ᐅ Emergency Overflow for Flat Roof Drainage

Hello everyone,

This is about the new construction of a single-family house with a flat roof. I have a main roof of 120 sqm (1,292 sq ft), a garage roof of 80 sqm (861 sq ft), and two balconies with masonry parapets. The roof slope is 2° and the main and garage roofs are covered with standard gravel. The construction company has installed three downspouts on the main roof and two downspouts on the garage roof for drainage. According to DIN standards, flat roofs require an emergency drainage system that is not connected to the sewer system. This is not present in my case, with the explanation that additional "regular" downspouts were installed. According to DIN, only one drainage pipe (DN100 or DN80) would be required for each.

However, emergency drainage is now being retrofitted on the two balconies—requiring drilling through the installed membrane waterproofing.

I am a bit skeptical regarding the main roof and garage. Does anyone have experience with whether this really works without emergency drainage?

Thanks for any input 🙂

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

No, the pipes do not have an emergency overflow.

The background is that, should the downpipes become blocked for any reason (dead bird, leaves, etc.), the flat roof will fill with water. The water cannot drain away. That is why the DIN standard requires flat roofs to be equipped with emergency drainage systems. These are not connected to the sewer system and drain onto open ground areas (on the property). However, the general contractor says this is managed through additional "normal" drainage pipes.

To me, this seems illogical because in the worst-case scenario, these downpipes could also become blocked, causing water to back up.

Before I consult an expert, I thought there might be various experiences shared here...

Yes, backflow would also be my concern. Adding more pipes won't help if they eventually connect to the same branch. I would not be satisfied with that.

How high is the edge compared to the gravel layer? Can water flow over it? Is it possible to install a gutter? A typical roof has one gutter per side and one downspout. If you have three in total, everything should be fine.

The height of the parapet is 75cm (30 inches), meaning that water can never overflow it. The downspouts are routed through the parapet, allowing for drainage. Therefore, gutters are not effective for flat roofs in this case.