Plywood roof sheathing bowing up between trusses. HELP!

[alrednek]

I have a problem with my roof that desperately needs fixing, but I don't know exactly what to do to correct it. I built/contracted my home a few years ago and didn't know a lot about building at the time. I contracted a friend to do most of the work. He is not a liscensed contractor but has done construction for many years. He wanted me to use wafer board sheathing but I used 1/2 inch plywood on pre-fab trusses 24 inches on center. I realize now that I should have put the trusses on 16 inch centers. My problem is that when the roof heats up the plywood bows up in places between the trusses. It really looks bad and can't be good for the shingles either. I want to fix the problem. I have been told by my friend that I'll have to replace the *bad* plywood sheathing with wafer board since *plywood just ain't as good as it used to be, and it always bucks up when you use it on the roof*. Is he right? In my opinion the problem is caused by not leaving expansion gaps between the ends of the sheets of plywood when it was originally nailed to the trusses. I know for a fact that the plywood was butted tightly together at the ends because I helped put it on. I used cheap extruded aluminum plywood clips (another mistake) which didn't do much to hold the panels in place. I can see from inside the attic that they have bent where the panels bow up. Assuming that my theory is correct, would sawing end gaps between the panels after removing the roofing materials correct the problem? I feel that by sawing a 1/8 inch or so gap at each end would allow the plywood to expand when it heats up without bowing up between the trusses.

Am I on the right track? Will my approach fix the problem? Do I need to replace the sheathing with wafer board? I desperately need some other opinions/advice on what to do. I can try to take pics and post if needed to diagnose the problem. Please help!! Any and all advice will be appreciated.

Mike :?: :( :cry:

[Rich]

Plywood is as good as any wafer board. I hate to say that your friend is wrong. 24" centers is industry standard for truss spacings. Were sheathing clips used between trusses at the edges?
I don't know if that's your problem (expansion/contraction) but it's a place to start. Some pictures would be helpful also. Grumpy could probably diagnose the problem.

[grumpydasmurf]

What is wafer board? OSB?

Plywood CDX is the strongest of residential roof sheating I know of. Certianly stronger than OSB.

Hmmm Is this the same friend that messed up in the first place? Seek a second opinion. Oh yeah! Your doing that right now! ;)

Yes expansion gaps are a major deal. You must leave 1/8. Most carpenters use clips.

Sawing end gaps might help the problem. We've done it on a few roofs, but not because they were bowing. We took off the roofs, saw no gaps, so we cut them. I think you may be on the right track with the saw cutting the expansion gaps.

24" OC is not uncommon for trusses. Your engineer/architect should have specified. Personally if I were to sheet a roof 24" OC I'd use 5/8" CDX instead of 1/2" but we've been on more than plenty roofs with 24" OC and 1/2". There is always a little sway under our steps but not a big deal.

Rich is right, pics can always help!

I've seen concave bowing caused by poor ventilation. I'm wondering if you have concave or convex.

[alrednek]

Thanks for the replies Rich and Grumpy. I'll try to get some pics tomorrow if possible. The plywood was put on without anything at the ends for spacing. We did put sheathing clips between each sheet of plywood between each truss, which gives about a 1/8 inch spacing along the sides of the panels. However, where the ends of the plywood sheets meet on the center of a truss, they are butted tightly together- no gap at all for expansion. And the sheathing clips I used were very flimsy extruded aluminum. I have since seen some made of stamped galvanized steel that look much stronger. The trusses were specified on the house plans to be 24 inches O/C as designed by to the architect. I now feel that it would be better to put them 16 inches O/C for a stronger roof. I'll try to clarify whether the bowing is concave or convex by saying that the plywood bows upwards between the trusses causing humps in the roof. There are no places where it bows downwards. I feel like the house has more than adequate ventilation with large gable vents AND ridge vents. I plan to remove the ridge vents when I re-roof since they are probably not needed anyway.

I appreciate the comments and as I said will try to get some pics tomorrow if it gets warm enough to make the plywood bow up.

Mike

[roger g]

You say this happens only when it heats up!!!!! Never heard of that. I've never seen a gap between plywood where the ends but up. Trusses are usually 24 inches apart which is why builder use trusses. (one of the reasons) Wafer board we used to call chip board. Made from big chips of wood 2-3 inches across and about a sixteenth to one eigth inch thick.
If the plywood is lifting there there must be some pulling elsewhere. Go in the attic and run a straight edge under the plywood and see how much it has lifted. Only when heated! wow!. I wonder if where you think it has lifted is really where the truss is. and the plywood has really sunk imbetween the truss. I really don't know.

Roger

[grumpydasmurf]

Gable vents mixed with a ridge vent is almost always a NO NO. Ventilation probably isn't what's causing this problem since it's convex (bowing upwards). But ROger brings up a good point too.

I'll re-diagnose when I see pics. Take attic pics if possible also.

[alrednek]

Here's a pic from inside the attic

[alrednek]

Here's another pic from a different place. I hope you can see the problem. The difference in height from one panel to the other is almost the thickness of the plywood- 1/2 inch. In these pics the sheathing clips are still there, but in some places they have been bent and have fallen out. Is there any easy way to put clips back in?

The outside pics I took didn't turn out well due to having to use a lower resolution setting in order to post. I guess I wasn't exactly right about the problem only happening when the roof heats up- it's raining today and cool and there are these two places where it bows up. However it does seem to be worse on a hot day.

Let me know if these pics help you any in diagnosing the problem. I can take more if needed.

Thanks much guys.

Mike

[roger g]

I don't know of any way to put the clips back in. Never had the problem. Still hard to tell from the pics. I noticed that there doesn't seem to be a lot of roofing nails coming through the plywood. I'm used to seeing thousands of the damn things and my head find the one I didn't see.

Roger