Addition roof
I’m designing an addition to an old house that has an unvented cathedral roof with (i’m sure) poor insulation. The eave of the addition aligns with the eave of the original house so the intersection is like a butterfly roof and we’ll have a sloped roof between them, perpendicular to the primary slopes, to shed water off. I’m concerned that roof melt from the old roof will run down to the new valley created by the addition and cause significant ice dam issues. I assume that to avoid this I should tell the owner we need to add insulation and proper air sealing to the existing roof above the addition. That means a fair amount of work on the existing house which I’m guessing they wouldn’t expect because it’s not part of the addition. Has anyone dealt with this before and does my concern seem reasonable? Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
John, here in ice dam country, the term "butterfly roof" gives me butterflies. Even with adding a large cricket as you describe, it is likely that snow will melt at the old roof and freeze at the cricket. I can't think of a solution other than improving the insulation and air sealing at the existing roof, or creating a vent channel so the sheathing stays cold.
Just no. It will always leak.