Materials - External rendering

Binders: cement and lime

Cement

The type of cement used will depend on the location of the work, the conditions of exposure, the nature of the background and the colour of the finish required. For most conditions, ordinary Portland cement (OPC) or masonry cement (which contains a plasticiser) can be used. Where there is a risk of attack from soluble sulfates, which may be present in some bricks or in the ground behind rendered retaining walls, the use of sulfate-resisting cement is advisable. Some white or light-coloured finishes will require white cement.

Lime

The addition of lime to a sand cement mortar has several advantages. It improves the workability of the mix which makes it easier to apply to the wall. It helps to reduce the amount of drying shrinkage and also improves the suction of the render (explained later), thus assisting the application of subsequent coats. Lime will also improve the vapour permeability of a render. Nowadays hydrated lime is supplied as a powder in bags and is usually non-hydraulic. Further information on lime can be found in Chapter 10 on plastering and dry-lining.

Aggregates

Fine aggregates: sand

A ‘sharp’ sand should be used for renders (i.e. a sand with angular particles); this is different from that used for masonry mortars, where a ‘soft’ sand, with more rounded particles, is usually specified. The sand should be clean and well graded (i.e. with a range of fine and coarse particle sizes). The grading is important as it affects the water/cement ratio: a badly graded sand requires a lot of water to achieve a workable mix. Shrinkage will occur as the excess water evaporates, causing stresses within the render which can result in cracking.

Coarse aggregates

Roughcast and dry-dash finishes (described later) require the use of coarse aggregate, which can either be mixed in with, or applied to, the finish coat. The aggregates vary in colour, shape and size throughout the country and are often referred to by traditional names. Examples include Canterbury spar, Dorset spar and Welsh spar.

Additives

These are often incorporated into render mixes to improve the working properties of the material or to modify its properties in the hardened state. Plasticisers may be used to improve the workability of a render, without the need to add lime or to increase the water content. Water-retaining additives may be used to prevent a rendering from drying out too quickly when applied to an absorbent (high suction) background and waterproofers may be used to make a rendering less absorbent to rainwater.