roofing: valley, ice dam questions

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.
TIA, George
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On Sat, 25 Oct 2003 14:01:41 -0400, ge wrote:

Perhaps you do have inadequate ventilation (re: rusty nails). Ice dams are caused by inadequate insulation and ventilation. You might want to address these issues while your tackling your roof.
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woven valleys only can be done with certain shingles. if you have architectural shingles a western cut valley would be better. install water and ice barrier in the valley and galv. metal or a copper valley.

real roofers NEVER use roofing cement that is just a cover-up a metal valley will NOT likely dam. whereas a weaved valley will dam. use lots of water and ice barrier. I do NOT recommend roofing cement otherwise you will be patching it forever and it will be very difficult to find the source of a leak. roofing cement tends to funnel water. and yes I am a roofer.

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