How many plies of coal tar built-membrane are needed to waterproof a wall when the hydrostatic pressure is 49 feet?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (C)

Waterproofing a wall against hydrostatic pressure relies on how many plies the coal tar built-up membrane uses to resist the pressure. Each ply adds a chunk of hydrostatic head resistance. For coal tar built-up membranes, a typical per-ply resistance is about 16 to 17 feet of hydrostatic head. So with three plies you’re around 48 to 51 feet of resistance, which covers a hydrostatic head of 49 feet. Two plies would only give roughly 32 to 34 feet, which isn’t enough, while four plies would be more than needed in this case. Therefore, the minimum number of plies to withstand 49 feet is three.