Roof Waviness New Construction

Judy Norris · 09-26-2010, 06:12 AM

What causes waviness on a new roof, new house construction? It is not wavy all over the roof, only over the garage area in between the dormer and higher roof section and over the rear master bedroom. The roof is asphalt shingles over I believe OSB with an underside of radiant barrier.

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**Bruce Ramsey** · 09-26-2010, 08:08 AM

OSB sheathing that is too thin and has absorbed mositure or trusses 24" o.c.

**Matt Fellman** · 09-26-2010, 09:56 AM

Also check for a lack of H-clips at the exposed (horizontal) edges of the OSB.

**Brandon Whitmore** · 09-26-2010, 10:47 AM

Here's a few:

Uneven framing.
Buckled sheathing.
Sheathing inadequately nailed down.
Unevenness in the felt paper prior to the shingle installation.
Etc. , etc.

**Bob Knauff** · 09-27-2010, 09:40 PM

I believe you are referring to a look of slightly sagging material between trusses as viewed across the roof plane. It isn't seen across the entire roof and may not even be seen evenly up and down one section between trusses.

The most common cause of this unsightly roof plane problem is improper installation of the sheathing.

The APA instructs installers of all plywood and OSB type roof sheathing to lay the panel horizontally across the trusses, stand on the panel directly at the truss rafter and then fasten the panel at that point. Repeat for the rest of the panel.

Standing away from the truss rafter can cause the panel to deflect slightly. Nailing a panel that is deformed like that holds the shape and when viewed obliquely a "sag" will be noticeable between trusses.

It is ugly and unfortunate because it most likely must stay in place until the next roofing job but it is only cosmetic. Also, most sagged areas are only visible when the sun shines across the roof field at an angle. Otherwise it's not usually too noticeable, at least from the ground.

**Jerry Peck** · 09-28-2010, 03:47 PM

Originally Posted by Bruce Ramsey

OSB sheathing that is too thin ...

That could do it, as could many other things, such as using small pieces of OSB/plywood and not spanning the required 3 supports (small pieces only span from truss to truss, which allows sagging).

or trusses 24" o.c.

Nothing wrong with trusses at 24" o.c., most trusses are set at 24" on center and there are no problems.

If the trusses are set properly, and aligned properly, then it is most likely a roof sheathing installation problem.