What each build era left in the ground
The lakeshore villages and the old Dundas corridor are where we still find clay laterals with roots in every joint, and the occasional galvanized supply line scaled shut from the inside.
Erin Mills and Meadowvale brought the first big wave of copper and cast iron drainage through the 1960s and 70s. The west end that filled in later has a feature most of the GTA does not: foundation drain collector systems, a third pipe carrying weeping tile water only, running under parts of Lisgar, Churchill Meadows, Meadowvale Village and Central Erin Mills.
Roof leaders are not permitted to tie into them. Connect a downspout to one and you have created a problem for the whole street, not just for yourself.