Best practice for asphalt shingles over new Zip roof – do I need an extra layer of underlayment?
alexdorf
| Posted in General Questions on
Building a new roof vented gable rood in souther California. In using Zip sheathing for the roof, would I need to add another WRB prior to installing the shingles? I know adding one is a belt and suspenders approach, but given the climate I’m curious whether I can skip it.
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Replies
Alex,
Although I build in the wet PNW climate, for some reason underlayment was rarely used here until about two decades ago. I've had occasion to re-do a few roofs done that way and my experience has been that the damage to the sheathing I saw was in the predicable areas - gable ends, penetrations, and poorly flashed intersections - not in the field where underlayment might be expected to make a difference.
Would a layer of underlayment yield a better roof? Sure, but I think more important is to deal with the problematic areas, insuring they get peel & stick membrane or the appropriate flashing.
Got it! Thanks Malcolm!!
Oh, what a loaded question.
While I am absolutely a fan of Zip as a WRB, I'd be less likely to use it as the exclusive roof underlayment on a structure. I realized a couple realities on my own home (which has 5/8" Zip sheathing on the roof):
1. Roofs take a beating where sometimes just taping seams is not sufficient
2. Roofers gonna do what roofers do
My whole house had been roofed/re-roofed in Zip. Seams taped, etc. It took a few heavy rains to track down all the sneaky leaks. My framers were good about taping and rolling seams, but there were still a few holes that missed framing or were there from staging that allowed some leaks. We believe we got them all, ultimately, but adding underlayment felt like a nice, cheap insurance policy.
Roofers are used to putting on underlayment; it's probably more confusing for them to *not* install it. It doesn't cost much, in the grand scheme of things. In fact, it's required for some shingle manufacturers' system warranties (should you be pursuing such a thing). I think it's squarely in the realm of "can't hurt".
I think the appropriate balance point is: use the sheathing of your choice, but no matter what, *tape the seams*. And then still put underlayment over it. Taping roof sheathing seams will dramatically increase the rogue event safety of the roof, even if you get some small leaks here and there. At the time I was doing this on my house, I considered 5/8" Advantech, but it was a special order only product and therefore cost more than 5/8" Zip. So, I did Zip and happily left the sheathing exposed to weather much longer than any sane person might, otherwise.