External rendering - Recent developments

Because of concerns regarding the consistency of mix, loss of traditional skills, long-term durability and the need to redecorate painted renders, a number of pre-mixed renders have become increasingly popular in the past decade or so. These were originally developed in mainland Europe, where cavity construction is rare and render is commonly applied to single skin concrete-block houses – with or without externally applied insulation.

There are two broad categories of products:

   -monocouche renders: ‘single coat’ in French, intended primarily for masonry backgrounds
   -acrylic or synthetic thin-coat renders, intended primarily for externally fixed insulation boards or timber-based boards (but can be used on masonry too).

Monocouche renders are supplied to site as a pre-formulated dry mix, requiring the addition of water prior to application. These are all cement/lime based, but they also have a range of other additives. Although these are dependent on specific backgrounds, conditions and requirements, they will all have polymers added to improve their workability and bonding capacity when wet and their long-term flexibility once set. Additionally, various weatherproofing additives, such as silicone, will be included. The term weatherproofing, rather than waterproofing, is deliberate as these renders are designed to allow moisture vapour to pass from inside to outside while stopping the passage of raindrops in the other direction – so called breathability. The bagged dry mix will also contain selected aggregates and tinting agents to ensure a consistent colour throughout the mix – avoiding the requirement to decorate the finished render. Some manufacturers supply these in hundreds of shades.

Although they can be manually applied, they are generally projected, i.e. sprayed, onto the façade, ruled flat and finished by hand. If used as a single-coat render, the typical finish is achieved by scrapping back by 2–3mm of thickness to expose the aggregate and produce a consistent surface. If used as a single coat, the render must achieve a minimum finished thickness of 15mm. This is necessary to guarantee weatherproofing as well as avoiding grinning. If a more textured finish is required, such as roughcast or dry dash, then single coats are still possible. However, it is not uncommon in these circumstances to require two coats.

Acrylic or synthetic renders have principally been developed for application onto surfaces other than masonry – though they can be used on them. These renders are nearly always applied in a minimum of two coats. Typically in housing these renders are used on externally applied, mechanically fixed rigid insulation boards. They could also be applied to the sheathing board on timber-frame panels and structurally insulated panels (SIPs).

The key, suction and movement implications of such materials require special binding agents: these are typically a combination of cements and synthetic polymers that impart a high degree of flexibility while maintaining weatherproofing. The base coat is typically 5–10mm thick and is almost always supported by embedding a synthetic mesh (nylon or glass fibre) throughout the area of the coat. Additional mesh is often required at movement-vulnerable points, such as above window and door openings. This reduces the risk of cracking and improves the bond at these critical points.

The base coat materials are supplied to site as a dry pre-formulated mix requiring the addition of water and are usually projected onto the surface to be rendered. Typically, the base coat will be self coloured and this should match the top coat, which is effectively a thick, highly flexible, breathable, specially formulated acrylic paint. Silicone and other polymers are included in the formulation and it is supplied in tubs (rather than as a dry mix) ready for application. This mix is generally too thick for brush or roller application and is commonly trowelled onto the surface 3–4mm thick. Typical finished thicknesses for these systems range from 8–12mm.

The detailing of all these contemporary render systems is the same as cement-based renders: stop corner and bellcast beads are required to protect the vulnerable edges. Most manufacturers insist on plastic rather than metal beads and, given that many of these systems are self-coloured, there is a range of colour-matched components.