We have a hip roof, with dormers. We put in open valleys when we did
the roof, about 10 years ago, and the exposed rolled roofing in the
valleys is deteriorating pretty badly. I've been told that standard
practice here is now to do a 'woven' valley, which is what we'll do.
My question is regarding the ice barrier for the valley. We sometimes
get fairly heavy snowfalls, and a fair number of freeze-melt cycles
over the winter, and ice dams are a problem. The valley ends halfway
down the roof. So, at its bottom, one side is over the dormer
overhang, and the other is just the main roof sheathing. The valley
discharge just flows onto the main roof.
Snow can accumulate at the end of the valley. I understand the
methods for lining the valley to deal with water backup above this
point. But, what do you do just below the end of the valley? It
seems like there is potential for water backup there, too. There are
two areas that concern me:
- Water could dam back under the main-roof shingles, particularly back
under the dormer overhang.
- Or, it could dam back up the centerline of the valley, between the
underlayment and the metal liner, since the liner can't perfectly
conform to the sharp angle where the two roofs meet.
All I can think to do is to use a lot of roofing cement. Other ideas
would be appreciated.