If using Z-section steel purlins in place of timber, how should these be attached to the wall ? The fixing must be on the same plane as the roof, so that they do not stick out above or below. In industrial roofs, the purlins are usually bolted on top of a structural steel member, but these buildings are usually steel frame. I get the impression some people here think my roof is a bit odd, but of course you can use any construction method, as long as it meets the building regs ! Nothing like innovation. Cheers, Simon.