2x6 T&G as roof diaphragm
2x6 T&G as roof diaphragm
(OP)
From what I understand, 2x6 T&G can’t be used as a means for diaphragm load transfer, so there has to be a layer of plywood or OSB over T&G to act as a diaphragm. The architect in his section is showing 2x6 topped with 3 layers of rigid insulation (3” thick) and on top of that he is showing plywood. Could I use this plywood as the diaphragm with insulation sandwiched in between?






RE: 2x6 T&G as roof diaphragm
RE: 2x6 T&G as roof diaphragm
RE: 2x6 T&G as roof diaphragm
The 2005 edition of NDS SDPWS does provide design values for T&G sheathed diaphragms, see Table 4.2C. The values are low but they are not zero. However if you need higher capacities and can get an added structural panel overlay directly on the T&G you should be able to show an adequate load path.
If that isn't cost effective, for the assembly you describe, it might also be somewhat analogous to a SIPS diaphragm. You could use long SIPS type screws through the all the build-up. ICC should have some report and testing data to see if you can meet the requirements for application.
regards,
Michel
RE: 2x6 T&G as roof diaphragm
RE: 2x6 T&G as roof diaphragm
I do not trust the longer nails through 3" of rigid insulation to be able to generate any reliable shear value.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: 2x6 T&G as roof diaphragm
RE: 2x6 T&G as roof diaphragm
RE: 2x6 T&G as roof diaphragm
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: 2x6 T&G as roof diaphragm
I agree, it would be impossible to cantlver a 8d or 10d nail 3". But just to clarify, the arch does not want to put a plywood layer over the T&G. He insists that he has seen the a project where it was done. Now when I questioned him more, the project was not designed by him and he can't provide any calcs for the design either (supposedly engineered by his in-house engineer at the time). There is actually a metal seam roof he wants to put over the plywood. His logic is that the reason he needs plywood over the insulation is so that he has something to nail the metal seam roof to.
RE: 2x6 T&G as roof diaphragm
How does the connection of T&G to framing work though. Do the nails for plywood go thru t&g into framing below, or should t&g be connection to framing be designed first and then connection of plywood-t&g. Like I said, I haven't done a t&g roof before so I don't know.
RE: 2x6 T&G as roof diaphragm
RE: 2x6 T&G as roof diaphragm
Be sure to apply all the required factors to the values.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: 2x6 T&G as roof diaphragm
And you know what - it usually does UNTIL it hits design load - which may only happen once or twice in the life of the building.
Your choice.....
RE: 2x6 T&G as roof diaphragm
Ask the Arch. if his profession licence expects that he follow the applicable building codes; and/or use skilled judgement which can be supported (defended) using established principals, when something is not specifically covered by the codes. Ask him if he expects you to follow the Structural building codes, or will he send you a letter showing that he assumes full responsibility, and holds you harmless, where he wants you to deviate. Then send him a letter showing your objections and an explanation of the code deviations he is asking for. He hired you for your engineering expertise and judgement, and should take it; or else explain to your satisfaction how his detail complies with today’s code.
He may well need the top layer of plywd. to attach his metal deck to, but that better be thick enough for the type of fastening system the decking requires. And, that plywd. support layer has to support all roof loadings, in particular uplift loads in many areas of the roof. That uplift does mean nailing or screwing down into the 2x6 structural deck with sufficient cap’y., but that’s different than any shear transfer. Woodman’s table 4.2D is about what I expected to see, but mine would have been an older version. We don’t know what shear cap’y. you need from this diaphragm, but you have to explain to the Arch. what the code allows you to do, or get that letter, or get him to use that other (magician) engineer. Obviously, you have to attach the 2x6 decking to the primary structural members sufficiently to transfer all the various roof loads, then maybe btwn. planks, and then maybe plywd. atop the decking for additional stiffening and strength. What the Arch. thinks he saw work/saw done 25 years ago may not meet today’s codes, we have much higher lateral load and wind load requirements with the latest rounds of code changes. This should mean that the newer buildings are being designed to a higher required standard, not that the old building is obviously at risk of failure, or that the earlier engineer didn’t do his job.
RE: 2x6 T&G as roof diaphragm
regards,
Michel
RE: 2x6 T&G as roof diaphragm
Also, I once framed a roof with 1 1/4" T&G sheathing for 4 foot oc exposed heavy timber trusses. We (the boss and I) found a reference for using staples at the panel edges to get the shear values we needed. Large firms have great libraries. I wish I could tell you where we found that info, but it was a long time ago and I haven't worked for that company in almost 9 years. Can't call either - they went out of business 2 years ago.
I'm wondering if there is a way to nail, or staple the T&G edges to provide the required shear transfer from one board to the other. It has to be done with care so as not to mar the interior surfaces.
If you can find the reference and/or make the numbers work, go for it.
LJ
RE: 2x6 T&G as roof diaphragm
RE: 2x6 T&G as roof diaphragm
RE: 2x6 T&G as roof diaphragm
Just never forget tht YOU are the engineer of record, not the Architect.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com