2x6 T&G as roof diaphragm 3

[WWTEng]

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?

 

[MiketheEngineer]

If you can make the connections!!?? and prove it!!

 

[WWTEng]

Mike, I think what you are saying is that it would be hard to tell where the framing members are, correct? I think length of the nails might be an issue too.

 

[Michel60]

Good morning WWTEng,

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

 

[frv]

I think what mike is referring to as well is that the Yield Line Equations would not apply so you wouldn't be able to calculate a lateral capacity for the nail.

 

[msquared48]

I would put an extra layer of 3/8" CDX over the decking and nail off at the regular spacing with 6d nails.

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

 

[ztengguy]

What if the foam was glued down, and then plywood glued to it?

 

[dhengr]

I don’t have the copies of the codes you are looking at, but I agree with Michel60. The 2x6 T&G deck does have some diaphragm strength, that worked for years, just not as much as when compared with well built osb or plywd. diaphragms. The plywd. has to be in direct contact with the 2x6 decking to be effective, and nailed directly to it. Glue and nails to each plank would be even better. The idea is that the plywd. and plenty of nails keeps the individual planks from slipping w.r.t. each other at their T&G joints. Plywd. thickness and many smaller sized nails and spacing are a function of the diaphragm forces you are trying to achieve. Edge screwing the planks together, as they are laid up, can work too. You can’t cantilever the nails 3" through the insul. to make a diaphragm. The Arch’s. top layer of plywd. is only there to protect the insul. from above. Don’t they sometimes apply roofing right over the rigid insul. without the plywd. protection? Just like SIPS the rigid insul. in a panel has substantial shear strength, but that has to be factory made (and one continuous layer) and tested to be proven. I don’t think any code will give you any credit for three 1" layers, laid up and glued in the field.

 

[woodman88]

I do not think you can get a Dowel Bearing Strength of the foam to be able to use the NDS equations for a shear value of the nails. And like dhengr states you can not cantilever the nails through the foam.

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.

 

[WWTEng]

Michel, dh,

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.

 

[WWTEng]

Woodman, thats what I thought. But do recommend using 2x6 T&G as a shear diaphragm and if so, can you share a load table with me here! I googled the shear capacity for T&G but couldn't find anything. I don't think I am going to use T&G as a diaphragm but I am curious.

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.

 

[TheBlacksmith]

There are so many potential slip planes regardless of the gluing and such a moment on fastners through 3 inchs of foam that the diaphram effect of this arrangement would be almost neglible.

 

[woodman88]

See Table 4.2D on page 22 in the attached pdf. It is a free download from

http://www.awc.org/publications/download.html.

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.

 

[MiketheEngineer]

Architects say all kinds of crap all the time and "IT ALWAYS WORKS"

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.....

 

[dhengr]

WWTEng:
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.

 

[Michel60]

One detail I've seen in the past for this application is T&G on roof framing with 2x on edge "egg-crating" above and then the structural panel. In this particular case the insulation was liquid expansion type but I guess you could do it with lay-in rigid too. Definitely not an inexpensive assembly (basically you build the roof twice) but there are lots of things you could work with in order to meet all the connection and capacity requirements others have pointed out.

regards,
Michel

 

[McLJ]

I have actually done this before - several years ago. I can't remember 100%, but I believe the T&G was used by itself for the diaphragm. I do recall that it was only for a specific area of the house and the rest was conventionally built. It is a very popular look. I would personally be more comfortable with a layer of OSB or plywood on top.

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

 

[WWTEng]

Woodman, thanks for the link. I didn't realize there would be such a big difference in capacities for horz vs diagonal layout.

 

[WWTEng]

dhengr, I was finally able to get the drawings for the project that the arch has been quoting. One glance at the note sheet made clear that it was not really engineered, the wind criteria indicated was incorrect (the project has been around for over 10 years). But looking at the dimenions of the where 2x6 T&G was used (60'x60'), I actually think that T&G would have worked as a diaphragm since the max shear is like 60 plf. In my case I am at 350 plf, there is no way that T&G would work, I am going to specify a layer of CDX over T&G.

 

[msquared48]

Good choice.

Just never forget tht YOU are the engineer of record, not the Architect.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com