Plywood Over Old Roof

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.

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. Today a shiplap roof is usually 60 years old. As the boards to do the roof deck were manufactured here in the northwest it was still more economical than plywood through the 60s. Both cdx plywood and osb tend to come in standard sheets of four foot by eight foot 4 8.

However you might find that osb can better suit your needs than plywood for the roof. Open spaces form between the once tightly fitted shiplap boards. One thing to consider is that the new plywood probably wants to be the same thickness as what would be used if it was used by itself on the rafters i e. Its really common here too to see 3 8 plywood used on roofs from the 70 s.

Another issue that many people encounter is what size plywood the roof may need. 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. We resheet with 1 2 osb right over top of the old all the time be it 3 8 ply or old t g boards.

Fortunately this is one issue that has primarily been standardized by the industry. If the old plywood had been removed. Exterior grade plywood was not available until the 1940 s.