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?
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.