Disconnection
Toronto requires downspouts to be disconnected from the sewer system where it is feasible, and the reason is basement flooding across the whole neighbourhood rather than just yours.
Short answer
A downspout connected to the sewer sends roof water into a system sized for sanitary flow, which during a heavy storm is what backs the system up into basements. Toronto has a mandatory downspout disconnection bylaw with exemption provisions where disconnection is not feasible. A compliant discharge goes to a permeable surface well clear of the foundation.
Why the city requires it
Roof water entering the sewer system during a storm adds enormous volume to a system not sized for it.
That volume is what causes the system to surcharge and back up into basements, which in Toronto has happened at scale.
The problem is cumulative across a neighbourhood, so one disconnected downspout helps everybody slightly and the whole street disconnecting helps a great deal.
It also reduces the load on the treatment system and the combined sewer overflows.
And on an individual property it moves roof water away from the foundation, which is the homeowner benefit.
The bylaw is mandatory with exemption provisions, and the city publishes the process for applying.
Where exemption applies
Where there is genuinely nowhere to discharge — no permeable surface, a lot line too close, a grade that cannot be corrected, a risk of discharging onto a neighbour property or a sidewalk. The exemption process exists for exactly those cases, and applying for it is better than leaving it connected and hoping.
What a compliant discharge looks like
Discharge to a permeable surface — lawn, garden bed, soakaway — rather than onto pavement.
Well clear of the foundation, onto ground that falls away from the building.
Not onto a neighbour property, a sidewalk or a driveway where it will freeze.
Not into a window well or a stairwell.
With the sewer connection properly capped, which is part of the job rather than leaving an open stub.
And with the grade corrected where it falls back toward the house, because an extension onto a reverse slope defeats the purpose.
Doing it well
Deal with the grade at the same time, because that is half the benefit.
Use rigid extension where it will be walked on, and keep the extension on rather than removing it for mowing.
Consider a rain barrel on one or two downspouts, which stores the first part of the storm and has an overflow — it is a supplement rather than a substitute.
Consider a soakaway or a dry well where the lot allows, which handles the volume without a surface discharge.
Check the winter behaviour, because a discharge onto a path or a driveway creates ice.
And check it in a real storm, which is the only test that matters.
Common questions
Is downspout disconnection mandatory in Toronto?
Yes, under a city bylaw, with an exemption process where it is not feasible.
Why does the city care?
Roof water in the sewer during a storm is what surcharges the system and backs it up into basements.
Where should it discharge?
To a permeable surface well clear of the foundation, on ground that falls away.