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Old 11-13-2018, 12:26 PM   #1
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Waves in roof


14 year old roof on a townhouse, 1/2 inch plywood that was new 14 years ago. Front of the house (which gets lots of sun) looks fine. Rear side of the roof (which gets lots of shade) is developing waves -- in between the rafters there are slight sags, almost like gullies. I was really hoping to hold onto this roof for another 5 or 6 years. Can I? Is this typical in a 14 year old roof or do I have a problem that needs attention now? Also, is there anything besides warping I should look for that would indicate an urgent problem? Something like: warping + this other bad thing = don't wait another five years, get a new roof now. Thanks!
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Old 11-13-2018, 12:35 PM   #2
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Re: Waves in roof


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14 year old roof on a townhouse, 1/2 inch plywood that was new 14 years ago. Front of the house (which gets lots of sun) looks fine. Rear side of the roof (which gets lots of shade) is developing waves -- in between the rafters there are slight sags, almost like gullies. I was really hoping to hold onto this roof for another 5 or 6 years. Can I? Is this typical in a 14 year old roof or do I have a problem that needs attention now? Also, is there anything besides warping I should look for that would indicate an urgent problem? Something like: warping + this other bad thing = don't wait another five years, get a new roof now. Thanks!
Have you checked inside for a moisture problem?
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Old 11-13-2018, 01:20 PM   #3
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Re: Waves in roof


No moisture problems I can see. No obvious water stains or mold.
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Old 11-13-2018, 01:23 PM   #4
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Re: Waves in roof


Pics would help.
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Old 11-13-2018, 01:27 PM   #5
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Re: Waves in roof


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Originally Posted by agd202 View Post
14 year old roof on a townhouse, 1/2 inch plywood that was new 14 years ago. Front of the house (which gets lots of sun) looks fine. Rear side of the roof (which gets lots of shade) is developing waves -- in between the rafters there are slight sags, almost like gullies. I was really hoping to hold onto this roof for another 5 or 6 years. Can I? Is this typical in a 14 year old roof or do I have a problem that needs attention now? Also, is there anything besides warping I should look for that would indicate an urgent problem? Something like: warping + this other bad thing = don't wait another five years, get a new roof now. Thanks!
Actual plywood or OSB?
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Old 11-13-2018, 01:29 PM   #6
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Re: Waves in roof


You should not have to change plywood when you re roof.
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Old 11-13-2018, 01:53 PM   #7
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Re: Waves in roof


It's 1/2 inch actual plywood. Why it's warping is a good question, but the question I'm most interested in is: Can it wait another 5 years or am I in trouble now?
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Old 11-13-2018, 02:13 PM   #8
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Re: Waves in roof


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It's 1/2 inch actual plywood. Why it's warping is a good question, but the question I'm most interested in is: Can it wait another 5 years or am I in trouble now?
I would not expect a sudden failure but it would be good to no why.
Rafters or engineered trusses, 16" or 24" on center?
Plywood, vertical or horizontal?
Heavy snow loads?
Clear venting from soffits to peak north and south?


A straight edge like a 4 ft level stretched across 3 or 4 rafters on the inside will show if one is sagging.
With a 2 ft level held against two rafters on the inside the sag of the plywood can be measured in the center.
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Old 11-13-2018, 02:36 PM   #9
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Re: Waves in roof


What directions do the two sides face? Is the sunny side facing south while the shady side faces north?

One thing that comes to my mind is condensation causing plywood failure. Sunny side gets enough heat to keep it dry while the shady side doesn't.
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Old 11-13-2018, 02:43 PM   #10
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Re: Waves in roof


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What directions do the two sides face? Is the sunny side facing south while the shady side faces north?

One thing that comes to my mind is condensation causing plywood failure. Sunny side gets enough heat to keep it dry while the shady side doesn't.
Maybe 3/8" plywood, unless the OP actually check what he got at the time.
I guess he could check that around a vent from the inside.
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Old 11-13-2018, 06:20 PM   #11
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Re: Waves in roof


As a matter of fact there is a real difference in terms of sun and shade on my roof. Front of the house (where there is no problem) gets lots of sun. Rear (where the waves are) gets lots of shade. Plywood is vertical, yes rafters, 16" on center. Moderate snow every winter, the soffits should be delivering enough circulation to the ridge vent -- though I can't swear to that, haven't crawled down there and looked.

How common is plywood failure due to condensation?
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Old 11-13-2018, 07:11 PM   #12
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Re: Waves in roof


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As a matter of fact there is a real difference in terms of sun and shade on my roof. Front of the house (where there is no problem) gets lots of sun. Rear (where the waves are) gets lots of shade. Plywood is vertical, yes rafters, 16" on center. Moderate snow every winter, the soffits should be delivering enough circulation to the ridge vent -- though I can't swear to that, haven't crawled down there and looked.

How common is plywood failure due to condensation?
Lack of ventilation and air leaks from the house cause condensation and mold. it would be the mold that damages the sheeting.


Vertical plywood. Sheeting is usually put on horizontally with the but joints two rafters apart to firmly hold the rafter in place. Wood can and will move when the sheets are vertical all the edge nails are with in 1/2" of the edge of the sheet.
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Old 11-13-2018, 08:14 PM   #13
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Re: Waves in roof


Verticle ply is much weaker, likely contributing to the sagging. The whole roof is sheeted verticle?
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Old 11-14-2018, 08:09 AM   #14
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Re: Waves in roof


Yes, whole roof, front and rear, is vertical plywood. Mold is an interesting idea. There is mold/mildew on top of the roof shingles, but there isn't any on the underside of the decking seen from inside the attic. Could mold be getting into the plywood from the top down (atmosphere, shingles, paper, decking) as opposed to the bottom up (attic, underside of decking)??
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Old 11-14-2018, 08:36 AM   #15
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Re: Waves in roof


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Yes, whole roof, front and rear, is vertical plywood. Mold is an interesting idea. There is mold/mildew on top of the roof shingles, but there isn't any on the underside of the decking seen from inside the attic. Could mold be getting into the plywood from the top down (atmosphere, shingles, paper, decking) as opposed to the bottom up (attic, underside of decking)??

I doubt you have mold under the shingles, and you wouldn't change the roof on a guess for that.


From inside the attic find a hole and measure the thickness of the plywood and count the number of plies. Plywood has plies that change direction but there are always more long plies than short ones, that is what makes it stronger in that direction.
We know they cheated on direction they could have cheated on thickness too. A 3 ply 3/8" plywood would only have one ply doing all the work.


Then look at the trusses or rafters for some that are not straight of they have bent under a joint in the plywood, that would rip nails side way out of one sheet while pushing the next which would cause a dip.


This all guess work
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