08:53PM | 07/05/04
Member Since: 07/05/04
1 lifetime posts
I am purchasing a home. After the purchase offer was accepted, the physical inspection revealed that portions of the roof sheathing were delaminated and the rafters were discolored due to significant moisture in the attic caused by a bathroom exhaust fan which was venting into the attic. I agreed to continue with the purchase after the owners agreed to an addendum to the purchase offer requiring them to install a new roof to include sheathing/decking. I recently got a copy of the contract with the roofer and discovered that new plywood will be installed over the existing damaged plywood. The damaged sheathing will not be replaced. Is this an adequate repair for this problem. What plywood thickness should be used. I am very concerned. The roof is due to be installed in 1 week. Help! I know nothing about these things. I'd appreciate any advice.


06:21PM | 07/06/04
Member Since: 01/28/03
693 lifetime posts
I agree with Grumpy's advice to correct the ventilation problems and address any mold problems, but...

And while it is probably best to remove the old sheathing before installing the new...

That does not really address the question of adding new plywood over old.

As long as your building code permits its and it does not add too much weight to the existing roof rafter system, there is no reason I can think of that would prevent the addition of a new roof deck over the old.

As long as the new plywood itself can span the distance and carry the roof, wind, seismic and snow loads for your region, adding new ply over old is just fine.

The new plywood thickness should be at least equal to the exusting plywood thickness.


01:03PM | 07/08/04
Member Since: 01/21/03
66 lifetime posts
It is relatively common practice to lay new sheathing over old, but I don't like it, and whenever we are faced with this, we remove the old and install the new. I'm going a reroof in two weeks time, where we are removing all the old roof covering, and the old sheathing (delaminated and rotted 3/8 plywood) and replacing it with new 5/8 T&G plywood.

My reasons were: 1) the old stuff has no more structural integrity, and at best, it is a spacer between the new sheathing and the rafters onto which the new sheathing is fasted. 2) The old stuff is full of mold. Why build a new roof with the mold-starter built in? 3) If and when you will sell the house, someone will go into the attic and see all the old rotten and delaminated wood. How are you going to convince them that you've got good stuff hidden away behind it?

While we've got the sheathing off, we are also going to ensure that the ventilation channels (from soffits to the main attic area) are clear and open. The homeowner wanted to have a proper solution, since the two roofs he installed in the twenty years he's lived in the house have failed prematurely.


07:54AM | 07/11/04
Member Since: 11/06/02
1281 lifetime posts
You have two reasons for replacing the plywood. One is the structural failure you see in the de-lamination. The other is that there is undoubtedly mold damage and mold spores in not only the sheathing but in other places also. Has your inspector raised this concern? I would expect from seeing this problem a dozen times ( why don't people hook up their vents and kleep them clean?) that you have mold problems in the attic insulation as well. You need to be sure that the vent is correctly installed, and ANY material harboring mold is replaced with new.

As to the structural issues, There may be local jurisdictions - mostly in seismic zones and hurricane zones that would have specific issues with nailing new over old that would probably be adressed with increasing the number of fasteners to overcome the lost shear strenth in offsetting materials like that. Other places might reject that option outright, requiring direct application to satisfy requirements for shear.

Here is another hint for your negotiations, If the current roof sheathing has only 3/8", they may be considering the same for replacement. that is still acepted in some places but is woefully inadequate for roof loads if you still want it to look good in a few years to avoid sagging between rafters. depending on the area of the country you are in, you would want 1/2" or 5/8" sheathing

Excellence is its own reward!


10:52AM | 09/04/13
"and the old sheathing (delaminated and rotted 3/8 plywood) and replacing it with new 5/8 T&G plywood."
Question? When you put tongue and groove on, what happens when the sheathing expands? I have seen some pretty bad damage from this type of installation in the past(torn shingles, plywood raised on all edges, nails pulled out of decking) has wood changed over the years or does it still expand abd contract. If it still expands get ready for some serious problems.
El Dude erino


11:34AM | 09/04/13
Member Since: 01/24/06
1626 lifetime posts
Replace the entire roof and use CDX or other treated 3/4 Ply wood


10:12PM | 11/20/15
Best to replace the sheathing in-kind, same thickness, except the first row of sheathing near the eaves should be done with exterior grade plywood. This is to address likely water and moisture originating from backed up gutters or ice damming. With the exterior grade plywood, you won't have to worry about the damage that invariably occurs. If doing an all new roof decking, definitely go with at least 5/8 thickness if you are spanning more than 16 inches between roof framing elements.
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