Membranes

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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A barrier on the outside of the wall and a drain at the footing work together to keep a basement dry.

The big idea

Concrete is not waterproof. It soaks up moisture, it cracks, and its joints leak. A barrier between the wall and the soil, plus a drain that takes water away, work together. A barrier with no drainage fails under water pressure, and drainage with no barrier leaves the wall damp.

Dampproofing or waterproofing?

They are not the same thing, and the code treats them differently (NBC Part 9, Section 9.13, as printed in the Vancouver Building Bylaw 2014).

• Dampproofing resists moisture from damp soil. Where the exterior finished ground is higher than the inside ground level, the outside of the wall below ground is dampproofed (9.13.2.1). Floors on ground are dampproofed too, unless they sit on at least 100 mm (4 in) of coarse granular material with 10 percent or less passing a 4 mm sieve, or are garage floors.
• Waterproofing resists water under pressure. Where hydrostatic pressure occurs, waterproofing is required on the outside of below-ground walls and floors on ground (9.13.3.1). The code example is built up from bitumen-saturated membrane layers, and in practice it is an engineered system.

If you expect hydrostatic pressure (high water table, a spring, poor soil), get the design from a professional. A basement that could float or flood is a structural question as well.

Types of membrane

• Asphaltic coatings (spray or brush, solvent or water based): the cheapest, but they do not bridge cracks and cannot stop water under pressure.
• Peel-and-stick sheets: rubberized asphalt on a plastic film, with consistent thickness. Some need a primer, and some are rated for cold installation. Check the maker.
• Liquid-applied membranes (rolled, sprayed or troweled): seamless, but the thickness depends on the applicator, so check it with a wet-film gauge.
• Dimple mat: a plastic sheet with the dimples facing the wall, which makes an air gap. A building-science article says it is not waterproofing on its own, because it is not sealed at the bottom of the wall.
• Drainage board (dimples facing out, with a fabric face): protects a waterproofing layer, and is not a stand-alone product. Which way the dimples face depends on the product, so follow the maker.

Surface preparation

• Strip the forms, then clean off form oil, dirt and loose material.
• Seal the form-tie holes. Part 9 requires it (9.13.2.4).
• Patch honeycombing, voids and holes with repair mortar.
• Let the concrete cure as the maker requires. Some products need a prime coat or a longer cure.
• The surface must be dry enough for the product. Moisture trapped under a membrane can cause blisters and failure.

Installation

1. Start at the footing Seal the cold joint where the wall meets the footing with a heavy bead of sealant, and work from there up to the grade line.
2. Lap and press Lap the seams as the maker says. Press peel-and-stick sheets firmly to remove air pockets and fish-mouths.
3. Detail the penetrations Seal around pipes, sleeves and corners with the maker's detail.
4. Terminate at the top Finish at the top with a termination detail, so water cannot run behind the membrane.
5. Protect it Fit the drainage mat or protection board right away. Rocks and debris tear membranes during backfill.
6. Inspect and photograph Check for tears, missed spots and short laps before the inspector comes and before backfill, and take dated photos.

The footing-to-wall joint is where leaks start. Water runs along it at footing level, so it needs to be sealed and drained.

The drain

From Part 9, Section 9.14 (as printed in the Vancouver bylaw):

• The bottom of every exterior foundation wall is drained by drain tile or pipe laid around the foundation, or by a gravel or crushed rock layer, unless it can be shown to be unnecessary (9.14.2.1).
• The pipe is at least 100 mm (4 in) in diameter, with its top below the bottom of the floor slab (9.14.3.2, 9.14.3.3).
• The top and sides of the pipe are covered with at least 150 mm (6 in) of crushed stone, with no more than 10 percent passing a 4 mm sieve (9.14.3.3).
• The drain must go to a sewer, a drainage ditch or a dry well (9.14.5.1). Where gravity drainage is not practical, a sump with an automatic pump is required (9.14.5.2).

Example. A house 10 m by 8 m (32 ft 10 in by 26 ft 3 in) has a 36 m (118 ft) perimeter. With 2.4 m (7 ft 10 in) of wall below grade, that is 36 × 2.4 = 86.4 m² (930 ft²) of wall to dampproof, and 36 m of drain pipe. With a 100 mm (4 in) pipe and 150 mm (6 in) of stone on top and both sides, the gravel bundle is 400 mm (15 3/4 in) wide and about 300 mm (11 3/4 in) high, if you allow 50 mm (2 in) under the pipe. That is 0.4 × 0.3 × 36 = 4.3 m³ (5.7 yd³) before the pipe and an allowance. The sizes and the bedding are made up.

The next pages cover sump systems and drainage corrections.

Radon rough-in

Part 9 also asks for a soil-gas rough-in. Houses get an air barrier on the assemblies separating the house from the ground, and a subfloor depressurization rough-in, unless the building is in a Radon Area 2 location (9.13.4.2). One option is at least 100 mm (4 in) of granular material under the slab with a vent pipe of at least 100 mm (4 in) diameter, labelled "RADON VENT PIPE" (9.13.4.3). Ask your inspector what your province requires, and plan the rough-in before the slab is poured.

Common mistakes

• Applying to a wet, dirty or uncured wall
• Short laps
• Air pockets and fish-mouths in the sheet
• No protection during backfill
• Missing coverage at the joint and penetrations
• Dampproofing where water pressure is expected