Before the ice-dam protection membrane is applied, we seal the roof sheathing to the fascia.
Before we installed the self-adhering membrane along the eave edges of the roof to resist ice-dam leaks, we installed a narrow piece of membrane to seal the roof sheathing to the fascia.
The building code calls for roof underlayment to be applied over the top of eave drip edge,, but there’s still a risk of ice dams that form inside gutters and back water up into the joint between sheathing and the fascia. I’ve seen cases where water that enters the eave through this joint can flow on top of the soffit board back to the wall. And depending on how far up the wall the housewrap WRB is installed, the water can get into the wall cavities. Or the water can weep between the housewrap and siding and form icicles along the butt lines of siding courses and pry the siding away from the house.
So to seal the sheathing to the fascia and still comply with the building code, we applied a 6-in.-wide strip of ProtectoWrap Universal Primer-free membrane first. We bonded the bottom of the strip about 3/4 in. over the top of the fascia and the rest onto the sheathing. Over that we applied the drip edge and then the ice-dam protection sheets.
Some membrane manufacturers’ instructions show a detail where the sheet is applied over the fascia before the drip edge to seal the joint. I think this is a good alternative to following the prescriptive building code. But sometimes it’s hard to align the membrane for an even overhang to seal the fascia, and some membranes are stiff and don’t bend over the fascia easily. That’s why I’ve taken to using a 6-in.- to 12-in.-wide strip of thinner membrane, such as ProtectoWrap’s, to make the seal.