Roof Sheathing Lifting After Roof Replacement

natewallFebruary 23, 2009

About two months ago I had a complete tear-off and roof replacement on my 1963 Cape Cod style home. This included replacing eighteen 4 X 8 sheets of plywood, due to what I believe is 45 years or so of moisture from the inside of the house breaking the plywood down. The wood replacement was on the north side of the roof only. All other roof areas had good wood. Shingles were Certainteed AR 25 (year warranty).

About a week after the roof was installed I noticed slight "bulging" on the surface of the roof in various places, only on the portion where the plywood was replaced. I figured I would give it a couple of months for the shingles to lay flat and then take a close look, but the bulging got a little worse.

I examined it closely yesterday. The plywood that was replaced is bowing, or bulging, lifting up the shingles in a few places. You could see it plainly at the edges of the roof. The bulge is about half an inch to an inch high and covers about five feet or so and is in several places across the roof.

I watched the roofers closely when they worked. The plywood is exterior grade, but no plywood clips were used to provide a gap between adjoining pieces, where installed. The plywood was installed without any gap (I think the shingle package stated to have a small gap).

I think the plywood must have expanded and lifted, or that it was not properly fastened -- perhaps only the corners were nailed down and its lifting in the middle.

I think the only proper fix is to tear off the roof and start over and correctly replace the wood on the north side. I plan to have the roofer come by and offer his explanation, but imagine he will tell me "Its OK, don't worry about it, it will be fine".

Is this a big problem that could shorten the life or compromise the roof, or just a slight cosmetic issue? Is the roofer at fault, or is this an unavoidable bowing or a defect in the plywood? I have never noticed this problem on any other roof I have seen installed. Opinions?



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The only thing outside of not nailing it down properly would be if the plywood was wet when they layed it down and they covered it while it was wet. How was the weather during the plywood process concerning rain and the materials getting wet?

There was a house in our area where the inspector passed the nailing inspection when the sheets were only tacked at the corners and around the perimeters on the rafters, but not in the field. The roofers covered it over and every sheet bowed up similar to your explanation. They had to tear it off and after nailing properly to code, re-roof.

    Bookmark   February 23, 2009 at 10:09AM
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I would say your roofer is in trouble. I hope they are still in business. I don't know what the rules are and as far as I know, my '80's-vintage british columbia home does not have the clips but it may not have been code there. I'm not happy with my roof either, some shingles still seem to be sticking up somewhat and I'm concerned they talked me out of proper underlay near the edges for ice damming "Oh, we don't need that in Kamloops" as it happens, this winter, we did.

I don't think there is any alternative to a tear off and redo, and I'd look askance at them if they say they can fix it any other way.

    Bookmark   February 23, 2009 at 1:08PM
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I agree with others that there's something wrong with your install, and that it needs to be fixed somehow.

If you can get into the attic and look at how the rafters are relating to the sheets, that might tell you a lot. (Be very cautious about puncturing your head on all the roofing nails that are sticking through. A hardhat would be a good idea.)

If you take something thin, like a ruler, you'd be able to pass that between the top of the rafter and the sheathing to feel if there are any nails there.

You might also take a pen-knife or something similarly pointy to see if the tops of your rafters are rotten. This could cause them to not hold nails/screws adequately. (And it could also point to more substantial structural problems that your roofer neglected to mention. The good news is that a few semi-rotten rafters are relatively easy to fix by "sistering" extra members.)

Good luck.

    Bookmark   February 24, 2009 at 6:30AM
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Thank you for your resoponses. Yes, something appears amis. This roofer got good ratings in Washington Checkbook too! Replacing plywood is something a roofer should be able to do.

Its difficult to examine the roof from the inside. Its a Cape Cod style house, and the majority of the wood was replaced on the portion of the roof that has on the inside only a stud cavity and then drywall for the upstairs rooms. There is insulation in the stud cavity and I think it DOES NOT have a vapor barrier, and I think that is why the wood degraded over the years. I may be able to go behind the knee wall upstairs and view from the inside one of the pieces of plywood that is lifting.

I do not think this is a moisture issue, because I noticed it within days of the roof being installed. Also, about 12 years ago the roof was replaced (by the previous owner) and some plywood sheets were replaced. Those replaced sheets are still in great shape and are not lifting a bit.


    Bookmark   February 24, 2009 at 7:37AM
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Call the roofer back and give him a chance to remedy the problem.
It could be that the sheeting was put on too tight. Was it plywood or or was it OSB. OSB must be left with a gap. It will expand when applied.

    Bookmark   February 24, 2009 at 9:07AM
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It was plywood. I'll call him back and ask his opinion. If its cosmetic, I am not too concerned.


    Bookmark   February 24, 2009 at 9:22AM
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Roof sheathing panel clips are not for the purpose of spacing the panels although they will force proper spacing by an incompetent carpenter; they support the panel edges midway between the rafters when the sheathing is less than 1/2" thick. No roof should be built with plywood thinner than 5/8" (for proper shingle nail holding) so panel clips should never be needed.

It seems most likely that your roofer didn't provide any gap between the panels and they are expanding from moisture. If you thought the roofer was a competent carpenter it appears you were mistaken. The only solution other than replacing the entire roof is to remove any unnecessary and/or excessive source of moisture.

    Bookmark   February 24, 2009 at 9:23AM
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Not sure what thickness it was, but I sheathed the roof of a carport (out in the sticks, no building permit) with OSB that was just under 1/2" with a big snow load - we used clips and it seems to have held together well, it was for my FIL and he was contemplating just using tarps over the rafters, we bought him the osb as a present (I was also feeling guilty after accidentally firing a nail into his finger, the nailer misfired and a second nail glanced off the wood and straight into his finger)

It's more than cosmetic. Something is wrong and it must be rectified.

    Bookmark   February 24, 2009 at 12:05PM
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"There is insulation in the stud cavity and I think it DOES NOT have a vapor barrier, and I think that is why the wood degraded over the years."

Not likely.
The Washington, DC area has pretty equal heating and cooling requirements, so what side does the vapor barrier go on?

It does not get cold enough to freeze moisture in insulation, and condensation during the cooling season is also pretty rare.

The Wood Engineering Handbook (USDA, Forest Products Lab) used to have a map of the US showing vapor barrier locations. A huge section was between the lines indicating a barrier was not really required.

    Bookmark   February 24, 2009 at 4:19PM
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