ᐅ New Flat Roof Construction: Internal Roof Drainage

Dear forum,
For our single-family house, we are planning a flat roof due to having 2.5 storeys. We have already discussed our plans with our trusted general planner. To get an independent opinion, we have commissioned an external architect to review and optimize the design. He has now recommended an internal flat roof drainage system. At first, I strictly rejected the concept. However, I must admit that it also offers (at least) aesthetic advantages. Perhaps there are also practical technical or structural benefits (no icing?), too...

Now to my main question: Do you have experience with internal roof drainage systems for flat roofs? Should we avoid them, or is proper installation not that difficult for a specialized contractor?

In our case, the downpipes would be routed down behind drywall partitions into the basement. Alternatively, would it be permissible or possible to "hide" the downpipes within the external insulation (ETICS 140mm (5.5 inches))?

It is an additional challenge for us that we must discharge surface water not into the sewer system but into an above-ground channel in the street. Since the connection point to the sewer is only 20cm (8 inches) above street level, I am wondering how to guide water from all four corners of the house to the street... The two rear corners are 15–17m (50–56 feet) from the street, while the two front corners are only 3m (10 feet) away. With external downpipes, I can somewhat imagine this, as there would at least be some slope. But how can this be implemented with internal downpipes? These only exit the building below the basement ceiling, so there is no longer any slope toward the street gutter...

On the left side, the path to the street is also "blocked" by a shed and, further on, by a carport. I simply cannot imagine how this could be laid out... Our flat roof will be greened, which will retain and delay rainwater discharge. Perhaps it would be better to aim for infiltration on our own property directly, or in combination with a soakaway?

I would be grateful for any creative tips!
Have a nice Sunday!

Regards
Stephan

StephanM schrieb:

He has now convinced us of an internal flat roof drainage system. I initially rejected the concept outright. Unfortunately, I have to admit that it offers at least some aesthetic advantages.

In my opinion, there are only aesthetic reasons and otherwise just risks.

StephanM schrieb:

Also tangible technical/structural advantages (no freezing?).

Not that I know of.
I have always wondered how an effective emergency overflow can be implemented with an internal drainage system. You can hardly design the roof slope to run toward an internal point and at the same time have an emergency overflow at the parapet. Or rather, it would require that water is significantly backed up on the roof before the overflow starts to function.

StephanM schrieb:

Do you have any experience with internal roof drainage on flat roofs?

We have a lot of flat roofs here and a few neighbors have done internal drainage. I haven’t heard anything negative. In my opinion, however, these were rather forced solutions because it was difficult to place downpipes on the facade (due to lots of glazing and such).

StephanM schrieb:

Would it alternatively be permitted/possible to “hide” the downpipes inside the external insulation (ETICS 140mm (5.5 inches))?

That would significantly reduce the insulation effect, but not necessarily make it impossible. There are certainly solutions with integrated downpipes in cavity walls (within the brick layer as a rectangular pipe).

Take a look around housing estates with flat roof buildings. There are definitely more options than the standard round folded pipe with a bulky inlet box.

StephanM schrieb:

Since the roof drain outlet (RDO) will only be about 20cm (8 inches) above street level, I wonder how we’re supposed to direct water from all four corners of the house down to the street...?

Why all four corners? If you really want, you can easily drain a flat roof at a single point.

StephanM schrieb:

The rear two corners of the house are about 15-17m (49-56 feet) from the street, while the front two corners are only about 3m (10 feet) away.

Then just drain the roof with 1-2 downpipes leading to the street 🙂

StephanM schrieb:

But how would that work with internal downpipes? They only exit the building below the basement ceiling, so there is no longer any slope toward the street gutter...

That’s a legitimate question. Ask the proponent how this can be practically implemented.

StephanM schrieb:

By the way, on the left side the path to the street is blocked by a shed and further ahead by a carport. I simply can’t imagine how this could be realized...

I can’t imagine the layout either.
Personally, I think (and did so on my own building) that a short overbuild of drainage lines is not a disaster. Especially not on paved areas/carports/sheds — these are not permanent structures and can be changed in case of a worst-case scenario.

StephanM schrieb:

Maybe it would be better to aim directly for on-site infiltration or combined with a soakaway system?

Also an option. A soakaway chamber would be cheaper if permitted. I still find an overflow to the public sewer system sensible.

StephanM schrieb:

Unfortunately, I have to admit that it also offers (at least) aesthetic advantages.

It can look better, but it doesn’t have to. It’s a matter of the overall design.

StephanM schrieb:

Maybe there are also solid technical/structural benefits (no icing up?)...

No additional benefits.

StephanM schrieb:

Do you have any experience with internal roof drainage on flat roofs?

48 years without problems in my parents’ house – internal drainage but a pitched roof.

StephanM schrieb:

To get an independent opinion, we hired an external architect to review and optimize the design. He has now recommended an internal flat roof drainage system.

The same person who already showed a lack of spatial awareness with the staircase layout, and who cannot convincingly demonstrate to the builder how to build out the top floor with a stepped design more easily? Then one might have to ask whether accepting further special features from him is still courage or already madness.

guckuck2 schrieb:

In my opinion, there are only aesthetic reasons and otherwise just risks.

hampshire schrieb:

48 years trouble-free in my parents’ house – internal but with a pitched roof.

I don’t know of any such cases personally, neither trouble-free nor problematic ones. However, I am old enough to remember how briefly this trend lasted back then.

---

https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/

11ant schrieb:

I don’t personally know of any cases like that, neither trouble-free ones nor those with problems. However, I am old enough to remember how brief this trend actually was back then.

The house was built in the early 1970s. There were inspections and repairs of supply and drainage pipes, but none of these involved leaks or issues with the internal soil stacks, which worked perfectly until the sale earlier this year. Since then, I have no further information. By the way, the drainage is connected to the sewer system.

StephanM schrieb:

Now to my main question: Do you have any experience with internal roof drainage for flat roofs?

Yes, we have the entire roof drainage system installed internally and have had no problems with it—neither noise from water running inside the walls nor issues with freezing or similar. We chose this for aesthetic reasons since the house is built in a Bauhaus style, and we wanted smooth surfaces without those unsightly boxes on the walls.

guckuck2 schrieb:

What I have always wondered is how an effective emergency overflow can be implemented with internal drainage.

An emergency overflow is not internal and probably wouldn’t work if it were. Unless you install all pipes twice over. But even then, there is a higher risk that both vertical downpipes could get blocked by leaves or debris. Our emergency overflows run horizontally (at a slight angle) to the outside but are barely noticeable. Due to slight slopes on the flat roof, hidden behind the parapet, water is directed toward the concealed emergency overflows on the roof. They exit subtly at points where they are hardly visible from outside. If someone walked around the house, they probably wouldn’t even realize they exist.

How to create the necessary slope in your case should be determined by your planner. There are certainly many solutions; the worst would probably be a sump pump system.