Flat roof drainage with emergency overflow

Hello everyone,

this is about the construction of a new single-family house with a flat roof. I have a main roof of 120 sqm, a garage roof of 80 sqm, and two balconies with masonry parapets. The roof slope is 2°, and the main roof and garage roof are normally covered with gravel. The construction company has now installed 3 downpipes on the main roof and two downpipes on the garage for drainage. According to DIN, flat roofs require emergency drainage which is not connected to the sewer system. This is not present in my case with the reasoning that additional "normal" downpipes were installed. According to DIN, only one drainage pipe (DN100 or DN80) would be necessary. However, an emergency drainage is now being retrofitted on the two balconies – drilling through the installed waterproofing membrane. I am now a bit skeptical regarding the main roof and garage. Does anyone have experience whether it really works without emergency drainage?

Thanks for any input :)

Do the pipes at least have an emergency overflow? What is specifically owed?

No, the pipes do not have an emergency overflow.

The background is simply that if, for whatever reason (dead bird, leaves, etc.), the downpipes become clogged, the flat roof will fill up. The water cannot drain away. That is why the DIN standard requires flat roofs to be equipped with emergency drainage as a rule. These are not connected to the sewer system and drain onto free ground areas (on the property). However, the general contractor says this is intercepted by additional "normal" drainage systems.
To me, this is illogical, since in the worst case a backup can also occur in these downpipes.
Before I hire an expert, I thought maybe there are various experiences here...

Yes, backflow would also be my issue. More pipes won’t help you either if they ultimately converge into one line. I wouldn’t be satisfied with that.

How high is the edge compared to the gravel layer? Can it run over it? Can a gutter be installed? Every normal roof has 1 gutter per side and 1 pipe, if you have 3 in total, then everything is fine

The height of the parapet is 75cm, meaning the water can never flow over it. The downpipes were drilled through the parapet and thus drainage occurs. So, the gutter is unfortunately useless in this case for flat roofs.