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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.

--- UPDATE FROM ORIGINAL POSTER - Fast forward 3 Years ---

I see this has been viewed quite a bit and I wanted to give a 20/20 hindsight update for anyone deciding on a new roof...

#1) Be there the day the roof is done.

#2) DO NOT TRUST THEM TO AUTOMATICALLY DO THE RIGHT THING. ***I can't stress this enough.

I was trusting the roofers as I had been promised over and over by several people in the company, "My guys will not lay a roof on a bad/compromised piece of plywood". As a battle hardened DIY'er, I have learned the hard way to keep a watchful eye on any and all contractors.

After they completly removed the old roof, I saw them pull out the sheeting/underlayment and tacking it down without replacing a single sheet of plywood. They also showed up with NO plywood. I got up on the roof and stopped them and starting telling them to replace this piece and that one... I walked the entire roof and had to use my best judgement as the crew didn't seem willing or able to tell me what needed replacement. In the end I had them use 9 sheets of new plywood (of the 25 or so that were on the roof). The roof is very solid now and I think it will be good for another 20-30 years with a little luck. I realize walking the roof is not an option for many, but do all you can to ensure you are getting what you are paying for. Make sure they have plywood at the ready and are actively using it if necessary. Had they replaced the roof with out any new plywood, it entire venture would have been a waste of time.

#3) Refer to rule #2... Give your complete and utter trust to NO ONE! Keep a keen eye on the crew during the process. Do not just let them "do their thing" and hope for the best. I know you've probably heard this 1,000 times, but you are your best advocate, especially in a contractor situation. Be as informed as you can be and never hesitate to call them if they do something that you know in your gut something doesn't add up.

#4) If I was going to be in the house for 20+ years, I would have replaced all the plywood, but doing what was about 1/3 of the plywood makes the roof feel rock solid and everything seems to be aging well so far.

Good luck and feel free to ask any questions.

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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, 2019 at 16:26
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    The sap sounds right. Looking up at the roof from the attic, are you seeing any evidence of water ingress whatsoever? Jun 3, 2019 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, 2019 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, 2019 at 17:10
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    I wouldn't be surprised if the initial non-replacement was due to lazy workers. The owner/manager/boss may be perfectly happy to replace plywood (normally it is part of the price - 'x' for the roof plus 'y' per sheet of plywood replaced) but the guys on the roof may see it as just slowing down the job and making their day longer (especially if they get paid by the day or by the job and not by the hour). Jul 12, 2022 at 14:57

1 Answer 1

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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 its Modulus of Elasticity (its 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, 2019 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, 2019 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, 2019 at 13:31

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