I am building a cottage that has two 6/12 shed dormers in a 12/12 roof. The cottage is in a cold/snowy climate in Canada so I am installing 2″ of rigid foam on the outside of the exterior walls including the dormer walls which are already sheathed with 1/2″ ply. I will be installing strapping on the foam and then wood siding, with “rainscreen” details. My question is regarding the step flashing for the shingles that needs to be installed up the side of each dormer wall. Should it be nailed to the plywood before the foam is installed, or nailed to the strapping that will go on after the foam?
I like the idea of nailing it to the sheathing so any moisture that get’s behind the siding will be diverting onto the shingles, however, I am concerned that this approach directs water to travel under the rigid foam. On the other hand, if I nail the step flashing to a horizontal piece of strapping; I am jeopardizing the rainscreen and potentially trapping moisture behind the siding.
I’m stumped on this one and have been getting differing opinions from roofer and other contractors that I have asked. Any ideas?
Thanks for you insight!
Replies
It was my understanding that step flashing is never nailed to the sidewall.
Always nail to the roof deck - ideally not to the side wall at all. Take care of the side wall with a counterflashing.
This will allow for shingle and flashing replacement as years go by without having to disturb the siding.
Terry
Thansk Terry; nailing to the roof deck is a new one for me but makes sense.
Like they say, nailtop of step flash to the roof, but that does not really answer your main Q, does it?
Here is how I would detail this -
Dry in the rof with RooftopgaurdII or whatever underlayment, rolling it up the dormer sheathing 10"
Install the foam board
Install the step flashing
Run the tarpaper or Typar on the dormer walls down over the steps.
install the strapping and siding over that, first piece of strapping to be angled at pitch of roof, just above the tops of the steps
Out of curiosity Piffin - why would you not roll the underlayment up the foam instead of the sheathing? (Presuming you would seal the underlayment to the foam with a Peel & Stick flashing of some sort)
That would work too as long as you lap down over it.
Mine was a practical concern - that I dry in th eroof as soon and it is sheathed, so why wait forwhen I do the rigid foam insulation to roll the underlay up that? Get it dry immediately, then do the faom when you are ready