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Water Control

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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Keep water out of the dig, get it out when it gets in, and send it somewhere legal.

I am a:

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

What it isKeeping water out of the dig, getting it out when it gets in, and sending it somewhere legal.
Why it mattersWater weakens trench walls, softens bearing soil and floats forms.
The hard ruleNever pour on standing water or softened ground, and never send muddy water to a drain or creek.
Who sets the rulesProvincial OHS regulators, municipal bylaws and the federal Fisheries Act.

By the end you can

  • Name where water on a site comes from.
  • Keep surface water out of the hole, and pump from a sump outside the footing line.
  • Work out how much water is in a hole and how long a pump takes to clear it.
  • Settle silty water before it leaves the site, and know when pumping needs a permit.
  • Know when water makes a trench unsafe, and when to call an engineer.
Stop and read

Never enter a flooded or wet trench at any depth, and never enter one over 1.2 m (4 ft) deep that is not sloped or supported. Wet ground is weaker than it looks, and water pressure behind a wall can bring it down. If water is coming in faster than you can pump it, stop, get everyone out and have the area assessed before anyone goes back in.

The big idea

Water is the thing that most often turns a good excavation into a bad one. It weakens trench walls, softens the soil under your footings, floats forms, ruins rebar and concrete, and if it leaves your site carrying mud it can bring a fine. Plan where the water goes before you dig.

Water control touches worker safety (provincial OHS rules), municipal bylaws and federal environmental law. The building code covers how water is kept away from the finished foundation, covered on the Waterproofing and Drainage pages.

Know where the water comes from

  • Surface water: rain, runoff from neighbouring lots, snowmelt
  • Groundwater: a high water table, or seepage through the walls and floor of the dig
  • Existing pipes: a leaking water main, drain or old tile line
  • Your own site: washing tools, concrete washout, pump discharge going in circles

Controlling surface water

  • Grade the ground so water runs away from the excavation, not into it
  • Cut a diversion ditch or build a berm upslope of the dig
  • Keep spoil piles from acting as dams or channels (see Spoil Pile Management)
  • Cover exposed slopes with poly or erosion blankets before a storm

How much water is that?

Rain on an open hole adds up fast. Area × depth of rain gives the volume. For a 10 m × 12 m hole: 120 m² × 0.025 m (25 mm of rain) = 3.0 m³, about 3,000 L. In feet, 1,292 ft² × 1/12 ft (1 in) = 108 ft³. A pump that moves 200 L/min (about 53 US gal/min) takes 3,000 ÷ 200 = 15 minutes to clear it, but only if nothing keeps flowing in.

Controlling water in the hole

  • Dig a sump pit at a low corner, outside the footprint of the footings, and pump from it
  • Use a pump rated for solids if the water carries silt. Clean intake screens often
  • Never leave a pump unattended near an unsupported trench without a plan if it stops
  • Never let water sit under a footing. Remove it and any softened soil before pouring
Water changes the trench rules

Water in an excavation is covered by your provincial OHS regulator. In Alberta, workers may not enter an excavation until hazardous accumulations of water are eliminated (OHS Code s. 445). Wet ground can make a trench less stable, so have a competent person reclassify the soil. Water pressure behind a wall is one of the things that brings an engineer in. See Trench Safety and Utility Trenching.

Confirm

BC's Part 20 wording on accumulated water, and the matching rule in your province, were not re-read for this page. Read the section in your regulation.

High water table and large dewatering

Well points, deep wells and long-running pumping change the water balance around neighbouring buildings, and can pull fines out of the soil and cause settlement. These are engineered jobs. If the water table is above your footing level, talk to a geotechnical engineer before you start.

Where the pumped water goes

  • Never pump muddy water into a storm drain, ditch, creek or onto a neighbour's lot.
  • Let silty water settle first: a settling pit, a filter bag, or a straw bale and fabric filter.
  • The federal Fisheries Act says no person shall deposit, or permit the deposit of, a deleterious substance of any type in water frequented by fish, or where it may enter such water (s. 36(3)). Treat silty water as a pollutant and keep it out of creeks and storm drains.
  • Many municipalities have erosion, sediment or storm-sewer bylaws with their own fines and permit conditions.
  • Ontario: a Permit to Take Water is required to take 50,000 L or more in a day, unless the taking is exempt or needs a registration instead. At 200 L/min a pump reaches 50,000 L in 250 minutes, about 4 hours 10 minutes (about 13,200 US gal).
Confirm

Whether dewatering needs a permit or registration, and how silty water must be treated, differs between provinces and municipalities, and between storm and sanitary connections. Ask your municipality before you pump. Ontario's separate rules for construction dewatering were not read for this page.

During concrete work

  • No standing water under footings, slab bases or in wall forms at the time of placement
  • Keep water off the rebar zone and out of the forms
  • Cover fresh concrete before rain. Rain on a fresh surface weakens it
  • Pumping next to a fresh pour can wash out cement. Keep discharge away

Common mistakes

  • No drainage plan before the excavator arrives
  • Pumping water to a place where it runs straight back in
  • Discharging silt to a storm drain or a neighbour's property
  • Pouring footings onto softened ground
  • A pump with no backup, or a clogged intake, running unattended
  • Spoil placed where it channels runoff into the hole
Pro note

Sketch the water path on the site plan on day one: where it comes from, where the sump is, where the discharge goes, and what happens if the pump quits. Put the sump in early. It is far easier to dig a sump in a dry hole than to chase water around a wet one.

Weather note

Heavy rain, as on the BC coast, can fill a hole overnight, so cover slopes and check pumps before leaving. Where winters are cold, spring snowmelt over frozen ground sends runoff straight to the lowest point on your site, which is usually your excavation.

Code note

Footings must sit on suitable ground, free of water and frost. How water is kept away from the finished foundation (drain tile or a granular layer at the bottom of exterior walls, a sump, surface grading) is NBC Part 9, Section 9.14. Your local building official may ask for a geotechnical report on wet or high water-table sites. See the Waterproofing and Drainage pages.

✅ Check yourselfTap a question
The excavation is 10 m × 12 m and it rains 25 mm (1 in) overnight. About how much water ends up in the hole, if none drains away?
10 × 12 = 120 m². 120 × 0.025 m = 3.0 m³, which is about 3,000 L. In feet: about 1,292 ft² × 1/12 ft = 108 ft³.
A pump moves 200 L/min (about 53 US gal/min). How long to clear those 3,000 L?
3,000 ÷ 200 = 15 minutes, if the pump keeps up and nothing keeps flowing in.
In Ontario, a permit to take water is needed at 50,000 L per day or more. How long does a 200 L/min pump take to reach that?
50,000 ÷ 200 = 250 minutes, or about 4 hours 10 minutes. So a pump left running through a work day can pass the threshold. About 13,200 US gal. Ask the ministry whether a registration or permit applies.
Water is standing at the bottom of the footing trench. May you pour?
No. Remove the water and any softened soil first. Never let water sit under a footing, and re-check the bearing soil.
Your pump discharge is muddy and the nearest low spot is the street storm drain. What do you do?
Do not send it there. Settle it first in a settling pit or filter bag, and check your municipality's sediment and storm-sewer rules. Sediment reaching fish-bearing water can bring Fisheries Act charges.
The pump stops overnight and the water rises on the trench wall. Before anyone goes in, what must happen?
A competent person re-checks the walls, and the soil is reclassified as wetter and weaker. If water is coming in faster than you can pump it, get the area assessed first.
🧰 Gear for this stepMust-haveRent itNice to have
  • Rent itPumping:
  • Must-haveSettling and filtering:
  • Must-haveDiverting water:
  • Must-haveChecking the base:
  • Must-haveCleanup: a wheelbarrow or buggy, a flat shovel and a broom to remove softened soil before placing concrete
  • Must-haveWet-weather PPE:
👷 Who can do thisTap your level
Home DIY
You can divert surface water, cut a shallow sump and run a small pump, and keep silty water on your own lot. Do not enter a flooded or wet trench at any depth, and never any trench over 1.2 m (4 ft). Get an engineer if the water table is above your footing level or the pumping will run for days.
Laborer or helper
Help keep pumps running, hoses laid and the intake clear. Never enter a flooded or wet trench at any depth, or any trench over 1.2 m (4 ft). Tell your lead if the pump stops, the water rises or the trench wall looks wetter than before.
1st-year apprentice
You can set pumps, clean intake screens and build silt filters under supervision. Learn where the water comes from, where it goes, and what to do if the pump quits.
Apprentice, 2nd to 4th year
Run the pumps and filters on your assigned area, and learn where the water goes and why. Check the pump and the screen at the start of each shift. Never enter a wet trench. Report softening soil to your journeyperson right away.
Journeyperson
You own the water plan and the call on whether the bearing soil is still sound. Stop-work triggers: water arriving faster than the pump removes it, softened bearing soil, or a water table above footing level. Remove softened ground, hold the pour and call the geotechnical engineer.
Lead hand or foreman
Set up the pumping plan, a backup pump and a place for the water to go before digging starts. Check the bottom for softening at every shift change. Stop work, clear the trench and call the engineer if the water beats the pump or the bearing soil softens.
Contractor or business owner
Check the municipal sediment and storm-sewer bylaws, and whether dewatering needs a permit. Price pumps, backups and cleanup. Large dewatering is engineered work. Keep pump logs and photos of where the water went. Ask your lawyer or insurer who carries the cost of a fine or of a neighbouring building that settles.
Starting a new business
Learn your municipality's rules on pumped water before you start, and ask the building department whether you need a dewatering permit. Rent pumps and backups, and plan fuel and monitoring time. Ask the engineer to review any long-running dewatering, and ask your insurer whether neighbouring damage is covered.
By Province:

Where pumped water goes is set mostly by municipal bylaws and the federal Fisheries Act, so this topic varies by city more than by province.

  • BC: worker safety is OHS Reg. Part 20; confirm the section on accumulated water. Ask your municipality about sediment and storm-sewer rules.
  • AB: OHS Code s. 445 keeps workers out of an excavation until hazardous accumulations of water are eliminated.
  • ON: a Permit to Take Water is required at 50,000 L or more per day, unless exempt or registered instead. Confirm with the Ministry of the Environment.
  • MB: confirm perimeter drain discharge rules with the provincial building code office.
  • Everywhere else: ask the local authority about dewatering permits and discharge.

See Canada by Province for the full comparison.

Sources:

Last reviewed October 2026. General information, not engineering or legal advice. Check the current regulation and your local bylaws.

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