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Dimple Board

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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Dimple board leaves an air gap along the wall and leads water down to the drain. It does not replace the drain.

I am a:

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

What it isA plastic sheet with raised dimples that leaves an air gap along the wall and leads water down to the drain.
Your jobFix it from the footing up to grade, seal the top, and keep the bottom open to the drain gravel.
The hard ruleIt does not replace the drain pipe. A mat with no drain behind it only moves the water.
The checkTop fastened and sealed, laps as the maker says, bottom meeting the gravel, photographed before backfill.

By the end you can

  • Say which way the dimples face for a mat and for a drainage board.
  • Fix and terminate a mat at the top and bottom.
  • Keep soil out of the dimples during backfill.
  • Say what a mat does and does not do.
Stop and read

You are working beside a tall wall in a deep excavation. Treat any excavation 1.2 m (4 ft) or deeper as a trench, and follow the Trench Safety page and your provincial OHS regulator. Do not work beside a cracked or leaning wall. Drilling concrete makes silica dust, so wet drill or use dust extraction and wear a fit-tested respirator.

The big idea

Dimple board, or drainage mat, is a plastic sheet with raised dimples. It protects the wall coating from backfill and leaves an air gap along the wall where water can run down to the drain. It is a common and useful part of a drainage system. It is not a replacement for the drain pipe at the bottom.

Which way do the dimples face?

It depends on the product, so follow the maker.

  • Dimple mat (an air-gap membrane): the dimples face the wall, which makes the air gap, and the smooth side faces the soil.
  • Drainage board (used to protect a waterproofing layer): the dimples face out, covered by a filter fabric.

A building-science article says a dimple mat is not considered waterproofing on its own, because it is not sealed at the bottom of the wall. A national evaluation (CCMC 12658-R) of one dimpled product accepts it as dampproofing for cast-in-place and concrete block foundation walls, as an alternative to the materials in 9.13.2.2, for walls up to 3.7 m (12 ft 2 in) below grade. That is the evaluation of one product against NBC 2015, so check the evaluation for yours.

A mat is not required in every Part 9 design. It is a common method, and some designers and inspectors expect it. Ask yours.

What it does

  • Protects the membrane or coating from backfill, rocks and soil movement.
  • Creates a drainage gap along the wall. A maker says the gap lets water behind the membrane flow down to the footing drain and breaks the capillary path between wet soil and the wall.
  • It does not stop water under pressure on its own. Where hydrostatic pressure is expected, get a designed waterproofing system.

Characteristics: a high-density polyethylene sheet, with the dimples about 8 mm (5/16 in) high on the product in the CCMC report. Some have filter fabric, and some are plain.

Preparation

  • The dampproofing or waterproofing is cured and has been inspected.
  • Check for tears or missed areas before you cover them.
  • Clear the wall of anything that could puncture the board.

Installation

  1. Start at the footing The CCMC evaluation says to cover the wall from the top of the footing to final grade. Work up from the footing.
  2. Face the dimples correctly Put the dimples the way the maker says: toward the wall for a dimple mat.
  3. Fix it Fasten it with the maker's fasteners or adhesive. Do not fasten within the maker's distance of corners, and use the maker's spacing, because wrong spacing can let dirt into the dimple space.
  4. Terminate the top Fasten and seal the top edge and the vertical joints so soil cannot get behind the board.
  5. Lap the seams Lap seams as the maker specifies and keep the board tight to the wall.
  6. Open the bottom Leave the bottom able to drain into the gravel around the pipe. Do not let soil, mortar, foam or debris block the gap.

If the mat will sit exposed before backfill, check the maker's UV limit. The CCMC evaluation of one product says to protect it from UV light within 30 days.

Example. A house has a 36 m (118 ft) perimeter and 2.4 m (7 ft 10 in) of wall below grade, so 36 × 2.4 = 86.4 m² (930 ft²). One maker's rolls are 20 m long and up to 3.0 m wide (65 ft 7 in by 9 ft 10 in). The wall needs 36 ÷ 20 = 1.8, so 2 rolls, before laps and waste. The house is made up.

Backfill

  • Use free-draining granular backfill against the wall.
  • Keep large rocks and debris away from the board.
  • Backfill in lifts and watch the board as you go.
  • Heavy compaction against the wall is a separate risk. The Backfilling section covers it.

Common mistakes

  • Dimples facing the wrong way for the product
  • No top termination
  • Gaps that let soil in
  • Puncturing the board during backfill
  • No overlap at seams
  • Stopping short of the footing
Pro note

The mat is only as good as its drain. Check that water leaving the bottom has somewhere to go.

Weather note

Cold stiffens the board and makes seams hard to seal. Heat softens it and it expands. Rain can leave water trapped behind it if the wall is not drained.

What the inspector looks at

Termination at the top, seam laps, the bottom connection to the drain, and the backfill.

✅ Check yourselfTap a question
A house has a 36 m (118 ft) perimeter and 2.4 m (7 ft 10 in) of wall below grade. Using 20 m by 3.0 m (65 ft 7 in by 9 ft 10 in) rolls, how many rolls before laps?
Area 36 × 2.4 = 86.4 m² (930 ft²). One roll covers 60 m², but it only runs 20 m along the wall, so 36 ÷ 20 = 1.8, which means 2 rolls. Add margin for the maker's laps. The roll size is one maker's, from a CCMC report.
Does the mat replace the drain pipe?
No. It leads water down to the drain. The CCMC evaluation of one product requires the wall to be well drained under 9.14.2, and drainage engineering is still needed.
Which way do the dimples face on a dimple mat?
Toward the wall, making an air gap, with the smooth side to the soil. A drainage board used to protect a waterproofing layer faces the other way. Follow the maker.
How high must the membrane go, and where does it start?
The CCMC evaluation of one product says it covers the wall from the top of the footing to final grade.
What must you do at the top edge?
Fasten it mechanically and seal it, so soil particles cannot get behind the membrane. The same goes for vertical joints.
A mat is left uncovered on a sunny wall for two months. Any issue?
Yes. The CCMC evaluation of one product says to protect it from UV light within 30 days of installation. Check your maker's limit.
🧰 Gear for this stepMust-haveRent itNice to have
  • Must-haveLayout and measuring:
  • Must-haveCutting:
  • Must-haveFixing: termination bar, masonry fasteners with a hammer drill or the maker's adhesive fasteners, and sealant for the top edge
  • Must-haveSeams:
  • Must-haveRecords: a phone or camera for photos of the board and bottom connection before backfill
  • Must-havePPE: hard hat (CSA Z94.1), eye protection (CSA Z94.3), green-triangle boots (CSA Z195), cut-resistant work gloves, and a fit-tested respirator (CSA Z94.4) with dust extraction or wet drilling when drilling concrete
👷 Who can do thisTap your level
Home DIY
A capable owner can fit drainage mat on a shallow wall after the dampproofing is inspected. Treat 1.2 m (4 ft) as a no-entry line, do not judge sloping or shoring yourself, and do not dig beside an existing foundation. Do not work beside a cracked or leaning wall. Hire a pro if the mat is part of an engineered waterproofing design.
Laborer or helper
Carry and hold the mat while the installer fixes it. Never enter an excavation 1.2 m (4 ft) or deeper, and never work beside a cracked or leaning wall. Keep soil out of the dimples and report any damaged dampproofing.
1st-year apprentice
Under supervision, cut, fix and seal the mat, and keep soil out of the dimples. Learn first which way the dimples face for the product and how the bottom must meet the gravel.
Apprentice, 2nd to 4th year
Cut, fix and seal the mat under supervision, and keep soil out of the dimples. Check which way the dimples face and how the bottom meets the gravel. Report damaged dampproofing. You do not judge an excavation safe.
Journeyperson
You own the termination, the seam laps and the connection to the drain gravel. Stop-work triggers: the dampproofing is damaged or not yet inspected, or the bottom edge cannot reach the gravel around the drain pipe. Confirm with the designer whether the mat is needed at all.
Lead hand or foreman
Check that the dampproofing is inspected and undamaged before any mat goes on. Enforce the stop-work triggers: no bottom edge reaching the gravel. Keep the 1.2 m (4 ft) no-entry line for unprotected excavations, and confirm with the designer whether the mat is needed.
Contractor or business owner
Price by wall area and include backfill care. Document the top termination and bottom connection with photos before cover. The mat does not replace the drain pipe, so do not sell it as if it does.
Starting a new business
Learn that the mat does not replace the drain pipe, and quote it that way. Photograph the top and bottom terminations before cover. Confirm design needs with the designer, the inspection stage with your building department, and excavation rules with your provincial regulator.
By Province:

The technique is the same across Canada. What changes is the code edition, whether your inspector expects a mat, and the inspection stage. The one product evaluation I read (CCMC 12658-R) also carries an Ontario ruling, which applies to Ontario only. I did not check the other provinces, so confirm with your local building department.

See Canada by Province for the full comparison.

Sources:

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

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