ABOUT RENDER

Renders are used to provide a durable and visually acceptable skin to sound but unattractive construction. Renders can reduce rain penetration and maintain the thermal insulation of walls. It is essential to ensure good adhesion to the background.

Where a good mechanical key, such as raked-out brickwork joints, is not present, an initial stipple coat of sand, cement, water and appropriate bonding agent (eg styrene-butadiene-rubber) is required to create a key for the render. Bonding is also affected by the suction or absorbency of the background; where suction is very high, walls may be lightly wetted before the rendering is applied. Metal lathing may be used over timber, steel or friable masonry to give a sound background for the render. Two or three coats of rendering are normally applied; in either case the successive coats are weaker by a reduction in thickness or strength of the mix.

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Smooth renders are not recommended for external work, as they tend to craze, particularly if finished off with a steel rather than wooden float. Generally, permeable renders are more durable than dense impermeable renders, as the latter may suffer cracking and subsequent localised water penetration. Sands for external renders should be sharp rather than soft.

The design detailing of rendering is important to ensure durability. The top edges of rendering should be protected from the ingress of water by flashings, copings or eaves details. Render should stop above damp-proof course level and formed into a drip with an appropriate edging bead. Rainwater run-off from sills and opening heads should be shed away from the render to prevent excessive water absorption at these points, which would lead to deterioration and detachment of the rendering. Types of render include:

Roughcast render. Roughcast render consists of a wet mix of cement (1 part), lime (half part), sand (3 parts) and 5-15mm shingle or crushed stone (1 and a half parts). It is applied to walls by throwing from a hand scoop.

Dry-dash render. A 10mm coat of cement (1 part), lime (1 part) and sand (5 parts) is applied to the wall and while it is still wet, calcined flint, spar or shingle is thrown onto the surface and tamped in with a wooden float.

Scraped finish. A final coat of cement (1 part), lime (2 parts) and sand (9 parts) is applied and allowed to set for a few hours, prior to scraping with a rough edge (eg saw blade) to remove the surface material. After the render has been scraped the surface is lightly brushed over to remove loose material.

Tyrolean finish. For a Tyrolean finish render, cement mortar is spattered onto the wall surface from a hand-operated machine. Coloured mixes may be used.

Painted rendered finishes. Most renderings do not necessarily need painting. However, smooth render is frequently painted with masonry paint to reduce moisture absorption and to give colour. Once painted, rendered walls will need painting at regular intervals.

LEADING UK RENDER SUPPLIERS

Sto Render is a leading supplier of render. For example their Rend Cote render is coloured throughout the render, and does not need maintenance. It is a factory batched, modified levelling coat offering consistent mix for reliable levelling. Sto finishes are designed to offer protection against weather, but to allow vapour to be transmitted through the render. The Rend Cote system is resistant to surface cracking, and does not require any decorative painting.

Decopierre, which started in Montpellier, France, specialises in interior and exterior wall decoration. Its product is a made from a mixture of limestone and powdered marble, coloured with natural pigments. This produces a finish which closely resembles natural stone. The coating is projected onto the wall surface, and is then carved by skilled Decopierre plasterers into the required shape and form. Examples include Devonian sandstone and Scottish potpourri.

Click here for an easy-to-use listing of leading UK suppliers of render, provided by Archbone.

Other useful links relevant to renders and render suppliers:
Gypsum Products Development Association

DETAILED INFORMATION RESOURCES ON RENDER

We recommend the following websites, with building product information including manufacturers' specs, standard specifications, and CAD details: Barbour Compendium, RIBA Product Selector Plus, ASC Bricsnet, Building Products Index, British Board of Agrément, British Standards Online.

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