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We have a 30 year old roof in New England, which we have begun to interview roofers for this spring/summer.

The topic of plywood has come up and 2 have said we definitely need to replace the plywood even if it looks good. As plywood lasts about 30 to 40 years and we run the risk of delamination and required replacement in a few years.

The other two have stated that as long as the plywood is good and stays dry, it can last forever.

At quoted on 55 dollars per 4X8 sheet, its a sizable cost and so I am wondering if the plywood needs to be replaced regardless of current state.

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The other two are correct. Wood, in general, has no particular lifespan in and of itself. It all depends on the original quality and whether or not, as they say, it has stayed protected from the elements. – DA01 Feb 9 at 15:35
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Is that plywood gold-plated? I can get a full sheet of 1/2" plywood for half that at one of the big box stores. – Snowman Feb 9 at 20:16
    
Likely an "installed" price. But it's definitely a profit center...I still consider half that highway robbery, but I haven't really gotten over the "Iraq war plywood price explosion" (where the price went way, way up, and never came down - kinda like airfares with jet fuel prices more recently. – Ecnerwal Feb 9 at 20:31
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Did the other two quote a cost in case the plywood does need replacing? (Or will they just decide it needs replacing later when you have less choice and give a cost then?) Get basic contingency costs up front when you can. – user2338816 Feb 10 at 4:43
    
@user2338816 All of them had a quoted price for one sheet of plywood. They all ranged from 40 to 60 dollars, i assume its so pricey due to installation costs and that I live in Massachusetts. – treeNinja Feb 11 at 14:19
up vote 10 down vote accepted

I would give your business to one of the "is good and stays dry" roofers who are not blatantly trying to fleece you.

Now, the other two may be working from the assumption that if you bothered to call a roofer, you have leaks. But they strike me a lying scammers if they are selling you new plywood with no idea if the roof deck is rotten or solid.

Plywood does not have an expiration date.

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Thanks that is what I suspected. So no concerns of the glue failing? – treeNinja Feb 9 at 15:36
    
No. At least, not unless you can see the glue failing and leading to delamination, particularly at the corners. If this isn't happening, it's fine. – iLikeDirt Feb 9 at 15:39
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None, if it's kept dry. Do you rip out your subfloor after 30-40 years? Of course not, unless it's the rotten subfloor around the leaky toilet in the bathroom... – Ecnerwal Feb 9 at 15:40
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@Ecnerwal That's a bad example, since subfloors don't typically endure the extreme temperatures that a roof deck would. – Tester101 Feb 9 at 17:19
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Have some actual research data: woodscience.oregonstate.edu/sites/default/files/gupta/pdf/… and fpl.fs.fed.us/documnts/fplrp/fplrp543.pdf IF (and that's a big IF for a single-family structure) the plywood happened to be FR treated there evidently is some cause for concern. That type of material would most likely be found on multifamily structures from the early 1980's per a quick scan of these documents. The second indicates a maximum roof temp of 76C but that less than 21 hours per year would be above 71C, which helps inform reading the first. – Ecnerwal Feb 10 at 0:25

It's a mistake for the roofers to say that the plywood will definitely have to be replaced. However, they may be familiar with homes in your area, that were built around the same time. So they may have experience, that tells them that the decking will likely have to be replaced. Sometimes in situations like this, contractors will pad the estimate with the cost of replacing the decking. When they do the work, if they don't have to replace the decking they'll remove/refund the cost.

All that being said. I don't know these contractors, and have not talked to them. I can't say for sure what their intentions are.

Comparing plywood used as subflooring, to plywood used as roof decking is a mistake. Roof decking bakes in the sun, freezes in the cold, and may be surrounded by moisture laden (potentially salty ocean) air . While the subfloor rests comfortably in conditioned space (typically).

I'd say guaranteeing that the plywood will need to be replaced, is just as wrong as assuming it won't.

If it were me, I'd bring the concerns right to the contractor. You might find that they have a rational explanation. I'd find out if they plan to simply replace the decking, or if they're going to inspect it first. I'd also ask them if they'll reduce the price, in the case that the plywood is in good shape and doesn't need replacing.

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Very good points. The environment in the attic is partly what caused me to ask this question. I have had to work up there in all 4 seasons sadly. – treeNinja Feb 9 at 22:41
    
@Ecnerwal My main concern would be salty moisture laden air, flowing along the underside of the decking. As I said, I'm not a material science expert. I also don't like to jump to the conclusion that everybody is trying to rip me off. – Tester101 Feb 10 at 0:54
    
@Ecnerwal I'll jump to the conclusion that air has moisture in it, and be concerned that it could possibly also be salty. – Tester101 Feb 10 at 1:00

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