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.
Best Answer
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.