ᐅ Flat Roof – Water Standing on It

Hello everyone,

We are currently having a single-family house built and have two small flat roofs. After some rain, I noticed that water remained on the flat roofs, as seen in the photos.
My father-in-law is a civil engineer and said that this should not be the case.
Our construction manager said it depends on the height of the standing water. He wanted to take a look himself but apparently hasn't had time in the last two weeks. :\
From the pictures and in person, it seems like the roofs slope slightly uphill towards the drains, which would mean that a bit of water stays there permanently during the rainy months. It doesn’t dry off quickly either. :\
This sounds a bit worrying to me...
How do you see this? Is there any guidance or standards to rely on? I only found a recommendation of 2% slope.

Thanks and best regards 🙂

Flat roofs are probably the ultimate challenge. This is what I see every day from my desk. They just can’t seem to get it right either.

No, that is basically not a problem. A 2% slope prevents water from rushing too quickly to the drain ;-) Neither on the flat roof, the garage floor, nor in the walk-in shower.
It is also possible to achieve a zero slope according to flat roof guidelines; with the proper construction, this is no issue nowadays.

How was the slope created in the first place? Sloped insulation, screed? Is there only one drain, meaning the slope needs to run entirely from left to right, or are there other drainage points?

Good morning, why is there a slope towards the house?

In principle, a bit of standing water is not a big issue, but the drains don’t look like they are nearby. If there is landscaping added later, it won’t be a problem at all.

And why are two drains placed right next to each other? Normally, houses also require a secondary emergency drain—where is that located?

In one of the pictures, it looks as if the membranes were not fully bonded and have already formed air bubbles at the parapet?

How high is the water level? Hold a meter stick (ruler) in and take a photo.
To me, it also looks like the drain slopes uphill, which is definitely a defect. What slope is planned for the roof?

Offtopic schrieb:

And why are there two drains right next to each other? Normally, houses also need a second emergency drain—where is that?

The left drain is higher; that’s probably the planned "emergency drain" 😉

Offtopic schrieb:

Good morning, why is the slope directed towards the house?

And why are there two drains right next to each other? Normally, houses should have a second emergency drain. Where is that located?

One possible scenario could be a slope from the top of the picture down towards the house, then branching left and right to reach two drains.
The higher penetration might be an emergency overflow.

Just a theory, of course, but drains are often adjusted according to the homeowner's wishes regarding glazing and downspouts that are not routed over the glazing.
We could speculate on what the bay window looks like from below/outside... I’d guess it has a lot of glass, and the covering at the top of the picture supports that idea.