Sump Systems

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 sump pumps away foundation drain water when gravity cannot. It is a mechanical system, so plan for failure.

The big idea

A sump collects water from the foundation drain and pumps it away when gravity cannot. It is a mechanical system with a pump, a switch and a power supply, so it can fail. Design for that.

What the code says

From Part 9, Section 9.14, as printed in the Vancouver Building Bylaw 2014:

• The pit is at least 750 mm (2 ft 5 1/2 in) deep and at least 0.25 m² (2.7 ft²) in area (9.14.5.2).
• It has a cover that resists removal by children and is airtight (9.14.5.2, with 9.25.3.3).
• Where gravity drainage is not practical, an automatic sump pump discharges to a sewer, a drainage ditch or a dry well (9.14.5.2).
• Where a stone layer is used instead of pipe, the excavation is graded so the whole layer drains to a sump (9.14.4.3).

A 0.25 m² pit is about 500 mm (20 in) square, or 565 mm (22 1/4 in) round. The text I read sets no pump size, alarm or backup. Those are good practice, so confirm what your province requires.

The pit

• Set it at the lowest point of the drain system.
• Size it to the code and the pump maker. A retailer's guide suggests an 18 to 24 in (450 to 600 mm) pit, to reduce short cycling.
• A liner on a stone base, with a watertight drain connection and a sealed cover.
• Leave a gravel and sediment space below the pump, so silt does not foul the float.

Pump types

• Submersible: quiet and efficient, and common in houses.
• Pedestal: the motor sits above the water, which is easier to service but noisier.
• Backup pump: battery or water-powered, for power outages.
• Two pumps: for high water tables or critical basements.

Sizing

Size by flow and head, not by horsepower. A bigger pump is not always better, because an oversized pump short-cycles and wears its switch, and an undersized one runs continuously.

• Flow: pit volume per cycle times cycles per minute in a storm.
• Head: the vertical lift from the pit to the highest point of the discharge, plus friction. A retailer's guide uses 1 ft of head per 10 ft of horizontal pipe, plus 1 to 2 ft per pair of elbows. That is a rule of thumb.
• Pump curve: read the maker's curve at your head, with about 25 percent margin. Box ratings are at zero lift and overstate real flow.

Example. A 560 mm (22 in) pit has 0.25 m² of area. With the float switching on 200 mm (8 in) above where it switches off, each cycle moves 0.25 × 0.2 = 0.05 m³, or 50 L (13 US gal). If it cycles every 2 minutes in a storm, the flow is 25 L/min (6.6 US gal/min), so look for about 31 L/min with a 25 percent margin. With 2.4 m (7 ft 10 in) of lift and 12 m (39 ft 4 in) of horizontal pipe, the head is about 2.4 + 1.2 = 3.6 m (11 ft 10 in). Every number here is made up.

Discharge and check valve

• Rigid discharge pipe, with a check valve above the pump so water does not run back. One municipal checklist says to confirm there is a check valve on the drain line just above the sump cover.
• A union or serviceable fitting, so the pump can be removed.
• Where freezing is a risk, follow the pump maker's advice on a relief hole.
• The discharge goes to a sewer, ditch or dry well (9.14.5.2). The Discharge Lines page covers where the water goes.

Backup, alarm and maintenance

• A high-water alarm, and a battery or water-powered backup pump, are good practice. Power outages come with storms.
• A municipal checklist says to test at least twice a year, preferably before spring thaw. Pour water in the pit and check that the float moves freely, the discharge line does not leak, the alarm sounds and the backup only starts if the primary fails.
• Clean silt and debris from the pit, because they block the float and clog the impeller.
• If a ground-fault device is on the circuit, test it too, because it can trip and disable the pump.
• I did not find a verified service life, so the page gives none. Ask the pump maker.

Common mistakes

• Pump too small or too big
• Pit too shallow
• No check valve
• No backup
• Open or loose cover
• Discharge too close to the house