How much should ice and water shield overlap?
Short answer. Follow the printed lap lines on the product, but the common standard is about 3 to 3.5 inches where rows overlap along the long edges (side laps) and 6 inches where one roll ends and the next begins (end laps). Stagger end laps between rows, lap upper courses over lower ones, and roll the laps firmly so they bond.
Side laps and end laps
- Side laps: where a higher row overlaps the row below it along the long edge. Many products call for about 3 to 3.5 inches, often marked by a selvage edge or printed line.
- End laps: where the end of one sheet overlaps the start of the next in the same row. Six inches is the common requirement.
- Stagger: keep end laps in neighboring rows apart so seams do not stack.
Direction matters as much as distance
Every lap should be shingle fashion: the upper sheet over the lower one, so water running downslope flows over the seam. A lap installed backward lets water run straight into it, no matter how wide it is. On end laps, some installers cut the corner of the lower sheet at an angle so water is not channeled along the seam.
Specific locations
- Valleys: a full-width sheet centered in the valley. If a second piece is needed, lap the upper piece over the lower by the end lap amount.
- Eaves: the first row over the eave drip edge; the next row laps over it.
- Where membrane meets underlayment: the regular underlayment laps over the top edge of the membrane, never under it.
- Around penetrations: the piece above the pipe overlaps the piece below.
Making laps stick
Laps must be clean and dry. Press them with a hand roller, especially in cool weather. Some products require primer or extra pressure below certain temperatures, and on steep slopes manufacturers may call for nailing in the selvage edge. More in ice and water shield in Georgia and synthetic underlayment vs felt.