10.1 Waterproofing, Dampproofing, and Below-Grade Protection

Waterproofing vs. Dampproofing

The International Building Code (IBC) Section 1805 governs below-grade moisture control. The exam expects you to distinguish two terms that sound alike but solve different problems. Dampproofing resists moisture (water vapor and capillary seepage) where no hydrostatic head is present. Waterproofing resists liquid water under hydrostatic pressure.

The decision driver is the groundwater table. Per IBC 1805.1.2.1, where a high water table or other hydrostatic conditions are known to exist, walls must be waterproofed and the floor must be designed to resist that pressure. Otherwise dampproofing of foundation walls enclosing habitable or usable space below grade is the minimum (IBC 1805.1.2).

Exam trap: A basement below the water table requires waterproofing, not dampproofing. Choosing dampproofing in that scenario is the classic wrong answer.

Dampproofing Materials & Application

IBC 1805.2 lists acceptable dampproofing applied to the exterior wall face below grade:

For masonry walls, IBC 1805.2.2 requires the wall be parged 3/8 in. (9.5 mm) with Portland cement, then coated with the dampproofing. Concrete walls may be coated directly.

Waterproofing Materials

IBC 1805.3 recognizes membranes that bridge cracks and resist head pressure:

• 2-ply hot-mopped felts (ASTM D226 Type I or II)
• 55 lb (25 kg) roll roofing
• 6 mil (0.15 mm) polyvinyl chloride (PVC)
• 6 mil (0.15 mm) polyethylene
• 40 mil (1 mm) polymer-modified asphalt

Joints in the membrane must be lapped and sealed. The membrane is applied from a point 6 in. (152 mm) above the maximum water-table elevation down over the footing.

Subsoil Drainage & Worked Example

IBC 1805.4.2 requires drains around foundations retaining earth and enclosing below-grade space (unless on well-drained ground per Table 1610.1, Group I soils). The drain (perforated pipe or gravel) sits alongside the footing, with the top of the drain below the bottom of the slab, surrounded by at least 6 in. (152 mm) of washed gravel extending 12 in. beyond the pipe.

Takeoff scenario: A rectangular foundation 40 ft x 30 ft needs a perimeter footing drain. Perimeter = 2(40+30) = 140 lin ft. With 10% waste for fittings and a cleanout, order 140 x 1.10 = 154 lin ft of perforated pipe.

Drainage Gravel Volume

Using that 140 lin ft trench at 12 in. wide x 12 in. deep of gravel surround: cross-section = 1 ft2, so volume = 140 ft3. Convert to cubic yards: 140 / 27 = 5.19 yd3 → order 6 yd3 (always round up for granular materials). Knowing 27 ft3 = 1 yd3 is tested repeatedly across the exam.

Dampproofing vs. Waterproofing — Not the Same

The exam draws a hard line. Dampproofing resists moisture/water vapor (capillary damp) but not hydrostatic pressure — a sprayed/troweled asphalt coating on a foundation wall above the water table. Waterproofing resists liquid water under hydrostatic head (below the water table, planters, plaza decks) — using membranes (rubberized asphalt, bentonite, sheet membranes). Per IBC, walls retaining earth with a high water table require waterproofing, not just dampproofing.

Below-Grade Drainage System

Waterproofing works with drainage: a foundation (footing) drain of perforated pipe in washed gravel wrapped in filter fabric, a drainage board/composite against the wall to relieve pressure, and a path to daylight or a sump pump. Backfill with free-draining granular material, not clay. Without drainage, water builds hydrostatic pressure and finds any pinhole — the classic basement-leak failure the exam attributes to omitted drains or relying on dampproofing where waterproofing was needed.

Protection and Detailing

Membranes are fragile during backfill; install a protection board before backfilling. Treat penetrations, cold joints, and the footing-to-wall corner (cove) as the leak-prone details — they get reinforcing fabric/liquid membrane. Tie the below-grade waterproofing to the above-grade air/water barrier so there is a continuous moisture line.

Common Exam Traps

• Trap: Dampproofing stops hydrostatic water. It does not — use waterproofing below the water table.
• Trap: Backfilling with clay against the wall (traps water). Use granular backfill.
• Trap: Footing drain without gravel/filter fabric (clogs).
• Trap: Skipping the protection board so backfill tears the membrane.

Membrane Types and the Slab Vapor Retarder

Below-grade waterproofing membranes include positive-side systems (applied to the wet face: sheet rubberized asphalt, HDPE, hot fluid-applied) and bentonite panels that swell on contact with water to self-seal. Under slabs-on-grade a vapor retarder (≥10-mil polyethylene, ASTM E1745) over the gravel base blocks soil moisture and radon from wicking into the slab and finishes. Lap and tape its seams and seal penetrations — a torn or omitted under-slab retarder shows up later as floor-covering adhesive failure and persistent slab moisture.

Crawl Spaces and Radon Mitigation

Vented vs. conditioned (closed) crawl spaces are a code choice: a closed crawl seals the vents, places a vapor retarder over the soil turned up the walls, and insulates the perimeter, controlling humidity better than venting in humid climates. Radon mitigation uses a sub-slab depressurization system — perforated pipe in the gravel base vented through the roof — tied to the under-slab vapor retarder. The exam links chronic crawl-space moisture and elevated radon back to a missing or discontinuous ground vapor retarder.