Download PDF version (134.2k) Log In or Register to view the full article as a PDF document.
Q.When using peel-and-stick roofing membranes, does it matter if the membrane goes under rather than over the metal drip edge at the eaves? In my work as a home inspector, I've noticed that these membranes get installed both ways, and that even manufacturers differ in their recommendations. Also, what about the rakes? Should the drip edge lap over the felt paper underlayment, or vice versa?

A.Paul Fisette, director of Building Materials and Wood Technology at the University of Massachusetts Amherst and a JLC contributing editor, responds: In practice, I think that either method will work fine, in most cases. But my preference is to install the membrane over the drip edge so that any water that has intruded beneath the shingles runs down and out over the top surface of the drip edge. If the self-sticking quality of the membrane is strong, the connection will be continuous and water won't have a chance to back up between the membrane and the sheathing, even if there is an ice dam.

Interestingly, I once worked as a consultant for one of the major peel-and-stick membrane manufacturers when it was first developing its product. At the time, the company's engineers felt that applying the membrane directly to the roof deck offered the best protection for the sheathing if water ever got underneath the drip edge. But when the membrane is installed beneath the drip edge, there's a good chance the metal won't lie perfectly flat with the entire top edge pressed into the membrane, in which case fish-mouth gaps in the drip edge could allow water to pass underneath and eventually wet the bottom raw edge of the sheathing.

Felt paper works a little differently than peel-and-stick membranes in that it isn't self-healing and doesn't seal surfaces it covers, which makes proper layering more important. So, at the rake, I think the best practice is to cover the roof deck with paper first (putting it over the top of the rake is fine, as long as it is trimmed neatly), and then place the drip edge on top of the paper. With this detail, leakage from wind-blown rain against the roof edge along the rake will be directed over the drip edge and onto the paper, rather than onto the roof sheathing.

Of course, remember that the ability of felt paper to provide protection is limited; that's really the job of the shingles.