even a coal scuttle shovel) It can also be applied by mortar spray or Tyrolene gun. It should then be left for at least 4 days to set hard before the application of the first scratch coat of render.

The harling material is a combination of aggregates and lime, mixed into a slurry consistency and applied by casting vigorously at the wall.

First Coat
The first coat of lime render is applied by use of a laying on trowel or float, the coat is applied to a thickness of approximately 8-10mm thickness, coats much thicker than this will result in shrinkage cracks, in 2 coat work, this coat will need to be straightened by use of plastering straight edges, once flat the coating should be left to stiffen up, sometimes known as tightening, as this happens, the render should be compacted and compressed by scouring the render with a wood or polyurethane float, i.e., rubbing up in a circular motion. The over working of the surface should be avoided as this will draw lime to the surface. During the scouring process any shrinkage cracks should be closed, by pushing the crack back with the edge of the trowel and rubbing new material into the depression.

Once this process has been completed the surface is keyed by use of a scratcher in a diagonal pattern. The inclusion of hair or other fibres in the backing coat mixes will greatly aid the minimising of shrinkage cracks and is strongly recommended.

Second Coat
Before the application of the second coat, the first coat should be left for a minimum of 4 days, during which time the first coat should be checked for shrinkage cracks and also kept damp to avoid rapid drying out. Before applying the second coat the first coat should be damped down, making sure the water is absorbed into the render and not sitting on the surface. The second coat is applied using the same tools as the previous coat.

The coat thickness should be 8mm and no thicker, the coat is applied left to stiffen and then scoured up to the required finish, as before over scouring should be avoided. Good curing once the work has been completed is essential if shrinkage cracks are to be minimised.

After the application of lime renders, controlled curing and protection will be needed to ensure maximum strength and durability are achieved. The lime putty will stiffen initially due to absorption of moisture into the wall and evaporation to the air and will cure and harden as a result of Carbonation’, (the re-absorption of carbon dioxide).

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The NHL 2 will set more quickly as a result of a chemical reaction with the clay content known as the ‘hydraulic set’. ‘Carbonation’, (the re-absorption of carbon dioxide), also takes place. This process is best achieved in warm and moist conditions, which allows the new works to dry slowly. Therefore, during and after completion of the work, it is essential to ensure ambient conditions.

Rapid drying by the sun, wind or artificial heat will all have a detrimental effect on the final outcome of the lime finishes. Temperatures below 5ºc will slow the carbonation and hydraulic setting process and frost conditions will damage un-carbonated areas, through the action of freeze-thaw (expansion/contraction) resulting in feeble and crumbly finishes.

Excessive shrinkage is a result of rapid drying, and this can lead to separation between coats and background and cracking. Rapid drying of the surface of new mortars, can also lead to the pores of the mortar becoming blocked with fine material, transported to the surface by the passage of water evaporation too quickly from the mix, this will inhibit the carbonation process taking place deeper into the new mortar.

The best way to control and protect the carbonation process is to form a microclimate for the new work. Where the new work is scaffold, this can be a reasonably simple job. Scaffold netting is very useful for reducing the effects of wind. In addition to this in warm or hot conditions, damp hessian can be placed against the new work and then covered by sheeting to stop rapid drying. New work should be damped down for a minimum period of 10 days after completion and longer if possible. The emphasis should be on damping down as opposed to saturating new work. Provision should be made for damping down over weekends, holidays etc. In cold weather, the work must be protected from frost attack, by using thermal blankets e.g. polystyrene sheets. Hydraulic plasters/mortars will stand up to cold conditions after 3-4 weeks of hardening. It should be remembered that prolonged periods of cold temperature will slow the overall hardening process and extended periods of protection will be called for.

Any cracking occurring after the first few days of application can be remedied by scouring the surface with the wood float around the area of cracking to fill and compact the crack.

Health and Safety

Harling by its nature of application carries a risk factor, and therefore personnel should wear protective equipment, particular attention to be given to EYE AND SKIN PROTECTION. Eye wash should always be on hand. Gloves should be worn when working with any mortars or plasters.

LIME PLASTERING

Preparation of the Wall Surface

The successful application, bonding and correct hardening of hydraulic lime mortars, requires that the background should be clean, free from vegetation, free of containments and reasonably dry throughout the wall mass. The wall should be structurally sound and the masonry and bedding mortars in good condition.

The application of various dubbing out coats should not be seen as remedial repair work to the masonry or brickwork background.  The replacement of missing or damaged masonry or open joints should be repaired before any plastering of dubbing out coats are applied. Where excessive hollow or unevenness is present the areas should be corrected by use of mortar and stone or brick slips bedded into the mortar, with the aim of presenting a reasonable flat background before plastering commences.

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Suction Control and Bonding

Before the application of any new lime coatings, hydraulic or non-hydraulic, it is vitally important to check to the degree of suction within the background.  Poor or excessive suction can result in a weak bonding with the substrate caused by rapid drying of the newly applied render, which will result and a weak and powdery interface which will lead to later failure and separation. Where there is little or no suction further action will be required to help bond the coating to the substrate.

In situations where suction needs to be controlled, wetting down will be required, on dense blocks or near impervious masonry.  Simply dampening the surface with a mist spray may be all that is required, but on very porous surfaces such as old brickwork considerable wetting will be required. Wetting the wall by use of a hose, working from the top of the structure, downwards, may need to be carried out the previous day or several times throughout the day before rendering commences.

The objective of the suction control is to achieve a thoroughly damp surface, but not wet, i.e., the surface must not have running or standing water remaining on the masonry or brick, this will form a barrier between the coating and substrate, also lime mortars adhere and stiffen through a certain amount of suction.

On dense or near impervious background, it may be necessary to apply a sand/lime splatter dash coat to the background to act as a mechanical key.

Salt Contamination

Where new lime coatings are to be applied to masonry which is salt contaminated, the masonry should be allowed to dry fully before applying new renders. This will allow salt to be detected on the masonry and mortar joint surfaces, if excessive salt is identified clay or lime mortar poulticing may be required.

Specialist advice should be sought, if the technique is considered where salt is detected on the mortar joints, rake out the joints to a depth of 50mm, as this is likely to be heavily contaminated and in a weakened condition and re point (See re pointing). Salt contaminates should never be washed from the surface, as this will result in the crystallized salt returning to a soluble state and retreating back into the pores of the masonry or brick. Where detected on the masonry surface, the salts should be brushed from the surface and cleaned away from the structure

Techniques of Hydraulic Lime Plastering

The techniques employed in the application of hydraulic lime plasters should be to ensure a correct bonding with the background while striving to minimise shrinkage and rapid drying.  These techniques should be followed throughout the plastering process.

Lime plastering is generally applied in 3 coats, but it is common to find 2 coats or even single coat work in vernacular or early structures.

In 3 coat work the first coat on masonry or brickwork is generally known as the scratch coat or render coat.  This coat is applied in a coat of approximately 10mm thickness. It can be applied by use of a steel trowel or thrown onto the wall by use of a harling trowel and then flattened in by the steel trowel. When this coat has firmed up but has not gone hard, the plaster is keyed or scratched up to produce a key for the following coats. The keying up is carried out by use of a lath scratcher or similar toothed instrument and care should be taken not to cut through the plaster coat back to the background. The keying up is generally in the shape of diamonds of approximately 30mm spacing. This coat should be allowed to harden for 72 hours minimum before further coats of plaster are applied.  Before applying the second coat, the first coat should be checked for shrinkage cracks. These should be filled with plaster before proceeding with further coats. The first coat should also be brushed down to remove any dust which may have blown onto the surface. The first coat should then be damped down to ensure that the second coat is applied to a damp but not wet surface.

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This second coat is called the floating coat and is the coat which is straightened to ensure a flat and even surface.  After this coat has been straightened, the surface of this coat is scoured up with a timber or polyurethane float. The purpose of the scouring is to compact the plaster and counteract shrinkage. This process is generally required to be carried out at least twice.  During the scouring up, any small holes should be filled before the finishing coat is applied. This coat should also be allowed to dry and harden for 72 hours before applying the final coat.

After once again damping down the floating coat, the final coat is applied. This coat known as the setting coat, is applied in two layers, coat on coat, with the trowel. This coat is applied in a way similar to skimming. This material should be of a wet enough consistency to allow a long and even spread. Once the setting has been applied, it is scoured in a similar manor as the floating, to counteract shrinkage and then the surface can be left with a rubbed up rendered type finish or polished smooth with a steel trowel

General Mix Guide

Backing Coats: 1st and 2nd, 1 part NHL2 or NHL3.5 to 2 parts or 2 and a half parts SHARP WASHED SAND. The use of fibre or HAIR in these coats is recommended. Finish Coat: 1 part NHL2 1 part Fine Silica Sand Note Highly polished surfaces will not allow the passage of moisture as well as an open textured surface.

Curing Lime Based Renders & Mortars

A lime binder must be cured once it has been placed because it requires time to fully hydrate before it acquires strength and hardness. Curing is the process of keeping the mortar/render under a specific environmental condition until hydration is relatively complete.

Good curing is typically considered to use a moist environment which promotes hydration. Increased hydration lowers permeability and increases strength, resulting in a higher quality material.  Allowing the mortar/render/concrete surface to dry out excessively can result in tensile stresses. The still-hydrating interior cannot withstand these stresses, causing the mortar/render/concrete to crack.

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Protecting Lime Renders

Standard practice for protecting lime renders would be in the form of Hessian sheeting draped over the scaffold in relatively close proximity to the render. This should be left in place for at least a week. This is a standard requirement for any kind of rendering and one that is regrettably seldom practiced. Once again it’s very important for the hydration/hardening phase of the binder.

Cornish Lime stock four grades of Hessian; 229, 273, 320 & 360 GSM (Grammes per Square) Metre where the weight is relevant to the weave and amount of fabric used per square metre. The most commonly used for curing is the 229 & 273 GSM, the heavier fabric is more generally used for frost protection.

When appropriate we would advise the addition of a proprietary polypropylene or fiberglass reinforcing fibres added to the mix as an aid to control shrinkage cracking in the base coats.

Issues

1, Drying Too Quickly

Where a render is allowed to dry out too quickly hydration and carbonation of the binder is inhibited, resulting in drying shrinkage. There are two principle types of drying problems both of which will be manifested as cracks. The first, plastic shrinkage is the consequence of the rapid evaporation of mixing water from the mix (while in its plastic state). This leads to increased tensile stresses at a time when it has not gained sufficient strength. Plastic shrinkage cracks will be manifested in the first 48 hours.

The second, drying shrinkage is from the effects of climatic conditions such as wind and high temperatures or exposure to strong sunlight (compounded during times of low humidity).  Cracking from drying shrinkage tend to take that much longer to manifest but the outcome is much the same. Another consequence of rapid drying is that the mortar may become friable.

2, Excessive Water

The consequences of too much water in a mix can compound the plastic shrinkage, which as previously mentioned is likely to be manifested in the first few days following application. Water in a mix takes up volume and is given up during the hydration process

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3, Moving On Too Quickly

The consequences of applying subsequent coats of render coats to soon onto the previous coat may result in stress cracking as a result of unequal contraction (differential drying) between the two layers. We would advise that the backing coat should be allowed to achieve a sufficient set prior to applying additional coats.

4, Thick Top Coats

The application of excessively thick top coats can result in stress fracturing in the coat as a result of unequal compaction when finishing the render coat.  The purpose of floating (rubbing up) is primarily decorative, however it performs a technical function in that by closing the surface will help reduce the Ingress of water. Also where a top coat is too thick it will be extremely difficult, often impossible, to compress the whole thickness to an adequate level.

As well as supplying Hessian, Cornish Lime also supply a Wintermix product as part of our Cornerstone range. Please Contact Us for further information on this product.

Adding Hair or Fibres to Lime Mortar

Hair or Fibres have traditionally been used as reinforcement in lime renders as a means of improving tensile strength and to reduce shrinkage cracking. Introduced into the mix by teasing the hair into the mix prior to the mortars use. This is a time-consuming job as it involves teasing the hair into the mix as it is mixed. Clumps of hair simply cannot be placed into the mixer as they will remain in the mix as large balls of hair and will not part no matter what type or method of mixing is used.

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The use of hair in renders is most definitely advised on walls with a lath background. Hair should be added to the first two coats but not the final setting coat. As for its use on a solid masonry background this is somewhat questionable (in our opinion) but does act as a good reinforcement by reducing the amount of shrinkage cracking in thick coats of render.

Cornish Lime supply a range of types of hair along with polypropylene / flock fibres and can give guidance on the quantities needed. We also supply CLM28F, a ready-mixed lime putty mortar with the addition of fibres included.

Fibres Vs Animal Hair
Fibres work nearly as well as natural animal hair but are cheaper and far easier to introduce into the mix. Rather than teasing in as for hair, simply sprinkle the fibres into the mix. This then requires only a few minutes mixing for them to be thoroughly distributed.

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The hair used would be any animal hair, rough in texture from goats, cows, horses (body hair not the mane or tail) amongst others. Evidence of other types of reinforcement has been found which include hemp or jute fibres among others, but the use of hair is fairly universal for this role.

Hair is indeed ‘the’ traditional and often preferred choice with fibres as a very modern yet ‘honest’ approach to a centuries-old practice.

How to Apply a Three Coat Lime Render

A guide by Cornish Lime, using NHL or Lime Putty

There are a number of different substrates you could be working with, from a simple masonry wall to a timber lath substrate, and we have tried to keep the following guide as generic as possible.

The following guide applies equally to both NHL and Lime Putty renders, and Cornish Lime stock an extensive range of ready-mixed base coat and top coat renders to suit all applications, supplied as both NHL drymix and lime putty. We also supply premium quality lime putty plasters for fine plastering work.

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Preparation is key

As with most things in life sufficient preparation is key, and when carrying out any type of rendering making sure the surface is thoroughly cleaned and free of all dust or debris is of paramount importance.

Also ensure the surface is not too smooth and, if so, first score or roughen the surface sufficiently to provide a good key for the first coat to adhere to.

Avoiding the pitfalls of lime rendering

Lime renders can be temperamental and do require due care during their application and their infancy, and can fail from excess shrinkage, drying back too quickly, or weather damage during the early stages of their set.

However, applied properly, they will provide both protection and decoration to virtually any structure.

Failure can usually be avoided through basic preparation and, when necessary, sheltering from poor weather. Simple wetting tests, observation and planning at the outset is also strongly recommended.

If in any doubt contact us.

Solutions to common problems

Shrinkage – as initial shrinkage takes place in the drying out phase, this can be beaten back by using a plasterers’ float and dampening the wall as required – pressing the float home evenly, and in a close circular motion but only if necessary.

Drying out too quickly – Lime renders should never be allowed to dry too quickly, and a render that is simply allowed to dry out too quickly is more than likely to fail.

There is a vast difference between a render that has been allowed to carbonate and one that was simply allowed to “dry out too quickly”.

Pre-wetting the surface

To better control potential shrinkage, we highly recommend pre-wetting the surface to avoid moisture being drawn out of the render coat and into the substrate. Try to avoid over-wetting – pump-up garden sprayers are well suited for this purpose, as a hosepipe will deliver too much water in most cases. In the case of very porous materials such as cob, chalks, and clunch, along with different types of soft brick or stone, the use of a hosepipe may indeed be appropriate.

Weather permitting

You should also pay attention to the weather, as strong sun, wind, frost and rain will all have a bearing on the overall performance of a long-lasting, defect-free lime render.

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Work needs to be kept dry enough to allow the lime enough time to set, but do not allow it too dry back too quickly. Try to shield work from direct or wind-driven rain, and where necessary use hessian curtains to stop the work drying out too quickly from wind or strong sun. It is also very important to avoid frosty conditions during the render’s early set, particularly within the first 14 days.

Filling large voids

As lime mortar is more expensive than the stone usually to hand, you can pack out large voids or hollows with a combination of lime mortar and stone.

Scoring / Scratching in

Once the INITIAL set has taken place, key the wall using a convenient tool to make a groove in the render of sufficient depth that will allow the subsequent coat something to grab, or hang on to, without over scoring or tearing the backing coat.

Diamond keying is recommended for scratching in, and a three-pronged lath scratcher is a simple tool to knock up.

HOW TO LIME PLASTER ONTO LATHS

Lath and plaster is a building technique used mainly for interior walls in period buildings. A lath is a narrow strip of wood approximately 2 inches wide which is nailed horizontally to each stud in the frame. Each lath is spaced approximately 1/4inch ( thickness of the average little finger ) away from the next leaving enough space for the mortar to push through and hook over the lath to form a key laths.

lath-plastering

Preparation

The LATHS must be free from dirt, grease and vegetation. These elements should be removed before damaged laths are repaired. Spray a clean cold water mist on to the laths prior to applying the scratch coat.

First Coat

Using a ratio of 1 part LIME PUTTY to 2 and a half or 3 parts SHARP WASHED SAND with evenly distributed HORSE HAIR throughout the mortar and working to a layer of 10mm thick, push the plaster into and across the pre wet laths at a 45 degree angle to the laths.The plaster should be left until set hard but protected from heat and draughts.

Second Coat

The second coat also known as the intermediate or float coat will be applied similar to the first coat only without any hair being added to the mortar. You must ensure you wet down the first coat with a fine mist before spreading a 7mm coat, before scratching up and leaving in preparation of the finish coat.

Finish Coat

For the final top coat use a fine mix of 1 part LIME PUTTY to 2 parts well graded super FINE SAND. Remembering to wet down the surface before application with a fine mist spray. Allow time for the water to absorb into the plaster then apply a coat of approximately 5mm. Rule off and leave for a couple of hours. When ready rub up the finish coat using a plastic or wooden trowel to bring back the fat until the required finish has been reached.  In the case of any light crazing this can be rubbed up and trowelled out.

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Health and Safety

Lime mortar by its nature of application carries a risk factor, and therefore personnel should wear protective equipment, particular attention to be given to eye and skin protection. EYEWASH should always be on hand. GLOVES should be worn when working with any mortars or plasters.

The Benefits of Lime Mortar

Natural lime was used traditionally as a binder in mortars, renders and plasters up until the 19th century when Ordinary Portland Cement was introduced. Ironically, modern insight has revealed enormous benefits of natural lime over the modern cements that dominate the building industry today.

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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.

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.

Environmental

It is more eco-friendly, the lime manufacturing process produces less carbon dioxide than ordinary cement. Furthermore, it re-absorbs carbon dioxide thus lowering its carbon footprint even further.

Aesthetic

Attractive traditional appearance, pale in colour. Natural limes tend to reveal the colour and characteristics of the aggregate with which they’re mixed.