Putting new plywood over existing plywood?

Hi guys, the roof on my house was put on in August 2004, and I bought the house in June 2004. It is new 1/2 4 ply, no H-clips. The problem is that the plywood is severely dipping/buckling between the 24" OC rafters, and it needs fixed. The consensus among some contractors i have had on site, is that it is stemming from inadequate nailing and probably “blow through” from the air nailer.

One contractor is proposing to replace the really bad sheets, and then place another layer 1/2" 4 ply across the entire roof, to save on ripping off the old sheets. Is there any concerns with having two layers of plywood on the roof? He offers a 20yr warranty. (seams would be staggered so as to not line up).

Thanks for your help!

If some of it is already buckling I would not take the chance. If you put new over the top and the old under buckles…guess what :twisted:

I’d tear it all off if it’s that bad just to have piece of mind. Half inch is ok I guess but normally we only do 5/8" or 3/4" decking.


Nailing does not cause the plywood to sag. Inadequate support causes sagging.

If you use 1/2" again, you still did not learn your lesson. You will get the same thing.

Tear it off and put 5/8" or 3/4". You need a stronger plywood to span the opening.

If the 2004 plywood is buckling due to solely inadequate nailing, would the nailing of the new plywood theoretically properly anchor it in place to prevent further buckling?

I also don’t know that the original left the 1/8" gap required…

I prefer not to take the chance either, but the next quote is 7G’s more, but with a new 5/8" deck. Both contractors offer a 20 yr warranty…

Lefty, thanks for the reply…

I would in fact prefer the heavier plywood w/ h clips. Surprisingly most of the contractor’s here in Syracuse tell me that 1/2" is more the norm, than the exception. Although i would prefer the 5/8.

OK so to add to the mix… lets say I put 5/8 over the 1/2…would the save the expense of pulling off the old 1/2?

Thanks for the replies, I am just trying to educate myself here, so I don’t get screwed over again…


i am also in upstate NY and 90% of all roofs here are 1/2 or 3/8 if you replace with 1/2" with plywood clips you will be ok. i tear off 60 year old roofs daily that have 3/8 with clips and no signs on buckling. i personally wouldnt add the 2nd layer.

I am trying to understand your situation. The way I understand it is the home was’nt built new in 2004 and the contractor who put your roof on in 2004 removed the existing primary decking and installed new 1/2 inch plywood on top of the rafters. This is unusual unless the existing plywood had a delamination problem or there was a major mold issue on the underside. Whats more unusual is 3 years later the deck is sagging between the rafters.There could be a major ventilation issue going on in your attic space. An attic inspection needs to be done to determine that there is proper intake and exhaust ventilation. When in the attic the deck spacing also needs to be checked for the proper 1/8 inch space and if the nails missed the rafters. When plywood in jammed tight together it tends to buckle. If the problem results from improper spacing of the plywood the best thing to do is remove the deck and replace it with a properly installed one. I’ve had to deal with this situation in the past and would not reccommed going over the deck with a new one. The two main reason is the capital investment in throwing good money after bad and the warranty. No roofing manufacture on the planet will warranty there product over an existing bad substrate or deck. What does there 20 year warranty cover? Will they be around to service you if you have a problem. A workmanship warranty isn’t worth the paper it’s written on if the contractor goes out of business. Demand a manufactures warranty that covers everything(labor material ect.) The two best in the industry are GAF Golden Pledge offered by a Master Elite contractor or a 5 star warranty from Certainteed offered by a Select shingle Roofer. My company holds both of these status and can offer you these warrantys. Call my Syracuse office anytime to schedule an inspection at 652-5151.

rip off the 1/2"and put 5/8" or better yet 3/4"—3/8th to 1/2"is for 16"on center spacibg,5/8th to 3/4"is for 24"on center spacing—oone ply over another can have nails not securely seated and possible nail pops from expansion/contraction issues

Honest Abe,

You are correct with your understanding. The house was built in 1962. In 2004 a contractor for the previous owner put a new roof on with almost all new plywood due to poor ventilation/mold ($15K).

The attic currently has a full length ridge vent, 3" or 4" circular soffit vents, rafter mates, and is open on one end to garage addition which isn’t heated. Now granted, it MAY not be the best system, but the previous roof lasted 42 yrs with a probably a much worse vent system. This plywood isn’t even lasting 2.5 yrs. I did have a roof inspector over, and he doesn’t suspect ventilation, but is digging deeper to confirm, prior to issuance of a report.

The one quote I have is 22K (approx 41 sq roof), and that is for the GAF Golden Pledge Warranty (new 5/8 deck).

The other guy who spoke of two layers of decking is for approx 15K and that is for the Certeenied Select 20yr warranty?..ARGH…if the previous owner just would have had a compitent contractor… Do you guys have any thoughts…7G difference…

Thanks for the help guys…much appreciated…