Plywood Performance as Roof Diaphragm

[SperlingPE]

I am being asked by a contractor to substitute OSB for plywood for a roof diaphragm. I have spec'd an APA rated plywood. I know that OSB is also rated by the APA and is designed to do the same job as plywood. The contractor is asking for the change because the supplier tells them that they will not stand behind plywood as roof sheathing. According to them, plywood is prone to warp and the supplier will not stand behind the plywood for this reason. Has anyone heard of this? Has the APA released a technical bulletin on this that I am not aware of?

 

[jheidt2543]

I can only give you my personal preference, I would use the plywood. They both do the job, but in my experianc, the plywood stands up to the weather exposure better if it isn't covered right away by the roofing. Once the roofing materials are installed there shouldn't be a difference. Call the APA and check with them too.

Plywood prices have really jumped the last couple of months, that may have a lot to do with the request too.

 

[SperlingPE]

My first thought was that the supplier was trying to pull a "fast one". But they came at me with not standing behind a product.
An additional note not included above. The supplier wants a letter from me stating that I would not let them substitute OSb for the plywood. Or something to that effect.

 

[jheidt2543]

What is sounds like to me is they are looking for backup for a change order request. IF you specified plywood, then there should be not change in cost to the owner. I would call a couple of local suppliers and get prices on plywood vs. OSB just to have the info. in your hip pocket. There is usually a pretty good price differential.

The other comment I would have is, I have seen 1/2" sheathing (OSB or plywood) specified for roofs and structurally it should should work (depending on snow loading required). However, I've never been too happy with the jobs I've used it on because I've see it curl at 24" truss spacing. I would hope you specified and/or use 5/8". Again, this is my personal preference. It all costs someone, but I hate to skimp on areas that matter.

 

[ERV]

Plywood does have a tendency to warp when exposed to moisture, whereas OSB doesn't. We have had instances where entire roofs have had to be re-sheated due to warping and the supplier would not stand behind the plywood. OSB swells when it gets wet, but will not warp, for that reason the manufacturer recommends min. 1/8" spacing at all edges between sheets.
Best to use OSB when prone to moisture prior to roof covering (even 1 night exposed to moisture can cause severe warping and buckling).
At this time there is almost no price difference between OSB and ply-wood.

 

[ERV]

For those not familiar with OSB, here are some facts:

http://www.apawood.org/pdfs/managed/TT-047_Ply-osb.pdf

 

[CSEllc]

In my area (NE - Connecticut), the contractor will substitute OSB for plywood on the walls, never on the roof. The reason is OSB is "more slippery" when wet or damp versus plywood.

Regarding warping, I have't had any problems with warping plywood. I can see it if you leave it exposed for a while, but if it covered in a reasonal amount of time it should be fine.

I have never been asked by a supplier to give them a "hold harmless" type letter. You might want to check with your (errors and ommmissions) insurance provider if you can do that. I think not and I would not. You are the PE on the project. Either they should supply what you specify or if they feel strongly that you are "wrong", decline to supply the plywood. The GC can purchase plywood anywhere from Home Depot to any local lumber yard. Don't be strongarmed.

 

[JAE]

I agree with CSEllc -

You spec'd plywood, plywood has always been recommended for use on roofs for sheathing (OSB is a late newcomer), even though OSB can substitute for Structural I plywood in the UBC tables it has the issues stated above, and finally, they are trying to scare you into changing your design to allow them to get more profit.

They are not required to stand behind YOUR design, only THEIR workmanship and compliance with the design plans.

You are NOT required to give them a letter of any kind. They can certainly write you a letter saying they have problems with your design, but you should never write some letter to the contractor defending your design. Your design is adequately defended by your seal on the plans.

 

[dik]

"the supplier tells them that they will not stand behind plywood as roof sheathing"

You may want the contractor to provide a letter stating that the material (plywood) to be supplied conforms to the specifications... You may also want to check that the material specified is suitable to task... you may want to look at the requirement for waterproof adhesives for the plywood...

I don't like using OSB for roofs, but have... For marginally overspanned conditions, OSB can 'delaminate' a bit... a problem with humidity and exterior exposure to rain... The orthotropic properties of OSB, generally, is less pronounced than plywood...

 

[boo1]

APA technical note titeled "Buckling of Wood Structral Panel Sheathing D481" lists recommendatios to avoid warpage.

They recommend using metal clips to create 1/8" gaps beween panels. The problem is edges of the seathing swell due to high moisture environment or when the seathing is installed without leaving expansion gaps between panels.

http://WWW.apawood.org