How to prevent basement flooding in Ontario

Outside the house

• Grading. Soil should slope away from the foundation on every side. Over time it settles and tips back toward the house. Top it up.
• Downspouts. Discharge at least a couple of metres from the foundation, onto ground that slopes away. Many municipalities require downspouts to be disconnected from the sewer.
• Eavestroughs. Clean them in late fall and spring. Overflowing gutters dump water right at the foundation.
• Window wells. Keep them clear of leaves and consider covers.
• Driveways and walkways that slope toward the house can channel water to the wall. Sealing the joint helps; regrading fixes it.

Inside the house

• Sump pump. Test it twice a year. Add a battery backup and a high-water alarm. See sump pump failure.
• Backwater valve. Stops sewage backing up from the municipal sewer. See the backwater valve guide.
• Floor drain. Keep it clear and know where it is.
• Foundation cracks. Have active leaks sealed.
• Storage. Keep valuables and documents off the floor, in plastic bins on shelves.
• Finishes. If you are renovating a basement that has flooded before, choose materials that tolerate water better.

Plumbing

• Replace rubber washing machine hoses with braided steel.
• Know where the main shut-off is and make sure it turns.
• Consider leak sensors or an automatic shut-off valve.
• Before a winter trip, keep the heat on and have someone check the house.

Seasonal checks

Spring: clear snow away from the foundation, test the sump, check that downspout extensions are attached.

Summer: watch the forecast for severe thunderstorm warnings. Avoid running water-heavy appliances during intense rain if you have a backwater valve.

Fall: clean eavestroughs, check grading, test the sump again.

Winter: keep the sump discharge clear of ice and insulate pipes on exterior walls.

Help paying for it

Several Ontario municipalities subsidise backwater valves, sump pumps and related work. Check your municipality’s website before starting, because some programs require approval first. Our service area pages link to the official programs we have verified.

Check your insurance at the same time

Ask your broker whether you have sewer backup and overland water coverage, and what the limits are. Prevention and coverage work together.

Start by knowing your risk

Three questions tell you most of what you need to know:

1. Has this house, or this street, flooded before? Ask neighbours and the previous owner, and look for stain lines on the foundation wall.
2. What kind of sewer serves the street? Older areas of some Ontario cities have combined sewers, which are more prone to surcharge.
3. Are you near a watercourse or in a low spot? Conservation authorities publish floodplain maps, and many municipalities map basement flooding study areas.

Your municipality’s water department can often tell you whether your area is in a flood study or a subsidy program.

The measures, in order of cost

Free or nearly free

• Clean eavestroughs twice a year
• Extend downspouts and point them away from the house
• Keep window wells and the floor drain clear
• Move valuables off the basement floor
• Test the sump pump

Moderate

• Regrade soil that has settled toward the foundation
• Add window well covers
• Seal active cracks
• Fit a high-water alarm and leak sensors
• Add a battery backup to the sump pump

Larger investments

• Install a backwater valve
• Disconnect weeping tile from the sanitary sewer and add a sump pump
• Repair or replace a damaged sewer lateral
• Waterproof the foundation from outside

Several municipalities subsidise the items in the last group. Check before you start, because some programs require approval first.

What to do when a severe storm is forecast

• Clear the area around floor drains and the sump pit
• Check that the sump pump runs and the discharge is clear
• Move anything valuable off the floor
• Avoid running the washing machine and dishwasher during the heaviest rain
• Charge phones, in case the power goes out

When prevention is not enough

No measure makes a basement flood-proof. Finishes that tolerate water, belongings kept off the floor and the right insurance reduce the loss when water does get in.