Dana,

The 2x6 T&G is the finished inside ceiling. This is what you see when you look up. All of the insulating must take place on the other side, this being the outside, of the T&G. The roof system is as follows from the inside out:

2X6 T&G then 2x6 rafters on top of the T&G (spaced off of the T&G 1.5") with 5" - 2 lb. SPF in between the 2x6 rafters then 5/8" plywood on top of the 2x6 then roofing (type tbd)

RGates I'm assuming that the t&g is spanning timbers for structural support. Therefore when you look up at the ceiling you will still see the nice big timbers with the t&g (2x6?) nailed to them. From the top side down you are suggesting sheathing nailed to 2x6 rafters that bear in 2x4 (or 2x2) on the flat which are nailed to the top side of the t&g ceiling probably over the timber purlins. Are you venting the space below the sheathing? If just considering the expense of rigid vs. spray consider laying out all the big areas with rigid and than letting the spray boys do the complex areas. With the low cost of lumber consider using 2x10 or 12 instead of 2x6 and spray an inch or two of foam and than filling with spray cellulose.

In an attempt to clarify the assembly here is a rough sketch. The sketch on the left is a side view, the right is an end view. The T&G bares on the timbers inside and is a finished ceiling.The depth of the SPF will ultimately determine the air space under the 5/8 sheathing, allowing a cold roof . The blocking under the 2x6 allows the SPF to flow under to help alleviate thermal bridging. The 2x6 rafters will be at least 24" o.c. The spacer block separation is TBD.

How do you intend to secure your exterior 2x6 rafters to the structural timbers inside? Just via the connection to 1-1/2" spacer? If that is all, I would be concerned with wind uplift pulling your roof off. While the SPF will provide some adhesion to the interior T&G 2x6 which is attached to the structural timbers, I'm not sure how much credit you can take for that in an uplift situation. Do you have any plans for metal strapping or Simpson ties to address uplift?

I think he was planning on fastening through the 1.5" isolators to tie the 2x6s into the timber frames.

Posted By WindowsonWashington on 03 Dec 2011 11:47 AM
I think he was planning on fastening through the 1.5" isolators to tie the 2x6s into the timber frames.

How?  Toe nailing or long timber screws?   Also, the timber frames are generally spaced much wider than 24" OC so they typically won't line up with his 2x6 rafters at 24" OC.  I didn't see any mention of this in the previous discussion.

Posted By arkie6 on 03 Dec 2011 02:07 PM

Posted By WindowsonWashington on 03 Dec 2011 11:47 AM
I think he was planning on fastening through the 1.5" isolators to tie the 2x6s into the timber frames.

How?  Toe nailing or long timber screws?   Also, the timber frames are generally spaced much wider than 24" OC so they typically won't line up with his 2x6 rafters at 24" OC.  I didn't see any mention of this in the previous discussion.

Not sure but you raise a very good point.  I would not want to count on the adhesion of CC SPF for my wind uplift resistance.

With the 2x6 t&g under the rafters (rather than half-inch gypsum,etc) you can't have to worry about uplift issues- if you have enough uplift to break through the t & g it means a bomb has gone off in you kitchen or something. (I assume you're using framing ties to bond the rafters to the structural wall for hurricane & earthquake-proofing?)

If you're really going with 5" of ccSPF it will be more structural than nails or timber screws. (At 2" maybe not.) At 3' the SPF is sufficiently structural from an uplift point of view, even with a gypsum ceiling- the stuff is essentially Gorilla Glue (tm) with a controlled density.

If going ccSPF you can nail the t & g, and let the additional grip of the foam keep it from shaking loose & coming down in a magnitude 5 earthquake, otherwise screwing them in place is advisable.

Arkie,

The roof system has timber purlins not timber rafters, so there will be chances to attach at approximately every 4' to 5' depending on the section of the roof. I have not worked out the attachment detail yet. I was considering a Teco or Simpson style fastener or maybe fabricating  up something like a joist hanger but elevated.

Dana,

The timber frame itself is attached directly to the foundation.

To ALL,

This system is a concept that I am considering using, it is not be any means set in stone. Please feel free to scrutinize and introduce any ideas, that is what this is all about. There is a ton of knowledge lurking around in these forums. Multiple minds are better then one mind all day long! I greatly appreciate all of the input.

Posted By Dana1 on 03 Dec 2011 03:25 PM
With the 2x6 t&g under the rafters (rather than half-inch gypsum,etc) you can't have to worry about uplift issues- if you have enough uplift to break through the t & g it means a bomb has gone off in you kitchen or something. (I assume you're using framing ties to bond the rafters to the structural wall for hurricane & earthquake-proofing?)

If you're really going with 5" of ccSPF it will be more structural than nails or timber screws. (At 2" maybe not.) At 3' the SPF is sufficiently structural from an uplift point of view, even with a gypsum ceiling- the stuff is essentially Gorilla Glue (tm) with a controlled density.

If going ccSPF you can nail the t & g, and let the additional grip of the foam keep it from shaking loose & coming down in a magnitude 5 earthquake, otherwise screwing them in place is advisable.

I think he referring to the 2x6s themselves and the non-structural roof deck at that point.

I know that SPF (especially CC) has great adhesion but I would not count on that and a screw every so often across the deck.  SPF does have a history of shrinking a bit overtime and that would cause it to pull away from the 2x6s. 

Shrinkage on cc SPF is usually installer error, far more likely when shooting more than 2" per lift (in contravention of manufacturer's specs) at less than optimal temperatures.

But screws are cheap insurance, to be sure!