Flat roof drainage

Flat roofs are not very good on water drainage, especially if you want to have a roof top terrace which is what my design is. If you don’t have a roof top terrace, you can have a consistent pitch on your flat roof and drain the water to one side. That’s proven to be pretty effective. You can always have a higher pitch if you don’t have a roof top terrace, since no one will be walking on it. But in my case because of the roof top terrace and the accompanied parapet wall, the roof has to be designed to drain into interior points. This creates the necessity of creating complex low pitches on the roof to drive water towards where you wanted. And sometimes these pitches might not work the way you wanted.

No that the roof top is pretty much completed, I need to make sure that any standing water on the flat roof is within an acceptable range. And just so happens that today right after the metal sidings are installed, a quick thunderstorm passed through. A good time to check to see if the drainage is working the way it should be.

So here’s what I have immediately after the storm. The south side is the low side and that’s where the roof drains are. The south side mostly drained well. But the two skylight curb created a bit of a high point and is preventing some of the water from flowing towards the low points. This is a view from the west side. As you can see there’s a small body of water right around the skylight curb.

Here’s a look from the east side. Again a small body of water right around the skylight curb.

The deepest point of these water puddles is about 9mm. And that’s about the same around either skylight on either side. So it’s not that much water that’s accumulating.

Now let’s look at the high points on the roof. This is the north east corner. No standing water here at all.

This is the north east corner of the 3rd floor. Here’s also where the 3rd floor roof drains through that downspout in the corner. It is also dry here.

This is the north west corner. Again dry, no visible water accumulation.

And this is the south west corner. It’s mostly dry. The skylight curb is holding just a tiny bit of water behind the south most skylight on the west side. But it is minimal.

I will check again after 48 hours if there’s no more rain to see if there’s still any standing water left. The amount of water remaining on the roof looked fine and I think they should all evaporate by then.