A waterproofing membrane is designed to keep water out of the building.
That does not mean good roof design should deliberately leave water sitting on it.
Drainage is part of the waterproofing strategy.
Falls, outlets, gutters, overflows and drainage paths all have to work together to move water away from the roof efficiently.
When they do not, standing water can expose the roof to conditions it was never intended to experience continuously.
Ponding water may result from poor falls, blocked outlets, structural deflection, badly positioned drainage or local low points in the finished surface.
And the effects go beyond simply having a puddle on the roof.
Standing water can increase loading, collect dirt and organic growth, repeatedly expose details and laps to water, make maintenance more difficult and highlight weaknesses around outlets, penetrations and terminations.
Different waterproofing systems also have different limitations regarding prolonged ponding water.
So the question should never simply be:
“Is the membrane waterproof?”
We should also ask:
Is the roof designed to drain?
Are the falls adequate?
Are the outlets located at the true low points?
Is there sufficient drainage capacity?
Are emergency overflows required?
Can outlets be inspected and maintained?
Could structural movement create new low spots later?
Waterproofing and drainage should support each other.
One should never be used as an excuse for poor design of the other.
A waterproof roof still needs to drain.