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Waterproofing Repairs

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 leak means the barrier or the drainage has failed. Find the source, then restore the whole system.

I am a:

Tap your level and the boxes below open to the advice for you. Remembered on this device.

What it isFinding why water is getting in, and restoring the barrier, the drain and the discharge together.
Your jobDiagnose first, fix the surface water, and only then dig, from outside, with the excavation protected.
The hard ruleNever dig below or beside the footing without protection, and call an engineer for a cracked, leaning or moving wall.
The checkRepairs are inspected like new work: coverage, drain level, stone and discharge before backfill.

By the end you can

  • Trace a leak to its source before you dig.
  • Plan a safe dig that does not undermine the footing.
  • Repair the membrane, mat, drain and stone as one system.
  • Tell a cosmetic crack from a structural one.
Stop and read

Never undermine the footing. Digging down to repair a wall exposes the footing and the soil that carries it. Do not excavate below the bottom of the footing, and do not leave the soil beside it unsupported. The footing may be carrying the house. If the dig is deep, close to the wall, or the wall is cracked or leaning, get an engineer involved before you start.

Treat any excavation 1.2 m (4 ft) or deeper as a trench, and follow the Trench Safety page and your provincial OHS regulator. Read the safety data sheet for every membrane and primer (WHMIS), because solvent vapours collect in a deep hole.

The big idea

A leak means water is getting past the barrier, or the drainage is not working, or both. The cure is to find the source, expose the problem and restore the whole system, not just patch what you can see. Repairs are also where trench and excavation hazards are highest.

Finding the source

A Canadian home-inspector association (OAHI) says the first step in curing chronic dampness is finding the source, and to check the surface first.

  • Rule out condensation and interior sources: washers, dryers, showers, and sweating or leaking pipes. Damp air is not always a leak.
  • Check grading, gutters and downspouts before you chase cracks. OAHI says improper grading and drainage cause the most common problems, and that problem downspouts show up as leakage next to the downspout.
  • Wall leaks: cracks, tie holes, the footing joint, or failed dampproofing.
  • Footing-level leaks: a blocked or high drain pipe.
  • Window well leaks: poor drainage or missing stone.
  • Floor leaks: water pressure from below the slab.
  • Signs: efflorescence (a whitish powder), musty odour, mould, and rain on frozen ground that cannot soak in.

OAHI says excavation, dampproofing and new drain tile are for chronic flooding that persists after surface improvements. A professional site inspection is advised.

Excavating

  • Dig to expose the whole problem area, down to the footing where needed, but never below it.
  • Dig by hand near the wall and any buried pipe, after locates.
  • Keep spoil back from the edge, and protect the trench.
  • Clean the wall so you can see what failed.

Membrane, mat and drain repairs

  • Replace damaged membrane rather than patching on top of it. Clean and dry the wall, lap new material over old as the maker says, and reinforce cracks and the footing joint.
  • Apply any coating at the thickness the maker specifies (see the Coatings page).
  • Replace torn or crushed mat, and rebuild the top termination (see the Dimple Board page).
  • Expose and replace crushed or blocked pipe, reset it level, and rebuild the stone and fabric (see the Drain Tile and Gravel Beds pages).

Example. A 6 m (19 ft 8 in) repair section of a 2.4 m (7 ft 10 in) wall is 6 × 2.4 = 14.4 m² (155 ft²) of wall. Re-laying the drain with a 0.4 m by 0.3 m (16 in by 12 in) stone bundle takes 0.4 × 0.3 × 6 = 0.72 m³ (0.94 yd³) of stone and 6 m of pipe. The section and sizes are made up.

Cracks

Find out why it cracked. Most foundation cracks come from shrinkage while the concrete cured. A crack that is growing, or that has moved, is a structural question for an engineer.

  • Epoxy is the structural option: it bonds the crack rigidly. A resin maker says low-viscosity epoxy suits hairline cracks under about 2 mm (5/64 in).
  • Polyurethane foam suits wet or actively leaking cracks and stays flexible, so it tolerates some movement.
  • Fit injection ports over the crack, seal the crack between them, and let the seal harden before you inject. Inject slowly at low pressure from the lowest port up, until resin shows at the next port.
  • Do not inject into frozen concrete. The same author says epoxy is typically installed at 40°F (4°C) or above.
  • OAHI says that if leakage continues after surface fixes, you can inject epoxy from inside or excavate beside the crack and patch from outside, and that the exterior repair is more often effective.

The resin source is a maker's article, so follow your product's data sheet and the engineer's direction.

Interior fixes

  • Interior sealers and paint do not stop water pressure from outside.
  • Interior moisture control, such as foam board on the wall, insulated cold pipes and a dehumidifier, helps with condensation, as OAHI notes.
  • Interior drainage can manage water, but it does not replace exterior work. If you cut the slab for it, seal it airtight, because an open cut can become a radon pathway.

Common mistakes

  • Patching the surface only
  • Not digging to the footing
  • Tar over tar as a repair
  • Leaving the drain pipe at the wrong level
  • Not replacing damaged mat
  • Ignoring dirty stone
Pro note

Fix the surface water first, then repair from outside, and do it once. Interior patches buy time, not a fix.

Weather note

Rain floods the excavation and can undermine the wall. Cold stiffens materials, and resin should not go into frozen concrete. Do exterior repairs in dry, stable weather.

What the inspector looks at

Repairs are inspected like new work: coverage, drain level, stone, and discharge before backfill.

✅ Check yourselfTap a question
A basement wall is damp in one spot beside a downspout. What do you check first?
The surface: grading, gutters and downspouts. A Canadian home-inspector association says improper surface grading and drainage cause the most common problems, and that downspout problems show up as leakage next to the downspout. Fix those before you dig.
Why rule out condensation before you excavate?
Washers, dryers, showers and cold pipes can wet a basement without any leak. Check interior sources first.
You repair a 6 m (19 ft 8 in) section of a 2.4 m (7 ft 10 in) wall. How much wall area, and how much drain stone at 0.4 m by 0.3 m (16 in by 12 in)?
6 × 2.4 = 14.4 m² (155 ft²) of wall. Stone 0.4 × 0.3 × 6 = 0.72 m³ (0.94 yd³), plus 6 m of pipe. The sizes are made up.
A crack is 3 mm (1/8 in) wide and wet. Which resin is usually suggested?
A resin maker says polyurethane foam usually works better for wet or actively leaking cracks, and epoxy suits narrower, dry cracks under about 2 mm (5/64 in). It also says cracks from movement or settlement should go to a structural engineer first.
What do you do if a wall is cracked, leaning or moving?
Stop and call an engineer. Do not dig beside it, and do not undermine the footing.
Why is an interior sealer not a fix for water pressure from outside?
It sits on the dry side, so the water keeps pushing at the wall and eventually breaks the coating or finds another path. Exterior repair is more often effective, according to the home-inspector association.
🧰 Gear for this stepMust-haveRent itNice to have
  • Must-haveFinding the source: a moisture meter, flashlight, a camera, a crack gauge to see if a crack is moving, and a hose to test grading and downspouts
  • Rent itDigging (machine): an excavator or mini-excavator
  • Must-haveDigging (hand tools): shovels and spades, to dig by hand near the wall and any buried lines
  • Rent itTrench protection (trench box): an engineered trench box or shoring, supplied and set by a qualified crew, for any dig past the trigger depth
  • Must-haveTrench protection (ladder): a ladder (CSA Z11) for egress
  • Must-haveWall prep:
  • Must-haveRepair materials:
  • Nice to haveChecking (laser): a laser level to reset the drain slope
  • Must-haveChecking (photos): a camera for dated photos
  • Must-havePPE:
👷 Who can do thisTap your level
Home DIY
A capable owner can fix grading, downspouts and window wells, and can patch a small, shallow area. Treat 1.2 m (4 ft) as a no-entry line and do not judge sloping or shoring yourself. An excavation beside an existing footing or foundation is not a DIY job, so do not dig below or close to the footing, and do not work beside a cracked or leaning wall. Get an engineer for any crack that is growing and a pro for full exterior waterproofing.
Laborer or helper
Clean up, haul away spoil and hand-dig only where a qualified person says it is safe. Never enter a trench deeper than 1.2 m (4 ft) unless it is sloped, shored or boxed. Stay clear of a cracked or leaning wall and speak up at once.
1st-year apprentice
Under supervision, hand-dig near the wall from outside any trench deeper than 1.2 m (4 ft), clean and prepare the surface, and install mat and stone. Never stand in a trench while a trench box is moved. Learn first the excavation rules, how to read a leak, and what undermining a footing looks like.
Apprentice, 2nd to 4th year
Prepare surfaces and place mat and stone only from outside a deep trench, or inside one that is protected. Keep out while a box is being moved. Report new cracks, wet soil or exposed footings before you carry on.
Journeyperson
You own the diagnosis, the dig plan and the complete system repair, from membrane to drain to discharge. Stop-work triggers: stop and call the engineer if the wall is cracked, leaning or moving, the footing is exposed or undermined, or the dig is past the trigger depth without sloping, shoring or a box.
Lead hand or foreman
Check the dig plan each morning, and enforce the stop-work triggers when the wall moves, the footing is exposed or the depth passes the trigger without protection. Keep crew members out of the trench during box moves. Call the engineer rather than judging a cracked wall.
Contractor or business owner
Price investigation separately from repair, and state the limits of the warranty. Document the failure and the repair with photos. Check the permit, locates, shoring and WHMIS, and insure for undermining and property damage. Patching only the surface is the common callback.
Starting a new business
Before taking repair work, learn the excavation rules, locate rules and WHMIS duties from your provincial OHS regulator and workers' compensation board. Subcontract shoring if you lack it. Ask your insurer about undermining claims, and have the contract spell out what the warranty covers.
By Province:

The technique is the same across Canada. What changes is the trench depth that triggers shoring or sloping, the permit rules for repair work, and who inspects. I did not verify provincial trench-depth triggers on this pass, so check them with your OHS regulator and the Trench Safety page, and ask your building department whether a repair needs a permit.

See Canada by Province for the full comparison.

Sources:

Last reviewed October 2026. General information, not engineering or legal advice.

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