TL;DR: Can I attach metal roof to dry underlayment but damp sheathing? Or should I wait until spring drying? 

Setting: Sonoma County, CA, zone 9, installing a metal roof, installed Tyvec 160 synthetic roofing underlayment. 4/12 hipped, 30′ eaves on most of the house, formerly shingled. With the “help” of my 6 yo son there are more staples in the underlayment than absolutely necessary. Most of his staples are not capped, all of mine were. Underlayment looks good, mostly smooth, capped staples in every spot called for. 

During the recent “atmospheric river” rain event here in Sonoma County, CA our underlayment leaked in several places. Significant leaks in one bedroom window (ironically, his room), and two other leaks inside the house. In the garage I can see wetting on the underside of the sheathing in a couple places, primarily at sheathing seams. I assume something similar is happening throughout the attic but haven’t looked to confirm. 

I was shocked that the underlayment leaked this much. It doesn’t seem like the staples really create that much of an opening. Also, the underlayment seems to be “soaked,” best way to describe it. A LOT of water came down in a couple days, so pretty much everything got soaked through. From comments I read here I shouldn’t be that shocked that we got leaks, as the underlayment is really only for handling minor leaks. It also occurred to me that this underlayment went through the super high winds during the Kincade fire which nearly burnt our little town. Some of those gusts were over 50mph and I can imagine all that buffeting and jostling loosening the underlayment around staples. 

All of this said, my local advisors (dad, mom, wife) all thought through the drying potential of applying the metal roofing in the next couple weeks with likely damp sheathing underneath the underlayment. Our thoughts were that there still exists significant drying potential even with the metal roof on and that the sheathing would be able to dry. We reasoned that the underlayment and the roofing assembly has plenty of permeability to water vapor. Assuming we do a good job attaching the standing-seam metal roofing no additional bulk water will be entering. Then the underlayment and the sheathing will be able to dry over time. Our summers have plenty of days over 100. Even in January we frequently have days in the 70’s, so that seems to support the drying out theory
. Also, the undersides of all of the sheathing is open to plenty of warm dry air. It occurred to us later that the sheathing is also perforated all over with holes from shingle nails. Lots of little bits of crumbled shingle fell through the holes into the garage, and presumably into the attic as well. All those perforations will certainly help the drying.

Sorry so long to tell the story, but the basic question is do you think we thought this through correctly? Will our underlayment and sheathing really dry out? Or should we wait until next summer and tear off the underlayment and do it all over again over total dry sheathing? (Of note, I’m not totally clear that all the sheathing is wet, just some that I can see.) I would hate to do that. But I’d also hate to screw down a 50-year metal roof to a slowly rotting patch of sheathing. Thoughts? Comments?

Related thought. If underlayment really leaks this much from staples won’t it leak at all of the spots you put a screw through it for the metal roof? Or does the tightness of the metal and the screw at that spot preclude much leaking? This experience has made we wish that maybe I had painted the roof with RedGuard or something like that to keep it waterproof and then thrown on a much less expensive underlayment that would protect the RedGuard and keep the metal from sticking to the RedGuard. But maybe that’s overkill, hard to think totally straight when there are active drips of water coming through your walls!

Many thanks in advance for any thoughts. 

Christopher Peck