The trigger: hydrostatic pressure
IBC 2021 Section 1805 decides between dampproofing and waterproofing by one condition: hydrostatic pressure, as determined by the ground-water investigation in Section 1803.5.4.
- Section 1805.2: where hydrostatic pressure will not occur, walls are dampproofed.
- Section 1805.3: where hydrostatic pressure exists and no Section 1805.1.3 ground-water control system is installed, walls are waterproofed.
Not the trigger: wall height, wall material or climate zone (which governs vapor retarder class under Section 1404.3).
Dampproofing walls
Section 1805.2.2 applies dampproofing to the exterior from the top of the footing to above grade. Unit masonry walls must be parged first. Listed materials:
- Bituminous material.
- 3 lb per sq yd (16 N/m²) of acrylic modified cement.
- A ⅛ in (3.2 mm) coat of surface-bonding mortar complying with ASTM C887.
- Any Section 1805.3.2 waterproofing material, or other approved materials.
Traps: 1/16 in names the right standard at half the thickness. ASTM E96 is the vapor retarder test, not a mortar standard. Acrylic modified cement is dampproofing-only; it is not on the waterproofing list.
Waterproofing walls
Section 1805.3.2 lists two-ply hot-mopped felts, 6-mil polyvinyl chloride, 40-mil polymer-modified asphalt, 6-mil polyethylene, or other approved materials that bridge nonstructural cracks.
Waterproofing runs from the bottom of the wall to not less than 12 in (305 mm) above the maximum elevation of the ground-water table. The rest of the wall is dampproofed.
The trap: stopping at the water table itself. The extra 12 in is the part people drop. The 12 in is measured above the water table, not above grade.
Worked example
A 9 ft wall has hydrostatic pressure, the maximum ground-water table is 3 ft 6 in above the bottom of the wall, and there is no ground-water control system.
- Waterproofing: 3 ft 6 in + 1 ft 0 in = 4 ft 6 in.
- Dampproofing above: 9 ft − 4 ft 6 in = 4 ft 6 in.
Stopping at 3 ft 6 in misses the 12 in. Waterproofing the full 9 ft is extra work, not the minimum.