Peel-and-stick membranes allow a level of protection and redundancy never before possible when flashing a dormer. The process involves some forethought, mainly because a portion of it has to be done before you're finished framing.
For example, the first pieces of membrane have to be installed after the wing walls are in place, but before the roof is framed. That’s because the rafters’ overhang makes it almost impossible to get the membrane in place at the critical juncture where the top of the cheek meets the main roof. I like to use Grace’s Roof Detail Membrane because it comes in a convenient 18-inch width and there are ripcords embedded beneath the release sheet which can be used to separate it into more manageable sections making it easy to peel only the paper you want to remove at a given time.
By following the steps below, you can help ensure that your dormers will be watertight.
After cutting an 18-in. square of peel and stick membrane, use a new blade and carefully score the release paper backing to leave an L-shaped, 2-in.-wide, strip of paper on two edges of the membrane.
Press the membrane into place at the top of the wing wall so that it overlaps the top plate and the roof sheathing. The strips of release paper should be on the bottom edge and the edge on the roof sheathing. Leaving the paper on the membrane for now makes it possible to slide the other membrane and other roof ing materials (felt underlayment, roof shingles) underneath the edges of this first piece later on.