ᐅ Water on flat roof – double layers of roof waterproofing

Hello,

On our attached structures (garage), we have standing water for an extended period (24-36 hours) after rain showers. I estimate about 5 cm (2 inches) and plan to inspect the roof soon.

The developer says this is completely acceptable for a flat roof or even good because of the waterproofing.

I’m having a hard time with that 😳

5cm (2 inches)? Are you sure?

I would be concerned about that too, even if it’s probably not critical yet.

1-2cm (0.5-1 inch) might be acceptable since there are double layers of roof waterproofing in some areas and small elevations can occur. But 5cm (2 inches)??? I can’t imagine that being normal.

Is this a prefabricated garage? These usually have a built-in slope for water drainage...

No, it was built with masonry....

What is the ceiling made of? I mean, 5cm (2 inches) must be quite a large puddle... and where is the drain located? With the assumed 2cm (0.8 inches) slope, this kind of issue shouldn’t occur at that scale, right?

I also believe that a significant dent would be required to create a puddle with a depth of 5cm (2 inches). I would be less concerned about the water itself (a bit of moisture is actually beneficial for bitumen, whereas direct sunlight is not), and more about the fact that water is pooling in an area where it shouldn’t be. If your flat roof were greened, the moisture would remain on the roof even longer, but green roofs are not more prone to water damage than non-green roofs—in fact, quite the opposite.

We also have two flat roof extensions without green roofs, where after a rain shower a small amount of water remains; however, this is only a few millimeters (fractions of an inch) deep, and the rest drains away.