Toronto's Wet Summer: How to Protect Your Basement from Flooding and Mould

We will start with the bad news: this summer in Toronto is shaping up to be a wet one.

The Weather Network's seasonal outlook calls it the "on-again, off-again summer". Heat that builds and then gets interrupted, more days with showers and thunderstorms, and cooler-than-normal Great Lakes. 

What does that mean for your house? Most years, your big summer worry is keeping the AC running. This year, the story isn't heat. It's water.

A wet, humid, storm-interrupted summer puts two specific risks on the table that aren't your usual seasonal concerns: basement flooding and humidity and mould. The good news is both are preventable, and most of the fixes are cheap. 

Here's how to know if your home is at higher risk, and what you can do now.

The quiet one: damp and mould

Frequent rain plus warm air means a damp summer, and damp is what grows mould, particularly in basements. This one is sneaky because there's no dramatic event. It just creeps in.

Is your home at higher risk for mould? 

• You have a finished basement (drywall, carpet, laminate give mould food and hide it)

• You've had water down there before, or notice a musty smell that comes and goes

• Your basement humidity runs above 55–60%

• You have an older or below-grade foundation that feels damp or shows white powder

• The space is poorly ventilated, with no working exhaust fan

Start with a hygrometer. It runs about $20, and you can't manage humidity you can't measure. You're aiming for 30 to 50 percent through the summer. If the reading sits higher, run a dehumidifier, and either empty it on a schedule or plumb it to a drain, because one that's left full stops working and you won't notice. While you're at it, check that the bathroom and laundry exhaust fans actually vent outside, not into a wall cavity or the joist space. Venting into the structure is a common way to move the damp from one room to another instead of getting rid of it.

One caveat: a dehumidifier manages moisture in the air and nothing else. If the musty smell keeps coming back no matter what the hygrometer says, or you see staining low on the foundation, that's water getting in, and drying the air won't fix it. At that point you're looking at grading, drainage, or the foundation itself, which is a bigger conversation than a $20 gauge.

The loud one: flooding

More frequent, heavier downpours are exactly the conditions that send water into basements. Especially in the older parts of Toronto where storm and sanitary sewers share one pipe and can back up during intense rain.

Is your home at higher risk for flooding?

• Your street or immediate neighbours have flooded before

• You don't have a backwater valve (or you're not sure you do)

• You have a floor drain that's ever bubbled or backed up in heavy rain

• Your downspouts dump right next to the foundation instead of away from it

• Your house sits at the low point of the street

Two devices do most of the heavy lifting, and they solve different problems, so it's worth knowing which one you actually need. A backwater valve stops sewage from flowing back into the house when the city main overloads. A sump pump deals with groundwater collecting under the foundation. A house can need one, the other, or both. Whichever you put in, a sump pump is only as good as its power supply, and the storm that fills the pit is usually the same one that cuts the power, so a battery backup isn't really optional in a serious storm. If you work with a licensed contractor on any of the above, Toronto’s Basement Flooding Protection Subsidy Program can help to cover the cost.

The rest you can handle yourself. Keep the eavestroughs and downspouts clear, then extend the downspouts so they discharge at least 1.5 to 2 metres from the foundation rather than dumping against it. Clear out the window wells and cover them so they drain instead of filling like buckets in a downpour. And where the ground allows, regrade the soil so it slopes away from the house. Even a gentle fall over the first couple of metres moves a surprising amount of water away from the wall.

The 20-Minute Weekend Check

You don't need a contractor to get ahead of most of this. Before the first long stretch of rain, walk the basement and the perimeter and do five things:

• Plug a hygrometer in and note the reading

• Clear the eavestroughs

• Point the downspouts away from the foundation

• Test the sump pump by pouring a bucket of water into the pit and confirming it kicks on

• Clear and cover the window wells