Inspecting a roof covering gets complicated because of access issues, snow, rain and an unlimited variety of roof and shingle designs. One thing a smart inspector should do is check the underside of the roof in the attic if it is accessible. In the attic, inspect the roof deck—the boards, plywood or oriented strand board (OSB) supporting the asphalt shingle assembly. We can’t see the roof deck from the outside because the deck is covered (Illustration: Metal Edge Flashings).
Most installation instructions for new asphalt shingles use some version of this statement: “Roof deck: The deck surface must be clean, bare, gap-free and flat.” Most manufacturers assume the use of a plywood or OSB deck. But what happens with older homes?
When you’ll encounter spaced boards
In older homes, asphalt shingle roofing is usually installed over OSB (Photo 1). From the attic side, you can often see the underside of the OSB placed over the spaced roof boards.
You will also find plywood placed over the spaced boards (Photo 2). In the case of OSB and plywood decking, decking should be nailed to roof rafters at the edges and there should be no gaps larger than 1/8 inch between the sheets.
What if you see black roof felt between boards?
When black roof felt or a synthetic underlayment is visible through the spaced roof deck board, this may indicate a serious problem (Photo 3). Try to reach the space between the roof boards; push on the felt to check for a roof deck of plywood or OSB. If there is no roof deck, you found a problem. You may also see the shingle, felt and nails pushing into the attic through the gap because there is no proper roof deck.
In the old days, some gaps were tolerated
Twenty years ago, gaps of up to ¾ inch between boards often were ignored and the asphalt shingle assembly was installed over the spaced boards. At that time, these smaller gaps seemed to cause no issues, but now the rules have changed. New asphalt shingles must have a roof deck that is “clean, bare, gap-free and flat.”
Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com, he provides high-quality marketing materials that help professional home inspectors educate their customers. Copyright © 2018 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.