Have you encountered problems with the absence of emergency drainage from a flat roof?

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I'm talking about building a new EFH with a flat roof. I have a 120 sq ft main roof, an 80 sq ft garage roof, and two balconies with a brick parapet. The roof pitch is 2° and there is regular gravel on the main roof and garage roof. The contractor has now installed 3 downspouts on the main roof and two downspouts on the garage for drainage. According to DIN, flat roofs need an emergency drain that is not connected to the sewer system. I don't have this for the reason that additional "normal" downspouts were installed. According to DIN, only one drainage pipe is required (DN100 or DN80).

On both balconies, however, an emergency drainage system has now been retrofitted – drilled through an attached foil seal.

Now I'm a little skeptical about the main roof and garage. Does anyone have any experience, does it really work without emergency drainage?

  • Do the pipes at least have an emergency overflow? What exactly is due?
  • No, the pipes do not have an emergency overflow. The only reason for this is if for any reason (dead bird, leaves, etc.) the downspouts get clogged, the flat roof will fill up. The water can't run off. This is why the DIN prescribes that flat roofs must always be equipped with emergency drainage systems. They are not connected to the sewer system and drain to the open ground (on site). However, the general contractor says that this will be compensated for with additional "normal" drainage.
  • This doesn't seem logical to me, because in the worst case scenario, even with these drainage pipes, there could be a backwater. Before contacting an expert, I thought maybe there are different experiences here...
  • Yes, backwater would be my problem as well. More pipe would not help you either if they ended up together on the same string. That wouldn't work for me.



Our pipe goes from the roof to the open drainpipe and only then to the downpipe. If the gutter is clogged, the box overflows. I know this from other flat roofs with outside gutters.

So maybe just retrofit the trap? It's unlikely that all 3 downspouts will get clogged.


How high is the edge compared to the gravel layer? Can it run over it? Can you install a gutter on it? Every normal roof has 1 gutter per side and 1 pipe, so if you only have 3, that's fine.

  • The parapet is 75 cm high, which means water can never overflow over it. The gutters have been drilled through the parapet and drainage occurs that way. So the gutters in this case are unfortunately not suitable for flat roofs.
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