Q&A: Roofing and Insulation for an Exposed Rafter Ceiling

A. If the framing won’t support the weight of concrete tile, my choice of roofing in a hot climate would be fiber-cement shingles. These are becoming more widely available and seem to provide a long-lasting roof without adding much weight. Fiber-cement shingles weigh just a little more than architectural-grade asphalt shingles (approximately 400 pounds per square when installed).

There are two ways to handle the insulation problem. The first method is to install a urethane foam insulation board and a new plywood deck over the existing sheathing (see illustration). In warm climates, a second possibility is to install a new roof deck on 2x2 sleepers over the existing sheathing, with a radiant barrier between the old and new roof decks. This would prevent the building from absorbing a significant amount of heat, providing the radiant barrier has at least a 1/2-inch air space on either side. Use either two layers of reflective foil insulation installed back to back, or one layer of a "double-polished" reflective material, which is shiny on both surfaces. The roof would then have about the same insulating value as R-11 fiberglass against downward heat flow. The space should be ventilated with continuous soffit and ridge vents. This would not be a good solution in a cold climate, where you would have to insulate against conductive heat losses from inside the building.