ᐅ Is water on a flat garage roof problematic or hazardous?

Created on: 14 Feb 2018 20:20
M
Marchonisch
Good evening

There is always water standing on our garage roof. In my opinion, the drain is incorrectly positioned.

The architect and the structural builder say this is normal and not a defect.

However, my roofer sees it as problematic. He says mold can develop...

What is your opinion or recommendation?

Flat roof with green roof waterproofing, construction wood remnants at the edge, autumn trees in the background.
L
LuckyDuke
1 Dec 2019 17:59
Editing is unfortunately no longer possible. I wanted to post the full response from the general contractor here:

“The flat roof construction is not yet complete. The lower edge of the garage roof drain is currently above the existing water-bearing layer. This drainage component is a prefabricated part and, due to its design, cannot be installed any lower. As a result, water currently remains standing on the flat roof. Since the garage roof will be covered with a gravel layer, additional protective mats (made of rubber) will be installed over the waterproofing membrane to protect it. These mats raise the water-bearing layer and thus largely prevent standing water. According to the flat roof guidelines, the remaining standing water on the waterproofing membrane does not constitute a defect but is rather unavoidable due to connection details, component overlaps, dimensional tolerances, etc.”

Regards
Christian
M
Müllerin
1 Dec 2019 19:49
Well, then just let him finish it properly. If there’s too much on it afterwards, you can still complain.
We barely had anything on ours, but by now it has grown over anyway.
The neighbors opposite intentionally don’t have a drain—for cooling. Uh, no comment.
H
HilfeHilfe
2 Dec 2019 06:21
Otherwise, green roofing
11ant2 Dec 2019 17:30
LuckyDuke schrieb:

This drainage element is a prefabricated part and, due to its design, cannot be installed at a deeper level.

However, the installation manual does not say "choose a product from our range intended for a different purpose or install a suitable product incorrectly." I don’t believe that the inventor of a drainage system would design a drain that guarantees residual moisture on a garage roof—unless the model is specifically intended for a green roof and takes into account that grass is less suitable for conventional hydroponics. An unplanted roof is not supposed to become moss-covered either, so zero-residual drains are used for that.
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M
Mottenhausen
3 Dec 2019 14:27
LuckyDuke schrieb:

"Additional building protection mats (made of rubber) are applied on top of the waterproofing to protect it. These mats increase the water-draining layer and thus largely prevent standing water."

I would translate that as: the water will then stand between the roofing layer and the building protection mat.

For all puddles applies: it doesn’t matter in summer, but in winter it freezes, expands, and gradually damages the surface of the roof. The drainage is simply too high; the explanation "it cannot be installed any lower" is a joke. Ask if it really can’t be done and whether you seriously need to bring in someone else who can do it.

PS. And what is that gap between the pipe and the roof for? Is it sealed, or does everything flow around the pipe? The roof waterproofing should be integrated into the pipe, not the other way around.
11ant3 Dec 2019 16:01
Mottenhausen schrieb:

The roof sealing belongs inside the pipe, not the other way around.
That’s true – in our world. But the contractor carrying out the work obviously has no (or a rather esoteric) understanding of physics.
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