Winter construction metal roof
Building a new house in Zone 7A Edmonton, Canada during winter temperatures (could be below freezing) and looking for which underlayment to use for the metal roof (24 guage, mechanical fastened, med/dark grey). The roof is vented/vaulted, 6/12. Electrical heater in the basement during construction to keep basement above freezing.
Roof Envelop = 5/8′ drywall+ resilient channel + smart vapor barrier + 24″ deep open web chord trusses with the full heal sitting on the wall + R60 Rockwool batts (16″) leaving an 8″ air gap + plywood + underlay + metal roof. It’s mostly all a gable roof with a ridge vent. The eave edge will be ice shield but…1) use mechanically fastened for the balance? 2)is permeability useful? 3)Considering the design, is there any further sound proofing needed? 4)Is 1/2″ plywood enough or 5/8″?
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Replies
vlahojacob,
1. Use a mechanically fastened synthetic roof underlay rated for use under metal roofing.
2. The sheathing can dry into the vent space below. Permeability is of no advantage.
3. I'm a bit unsure as to whether the res bar will act as you want it to. It is typically used on one or both sides of an acoustical assembly to mitigate noise transmission. But in your case the transmission will be between rooms that are both under the same roof, and the air-space where the res bar is may act as a bypass route for sound to move laterally.
4. If you can afford it, go with 5/8"
Thanks Malcolm, the winter construction was a bit unexpected but it's looking doable with a few product switches here and there and some deeper thinking on construction sequencing but no biggie. Your items 1,2,4 will go as you advise. For 3) all vaulted areas will be R-bar and then I have an acoustic plan that considers flanking, penetrations, isolation, attenuation, etc and heck it makes the thermal envelop thinking look easy!
vlahojacob,
"I have an acoustic plan that considers flanking, penetrations, isolation, attenuation, etc and heck it makes the thermal envelop thinking look easy!"
Excellent!
you already have an incredibly expensive roof, i tend to gravitate to peel and stick everything, especially if its not standing seam. but its a little more. definitely use 5/8, esp if you are spacing at 24 with truss.
Thank You! 5/8" it will be...a bit of a detail perhaps but, is there a grade of plywood beyond CDX you would use?
For a roof like this in Edmonton, I’d be more concerned with getting the whole assembly right than choosing a particular underlayment brand.
With the 6/12 slope, R-60 Rockwool and an 8" ventilation cavity, the setup sounds pretty good on paper. The important part is making sure that ventilation path stays continuous from the soffit to the ridge and isn't restricted around the heel/eaves.
For the underlayment, I'd use ice-and-water protection at the eaves and other vulnerable areas, with a good synthetic underlayment over the rest of the deck, assuming that's compatible with the specific metal roofing system. Since you're dealing with Edmonton winters, I'd make sure whatever self-adhered product you use is actually rated for the temperatures and installation conditions.
Permeability is useful, but I wouldn't make that the main deciding factor. In this assembly, proper air sealing on the warm side, the smart vapor-control layer, insulation and the continuous ventilation space are more important for managing condensation.
For sound, I don't think you necessarily need to add anything else. 5/8" drywall + resilient channel + R-60 mineral wool is already a pretty substantial interior assembly. Metal roofs can obviously be louder during heavy rain or hail, but that combination should help considerably.
For the plywood, I'd personally lean toward 5/8" rather than 1/2", particularly if the framing spacing and the metal panel manufacturer's requirements support it. But I'd follow the actual roofing manufacturer's deck requirements rather than choosing based on thickness alone.
One thing I'd definitely confirm before ordering materials is the exact metal panel system. Exposed-fastener panels and standing-seam systems can have different requirements for underlayment, fastening and movement.
Overall, your proposed assembly sounds reasonable. I'd have the roofer/engineer specifically check the eave/heel detail, ventilation continuity, snow-load requirements and the manufacturer's installation requirements before locking it in.
I've spent some time looking at roofing assemblies myself, and companies that handle the roofing, waterproofing and ventilation details together tend to be the ones worth talking to. That's one thing I like about the approach used by Asbury Home Improvements — they cover more than just the roof covering itself, which is important on a cold-climate project. Their site is asburyhomeimprovements.com if you want to see the type of work/services they provide.
I suspect this is AI spam.
Shucks fooled me, thanks for the extra learning.
Spot-on thanks! I'll go with ice & water (with primer + heat conditioned rolls + electric heat in the basement) and synthetic for the majority on top of 5/8". As you advise, I've been talking to contractors & suppliers and all roof fasteners will be concealed but, there's a lot more to resolve such as spacing between the 5/8" plywood sheets, how taut to pull the synthetic underlay during cold install, types of capped staples/nails.... I thought i was smart learning about wood pecker proof stucco and now I realize even roofing is complicated!
I prefer self adhered high temp ice and water over the entirety of the roof, fasters can radiate and show through the panels. Not too brand loyal, shop around for price. Also, 5/8 t+g will help prevent deflection at the seams between the 24in oc framing members. Advantech is great here (plywood clips will radiate too).
CC,
One of the nice things about metal roofing is that unlike with shingles, it doesn't matter much if the horizontal seams between the sheathing stay co-planar in between the framing, . So H-clips aren't practically necessary.
Oh, I didn't think about T&G, thanks for that point. Is a 1/8" spacing between sheets reliably achieved in real world? I also picked up on Malcolm's point below about not needing H-clips on the horizontal.
Also meant to ask you, for winter install potentially below freezing, would you then use a primer vs a self adhered (will have to put ice & water on at least the eaves etc). For fasters radiating through, do you mean for synthetic underly - aren't the staples or nails capped to prevent that?
vlahojacob,
The Alberta building code doesn't require H-clips or t+g for 1/2" and thicker roof sheathing at 24"oc. spacing.
Most nominal 4ft x8 ft commodity sheathing at lumberyards is 1/8" smaller and marked "sized for spacing".
Staples don't telegraph through. The odd cap-nail can sometimes show on the panels - but it's nothing serous.