External rendering - Smooth and scraped finishes

The topcoat is applied to a thickness of about 6mm and finished flat. It should not be over- trowelled as this can bring a thin film of water/cement paste to the surface, which forms tiny hairline cracks as it dries. Before the top coat has set it can be ‘rubbed up’ with a wooden or plastic float to roughen the surface slightly, providing an even finish.

If a scraped finish is required, it will be created by scraping or rubbing it with a ‘Devil float’ or a wooden float covered with expanded metal. The scraping should be firm enough to remove the surface skin of mortar and expose the aggregate. It is not uncommon to paint smooth and scraped renders, but the type of paint should be chosen with caution. Conventional oil-based paints are unsuitable, as they are not compatible with the alkaline nature of the cement and also form an impervious layer, which prevents evaporation of moisture from the wall. External grade paints are better alternatives, although painting is not a ‘once only’ treatment and will require periodic renewal. This issue partially accounts for the increasing use of pre-mixed renders (described later), which are ‘through-coloured’.

Roughcast

This is a mix of cement, lime, sand and small stones, which is thrown onto the undercoat to give a rough textured finish. The topcoat is quite strong, which makes this form of finish unsuitable for weaker backgrounds. It requires great skill to obtain a uniform finish but does provide a very hardwearing surface. However, like most coarse finishes, roughcast will gradually accumulate dirt in an urban environment.

Dry dash or pebble dash

In roughcast, the coarse aggregate is mixed in with the final (top) coat before it is applied to the wall, but in dry dash the aggregate is thrown onto the topcoat after it has been applied and while it is still wet. Typical stones include small pebbles and various coloured chippings. Once the aggregate has been applied, it should be lightly ‘tamped’ (pressed) into the topcoat with a wooden trowel to ensure a good bond. It is usual to incorporate a waterproofer in the undercoat to reduce suction, i.e. to reduce the speed at which the top coat sets.

Tyrolean

This finish is usually applied by a hand-operated machine. Most builders use a pre-mixed material which contains fine aggregate, cement and coloured pigments. After the topcoat has been finished level, the Tyrolean is mixed with water then ‘flicked’ onto the wall. By varying the amount of water and the number of passes with the machine, different textures can be produced. The finished texture is not dissimilar to roughcast but has a more even appearance.