Where It Lands
The single most common cause of a wet Toronto basement is a downspout discharging a metre from the wall.
Short answer
A roof collects an enormous volume in a storm and a downspout concentrates it at one point. Discharged next to the foundation, it saturates the soil against the wall and finds its way in. Extending the discharge well clear of the building, onto ground that slopes away, is the cheapest basement waterproofing available.
The volume involved
A single downspout on a typical Toronto house serves a large roof area, and a heavy summer storm delivers hundreds of litres through it in an hour.
All of that arrives at one point on the ground.
Discharged at the wall, it saturates the backfill, which is the loosest and most permeable soil on the property.
That saturated backfill sits against the foundation wall and against any crack, form tie or window well.
In winter the same water freezes in the soil and the expansion works on the wall.
And it is the most easily fixed water problem a house has.
How far is far enough
Further than people think, and onto ground that slopes away rather than back. A short splash block is not an extension. Two metres is a reasonable minimum on a level lot and more where the grade is poor. The test is simple: watch it in a real downpour and see where the water goes.
What goes wrong
An extension removed to mow the lawn and never put back, which is extremely common.
A splash block that directs water a foot from the wall, which does almost nothing.
Discharge onto a paved surface that slopes back toward the house.
A buried extension that has collapsed, blocked or disconnected underground, which is worse than no extension because nobody knows.
Discharge into a window well, which puts the water directly against the window and the wall.
And discharge over a property line, which is a neighbour problem and in some cases a bylaw one.
Doing it properly
Extend well clear of the building, onto ground that falls away.
Fix the grade if it slopes back, because an extension onto a reverse slope just moves the problem.
Use rigid extension rather than flexible corrugated where it will be walked on or mowed around, because the flexible ones crush.
If burying it, use proper pipe with a positive slope and a visible outlet, and keep a record of where it runs.
Check all of them in a real storm rather than assuming, which is the only test that counts.
And address the downspout disconnection question separately, because Toronto has specific rules about discharging into the sewer system.
Common questions
How far should a downspout discharge from the house?
Further than most people think — a couple of metres minimum, onto ground that falls away.
Is a splash block enough?
No. It directs water a foot from the wall, which does almost nothing.
How do I test it?
Watch it in a real downpour and see where the water actually goes.