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My Question(s) - : Is spongy plywood on a 20 year old roof OK or does it need to be replaced? This plywood was solid 5 years ago.

More background (edited based on comments):

I was walking on my roof last week and it is spongy all over (like walking on carpet with a reallllly good pad). 5 years ago it was solid (far more solid - definitely had some flex). There are no leaks to speak of (that I know) except a gasket that was worn around the sewer air intake drain that I corrected as soon as I found it 3 weeks ago, but not sure how long it was happening. It's a split level and the other half (that is dry) is equally spongy). I got a quote and the guy walked the entire roof and acknowledged that it was spongy, but said 3 pieces of plywood would need to be replaced. Can a 20 year old roof that was solid 5 years ago and is spongy now only need 3 sheets of plywood. Another estimator told me the golden beads along my joists in the attic were sap from the plywood. My guess is that I need to replace most if not all of the plywood.

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  • Depends on what "spongy" means. I very much doubt that your roof deck had no flex five years ago. Half inch plywood or OSB is standard since the 1960s, and that most definitely flexes under the weight of a man in between 24" on-center rafters, especially near seams. Have you checked for water damage? – isherwood Jun 3 '19 at 16:26
  • The sap sounds right. Looking up at the roof from the attic, are you seeing any evidence of water ingress whatsoever? – Harper - Reinstate Monica Jun 3 '19 at 16:56
  • @isherwood The roof had a little flex to be sure, but much more now. The house born on date was 1960, the latest roof was put on around 1999 and I wasn't there, so I don't know the state of the t&g/plywood/osb? – Dan B Jun 3 '19 at 17:10
  • @Harper The only water is from a leaking sewer gasket , that I recently discovered and corrected 3 weeks ago... might have been leaking for some time, but I just noticed it. Very minor leak I think. It's a split level and the other side of the roof is bone dry as far as I know. – Dan B Jun 3 '19 at 17:10
  • For what it's worth, I'm voting to close because we can't possibly answer this question with confidence. We don't know if there's water or termite damage, and we don't know whether the deflection you're seeing is outside the range of normal. Best to have a local objective party take a look. It's very rare that a modern roof would need new sheathing without some sort of problem having occurred. – isherwood Jun 3 '19 at 18:24
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Yes, the roof sheathing could go bad (weaken) after a few years due to 1) moisture, 2) extra heavy snow loads

1) Moisture is the most likely problem. Plywood is made from wood veneer plies that are glued together. When moisture seeps into the plies, they will delaminate. The plywood will probably look fine from the attic (looking up) but it will become spongy.

2) If the plywood is slightly undersized, (half inch thick on joists spaced 24” spacings) it could feel spongy, especially if there have been several extra heavy snowfalls.

The plywood will loose it’s Modulus of Elasticity (it’s ability to recover from deflection) if loaded to its capacity several times.

Generally we require 5/8” thick plywood for joists at 24” on center. (1/2” is rated for 24” spacing, but will deflect significantly.) If you live in a “high wind” area or a seismically active area, 5/8” is required. (High wind areas will suck your shingles off your roof without an additional holding power of a minimum of 5/8” thick plywood.)

Note: I doubt only 3 sheets are required. I also doubt a whole sheet will need to be replaced at each problem location. The roofers are going to find more problems and charge you accordingly once the roofing is removed. (Old common roofers trick.) I’d get a unit price for each 4’x8’ sheet and 4’x4’ half sheet that needs to be replaced.

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  • I think moisture is likely the culprit.. and I agree that more than 3 sheets of plywood are going to be needed. I have two quotes that are $2000 apart. The lesser includes the 3 sheets & $65 for each additional 4X8 sheet, the greater will replace any needed plywood. It's a tough choice as I want all new plywood. – Dan B Jun 3 '19 at 20:05
  • @DanB “All new” as in remove and replace? Or, “all new” as install new over existing? If you add new over existing, I’d check the joists to make sure they’re still adequate. (What is the size, spacing and span? ...face of support to face of support on horizontal.) – Lee Sam Jun 3 '19 at 20:57
  • Remove and replace. 24" on center. The timbers are birch and amazingly hard, sound, and it's one of the best constructed houses I've seen. – Dan B Jun 4 '19 at 13:31

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