ᐅ Rainwater from the roof – underground drainage for disposal?

vaderle schrieb:

Water should generally drain away well.

Have you ever experienced heavy rainfall? Otherwise, you would know that what you imagine is not accurate.

nordanney schrieb:

Have you ever experienced heavy rain? Otherwise, you would know that what you are imagining doesn’t make sense.

But then it would seem like every other garden should be flooded.

Water from the roof must be directed to the building’s drainage system, as the roof area is significantly larger than the footprint of the house, and there is no area around your house for infiltration. That’s how I see it. If you are already in the construction phase, this should no longer be an issue, as it is normally addressed during the planning phase.

vaderle schrieb:

But then it would feel like every second garden would be flooded.

In Germany, there is something called a sewer system. One part is designed to carry away your sewage and similar waste, and another part is for draining rainwater. That’s why gardens don’t end up underwater.

If you don’t have a sewer system, you need to create something like an infiltration basin. The basin should have about 10–20% of the roof area (if designed shallow). Alternatively, it can be deeper but less extensive. This depends on the soil and the groundwater level (which in your case is quite high).

What does the drainage plan look like that you had prepared as part of your building permit / planning permission application?

P.S. I’ve already told you how to do it:

vaderle schrieb:

“Rainwater infiltration is possible starting from the bottom edge of the future topsoil (for example, infiltration shafts or infiltration basins). The bottom of the infiltration system should be above the highest groundwater level. It would also be possible to collect rainwater in a cistern for further use and only let excess water infiltrate.”

By the way, “Sickerschaft” is probably meant to be “Sickerschacht” (infiltration shaft). You can buy those.

Otherwise: just Google “Sickerschacht” and “Versickerungsmulde.” That way, you’ll understand what they are. Simply letting water run onto your property is a) not allowed and b) will flood your and possibly your neighbors’ gardens. If just 20 liters per square meter (about 0.7 gallons per square foot) fall in an hour, that means a neat 4 cubic meters (about 140 cubic feet) on a 200m² (about 2,150 ft²) roof—water that your garden has to handle in addition to the usual rain. That just won’t work – it’ll look like farmers’ fields after a flood. A muddy wasteland!

If you have enough garden space, infiltration (or collecting and using the water during dry periods) is always better than sending it to the sewer. Usually, municipalities also offer a fee reduction if you use infiltration.

There is no other opinion than that the topic belongs to drainage planning, as already mentioned. Whether as a cistern, soakaway shaft, or infiltration basin is almost irrelevant, but it definitely must be planned. And as long as you are still in the construction phase, you should ask the earthworks contractor for a quote. These are matters that belong in professional hands.