Roof Diaphragm - Sheathing over T&G decking
Roof Diaphragm - Sheathing over T&G decking
(OP)
There's been one other post before this on this topic (thread507-106592: T&G Decking as diaphragm) but we just faced this situation and I thought I'd ask out there if there are further comments or input.
The IBC doesn't recognize 2x tongue and groove decking (we have 2x6's) as providing roof diaphragm shear behavior. I've heard in the past that placing Structural I plywood or OSB over the top of the decking can provide the necessary diaphragm action.
My questions, however, are this:
The IBC recognizes the sheathing but specifies nailing on the "supports" - edge nailing and field nailing. We have gluelam beams at 6'-9" o.c. and thus there are no roof members spaced at 16" or 24" o.c. to specifically provide for the nailing.
If we use the 8d nailing with 1 3/8" required penetration, there is no standard length of nail that works - 8d or 10d nails are too long and would poke through the decking - ticking off my architect.
Is the sheathing considered as "blocked" under the IBC diaphragm capacity table?
The IBC states that we can use the NDS nail capacities and manually calculate diaphragm shear capacities from them. Is there a specific technique that shows how to do this?
What have any of you done with respect to this?
The IBC doesn't recognize 2x tongue and groove decking (we have 2x6's) as providing roof diaphragm shear behavior. I've heard in the past that placing Structural I plywood or OSB over the top of the decking can provide the necessary diaphragm action.
My questions, however, are this:
The IBC recognizes the sheathing but specifies nailing on the "supports" - edge nailing and field nailing. We have gluelam beams at 6'-9" o.c. and thus there are no roof members spaced at 16" or 24" o.c. to specifically provide for the nailing.
If we use the 8d nailing with 1 3/8" required penetration, there is no standard length of nail that works - 8d or 10d nails are too long and would poke through the decking - ticking off my architect.
Is the sheathing considered as "blocked" under the IBC diaphragm capacity table?
The IBC states that we can use the NDS nail capacities and manually calculate diaphragm shear capacities from them. Is there a specific technique that shows how to do this?
What have any of you done with respect to this?





RE: Roof Diaphragm - Sheathing over T&G decking
Technically, in normal wood construction, wherever there is nailing, there is blocking. Just use the same nail pattern you would use for normal construction here, but with 6d for either a blocked or unblocked diaphragm.
I've done this in the past and it has worked for me.
Mike McCann
MMC Engineering
RE: Roof Diaphragm - Sheathing over T&G decking
Per IBC table, 6d nails are required to have 1 1/4" penetration into the support members. 6d nails are 2" long which is too long.
RE: Roof Diaphragm - Sheathing over T&G decking
RE: Roof Diaphragm - Sheathing over T&G decking
Mike McCann
MMC Engineering
RE: Roof Diaphragm - Sheathing over T&G decking
Mike McCann
MMC Engineering
RE: Roof Diaphragm - Sheathing over T&G decking
Ron, how do you "justify" the adhesive under the IBC? What are the load values (plf)?
RE: Roof Diaphragm - Sheathing over T&G decking
It is interesting to note that a similar section is not found in the structural diaphragm section of the IBC (2305.2), which to me would imply two things: either
1. It is not to be relied upon structurally in structural diaphragms, or
2. By omission from the section, it is permitted if it is tested and verified.
Perhaps there is more guidance in ASCE7 or the NDS...
Mike McCann
MMC Engineering
RE: Roof Diaphragm - Sheathing over T&G decking
The general consensus was that diaphragm deflection was decreased with the glue used, Scotch 5230 (which I believe is no longer available), but I could not find any quantification of any load per foot with the glue. It only listed the applied test load of 200#/ft used in the test.
I thought there was an old ICBO Report on Scotch 5230 that gave the application rate and some tested values. Similar adhesive manufacturers today should have similar data.
Mike McCann
MMC Engineering
RE: Roof Diaphragm - Sheathing over T&G decking
Mike...I searched the Evaluation Reports but didn't find anything on that adhesive under either "Scotch" or 3M. All of the Eval Rpts on adhesives apply to manufactured panels that have separate Eval Rpts.
I would agree with both mechanical and adhesive fastening. I'm not sure how much area you have to deal with, but you might consider screws instead of nails. You can, obviously, get the screws in any length you want. Automated screw guns work well for large areas. Adhesive fastening will cut down on squeaking that you'll get under load.
As for the adhesive, I would suggest an epoxy, since it is not solvent cured. Solvent cured adhesives will maintain an odor longer than epoxy. You could use an acrylic adhesive, I'm just not sure you can get the strength you want without creep; though for diaphragm action, the load is not continuous so it probably doesn't matter.
RE: Roof Diaphragm - Sheathing over T&G decking
You might go to www.apawood.org and find Form No. TT-097B "Designing Diaphragms over Existing Board Floors or Roofs". The three page article has some suggestions about the subject of your post.
I hope that this helps.
RE: Roof Diaphragm - Sheathing over T&G decking
Just labor $$$ here I guess.
Mike McCann
MMC Engineering
RE: Roof Diaphragm - Sheathing over T&G decking
JAE...the paper noted by OPM is a re-statement of the code requirements. It addresses the nailing pattern as you previously noted, but does not address adhesives.
RE: Roof Diaphragm - Sheathing over T&G decking
Found this by googling Scptch 52130 Adhesive. Appeared to be an Australian site, but it seems like it is still around.
Product Catalogue for Maintenance and Repair (MRO) Solutions Adhesives 3M™ Miscellaneous Adhesives 3M™ Scotch-Grip™ Wood Adhesive 5230
3M™ Scotch-Grip™ Wood Adhesive 5230 Printer-friendly format
Additional Information
Meets the requirements of plywood fabricator service specification AFG-01 and complies with the provisions of FHA "Use of Mateirals bulleting UM-60". Conforms to I.C.B.O. Research Recommendation No. 1379 concerning diaphragm construction when using "Lock-Deck" (Reg. T.M. of Potlach Corp.) Laminated decking, roofing and
flooring system or equivalent; to I.C.B.O. Research Recommendation No. 2964 for attaching plywood sheathing to support members prior to nailing in accordance with Chapter 25 of the Uniform Building Code. GALLONS AND LARGER - NOT INTENDED FOR CONSUMER SALE OR USE.
Note that there are two reports - ICBO Research Recommendation #2964 and #1379.
Mike McCann
MMC Engineering
RE: Roof Diaphragm - Sheathing over T&G decking
Mike McCann
MMC Engineering
RE: Roof Diaphragm - Sheathing over T&G decking
You can obtain a small amount of diaphragm using the NDS guidelines for a horizontal lumber diaphragm, but it ignores any contribution from any toenailing or shear friction in the tounge & groove.
Why use a plywood or OSB thicker than 5/16"? Anything thicker will require a larger nail and more nail penetration....penetration you can't obtain with a nominal 2" T&G decking unless you special order some 1 3/4" long 8D nails.
RE: Roof Diaphragm - Sheathing over T&G decking
RE: Roof Diaphragm - Sheathing over T&G decking
RE: Roof Diaphragm - Sheathing over T&G decking
RE: Roof Diaphragm - Sheathing over T&G decking
I'm not trying to be arguementative, I'm just trying to understand too. I may be looking at a different chart than you.
RE: Roof Diaphragm - Sheathing over T&G decking
We assumed that there is a natural variation in the positioning of the 8d nails in 2x framing such that the nails can get as close as about 3/8" from the 2x edge - thus per NDS there is a 4D loaded edge distance requirement that must govern. With 3/8" we have less than 4D (.375/4D = .72 reduction factor on the 8d nails).
This equates the two nails.
Now I'm not 100% sure that the diaphragm tables relied on a variation of nail placement like that but the fact that the diaphragm values vary with 2x vs. 3x framing suggests an edge distance issue.
It would be nice if the IBC or someone could produce an example of how the diaphragm table values are calculated.
RE: Roof Diaphragm - Sheathing over T&G decking
Might be available in your local engineering library.
Can still be purchased.
I thought that T&G paneling (I know not 2x6's) was allowed by IBC.