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...

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