Smoothing over the cracks

Cracking render, as any plasterer will tell you, is a constant hazard. Usually a mix of sand and cement plaster, render is applied to the frame of a building and can crack for many reasons: the quality of the surface it's applied to, drying time, the mix, how it's applied and even the direction the wall faces. But is it acceptable in an industry working toward zero defects? One leading housebuilder has had enough, and is looking at alternatives.

The housebuilder, which wants to remain anonymous because it doesn't (naturally enough) want to be associated with cracking render, is looking at renders bound with plastics that are designed specifically not to crack. One alternative render system known as external wall insulation (EWI) is used on commercial buildings in Britain and all types of construction in Europe.

EWI can use sand and cement but tends to use mineral, acrylic or silicon renders, which are more flexible and less likely to crack. It is also claimed to be energy efficient. Changes to the Part L regulations and problems with cracking have led this top-10 housebuilder to look for alternatives to sand and cement.

Sand and cement render has always had its problems. "Some people thought moving from lime render was a retrograde step," says the housebuilder's group technical manager. "Although lime is very porous, it breathes with the building. Sand and cement render is very rigid and if the wall behind it moves, cracks can appear."

As sand and cement has become the favoured type of render, the housebuilder has tried to minimise the risk. "We try to reduce the risk of the render cracking," he says. "There are best practice guides available from the British Cement Association, and we give our own specifications to the plasterers. We also send the render away for analysis sometimes, but there are no guarantees it won't crack."

Pre-mixed sand and cement render that you just add water to is one way of reducing that risk. "We have tried using ready-made render recently, but have no feedback as yet. But it's still sand and cement, and will still be rigid when you apply it to the wall," says the technical manager.

Cutting down on drying time

Cracking isn't the only drawback to using sand and cement. "It takes a long time to dry. If it is put on blockwork, two coats are needed, and on timber three. This really slows down the time to market for the house. With the new part L regulations coming into force, more and more houses are going to be constructed with timber frames, so sand and cement render becomes even more of a problem."

So is the answer to return to lime render? Not according to the technical manager. One way to solve the difficulties may be EWI. But it would mean a different style of working for UK housebuilders.

"In Europe, and Germany in particular," he says, "they use a different method of insulation to the UK. They tend to use EWI, which puts insulation on the outside of the wall and the render on top of that."

The advantage of using acrylic, mineral or silicon render is clear, according to the technical manager.

"Only two thin coats need to be applied and therefore it dries a lot quicker, which means the house is quicker to market," he says. "There is also less risk of cracking as it is much more flexible. The downside is that it is more expensive, more difficult to install, there is less variety in the finish and patch repairs can be difficult as it tends to be self-coloured."

Is it more expensive? Yes and no

Whereas traditional sand and cement render can be mixed and applied by plasterers, EWI does require specialist subcontractors to install it. And supply and installation can cost between £55 and £100 per m², depending on the system. This is compared to £50 per m² for entire wall construction using sand and cement, the render representing about £18 per m² of that cost, including labour.

Despite these difficulties, it seems the construction industry is willing to give EWI a try. Copthorn Homes is using the system for the first time at a residential development on Mile End Road in East London. Although the system is more expensive, development manager Peter O'Sullivan is happy with his choice.

"We were forced to use EWI because of the design of the building," he says. "But it means you can be a lot more inventive than with traditional materials. The development includes a round building and lots of elevations.

"EWI also made the construction easier, for this reason. The architect wasn't keen on lots of cracks, and neither were we, from a maintenance point of view." The main manufacturers of EWI in the UK are Sto; Weber and Broutin; Alsecco; Marmorit and Rockwool.

Mitchell Gee, external wall sales manager for Rockwool, says the render market is set for a revolution. "The whole idea of wall construction is in a state of flux, with the need for greater energy efficiency. The new build sector is booming for EWI. With the new part L regulations, that seems set to continue."