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 are 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 topcoat 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  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.

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 materials. 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 a large-scale failure, especially when it rusts.

For plastering onto existing wooden laths check that they are firmly fixed and free from lumps and 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 8-10 mm. 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 hosepipe. 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.

Apply and render

There are many different ways of applying render depending on the types 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 common procedures but it is by no means a final word.

Rendering or plastering is not something that can be easily taught without 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 always 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 plasterer’s trowel the best results are 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 scratch coat or dubbing-out coat, and involves less physical effort than using a trowel. This is particularly important if you’re 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 plasterer’s 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 easier to throw it on.

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

For plastering onto wooden laths a plasterer’s 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 a 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 reuse 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 experimenting 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 the steel tool will draw the lime to the surface creating a lime-rich layer over a lime depleted layer which can lead to permanent failure of the render. Simply apply with one stroke, pushing hard or throwing  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 of 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 with horizontal lines using a lath scratcher. Do not use the edge of the 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 a 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 is a huge hump 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 is firmed up sufficiently to resist indentation with a thumb but is still soft enough to scratch with a fingernail 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 the 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 the 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 from 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 the conditions.

Always ensure that the previous coat is damp before applying the next one. If applying a fairly fine topcoat, keying of the previous coat is best carried out using a comb scratcher or a devil float which creates a finer key than the lath scratcher. If the keying is too coarse it may well result in cracking of the topcoat 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 topcoat 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 plasterer’s trowel can be used to create a smooth polished finish. A traditional type of external render finish for simple cottages and farmhouses is known is roughcast. A slurry or mortar containing some quite coarse particles of gravel is thrown onto the topcoat 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 that the upper parts of the 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.

Case Study: Hard external render to a domestic dwelling circa 1800

Discussion document.

Hard external cement render applied to a dwelling circa 1800 constructed of solid random rubble stone in lime mortar can prove to be problematic.

It is not uncommon for cracking to occur on the external face with the result that moisture enters the substrate through the cracks and cannot readily evaporate, because this type of external wall finish and when painted with an external stone paint is virtually waterproof, with the result that the stone substrate maintains a high moisture content. This phenomenon can be proved by the condition of the external paint. If the paint remains in very good condition it means that it is indeed waterproof and cannot be pushed off the face of the wall from the inside.

This type of building benefits from external walls having the ability to breathe. In extreme cases with exposed external walls facing prevailing weather, other forms of protection must be considered i.e. Hung tiles or natural slate or similar. Further consideration may be given to a membrane system, fixed on the external wall which has the benefit of ventilation top and bottom and finished with lime/cement render.

If internal dampproofing is required, where the outside ground level is much higher than the internal floors, consideration can be directed towards a cavity drain system (membrane). If this type of system is employed on the internal face of the external walls, together with a chemical DPC at least 150 mm above the outside ground level, it is essential that the external face of the walls is allowed to breathe naturally.

In extreme cases, if the inside and outside of the wall is provided with waterproof hard cement render and the wall floor joint is sealed, the result can often be that moisture will rise to at least the level of the first-floor joist, putting the joist ends at risk of rotting

It is therefore essential to install a chemical injected damp proof course and remove the external render to prevent bridging and to allow the wall to breathe. It is generally accepted that all hard cement render applied to the external walls should be removed and re-rendered with a softer lime-based render. In this case, great care must be taken during the application to ensure that the lime-based render is not allowed to dry too quickly during the application and during curing.

Care should also be taken to ensure that the area of external wall finish below the DPC line is not covered with a render coat and is left exposed to allow the wall to continuously breathe.

The brick or stone finish can be cleaned and brushed to remove all loose material and repointed with a lime based mortar.

Case study. December 2014

DSC_2092

We took instructions from an owner/occupier wishing to sell a Grade II listed property in the centre of Taunton.

We were instructed to carry out tests on the ground floor walls because a prospective purchaser had previously invited a contractor to carry out a damp inspection on the ground floor walls before the purchase commenced. The contractor’s report stated that rising damp was detected using a conductivity meter (damp meter). The contractor suggested that the rising damp was due to the absence of a physical damp proof course and had therefore recommended the removal of the internal plaster to at least 1.2M high, followed by a chemical injected damp proof course and the subsequent re-plastering with hard sand cement render.

As Independent Specialist Surveyors, we have a variety of tools at our disposal to identify dampness in properties. One such tool is the Speedy Carbide Test (SCT). The SCT is an item of apparatus used by specialist surveyors to determine the moisture content within the walls and is an accurate diagnosis of dampness or otherwise within the property.

The SCT has a greater degree of accuracy which includes taking a sample of masonry, as compared to the Protimeter Moister Meter (conductivity meter) which will only give an indication of moisture on the surface of the walls and could be confused with condensation. The apparatus itself contains a sealed vessel that is thoroughly cleaned before each test is carried out. This prevents any discrepancies with the result that it provides an accurate reading. We drilled a number of holes in the ground floor walls with a slow percussion masonry drill to get samples to test the moisture content. The samples were weighed using accurate scales and poured into the flask standing vertically. Once the sample was placed carefully within the flask, calcium carbide was measured and placed into the lid of the flask, ensuring the two substances did not mix. The lid was then securely placed on the flask, still being held vertically and then shaken vigorously to mix the substance together. By mixing the sample with the calcium carbide, any moisture within the sample reacts with the calcium carbide and releases an acetylene gas. This gas builds up within the flask turning the dial at the bottom of the flask, allowing the surveyor to calculate the percentage of moisture within the sample. If the total moisture content reading is above 4% in traditional brickwork within the walls, it would suggest that the moisture content is higher than that which could be regarded as dry. It must be stressed that these figures are dependent on the porosity of the brick substrate. In this case, the reading was very much lower than 4% and it was our recommendation that remedial works suggested by the contractor were not necessary. We were told that this had saved the client many thousands of pounds during the final negotiation.