barrier between metal roof & rigid
crawfordesquire
| Posted in General Questions on
new residential construction in 5b. bottom to top unvented assembly: plywood, sa membrane (adhero), 6″ polyiso, then metal roof. then likely solar on top of metal.
is it advised to install a separate layer of some sort to serve as a buffer of heat between metal roof and insulation? or just go metal right over polyiso?
thanks
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Replies
crawfordesquire,
Are you missing a layer in your description? What is the metal roofing fastened to?
so i believe this will be ss screws [and washers] through sheathing. but not certain.
crawfordesquire,
If you are using plywood or OSB sheathing you need a sheet underlayment rated for metal roofing. If you are using ZIP you maybe able to forgo that, but depending on your code may still need a membrane at the eaves.
I prefer to install metal roofs directly on the sheathing rather than on strapping for several reasons.
- It reduces the risk of condensation on the underside. No air-space means no moist air to condense.
- Having furring also makes the panels load-bearing in a way they are not when backed by sheathing. That makes them more vulnerable to damage from debris or workers.
With 6” of polyiso you need some sort of material to fasten to outside of the polyiso. This can be an entire layer of sheathing or furring strips. Do you have 16” or 24” o.c. rafters? Vertical furring strips can work with 16”, but if you have 24” most metal roofing manufacturers require fastening every 16” so you would have to run them diagonally to satisfy that requirement. The furring strips are nice also because they create a rainscreen and vent channel.
mathew25,
Metal roof panels rely on their ribs (that run vertically) for their structural rigidity. I can't think of any that could be laid over vertical strapping, no matter what the spacing.
The OP did not specify what type of metal roofing. Standing seam metal roofing can certainly be installed over furring strips without the need for a continuous sheet of sheathing. This is usually the product of choice for post frame construction. Kyle Stumpenhorst and Matt Risinger both install standing seam over open rafters or furring strips. With some metal roofing types you may not be able to do this. Here is just one example I found with a quick Google search:
Page 8, bullet 3C
https://www.bestbuymetals.com/wp-content/uploads/2019/02/standing-seam-architectural-install-guide.pdf
matthew25,
Standing seam can be installed over strapping, my own house is built that way. Maybe I have misunderstood your post about spacing and direction, but my point was that you seem to be suggesting the strapping can be installed vertically, which it can't.
Okay perhaps we are both confused. My proposed assembly from inside out was:
- Sheathing
- WRB
- Polyiso
- Furring strips installed vertically (screwed into rafters or trusses)
- Standing seam metal roof
Hopefully that clears it up. Furring strips (over the polyiso) can be “vertical” in this instance, no? If they were horizontal they would act as water dams. Now of course they will really be at the same angle as the roof slope, not literally vertical as in plumb.
Matthews25,
No, unfortunately you can not install the furring strips in the same direction (vertically) as the panels. As I said in my post above (#5), metal roofing relies on the ribs for its structural integrity. It can not span in the other direction. If you want a ventilated gap you would have to use two layers of strapping.
You can test this quite easily. Take a panel of any profile. You can bend it easily into a circle parallel to the ribs, but not in the other direction.
Okay, I am seeing my mistake now. I guess the videos I’ve seen are at an angle, not truly parallel with the rafters. Like this Risinger video.
https://m.youtube.com/watch?v=OB94meyqZNk
matthew25,
You can use the manufacturer's spacing requirements to see how close the strapping has to be as you angle it. So for horizontal strapping most start at 16" oc - and you have to maintain that same spacing measured along the length of the panels as the angle increases. At 45 degrees they would need to be 11 1/4", and so on until when they were completely vertical they would need to be a solid substrate.