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Exterior wall Render info

Here is some handy information about exterior wall renders; their differing types, and typical uses. Renders are used to provide a durable and visually acceptable skin to sound but unattractive construction such as “breeze” blocks or poor quality stone or brick. In the short term, render may reduce rain penetration whilst maintaining a certain degree of thermal insulation of external walls.

If a render is to last for many years, it is essential to ensure good adhesion to the background surface or substrate.

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 can be lightly wetted before the rendering is applied. Expanded metal lathing or EML, 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 by the builder or plasterer; in either case the successive coats are weaker by a reduction in thickness or strength of the mix and skill is required to get the right thickness.

Very 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. It is important not to rely on uncoated or untreated render to last as experience has shown that render that allows moisture content within, will eventually crack due to the moisture freezing in cold weather, expanding and then pushing the render coat away from the wall. An ideal way to eliminate the potential of this is to have a “Never paint again” exterior spray applied masonry coating.

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.

Some rendered walls 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 unless you choose a long-lasting weatherproof high performance exterior wall coating which can last for up to 25 years if applied by a reputable, professional company.

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.

The above was sourced from the public domain and (heavily) edited by G Bell.

The picture above is of the delighful "fairbank hotel", near Newquay in Cornwall which was refurbished by one of our skilled contractors recently. Image copyright Fairbank Hotel.