20 year old "Spongy" Roof - It was solid 5 years ago - Do I need new plywood? (with update 3 years later)

Question

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.

Answer 1

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.