How do you calculate the number of structural screws that would hold down osb over 4″ of polyiso screwed into a TJI Roof?
MikefromtheMountainsofUtah
| Posted in General Questions on
My engineer is not sure about how to calculate this, which is perplexing because I’ve seen the concept mentioned multiple times here.
The low slow (1.5/12) roof is TJI’s. There is a layer of sheathing and then 4″ of insulation, another layer of OSB, and that will be structural screwed downward into the flange of the TJI. Per Boise’s engineers and their screw document the holding force per screw is a couple hundred pounds. How am I to calculate how far apart the screws should be?
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You have to calculate the uplift on your roof, which is dependent on slope, wind speed, exposure, surface area. The uplift will be generated by aerodynamic forces of wind over your roof that causes pressure differentials. I bet if you look in ASCE you will find an equation
IRC table 301.2(2) shows the positive and negative pressure for claddings. The negative value is the uplift force. "Zone" refers to figure R301.2(8) which is hard to read but you can find clearer versions online. https://codes.iccsafe.org/content/IRC2018/chapter-3-building-planning#IRC2018_Pt03_Ch03_SecR301.5
Mike,
This was the advice Martin gave from this blog:
https://www.greenbuildingadvisor.com/article/installing-rigid-foam-above-roof-sheathing#comment-267412
"When fastening the upper layer of plywood, OSB, or 2x4s above the rigid foam, use the same number of screws that you would use for furring strips installed on walls—basically, one screw every 24 inches along each rafter, with a minimum penetration into solid wood of 1½ inch. That fastening schedule is more than enough, with one exception: climates with lots of snow. If you live somewhere with lots of snow, you should consult an engineer."
I think this is very dependent on what screws you use….
freyr-design,
The link Akos posted has some useful information on that.
It's not something I know anything about beyond what I've read here.
That is a good resource.
I only mentioned that as company to company and size to size there can be large difference in pull out and pull through resistance.
Exactly. And per BCI my joist is spec'd to take some decent sized ones. https://bcconnect.widen.net/s/kwjrrcrtkz/ij-34_flange_screw_provisions_2021_02
1/4" lag - 150 lbs
3/8 - 205
That's exactly what I have been planning from and the reason I contacted the engineer. I am certainly in an area of high snow load but I believe that would be 100% irrelevant on a low slope roof. The snow will help, not try to shear off the upper surface as it would on a high slope roof.
This would be compounded as the shearing forces on the screw would actually become tensile and compressive forces as it turns into a truss with the foam. But as you say, that low of a slope probably doesn’t matter…
The fastening schedule for a nail base panel is probably a good starting point ie:
https://hunterpanels.com/wp-content/uploads/2023/06/UPDATED-H-Shield-NB-Installation-Guide-Web-File-Version-7.7.23.pdf
Dang. They aren't even sending the screws into the TJI's. That makes me think I'm over killing it.
It also makes sense if you look at how membrane roofs with rigid are installed, there are a handful of fasteners per 4x8 insulation sheet and they are just driven into the roof deck bellow. Those hold up just fine and this is not that far off. Note that the fastener needs to poke through the roof deck.
That's great to hear. Thanks.
With such a low slope roof, what roofing material do you plan on using? Rubber? Or a low-slope metal? With a low slope metal, depending on manufacturer specifications, you might be able to install strapping instead of another plywood layer.
Strapping might be worth considering anyway as the OSB ends will crush the foam with structural screws- that's assuming 5/8 T&G. If no T&G strapping is a no-brainer.
Strapping can be laid-out on the ground, pilot holes counter-sunk for the structural screws. This sounds like more work, but it'll save you a boat load of time as you can then gun nail the OSB. I typically use rough sawn pine 1x4 on 24" centers.
Interesting idea. I read that the use of metal (in my case standing seam is the only approved that low) will move air anyway so I was staying away from furring out. I am surprised that the force from the screw onto the osb will cause a collapse in the foam. I certainly hadn't thought of that.
Mike,
" I am surprised that the force from the screw onto the osb will cause a collapse in the foam."
Only if you overdrive them. Sheathing is routinely fastened over foam on roofs (as you can see from the details in Akos' link), and over slabs in basements without problems. Exterior insulation is an increasingly common assembly on single family residential roofs now, and has been used on multi-family and commercial buildings for decades.
Sounds good. Thanks
Sometimes you can screw standing seam clips directly onto foam with long screws and omit the osb on outside.
Pretty good specs from the rubber roofing companies.
foam screws are so cheap it is silly
A couple ideas about the standing seam metal roof. Standing seam moves very little air, not sure how much you are needing to move. Corrogated metal moves lots, but at 1.5/12 it will leak. Protrusions are a big deal on standing seam below 3/12 pitch. A simple overlap of metal, won’t cut it, water will move uphill a long ways. Like was mentioned, clips are available for fastening over foam. Bpdclips has some plate options. Bjarnes clips available at Steetz Copper Craft has some really cool clips for eliminating thermal bridging. Mechanical lock standing seam with traditional details is the best system at this pitch, in seam tape, or sealant might be a good idea also.
Have you contacted the structural screw manufacturers?
Fastenmaster for example has a Technical bulletin for furring strips over insulation on walls. They may have one for insulated roofs as well.
Walls: https://d3g5z6m6vcoqo2.cloudfront.net/resources/HEADLOK-TER-No.-1009-01.pdf