Ice Dams: Standing-Seam Metal vs. Composite Shingles
Standing seam or composite shingle on eaves with vented over-roof in Zone 7?
Before getting educated by Lstiburek on how to build a roof to prevent ice dams, I installed standing seam on the eaves to help the ice dams slide off.
Well, you can see how that turned out (pictures are worth a thousand words). Photos are today’s latest ice dam on the east side of the building.
I think the metal roofing helps accelerate the thaw and freeze cycle, and might make the ice dams worse.
With a properly vented air space just below the roofing, is there any benefit to having standing seam on the eaves? Or might composite shingle actually be better because it is less thermally conductive?
We plan to upgrade our open cathedral ceiling to a “perfect roof” with a vented “over roof”. Today, we have an R-4 roof with 4-foot overhang on the eaves (they are strong eaves, for sure).
From outside to inside, the proposal for a new roof is:
* 430 lb/sqf ground snow load on 6.5:12 sloped roof
* roofing material (composite shingle + standing seam over eaves)
* water barrier
* 2×4 stud bays for vented air space
* vapor barrier
* thermal barrier (polyiso)
* air barrier
* 4″ solid wood deck
* interior conditioned air
GBA Detail Library
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