Ziegel's feat of clay

Rob Woodhouse's mission in life has been to get the perfect render. It seemed easier in Germany, where he spent 10 years on building sites. But when he came home he felt British workers were not as interested in quality, perhaps because manual trades here have less status. For a time he had a job training workers in machine rendering, but achieving quality was still a struggle. "They weren't interested in getting the detail right, just the job done," he said. Then he had a flash of inspiration.

"I realised I'd got it the wrong way round," he says of his first British venture. "I needed to start with the substrate, not the render." His solution is fired clay blocks, which meet the Regs' 0.35 U-value for walls without the aid of a cavity or extra insulation. You can't do that with concrete. The 365mm-thick Ziegel blocks require just external rendering and an internal finish to create a load-bearing wall that's the thermal match of brick and block construction. "It's the quickest way to 0.35," says Woodhouse.

He's set up Ziegel UK to import the blocks from Germany. Not only is a single solid wall of the blocks loadbearing and thermally sufficient, it is also easier to machine-render. The blocks create a shell that is terracotta and nothing but – no steel lintels or expansion joints interrupt the expanse of clay. As Woodhouse had realised, machine-rendering lives up to its reputation of ease, speed and quality only when used on a continuous, uniform substrate.

"The building shell is the same material throughout, so it's much easier to render as there's no differential movement, and the insulation value is constant," says architect Dan Usiskin, who has awarded Ziegel its first big UK project with a Surestart centre in Sheerness, Kent. Usiskin says the thermal requirements of the Regs demand increasingly complex detailing and insulation for cavity walls. "We want to regain the simplicity of solid wall. Ziegel gives insulation and structural performance."

The insulation comes from the large number of air pockets in the blocks. Air channels run vertically through the blocks, while the clay itself is riddled with tiny holes created when the sawdust added to the clay mix burns off in the kiln. The trapped air insulates the block.

The advantage is speed at low skill levels. you don’t have to bodge it

Neil Roger

Although the blocks are as much air as clay, the density of the material gives the blocks a high thermal mass. And as the fired clay doesn't expand or contract, Ziegel walls need no expansion joints, although an external render is essential to prevent rainwater soaking through.

Fast and accurate
Builders can also put up clay block walls faster than traditional British brick and block, according to Usiskin. German manufacturer Bayosan planes the top and bottom of the blocks after firing to ensure they are all precisely 294mm high. Builders just slot them next to each other as interlocks on both perp-ends hold the blocks in line without mortaring.

The accuracy of the blocks means there is no need for a thick mortar joint between courses to iron out variations in block heights. A 1mm layer of cohesive thin-joint mortar divides each Ziegel course. "The main advantage is speed at relatively lower skill levels," says Neil Roger of Natural Building Technologies (NBT), which is also distributing Ziegel fired clay blocks and developing its own unfired clay blocks. "You don't have to bodge it."

You do, though, need to have an absolutely level base course to start off with, and that means using a laser-level and metal adjusters to position the first course. "It's very quick, but setting out is everything," warns Woodhouse.