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I just had my roof replaced and the contractor said that the old plywood under the two sets of shingles were separating and rotted in some places. He also let me know that he installed the new plywood over the old plywood to save the cost of removing the old. Will this be sufficient for the new plywood to hold? And will it affect the life of the new shingles?

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  • Sounds like you are going to have a lump in the roof where the added layer of plywood is. Kinda tacky and not very professional. Or is the whole roof getting a new layer over the old? – Jack Feb 11 '18 at 22:59
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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. 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. if the old plywood had been removed). This will ensure that the strength of the plywood is enough to bridge spots on the roof where the old plywood was weakened to the point that it would sag under the load on the roof.

You also should consider the design parameters of the roof to ensure that the extra weight of the added plywood is well within the static load capability of the roof support structure.

Lastly it would be best if the new sheets of plywood were offset from the seams of the existing plywood but at the same time placed so that the new seams can be properly nailed into the rafters and other roof structure members. It is unlikely that seams of new plywood simply nailed at random locations into the relatively thin old plywood will be assured to keep the plywood held down over time.

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    I agree with Michael Karas, but make sure 1) the moisture barrier was removed from the original plywood, 2) nails should be at least long enough to penetrate 3/4” into the roof rafters, and 3) the nails must penetrate into the joists...not just into the old plywood. You can look up in your attic to see if the nails missed the (joists)rafters. (If they did, measure the length and see how far the nails could have penetrated into the rafters.) – Lee Sam Feb 11 '18 at 3:40

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