When you drive by a home under construction and see bare sheets of flat material on top of the house, that’s the roof sheathing. It sits on top of the roofing structure and is secured directly to it. The most common material used is oriented strand board (OSB) because it’s lightweight, affordable, and available. Plywood is also used but mainly for roofs that will hold more weight, such as concrete tiles. Older homes (1900 and older) usually have skip sheathing. Skip-sheeting, or spaced sheathing, is just as it sounds: sheathing with gaps between. Traditionally, skip-sheathing consists of 1 inch by 4 inch boards nailed to the rafters with gaps between the boards. Spacing between boards is optimal for wood shakes but cannot support asphalt composite shingles. By RI Building Code, this sort of roof must be re-sheathed with plywood to support the new system.