purlins, continued

Our erector started installing the purlins this week. The purlins are z-shaped steel sections that span between the beams and support the weight of the roof panels. Our purlins are a little unusual in that they pass over the beams, rather than sitting in the same plane as the beams. This, along with the use of corrugated steel decking between the purlins and roof panels, is necessary for the roof to resist hurricane-strength winds. (The corrugated decking is hiding under the blue tarp at right).

Pieces of light-gauge steel (the short, silvery pieces in the photo at left) act as blocking to keep the purlins from twisting or rolling over. Each piece of blocking is attached to two purlins with galvanized angles and a bunch of screws, which is somewhat time consuming. Still, we expect the purlins, blocking and decking to be in place by the end of the week.

The purlins were made to order for our house. Each one passes through a series of computer-controlled rollers that bend the steel to the precise shape and dimensions needed. In our case, some of the z-purlins are bent so that the web stands upright even as the flanges remain parallel to the slope of the roof. Each purlin also has a piece number printed on it - corresponding to the erection drawings - to make installation easier. They were also supposed to have bolt holes drilled in them, however, the fabricator dropped the ball on that. We ended up with a team of two really nice guys (Lee and Tito) who spent a week drilling over 600 holes in the purlins. Actually, they drilled some extra holes by mistake, but the purlins should still function just fine. Nonetheless, this illustrates one of the problems with prefabrication, which is that computer-controlled equipment is only as accurate as the person operating the machinery or programming the computer.