Roofing Fastening Detail - Loads and General Critique 2

[KootK]

I'm hoping to evaluate the uplift capacity on the re-roofing detail shown below. I don't have a great depth of experience in this space and could use some bounce. Feel free to answer any of the questions below with no obligation to answer them all. US / Canadian / Other Places answers welcome. This is Canadian work FWIW.

1) Other than testing, how does one go about calculating uplift loads on roofing? This is related to a recent thread by @bookowski: Roof Topping Slab in High Wind.

2) Should I be worried about screws hitting the T&G joints?

3) Is there a way to install this and avoid hitting the T&G joints with the screws?

4) Do I have to satisfy that NDS thing where connections have to extend at least to the centroid of the supporting thing (deck boards)? That could get tight for tolerances on a 1.5" thick substrate. Obviously, it won't be cool if the screws are sticking out into the interior space below. The soffit is exposed.

5) Is this a generally sketchy setup?

6) Got any better / more conventional ideas? We're exploring adhered but not loving that on a pitched roof.

 

[RFreund]

If this were a wall cladding assembly like a rainscreen (so vertical application instead of a slope), the most common solution would be the z-clips as mentioned. Yes, this has some thermal bridging, but the idea would be to space them out to minimize the effect. Say a 6" long z-clip spaced 48" in one direction and 24" in the other.
Having said that some claddings or furring strips are fastened through thick insulation into wood or metal studs which rely on the truss action you alluded to.
The tricky part of the originally proposed detail is making sure the screw has enough penetration but not too much to penetrate to the interior side. Technically I think it could work, just not sure on the installation.

 

[jhnblgr]

I had a client in awhile back that wanted something similar. This is what he used https://www.fastenmaster.com/resources/download/headlok-technical-evaluation-report-1009-01-2

 

[jayrod12]

We do a whole bunch of building envelope stuff in our office. When having the discussions with them, the Z-purlin plan is typically used. Full elimination of thermal breaks is not generally feasible in all scenarios but the reduction of the thermal breaks is typically the goal. Therefore the small amount of thermal bridging is considered acceptable. The Z-purlins would run perpendicular to your roof rafters and therefore the actual thermal bridge would be a small area.

An improved detail is a grillage of z-purlins, the lower level perpendicular to the roof rafters, and the second level parallel to rafters. Then the contact thermal bridge area is only 2"x2". But that's double the pieces and double the fasteners, so it's expensive.

Wood is also a natural insulator in that regard, so maybe the z-purlins full depth isn't as big of a deal as you're making it out to be.

Lastly, I've used the headlok tech paper for wall cladding designs. In that case a bit of vertical deflection is anticipated and accepted to allow the screws to work in tension like a truss. I've never done it for 8" of insulation however, only up to 5"