Commercial Drainage
On a commercial building the drainage is a bigger system with a bigger consequence, and it fails at the parts nobody has on a maintenance schedule.
Short answer
Commercial roof drainage means larger troughs and downspouts, internal drains and scuppers on flat sections, and in Toronto an overflow provision that works when the primary is blocked or frozen. The failures are blocked drains, frozen downspouts and overflow routes that were never provided.
What is different
Larger areas and larger volumes, so the system has less margin.
Flat and low-slope sections with internal drains or scuppers rather than eavestrough.
Internal downspouts running through the building, which when they block or freeze leak inside rather than outside.
Overflow provision, which is the second line of defence when a primary drain blocks and which on an older building may not exist.
Roof-mounted mechanical equipment and curbs, which obstruct flow and create ponding.
And the consequence, which on a commercial building is stock, equipment, tenant space and business interruption rather than a wet basement.
The overflow route
Every flat roof needs a way for water to leave when the primary drain is blocked — an overflow drain, a scupper through the parapet, or at minimum a known low point in the parapet. Without one, a blocked drain turns the roof into a pond and the load becomes structural. It is worth confirming yours exists.
Where it fails
Blocked drains and strainers, which is the most common and the most preventable.
Frozen downspouts, particularly internal ones running in an unheated chase or on a cold exterior wall.
Ponding from inadequate slope or from equipment obstructing the flow.
Scuppers blocked with debris or ice.
Troughs on the sloped sections undersized for the area, which is common where a building has been extended.
And discharge that has no capacity at ground level, which backs the system up from the bottom.
The maintenance that prevents it
Clear every drain, strainer and scupper after leaf fall and before the first freeze. The single highest-value task.
A spring visit to find what the winter did.
Clear the whole roof field of debris, particularly against parapets and curbs.
Confirm the overflow provision is clear and functional.
Keep a roof plan with the drains and the downspout routes marked, which makes every subsequent visit faster.
And a documented inspection record, because on a commercial building that is what an insurer asks for after a loss.
Common questions
What is the highest-value commercial maintenance task?
Clearing drains and scuppers after leaf fall and before the first freeze.
Why do internal downspouts matter more?
When they block or freeze they leak inside the building rather than outside it.
What is an overflow provision?
A second route for water out when the primary drain blocks. Confirm yours exists.