How to Slip Peel-and-Stick Roof Membrane Under Existing Step Flashing

Covering a roof with full-width peel-and-stick membrane is a relatively straightforward task, although it’s arguably easier for two people working together. But when that roof meets an existing wall, slipping the sticky membrane underneath existing step flashing is a little trickier. Lynn Hayward, a builder and remodeler in Northport, Maine, has figured out a way to get it done with minimal fuss.

The main problem is how to lift and hold the step flashing out of the way while you install the sticky membrane. Hayward’s solution is a wedge-shaped stick about ¾-inch wide on each side and about 36 inches long [1]. The triangular shape means that regardless of how you orient the stick, a thin edge faces the wall and will easily slip under the step flashing. The fat edge of the wedge shape holds the flashing up off the roof to give you room to work.

Hayward also uses narrow strips of membrane instead of trying to wrestle with the entire roll. An 8-or 10-inch-wide strip is easy to work with and wide enough to both extend under the step flashing and allow enough surface area for good overlap with the membrane that completes the course.

Step-by-Step

Starting at the bottom, Hayward inserts the triangular stick under the step flashing, pushing it in as far as he can [2]. This lifts the flashing up off the sheathing without creasing it. Next, he peels back a few inches of release paper from the upper edge of a narrow strip of membrane and slides as much as he can under the flashing [3]. To maximize coverage and keep the strip parallel to the wall, he rolls the inside edge of the membrane up onto the stick a little bit [4]. Finally, he presses the membrane firmly into the sheathing, being careful not to adhere it to the stick, which would make it more difficult to remove the stick later.

With the top few inches of membrane stuck in place, Hayward works his way down, releasing paper and pressing membrane into the sheathing as he goes [5]. When he gets to end of the membrane strip, it’s time to pull out the stick. Working from top to bottom, Hayward gently tugs the stick up and away from the wall [6], then reaches back underneath to seal the inside edge against the sheathing.

Rinse and repeat all the way to the top of the wall. The rest of the roof can now be covered using a full roll, overlapping the strip of membrane where it extends beyond the step flashing [7].