Roof sheathing material – Zip or not
I have a hunch that you guys will probably say go with Zip for the roof sheathing but for good measure I am going to put this out anyway since this is a legit question of mine.
CZ2, all-brick one story house. We will be using Zip sheathing of some sort (regular or Zip R3) to match with our open cell spray foam cavity insulation on a 2×4 stud wall. Should I do Zip sheathing on the roof deck as well, or is it ok to do the traditional OSB they use? I am also considering something like Sharkskin roof underlayment or the Grace Ice & Water Shield, but have not done the research on which one would work best for our climate zone (feedback would be so appreciated on this too). Unfortunately, budget is a big factor. Wanted to do something like the monopoly framing Matt Risinger did in some of his projects but nobody here in Houston TX so far is familiar with it to be open to do it. Would be risky to expect contractors who are not used to it to be able to install correctly as they are super busy and want to just get each project done quick to move on to their next ones. Thanks in advance!
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Replies
While zip sheathing is nice to have, I wouldn't call it a requirement. There's nothing wrong with osb/plywood and tar paper. If they get the decking on, and get the shingles/tiles/metal on soon after, there is little benefit to the zip layer, which water shouldn't hit. Rather, I would focus on taping the seams with a good quality tape, either zip or the 3M 8067, and really nail down the details on the starter course, eaves, and underlayment / single fastener pattern.
If it is going to be exposed for some time, I might change my tune. Check out the fortified roof guide, its a nice read.
https://fortifiedhome.org/roof/
I think I posted something about this before, but, who knows...
Zip sheathing is nice, but by no means necessary. I was also looking at 5/8" Advantech for my roof, but it was special order and therefore more expensive than Zip.
I ended up with Zip, but still put underlayment over it. I trust Zip as an underlayment, but I don't trust all the abuse roof sheathing might go through before it gets covered up. My roof was exposed with just Zip for a long time. For that, Zip served its purpose and was worth the premium over commodity products.
If you plan to quickly proceed from sheathing to roofing, worry less about the product and more about the attention to detail (taping seams of the roof, in a lot of cases) and the roof installation.
I just installed zip. Considering an additional underlayemt as well. What did you go with? #30?
deaneasy,
What does your roofing manufacturer require?
A breathable underlayment or asphalt saturated roofing felt if I go with the Pabco composite I’m looking at. Guess that rules out any of those self adhesive unbreathable products.
Is the roof also an air barrier layer that will be sealed? Yes, then Zip makes sense but isn't the only choice. No, then it doesn't matter what the sheathing is if it isn't the primary air barrier.
If this is going to be over a vented attic, there is really no point to using Zip here. The big thing about Zip is that it has a built-in WRB — the green coating. On a roof, you don’t really need a WRB because the shingles are supposed to shed the water. I wouldn’t consider the coating on Zip sheathing to be a replacement for ice and water shield, either, so Zip is really of no benefit on a roof. If this is a conditioned attic, with spray foam on the underside of the sheathing, Zip is again of no real benefit.
Zip won’t hurt you here, but it’s an added cost for a premium product in an an application where the extra features of the premium product don’t gain you anything. If you want to upgrade your roof sheathing, consider using plywood instead of OSB. Plywood holds up better if it ever gets wet (from a leak) compared to OSB, and that is a potential benefit on a roof.
Note that regular OSB, plywood, and Zip can all act as air barriers if detailed as such. You don’t need Zip if you only need an air barrier.
Bill
So Bill are you really saying that the only positive for Zip on the roof is the extended exposure time to the elements while other things are being worked on?
Can you elaborate on why Zip isn't a real benefit in a conditioned attic scenario?
Here is what I love about continuing the Zip onto the roof system, there will always be some lag time before the roof goes on. Plumbing vents, maybe you want to pre paint eaves and fascia (spray), etc.. At the point the seams are taped I am dried in. Also, if it rains and I get a leak, at this point I can trace that leak and fix it before roofing goes on which will fail at the end of it's life and then the underlayment leak will be a real problem. Also, I am often doing a flash and batt insulation on sloping ceiling planes so I want to be sure that there is not a leak in the underlayment before the foam is sprayed.
For those reasons I think it pays off. Based on the alternative of stapling down building paper and having the wind rip it up a few times before the roof goes on, Zip wins.
gawdzira,
Building paper is a poor alternative, but sheet underlays provide the same protection and can be left exposed for months.
Malcolm,
Can you elaborate on that? I'm considering Zip for my roof shingle base (OSB structural sheathing, foam, 7/16 zip top layer) for the reason gawdzira noted: it's going to be awhile before I can get underlayment/shingles on it. I want to make sure it doesn't leak in the meantime and that the sheathing is dry when I'm ready for roofing. Even if I tape OSB/plywood, I'd be worried about it soaking up the rain. I figure Zip will ensure a dried structure come roofing day.
gary_b,
The sequence of construction I use is installing a synthetic underlayment as soon as the roof is sheathed. The ones I use are rated for 6 months exposure, and provide a good textured surface to make walking the roof easier.
Is there a particular synthetic underlayment you like under polyiso foam + EPDM on a low slope roof?
kelly,
Sorry, I haven't done a low slope roof in three decades. Hopefully someone else will know which work well.