Ice dam- leaking boot or bathroom vent design – GreenBuildingAdvisor

Hello,

I’m Andy

I am located in southern Ontario, south of Ottawa. I’ve been trying to diagnose an ice dam issue on a home I built last year. There is a roof leak in the area of the vent stack. I did not install the vent stack and boot, so I’m unsure if there was an error made there. However, there is extensive melting and ice damming in one rafter bay (which happens to be the same location as the vent stack). 

I’ve attached some photos showing melting and the large ice dam last winter.

My question is: is my bathroom fan, which is vented below the eave (2 ft overhang) through the wall blowing/allowing  hot air into that rafter bay, causing the ice dam? Perhaps the leaking boot is secondary? No leaks detected with rain, only seems to be cold weather months.

The house has a mono pitch roof with open soffit at the bottom and top for ventilation. 3/12 slope. Rafters are 2×12 with R31 batts (about 2 inches of vent space under roof deck to insulation). Vaulted ceiling, no attic.

Thanks in advance