External Lime Rendering / Plastering

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

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Preparation is essential :‍

As with most things in life sufficient preparation is key, 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.

Where natural weathering or incomplete repair works have previously been carried out, new repair work will be required to correct these defects. In masonry the natural weathering process can result in the loss of small stones, known as pinnings, which are traditionally placed into wide bed joints between larger stones, where these are missing their replacement should be carried out with any general repointing work which needs to be undertaken. Read our lime repointing blog here.

Where walls are covered in vegetation, lichen or moss, these should be removed, by use of cutting stems, treatment with biocides and eventual cleaning down with brushes, any remaining biological growth can retain water and may in time grow back through the new lime coatings.

The application of various coats of hydraulic lime should not be seen as remedial repair works to the masonry background, the replacement of loose or defective mortar, replacing missing stone pinnings, repairing damaged brickwork or stonework are distinct separate operations and there eventual outcome should be to present a reasonably flat and even surface, which is structurally sound and clean ready for the rendering application.

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.

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Suction Control and Bonding:‍

Before the application of any new lime coatings it is vitally important to check the degree of suction within the background. Poor or excessive suction can result in a weak bonding with the substrate caused by rapid de-maturing of the newly applied render. This will result in a weak and powdery interface which could lead to later failure and separation.

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.

On very porous surfaces such as old brickwork considerable wetting will be required. Wet the wall with a hose, working from the top of the structure, downwards, this may need to be carried out the previous day and several times on the day before rendering commences depending on the substrate.

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 could form a barrier between the coating and substrate.

A certain amount of suction is required for lime mortars to adhere and stiffen so the exact amount of wetting down is decided by the plasterer’s experience.

Weather:

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.

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.

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Render Application: ‍

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.

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First coat: ‍

Lime rendering 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 at a maximum thickness of 10mm and is 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 the 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, and these should be filled with plaster before proceeding with further coats. The first coat should be brushed down to remove any dust, which may have blown onto the surface.
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It is of the utmost importance that an adequate set has taken place in the base coat. To follow on too soon with subsequent coats will result in much greater shrinkage problems, as the individual layers will shrink back at differing rates.

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Second coat: ‍

This second coat is called the floating coat and is the coat, which is straightened to ensure a flat and even surface. Once sufficiently set the render should be rubbed up with a normal float and finished with a devil float to slightly score, forming a key for the topcoat of plaster.

The second coat should be treated the same as the first, and applied before the first coat has developed too much of a set. In normal conditions this should be about one week, but there is no hard and fast rule to the time it may take; Surfaces that are very damp will take longer to harden up.

When the 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, and these should be filled with plaster before proceeding with further coats. The first coat should be brushed down to remove any dust, which may have blown onto the surface.

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Final coat:‍

The final coat is treated much the same as the previous coats, assuming any straightening required has been carried out prior to this point.

Where possible the texture of the finished surface should be left slightly open or coarse, as this will allow better evaporation of moisture from the surface, the finished work should not be closed or “trowelled” up with a steel trowel.
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Once the surface has been laid, avoid rubbing up the work too soon, leaving it for as long as is practically possible.
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Top coat plasters will normally have a greater lime content and use a finer sand, so will be more prone to shrinkage problems. Working on lime mortars too soon results in free lime being pulled to the surface (Case Hardening), which affects the properties of the material and can sometimes lead to failure.

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Day Joints‍

Where day joints are unavoidable they should be hidden wherever possible behind down- pipe runs or over the shortest possible areas. When working with Roundtower NHL2 or NHL3.5 mortars the reworking of the joint is usually achievable the next day by lightly spraying the surface with water and rubbing up the joint with a wooden or polyurethane float, if this process proves difficult cut away approximately 50mm of the previous days render, it is this perimeter zone which is most open to the air and will therefore dry first.

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