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I had some dry rot and moisture issues in my attic because ventilation was not installed correctly 20 yrs ago when the roof was done. I am a new owner (10 months) and decided to replace my roof.

Yesterday the roofer called and said the entire North side of my cape cod needed new OSB. I said ok and when I showed up 2 hours later the new OSB had been put down and they were starting to shingle. The roof was finished yesterday, when I got home from work I went into my attic and I noticed the OLD OSB was still there. They did not replace the OSB, they just went over it.

My concern is the old osb dry rot affecting the new osb. Also, is there now a weight concern with the two layers of 1/2" OSB and shingles?

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  • @hubbhunter, please edit your post to describe the nature and age of your roof framing.
    – isherwood
    Commented Mar 10, 2017 at 17:10
  • If it was true dry rot it should have been removed but if just puffed up from moisture it may not be a problem and will have some strength.
    – Ed Beal
    Commented Mar 10, 2017 at 17:48

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Dryrot, unlike bug infestation, will stop when moisture stops. Plywood weighs 3.0 lbs. per square foot per inch of thickness. (OSB board could weigh slightly more because it has fewer voids and significantly more glue.)

So, adding a layer of 5/8" OSB board might add 2.0 lbs. per square foot at most...not significant, unless you live in a high snow zone or your roof structure has decayed too. (By the way, the Building Code allows up to 2 re-roofs before they all need to be torn off, which weigh about 4.0 - 6.0 per square foot depending on the type of shingle.)

A bigger issue for me would be what thickness of OSB board did they use? All roofing manufacturers require a minimum of 7/16". Less than that and you have no warranty...where the dryrot is located.

Also, do the nails penetrate both layers of roof sheathing or extend a minimum of 3/4" into solid wood? All this info is on each shingle wrapper.

In most jurisdictions, no Building Permit is required for re-roofing, (it's considered maintenance...that's why you don't need a permit for carpeting, painting, etc.) However, now they've replaced the roof sheathing (a structural item) and I'll bet they did not get a permit. So, who inspected the nailing of the new OSB roof sheathing? Big problem.

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  • When he dropped off the bill, he stated no rotting or mold but the plywood was delaminated so they just went over top. It's a cape cod, steep pitch so snow cover isn't an issue. During my complaint he did state he would discount the normal $40 per sheet for replacement to just $35 a sheet since he didn't replace anything, just we t over it. I did advise him I would be speaking with the local building code enforcement Monday before any money exchanged hands.
    – hubbhunter
    Commented Mar 11, 2017 at 17:19
  • Being puffed up from moisture does not require removal and there is still some strength in the original layer.
    – Ed Beal
    Commented Jul 27, 2017 at 22:50
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    @EdBeal I'd be cautious about "trapped" moisture. I think it could cause a problem, (I.e.: dryrot, mold, etc.) But if it's "dry", I agree, I see no reason to remove "old" sheathing. However, there is a limit to the length of nail to get the same shear diaphragm loads.
    – Lee Sam
    Commented Jul 28, 2017 at 1:54
  • No, dry rot does not stop when you remove moisture... that's why it's called dry rot, the fungus can live in dry timber. Wet rot on the other hand stops spreading when the affected timbers dry out... the fungus needs the timber to be wet to survive.
    – houninym
    Commented Nov 5, 2019 at 9:21
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It depends what you paid for and even what the bill says. But, I'd be very concerned as well or maybe I missed something. To me, it sounds like they put a new deck and shingles over the old shingles and deck.

I've never seen such a thing done and have only seen new shingles go over the old shingles. In that normal case, the rot is always completely cut out and replaced. The problem I have, if my presumption is correct, is if the new deck is nailed into the rafters or just the old OSB.

If it's just the old OSB, then you have a very good chance of wind ripping the new roof and deck entirely off. The roof deck must be securely fastened directly to the building's structural members.

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