Plywood Over Old Roof

As the boards to do the roof deck were manufactured here in the northwest it was still more economical than plywood through the 60s.

Plywood over old roof.

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. After the 1960s the predominant residential roofing substrate became 1 2 cdx plywood. If your home was built prior to 1970 there is a good chance it will have board sheathing.

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. Fortunately this is one issue that has primarily been standardized by the industry. Open spaces form between the once tightly fitted shiplap boards. 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. Exterior grade plywood was not available until the 1940 s. If the old plywood had been removed. 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.

Another issue that many people encounter is what size plywood the roof may need. 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.