Roof sheathing question ?

[Arky]

I can get 5/8" plywood sheathing for about $17.
However, I have found a source of 1/2" plywood sheathing for just $6.50, (it has been rained on, but is still sound)

For trusses on 24" centers, this begs the question, would 2 layers of 1/2 plywood (installed with the edges of one layer offset from the other) be as strong in bending and deflection as one layer of 5/8" ?

Or if not, what about if glue were applied between the two layers ?

[Richard A Hetzel]

Your subfloor load test gave you the answer. Install the sheets perpendicular to the first layer, and for the roof, I would use construction adhesive between the layers.

[Joe Carola]

I can get 5/8" plywood sheathing for about $17.
However, I have found a source of 1/2" plywood sheathing for just $6.50, (it has been rained on, but is still sound)

For trusses on 24" centers, this begs the question, would 2 layers of 1/2 plywood (installed with the edges of one layer offset from the other) be as strong in bending and deflection as one layer of 5/8" ?

Or if not, what about if glue were applied between the two layers ?

Why would you sheath a roof with two layers of 1/2"? Your wasting your time. Use h-clips with 2' centers and be done with it.I never sheath with 1/2" anymore, but there are millions of peopel sheathing with 1/2" using h-clips.

[Richard A Hetzel]

Yes, on 16-inch centers, but 1/2-inch on 24-inch centers is another matter.

[Don_P]

Actually a 3/8 panel is the minimum on a roof at 24" spacing with edge support. A 7/16" panel is not required to have edge support (clips) although you would be insane not to use them. 15/32 and 1/2" are allowed 32" span if clipped, 28" unclipped, again I would clip them.

This is for sheets stamped respectively;
3/8- 24/0
7/16-24/16
15/32,1/2-32/16
The first number on the stamp is the maximum rafter span, the second is the max floor span. This is table R503.2.1.1(1)

It is also worth remembering that a floor does not require sheathing under a solid wood 3/4" thick T&G finish floor under certain circumstances. Do remember these are all minimums not best or even good practice. I've worked on a 3/8 roof on 24's and expected to go through at any moment.

[Joe Carola]

Yes, on 16-inch centers, but 1/2-inch on 24-inch centers is another matter.

1/2" on 24" centers requires h-clips, that's all.

[Richard A Hetzel]

Do remember these are all minimums not best or even good practice.
Exactly the point. The problem with a code which sets a minimum for anything is that the minimum tends to become a maximum. The code is not a textbook on how to build a good solid serviceable building. The code is a book which explains how to get away with the bare minimum. Problem is, bare minimums do not a good building make. With all that being said, yes, 1/2-inch OSB will work (barely) on 24-inch centers with H-clips on the unsupported sides. Depending on the building, its purpose, and its expected life, that may or may not be adequate. Someone somewhere must exercise judgement, and select components and methods that suit the purposes.

That's what I think of when people jump up and down and exclaim that their work passed inspection. All that means, and then only if the isnpector was sharp, is that what he saw was a bare minimum. That's all he cares about. Whether the construction suits a client's needs is another issue altogether, and the inspector couldn't give two...um...farts about that.

[Don_P]

3/8 is the minimum on 24" centers. You were trying to tell the questioner that the 1/2" he had was not acceptable, that is your opinion, which is fine, I don't necessarily disagree. The fact is many homes have 1/2" and it suits their needs fine. I'll quote the code and my reservations which I did. I was confirming and showing that Joe was correct. I worked for one couple that had a roof designed on 12" ctrs, they had me put 3/4" t&g osb up there.

[Joe Carola]

Exactly the point. The problem with a code which sets a minimum for anything is that the minimum tends to become a maximum. The code is not a textbook on how to build a good solid serviceable building. The code is a book which explains how to get away with the bare minimum. Problem is, bare minimums do not a good building make. With all that being said, yes, 1/2-inch OSB will work (barely) on 24-inch centers with H-clips on the unsupported sides. Depending on the building, its purpose, and its expected life, that may or may not be adequate. Someone somewhere must exercise judgement, and select components and methods that suit the purposes.

That's what I think of when people jump up and down and exclaim that their work passed inspection. All that means, and then only if the isnpector was sharp, is that what he saw was a bare minimum. That's all he cares about. Whether the construction suits a client's needs is another issue altogether, and the inspector couldn't give two...um...farts about that.

Richard,

No matter what way you look at it 1/2" sheathing works osb or fir with no problems @ 24" centers with h-clips. I don't frame 24" centers and I don't use 1/2" on roofs anymore. I've also never once used osb on a roof, but people are using it all over whether it's bare minimum or not.There is no need at all anywhere that someone would have to double sheath a roof using 1/2" like what the OP started this thread about..

[PLANS]

I use 1/2" plywood sometimes with no problems.

[mjpliv]

Although most builders will stick to the minimum code requirements (they have to to remain competitive - I know - We are one of those builders) most will not compromise the integrity of a structure in the process. We have been successfull using 1/2 OSB (a product design for roof decks) and all of our roof trusses are at 24" OC. We do use H-clips and block where necessary. This assembly provides more than adequate support for construction and roofing/snow loads. We have never had a call back due to a roof failure or sagging. As they say "The proof is in the pudding!"

Would I sheet a roof with 1/2" standard 4 ply plywood? I personally do not like using the product because it is not nearly as stiff as the OSB sheathing. Does it meet code requirements for 24" OC framing? Yes it does. But so do single glazed windows and I wouldn't use those in a house either. I might consider it for 16" OC though.