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Drainage Corrections

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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Much of the water in a basement never touched the drain. Fix the grade, the roof water and the window wells first.

I am a:

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

What it isFixing the surface and backfill problems that send water to the foundation: grading, roof water, window wells and a drain that is not doing its job.
Your jobMove water away from the wall on every side, keep roof water on its own route, and expose the drain only if the surface is right and water still gets in.
The hard ruleNever send water onto a neighbour, a sidewalk or the street, and never bury the foundation wall's height above ground.
The checkWater leaves the wall after a hose test, the window wells drain, and the downspouts end well away from the house.

By the end you can

  • Regrade so water falls away from the wall without burying the foundation.
  • Route roof water, window wells and driveway runoff away from the house.
  • Tell when surface fixes are not enough and the drain must be exposed.
  • Find the slope numbers that apply to your own lot.
Stop and read

Do not dig beside the footing, and do not work in a trench 1.2 m (4 ft) or deeper unless it is sloped, shored or boxed. Surface regrading is shallow work, but a drain correction is excavation, so follow the Trench Safety page and your provincial OHS regulator.

Call locates before you dig. Keep machines and compactors back from the wall, because heavy compaction against a foundation wall can push or crack it.

The big idea

Much of the water that ends up in a basement never reached the drain. It came off the roof, ran along a low spot, or collected in a window well. The code text I read is short and plain on this point: the building site shall be graded so that water will not accumulate at or near the building (NBC Part 9, Article 9.14.6.1, read in a BC municipal reprint). Fixing surface water is often the cheapest and most effective step, and it comes before anything below grade.

A cross-section of a house and the ground around it. The ground slopes down and away from the wall on both sides. On the left a corrugated window well sits over a basement window, with a gravel path from its base down to a perforated drain pipe at the footing. On the right a downspout turns into a long extension that runs out along the ground to a splash pad, and a separate perforated drain pipe sits beside the footing.
Grading, window well and downspout. The ground falls away from the wall on every side. The window well drains through gravel to footing level. Roof water leaves through the downspout extension and stays separate from the foundation drain. Not to scale. The slope is not a code number, because it varies by municipality. The extension is drawn lying shallow in the ground, and the wall height above ground is not shown, so keep 150 mm (6 in) of wall above finished grade as the text says.

Grading

  • Slope the soil away from the foundation on every side. Fill low and settled spots, then check the result with a string line and level.
  • Keep the finished grade below the top of the dampproofing and well below siding and sill. The Part 9 text I read says exterior foundation walls shall extend not less than 150 mm (6 in) above finished ground level (Article 9.15.4.6), so check the clearance before you add fill.
  • Do not pile soil, mulch or planters against siding. The City of Edmonton also says to remove decorative mulch before regrading.
  • Use fill that sheds water at the surface. Edmonton does not recommend topsoil for rebuilding foundation grade because it does not shed water the way clay does.

What slope?

There is no single national number in the code text I read. It only says water must not accumulate. The numbers come from municipalities and guides, and they differ:

SourceWhat it saysIn feet and inches
City of Edmonton homeowner sheet (April 2018)10% slope away from the building1.2 in per ft (about 100 mm per m)
City of Toronto, infill housing grading rulesSide yards: at least 1.5%. Rear yard: not more than 10% unless existing grades are not changed0.18 in per ft at the minimum
Older guide values (about 5% over the first 3 m)Not confirmed. Do not use without checking your own municipality6 in over 9 ft 10 in

Treat the table as examples of how much the rules vary, not as your rule. Ask your building department what applies to your lot, and what they accept for the slope next to the wall.

Example (numbers made up). A settled strip runs 10 m (32 ft 10 in) along a wall. You build a wedge 150 mm (6 in) thick at the wall tapering to nothing over 3 m (9 ft 10 in): 0.5 × 0.15 × 3 × 10 = 2.25 m³ (2.9 yd³) of fill. First check that 150 mm of new fill still leaves the wall 150 mm above ground.

Backfill corrections

  • Soft or clayey backfill beside the wall can hold water against it. Where you have to open it up, replacing it with free-draining granular fill is common practice. Keep new fill out of the stone around any drain.
  • The Part 9 text I read says that where crushed rock or mineral fibre insulation is used against a foundation wall, it shall extend to the footing level so groundwater can reach the drainage system (Article 9.14.2.1). That is a design reason for free-draining fill at depth.
  • Compact in lifts, and keep heavy compactors off the wall. See the Backfilling pages.

Roof water and surface drainage

  • Downspouts: extend them well away from the wall, onto a splash pad or into a separate system. Edmonton says extensions should reach about 1.2 m (3 ft 11 in) past the excavation zone and stay within your property. Part 9 points to its roof-drainage article for downspout requirements (Article 9.14.6.5 refers to 9.26.18.2), which I did not read.
  • Gutters: keep them clean and working. Never let a downspout empty into a window well.
  • Keep roof water separate from the foundation drain. The foundation drain must drain to a sewer, drainage ditch or dry well (Article 9.14.5.1). Whether your municipality lets roof leaders connect to a storm sewer varies. Toronto, for example, requires written approval for some lots and says storm drainage must never go to a sanitary sewer.
  • Neighbours: Edmonton says extensions must not discharge directly onto adjacent property. Toronto says downspouts must not be directed onto adjacent property or make a sidewalk impassable.
  • Swales carry surface water around the house. Toronto sets longitudinal swale slopes at 1.5% or more, with side slopes no steeper than 33%. Those are Toronto infill numbers, so confirm your own.
  • Catch basin: where runoff from a driveway is likely to accumulate or enter a garage, the code text I read requires one (Article 9.14.6.4).
  • Window wells: every window well shall be drained to the footing level or another suitable location (Article 9.14.6.3). Keep wells clear of leaves and debris so the water can reach the drain.
  • Wells and septic: surface drainage shall be directed away from a water supply well or septic disposal bed (Article 9.14.6.2).
  • French drain (a gravel trench with pipe) can handle a persistent wet area, but it needs an approved outlet.

When the drain itself needs correcting

If water still reaches the wall after the grading, gutters and window wells are right, the foundation drain may be too high, flat, crushed or plugged. That means exposing it, which is excavation, so see Waterproofing Repairs. Rebuild the stone and fabric, check the pipe fall, and confirm it discharges to an approved place (Drain Tile, Gravel Beds and Discharge Lines pages).

Common mistakes

  • Regrading only, with a failed drain below
  • Adding fill until the foundation wall no longer stands 150 mm above grade
  • Clay or topsoil banked against siding
  • Downspouts ending at the wall or in a window well
  • Piping water onto a neighbour
  • Treating a number from a different city as your rule
Pro note

Water takes the easiest route. Your job is to give it a better one than the foundation.

Weather note

Heavy rain shows every weakness. Make corrections in dry weather, and walk the site during the first big storm afterward. Frozen ground cannot soak water in, so winter thaw and spring melt are the real test.

What the inspector looks at

Final grade and where it leaves the wall, drain corrections before they are covered, the discharge location, and window well drainage. Confirm what your building department checks.

✅ Check yourselfTap a question
Water is pooling on one side of a house after rain. What do you look at before digging up the drain?
Grading, gutters, downspouts and window wells. The code says the site must be graded so water will not accumulate at or near the building, and surface fixes are the cheapest and most effective place to start.
A window well has no drain. What does the code text I read say?
Every window well shall be drained to the footing level or another suitable location (NBC Part 9, Article 9.14.6.3, as reprinted in a BC municipal bylaw). Check your provincial edition.
Three cities give three different grade numbers. Which one is the national rule?
None of them. The code text I read gives no percentage, only that water must not accumulate at or near the building. Slope numbers come from municipal rules and guides, and they differ, so ask your building department.
You plan to add 150 mm (6 in) of fill against the wall. What do you check first?
How much foundation wall will still show above the ground. The Part 9 text I read says exterior foundation walls shall extend not less than 150 mm (6 in) above finished ground level, so raising the grade can break that rule and bury the sill.
A regrade wedge runs 10 m (32 ft 10 in) along a wall, 150 mm (6 in) thick at the wall tapering to nothing over 3 m (9 ft 10 in). How much fill?
0.5 × 0.15 × 3 × 10 = 2.25 m³ (2.9 yd³). The size is made up, and the number is loose volume to buy before compaction allowances.
Why must a downspout never empty into a window well?
The well then collects roof water right at the wall. Roof water needs its own route away from the house.
🧰 Gear for this stepMust-haveRent itNice to have
  • Must-haveLayout and measuring: a laser or line level, string line, stakes and a tape measure, to check the slope away from the wall
  • Must-haveDigging and moving fill (hand tools):
  • Rent itDigging and moving fill (machine): a skid steer or mini-excavator for large jobs
  • Must-haveCompaction (hand): a hand tamper
  • Rent itCompaction (machine): a plate compactor, usually rented, for fill beside the house
  • Must-haveSurface drainage parts: downspout extensions, splash pads, a catch basin and solid pipe, and perforated pipe with clean stone and fabric for a French drain
  • Must-haveWindow wells: clean stone and a drain connection to footing level
  • Must-havePPE:
👷 Who can do thisTap your level
Home DIY
This is the most DIY-friendly step: fix grading, extend downspouts, clear gutters and fill low spots. Treat 1.2 m (4 ft) as a no-entry line, do not dig beside the footing, and do not send water onto a neighbour, a sidewalk or the street. If water still enters after the surface is fixed, call a pro, because the drain may have failed.
Laborer or helper
Spread fill, extend downspouts and clear low spots as the crew directs. Do not dig beside the footing or cut a trench on your own. Tell the lead if water pools at the wall after you finish.
1st-year apprentice
Under supervision, regrade, tamp fill, extend downspouts and place catch basins. Learn first how water moves across a lot, and why the ground has to fall away from the wall on every side.
Apprentice, 2nd to 4th year
Regrade, tamp and place catch basins under the journeyperson, and check the slope away from the house with a level and string line. Keep out of any deep or unprotected excavation. Report standing water at the wall.
Journeyperson
You own the grading plan, the roof and surface water routes, and the choice between surface fixes and exposing the drain. Stop-work triggers: the new grade would send water onto a neighbour, a sidewalk or the street, the new grade would bury the foundation wall's required height above ground, or water still reaches the wall after regrading (then expose the drain).
Lead hand or foreman
Walk the finished grade before you leave and again after the first heavy rain. Hold the stop-work triggers: stop if water runs toward a neighbour or the street, or if water still reaches the wall after regrading, and bring in the drain-repair crew.
Contractor or business owner
Price by area and say plainly in the scope that surface work will not cure a failed drain. Photograph before and after, and record grades and the discharge point. Read your municipality's lot-grading and storm rules before you change drainage. Water on a neighbour's land is a claim risk, so ask your insurer or lawyer who carries it.
Starting a new business
Learn your municipality's lot-grading and storm rules before you quote, and confirm them with the building department. Write the scope so owners know surface work will not fix a failed drain. Photograph before and after, and ask your insurer about water claims.
By Province:

The national text is short: grade the site so water does not accumulate at or near the building, drain window wells, and use a catch basin where driveway runoff could enter a garage. Slope numbers, downspout distances, discharge rules and permits are mostly municipal and vary widely. The text above is from a municipal reprint of the national code and may be older than your province's edition.

I did not verify each province's current lot-grading rules on this pass, so ask your municipality for its grading guide before you change drainage. See Canada by Province for the full comparison.

Sources:

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

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