Choosing and maintaining tools for a lime render
You are about to apply a 15 mm two-coat lime render to a 40 m2 external wall of a heritage building. The weather forecast is 4 degrees C overnight with showers. Your tool kit includes a steel trowel, a plastic trowel, a wooden float, a polyurethane float, a stainless steel hawk, and a mixing paddle. The lime mortar is supplied in 25 kg bags and must be mixed with a measured amount of water. A colleague suggests adding a small amount of cement to speed up set because of the cold weather.
1. Which tools are appropriate for this lime render, and how should they be maintained between coats?
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A wooden float is generally preferred for finishing lime because it opens the surface and helps carbonation, while a plastic or polyurethane float can be used for rubbing back. A stainless steel hawk and trowel are suitable for applying and laying on, but a steel trowel should not be used to burnish lime as it can close the surface and trap moisture. Tools should be cleaned with water after each mix and kept free of set mortar, because lumps of set material will drag and cause defects. The mixing paddle should be cleaned before the next batch to avoid contamination.
2. Evaluate the proposal to add cement to the mix.
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Adding cement changes the mix design and is not permitted unless specified. It makes the render harder and less vapour-permeable, which can trap moisture in a heritage wall and cause failure. It also changes the colour and working properties. The cold weather should be managed by protecting the work, not by altering the mix. The correct action is to follow the specified mix, protect the render from frost and rain with sheeting, and monitor temperatures. Any change to the mix should be agreed in writing by the specifier.
3. What protection measures are needed given the forecast?
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At 4 degrees C overnight, frost protection is essential. Cover the wall with breathable sheeting or hessian and check that it does not touch the wet render. Keep bags of lime off the ground and dry. Avoid applying render if frost is likely before it has set. Provide temporary protection from showers so rain does not wash out the surface. Record temperatures and protection measures so there is evidence if a defect is later alleged.