If only Kevin Hoene’s choices for a new roof boiled down to a choice between an EPDM membrane and metal, his life would probably seem a whole lot simpler.
But Hoene, building a new home in Illinois and on the boundary between Climate Zones 4 and 5, will soon be weighing the pros and cons not only of different roof coverings, but also of what type of insulation to use, whether it should go above or below the roof sheathing, and whether the roof should be vented or unvented. In other words, nothing seems off the table.
“Our house is being built near the boundary of Zone 4 and 5 in Illinois with a 1:12 pitch metal roof,” Hoene writes in a Q&A post at GreenBuildingAdvisor. “I’ve done a lot of research on low-slope roofs because our builder does not have a lot of experience with flat roofs.”
After reading a couple of articles on the topic by GBA Senior Editor Martin Holladay, Hoene is leaning toward an unvented roof assembly (see the sketch above), which would include cellulose insulation in the rafter bays and 4 inches of extruded polystyrene (XPS) rigid insulation above a layer of airtight sheathing. That would be followed by a second layer of OSB or plywood sheathing and, finally, the metal roofing.
“Does the diagram look like a good approach?” he asks. “Any details that I should pass along to our builder about the roof trusses?”
Those questions are the start of this Q&A Spotlight.
Choosing the type of roofing
A first step, Holladay suggests, would be finding out whether the type of metal roofing Hoene is considering can be installed on the roof he’s planning. The pitch is very low — rising only 1 inch for every 12 inches of run — and some…