Plywood Over Old Roof

Open spaces form between the once tightly fitted shiplap boards.

Plywood over old roof.

Exterior grade plywood was not available until the 1940 s. Over time shiplap fir wood resin dissipates causing the boards to shrink crack form knot holes and become extremely brittle. The new plywood will have to be nailed to the rafters or roof trusses. Its really common here too to see 3 8 plywood used on roofs from the 70 s.

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. However you might find that osb can better suit your needs than plywood for the roof. Another issue that many people encounter is what size plywood the roof may need. But with the old plywood left in place it will be considerably more difficult to locate the rafters to ensure proper nail.

Both cdx plywood and osb tend to come in standard sheets of four foot by eight foot 4 8. Fortunately this is one issue that has primarily been standardized by the industry. 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. As the boards to do the roof deck were manufactured here in the northwest it was still more economical than plywood through the 60s.

After the 1960s the predominant residential roofing substrate became 1 2 cdx plywood. 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. 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.