Is backflow protection required in construction projects?

Hello everyone,

I am currently unsure whether we need a backflow prevention device for our construction project or not. Our architect was only somewhat helpful and said that we "actually" don't need one, but if we wanted to be on the safe side, of course, we could have one installed.

The following key data:
The property is connected to a separate sewer system.
We are building without a basement and there is no shower on the ground floor, meaning the lowest wastewater connection in the house should be the toilet on the ground floor.
The property is located on a slightly sloping street (on the plan sloping from top to bottom). Attached are the city's drainage plan as well as a section of the planned house, both with height indications.

The city’s drainage ordinance contains no information about the backflow level, which, according to my understanding, means that the street level is the backflow level (if this is wrong, please correct me).

From this, I derive the following picture:

- The nearest manhole cover above our property is at 163.52m NHN (according to the plan our surveyor created for our building application, at 163.48m NHN), the nearest downstream manhole cover at 162.23m NHN.
- Top of the floor slab is at 163.30m NHN, plus 0.18m floor construction, plus approx. 0.20m to the toilet drain = 163.68m NHN

As a layperson, I would conclude from this that a backflow prevention device is not necessary, since the lowest drain is 16 (or 20) cm higher than the nearest upstream manhole cover. Correct?

However, I wonder why the architect said that I could "of course install a backflow prevention device to be on the safe side." Sure, I can simply ask him, but I first wanted to educate myself here so that I can better assess his answer.

Thank you all in advance and have a nice 2nd Advent!

Ask the neighbors if the [Strassenkanaldeckel] have ever come up during heavy rain and those with basements if they have ever had backed-up sewage water in their basement, then make your decision.

I wonder then, however, why the architect said that I could "to be on the safe side" of course install a backflow prevention device.

Well, let's put it this way, I know someone who lives on a really steeply sloping street. Due to the hillside location, their entire basement on the street side is 100% above ground. The sewer is buried deep in the middle of the street in front of the house. Because of the height difference and the steep slope, where everything naturally drains away, basically everyone feels quite safe ... ... and then it still happened: a really heavy downpour brought such huge amounts of water that above ground a whole torrential stream washed down the slope and underground the sewer was so overloaded that it pushed water out everywhere – manhole covers, drain covers, and indeed at the said acquaintance a fountain of water spurted out of the floor drain in the basement. Even more than a 1 meter difference in height was no longer safe. And what ended up swimming in the basement afterward, nobody wants to have there ... ... so: long story short: I would definitely install a backflow prevention device.

Ask the neighbors if the street manhole covers have ever come up during heavy rain

Good point. Unfortunately, the whole area is a new development, so there is little experience so far with the existing neighbors...

[...] I know someone who lives on a really steep street. Because of the hillside location, their entire basement is 100% above ground on the street side. The sewer lies deep in the street in front of the house buried in the street center. [...]

On a steep street, the risk is generally rather higher than lower, isn't it? Because on a steep street the next manhole uphill (and that's what matters) is typically located higher than the lowest shower/toilet – especially if you build with a basement. Or am I mistaken here?

[...] and underground the sewer was so overloaded that it pushed up everywhere – manhole covers, drain covers, and also at the aforementioned acquaintance’s place, it gushed out like a fountain from the floor drain in the basement. Even more than a 1-meter height difference was no longer safe. [...]

What does the 1-meter height difference refer to? Was the next higher manhole/drain cover really 1 meter lower (!) than the floor drain in the basement?

In our case, the street only slopes very slightly (according to the plan about 1m over 30m length)

What exactly does the 1 meter height difference refer to? Was the next higher-level sewer/manhole cover really 1 meter lower (!) than the floor drain in the basement?

I cannot answer that like this and would have to take a look at it on site at some point. But from my own experience, I know that a backwater valve really does not cost the earth. Our house has a sewer inspection chamber anyway, and the backwater valve cost just €268.50 net including installation. Having water in the basement once will certainly be more expensive ...

I can’t answer that like this and would have to take a look on site when I get the chance. But from my own experience, I know that a backflow prevention device really doesn’t cost a fortune. Our house already has a sewer inspection chamber anyway, and the backflow valve with installation only cost €268.50 net. Water in the basement once will certainly be more expensive ...

Hm... So we don’t have a basement, but I thought that if the relevant wastewater contains feces (which would be the case with our toilet), you would need a backflow valve type 3F – which I see on the first Google search for over 1000 euros. But since that is still considerably less than the cost of a flooded ground floor, I will probably invest in it so I can sleep peacefully at night.