- Q.When building with structural insulated roof panels, how far is it safe to cantilever (or project) the ends of a panel past the top plates? I like the idea of creating a protective overhang for the siding and windows.
A.Paul Malko, an engineer with Foard Panel in West Chesterfield, N.H., responds: Unless the snow, seismic, or wind loads are unusually high, roof SIPs can be cantilevered for a third of their dimension. This means that if you're running 4-foot-wide panels horizontally, your maximum eaves overhang would be 16 inches, while your gable overhang could be several feet, depending on the length of the panel. Many SIPs manufacturers make 8-foot-wide panels; running an 8-foot panel along the eaves allows you to extend the eaves overhang up to 32 inches. That's why it's common in a SIP roof package to have a course of 8-foot-wide panels along the eaves and 4-foot-wide panels on the rest of the roof.