Fastener spacing for roof deck over 9″ of rigid insulation
I have two questions: one is spacing, one is product.
Working on details for my roof and can’t seem to find a solid answer on spacing for the super long (13-14″) screws that will go through about 10.25 inches of material and then into my rafters.
The assembly from bottom up on my shed roof at 3/12 pitch:
– 3.5×9.5 LVL Rafters, 24″ OC
– 1×8 pine shiplap nailed with 8D galvanized nails
– Sharkskin SA roof underlayment
– 3 layers of 3″ polyiso (9″ total)
– 1/2″ CDX nailing deck (THIS IS WHERE THE SCREWS GO)
– Ice/Water Shield (applied by roofers)
– Standing seam metal roof
Also lookin for recommendations on what screws to use. I’ve found these 14″ for instance: https://www.fastenersplus.com/products/275-x-14-conquest-structural-screw-t-30-star-drive-pkg-25
Thanks in advance! ~ Seth
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Replies
Heco Topix stainless screws. You want stainless over carbon steel since they only conduct 1/3rd the heat of steel.
Consider skipping the second roof deck and only using furring strips. If your metal roof panel does not support 24” oc supports you can lay the furring strips at an angle. Matt Risinger has some old videos using this method.
Thermal conduction of the fasteners is not a significant contributor to the overall heat loss of a structure. I actually showed this mathematically in the GBA forums some years ago, where I calculated the heat loss in BTUs due to the fasteners. I worked the numbers on a wall, but the principle (and result) is essentially the same for a roof.
If the stainless screws are significantly more expensive then regular steel fasteners, then I would not spend the extra money for stainless if the concern is thermal conductivity. If there is only a small cost premium, or none at all (which I doubt is the case), only then I would I consider stainless, and more for longevity than thermal performance.
While it is true that steel fasteners conduct heat, they are a very small overall cross sectional area of the structure, and they are embedded into a thermally insulating material (wood), so they do not conduct heat out of the structure as badly as people may think. The BTU loss of the structure contributed by the fasteners is minimal compared with everything else. Your efforts and money are much better spent on air sealing details than on trying to minimize thermal losses through fasteners.
Bill
matthew,
Angling the furring helps provide a drainage path, but actually increases the span for the same spacing.
There are a couple of disadvantages to using furring over a solid substrate:
- The furring creates a cavity where moisture can accumulate - which then needs a drying path. No cavity means no humid air to condense.
- Using furring transfers the load-bearing from the sheathing to the metal roof panels, making them more susceptible to damage from workers or debris.
Thanks for the replies and I appreciate the input regarding the assembly and materials. It took a good deal of research and some time with building science consultants to reach my several layer approach. My control layers are pretty thought through too. Obviously nothing is perfect, but I’m feeling pretty confident and only time will tell.
In any case, I’ve actually landed on 12” Fastenmaster HeadLok screws and feel that going through the 3/4” shiplap and then another 1.75” penetration into the LVLs should be solid against uplift. I hope. What I I’m still trying to dial in is how many per 4x8 sheet of 1/2” CDX.
Thanks again!!
NewSkills,
From this article: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
"According to building scientist Joe Lstiburek, you don’t need as many screws to hold down these 2x4s as you would need to fasten furring strips to a wall. (For information on wall screws, see Fastening Furring Strips to a Foam-Sheathed Wall.)
Here’s what Lstiburek has to say: “How many [screws] and how far apart? The good news is that this is less complicated than installing continuous insulation on walls. … Less gravity, better friction. Uplift is the problem, not slippage and bending moment of the fasteners. The folks who do commercial flat roofs have this uplift thing dialed in. The same fastening requirements for uplift for flat roofs will work for these roofs while simultaneously handling the slippage issue. Except where it snows a lot. Snow likes to stick to sloping roofs. With lots of snow things get very complicated. The good news is that structural engineers have a good handle on this — especially ones who work in ski resorts — particularly Swiss and Austrian engineers.”
To summarize: if you don’t expect a lot of snow, you can install the same number of screws (for the upper layer of plywood or OSB, or for the 2x4s, if any, installed above the rigid foam) that are recommended 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 — and you’ll have more than enough screws. If you expect a lot of snow, however, you should consult an engineer."
Thank you as always Malcolm. My thought was 18” and with your input I’m feeling solid in my plan. ~ Seth
Seth,
Practically, you are looking at a spacing 24" oc along the roof to hit your LVLs, and either three fasteners (22.5" oc), or four (15" oc) up the roof slope.
That makes sense, of course! Thanks!
First question to answer is if you actually need 9" of rigid.
When building with expensive insulation like exterior rigid there is no ROI going above code min as the operating cost saved is very small. Also when you have continuous insulation most codes allow for U factor based compliance which generally works to R38 to R42 of assembly. U factor compliance also gives you a bit of credit for the rest of the bits in the assembly like the T&G and the roof deck so you get about an R3 for free.
If you also bump up the T&G deck to 2x6 and you use the right length fasteners, the fasters can now go into the T&G anywhere without any worry of shiners. You sometime have to adjust the rigid or the top roof deck thickness a bit to work with standard roof fasteners.
As for fasteners look for roof deck ones. These are used for holding rigid down on commercial roofs. They can be had in many lengths in come by the pail and much cheaper than anything else.
Seamed metal roofs are liquid tight, so that I&W of the 2nd roof deck is not buying you anything. I would only install it where code requires it or you have some ugly intersections. For the rest use regular high temp synthetic underlayment.
Thanks Akos. I found a great deal on recycled Polyiso through GreenInsulationGroup.com, but worried about the degraded R value so opted for 3 layers of 3”. Funny enough, they had to substitute one layer of 3” for 2 layers of 1.5” so now I’m thinking 7.5” will be safe. And I especially like the idea of the 2x6 T&G so I can stress less about shiners. Question now is, how deep do the fasteners need to penetrate into the T&G? Would 1” be enough? 7.5” polyiso, .5” CDX with 9” fastener.
R value doesn't really degrade continuously over time, it just goes down to a sort of stablized value and pretty much stays there. In the case of EPS, R value is pretty much constant, since it doesn't have blowing agents diffusing out over time. With polyiso, you start higher than the rating, and eventually will get down more around R5.5/inch or so. It's common to derate polyiso to R5/inch in very cold climates, but there isn't really any need to worry about it performing any worse than that unless it's waterlogged or otherwise severely damaged in some way.
Bill