What is the lap requirement for WWM?

• A. One Grid + 2 Inches
• B. One Grid + 4 Inches
• C. Two Grids
• D. One Brick

Answer: (A)

The key idea is how much overlap is needed along seams to keep the waterproofing membrane continuous and watertight. For WWM, overlaps are measured using the membrane’s grid pattern, so installers can reproduce the same seam length every time. The required lap is one full grid plus 2 inches. The extra 2 inches over the grid ensures there’s adequate margin to account for alignment, movement, and bonding of the seam, so water can’t leak through.

If the membrane’s grid is, say, 12 inches wide, the seam should be 14 inches long. A lap of just one grid would risk insufficient overlap and potential leaks; stretching to two grids would be longer than standard practice; and a unit like one brick isn’t related to sheet membranes. This one-grid-plus-2-inches rule gives a consistent, reliable lap for effective waterproofing.