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Last edited 06 Jan 2020
The main components of renders are cement, sand, lime and water. A common mix ratio used for rendering is 6 parts sand, 1 part cement and 1 part lime. Any general purpose cement can be used, although the sand should be fine and clean of impurities. Coarser sand is usually used as the base layer and slightly finer sand for the top layer.
Additives can be included in the mix that help with bonding or slowing the drying process, as well as pigments that alter the colour depending on the aesthetic requirements. By adding lime the render can be given a smoother texture, increasing its elasticity and making it less likely to crack after drying.
Render is usually mixed with a cement mixer which ensures an even mix with the addition of the correct amount of water. Render is then applied using a trowel in thin, smooth coats. A thin finishing top coat or finishing wash can be applied to achieve decorative effects. Depending on the surface texture finish required, top coats may be applied using a trowel, brush or sponge. There is considerable skill involved in achieving different textures, finishing styles and decorative effects.
Acrylic, pre-mixed rendering products have the advantage of superior water resistance and strength, allowing application to more challenging surfaces, such as concrete, cement sheeting or cladding such as expanded Polystyrene. Some types can also be applied using special spray equipment. Whilst traditional renders can take up to 28 days to fully set, acrylic renders can set in 24-48 hours.
A lime mortar (in which lime is used rather than cement) may be used for rendering older buildings, particularly if lime mortar was used in the original construction. This has greater flexibility and so is less likely to crack, and is ‘breathable’ allowing moisture to escape from the wall, but it takes longer to dry
A more modern render which combines all of the above properties is a silicone enhanced render which is water repellant but where an acrylic render isn't breathable a silicone render is breathable like a lime mortar. In a range of colours and with its excellent breathable and water repellant properties, this is becoming a render of choice.
Building regulations may apply depending on the extent of the rendering work to external walls. The Planning Portal states that where 25% or more of an external wall is re-rendered, regulations will normally apply.
 Related articles on Designing Buildings Wiki
- Brick veneer.
- Cement mortar.
- Lath and plaster.
- Lime mortar.
- Parge coat.
- Polished plaster.
- Roughcast and pebbledash.
- Wet trades.
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