Plywood Over Old Roof

If the old plywood was clean and not infested with dry rot or mold it should be ok to cover it with the new plywood.

Plywood over old roof.

Fortunately this is one issue that has primarily been standardized by the industry. If the old plywood had been removed. Today a shiplap roof is usually 60 years old. However you might find that osb can better suit your needs than plywood for the roof.

If your home was built prior to 1970 there is a good chance it will have board sheathing. Over time shiplap fir wood resin dissipates causing the boards to shrink crack form knot holes and become extremely brittle. As the boards to do the roof deck were manufactured here in the northwest it was still more economical than plywood through the 60s. Exterior grade plywood was not available until the 1940 s.

Another issue that many people encounter is what size plywood the roof may need. Its really common here too to see 3 8 plywood used on roofs from the 70 s. In the case of the old t g boards it makes for a flatter surface plus greatly lessons the chance of nailing into the gap between boards. Both cdx plywood and osb tend to come in standard sheets of four foot by eight foot 4 8.

Open spaces form between the once tightly fitted shiplap boards. But with the old plywood left in place it will be considerably more difficult to locate the rafters to ensure proper nail. After the 1960s the predominant residential roofing substrate became 1 2 cdx plywood.