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T&G Lumber Diaphragm

T&G Lumber Diaphragm

T&G Lumber Diaphragm

Referencing thread337-254770: Roof Diaphragm - Sheathing over T&G decking

I am designing a lumber diaphragm using NDS Table 4.2C for 2x6 T&G Horiz Lumber Sheathing and seeing an allowable shear value of 50 plf. This is fine with me, but there is no indication anywhere in the NDS that I can find that dictates the maximum spacing of the supports. Naturally there is a limit for vertical loads, but none is given for lateral. Mine happen to be about 6' apart, some smaller. I only have a 25 psf snow load.

I would rather not put plywood on the top because I think it is a waste of money for this project. It is a small residential building, roof only. We are adding on and matching the existing.

Any thoughts?


RE: T&G Lumber Diaphragm

Call up Simpson Strong-Tie, Engineering Dept.

Personally, I think you are going to get sued for using 6' on center.

RE: T&G Lumber Diaphragm

Simpson? Sued for what?

RE: T&G Lumber Diaphragm

1) Simpson may have board-to-board connecters or fasteners (screws) to improve shear value.

2) Sued for a sagging roof in about 5 years. Like a ticking time bomb.

RE: T&G Lumber Diaphragm

Several different tables tell me that 2x6 DF T&G spanning 6 feet between supports (4x10 rafters) will EASILY handle 25 psf Snow and 15 psf Dead Load. If you have some helpful information on my original question I would appreciate it.

RE: T&G Lumber Diaphragm

OK, let me check my IRC. Give a few minutes.

RE: T&G Lumber Diaphragm

It is OK (sort of) according to the IRC

Their table says MINIMUM thicknesses, without no mention whatsoever of type of loads, duration, or intensity. IMO, worthless information.

What table(s) do you have?

RE: T&G Lumber Diaphragm

Sounds reasonable to me on a small residential roof diaphragm. As long as you don’t have wind or EQ lateral loads which are killers. The best diaphragm values usually involved edge nailing or screwing the pieces together to improve the shear btwn. the individual pieces (min. slip), and then for the total diaphragm. You gotta read ever single footnote in those tables; and/or the total report to get the full understanding of how they tested and got their values. The plywood overlay is intended for larger diaphragms where your table values are not enough, due to size and total diaphragm loads.

RE: T&G Lumber Diaphragm


It is not sort of OK. It is fine. Ive done this before and this building has been there since the 1970's and the roof has not sagged a bit. They even have some butt joints falling between supports, which we will not do on the new portion. The owner may even pull up all of the old and re-lay it with the new with joints on the supports for aesthetics and greater strength. I have a few copies of tables from different agencies like APA or AF&PA. I also have a pdf from AF&PA called Tongue and Groove Roof Decking, attached. The IRC is meant for lay people and I find it nominally useful as a structural engineer.

My question remains, does anyone know of a maximum spacing for lumber diaphragm supports? I cant believe it is not front and center on the NDS Table. Providing a nailing schedule at each support seems useless without an indication of support spacing. I must be missing something. I even checked all of the referenced sections in the footnotes with no luck. I know this table is conservative for diaphragm value because it does not consider any help from the T&G. It is for straight lumber sheathing.

Please, with all due respect, if anyone has good info I would appreciate it. I'd rather not waste any more billable time debating side issues.


RE: T&G Lumber Diaphragm

OK I would help if I could, but this is all news to me.

RE: T&G Lumber Diaphragm

I don't think there is a limit for lateral loads. For a building 100' x 100', I've always thought that if I could get a single sheet of plywood 100' x 100', I'd be good for lateral (uplift is another issue of course, and, I suppose, some weird kind of buckling might enter into it....). I think of all the detailed nailing as being a means to create continuity between 4x8 sheets (or whatever) because 100'x100' monolithic (what an interesting word to use with wood, but effective nonetheless, I suspect) continuity is not practicable.

And by the way, doesn't eng-tips pay your invoices?

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