1. What backflow is — and what the backflow level means
In heavy rain the public combined sewer can fill up so much that water no longer drains away but pushes back into the house connection. Everything below the so-called backflow level is then at risk: basement toilets, floor drains, washing-machine connections, downstairs showers.
The backflow level is the height to which water in the sewer can rise and push back. Unless the municipal statute states otherwise, the top of the street at the connection point generally counts as the backflow level (DIN 1986-100). Outlets above it are usually uncritical; everything below must be protected against backflow.
2. Who is liable? Usually you
The common misconception: "If the municipality’s sewer overflows, the municipality is liable." In fact, every connectee must protect against foreseeable events such as heavy rain in their own interest. If protection is missing, contributory negligence under § 254 BGB can apply to the owner — compensation may then be reduced.
How a specific backflow case is decided in liability terms is for the courts to judge case by case — sometimes the owner bears the loss, sometimes the sewer operator. Do not rely on it: protecting against backflow is first your own task, and it protects you before any dispute even arises.
3. And the building insurance?
Many residential building policies require, as an obligation, that backflow protection be kept "functional". Such clauses are not always valid: the OLG Frankfurt (7 U 71/21) held a clause invalid because it named no specific maintenance intervals and was therefore intransparent (§ 307 BGB) — the insurer had to pay.
The practical advice is unchanged, though: do not rely on a clause being struck down in a dispute. A correctly designed and maintained backflow system protects you first from the damage — and only secondly in a dispute with an authority or insurer.
4. The solutions — what fits when
The choice of protection follows DIN 1986-100 and the local conditions — not habit:
Two routes per DIN 1986-100 — and what needs no protection
- 1Wastewater lifting station with backflow loop: drains for wastewater below the backflow level must, per DIN 1986-100 (clause 13.1.2), generally be protected by an automatic lifting station whose pressure line is routed as a backflow loop above the backflow level. The most robust solution for low-lying, constantly used rooms.
- 2Backflow valve (to DIN EN 13564-1): permitted only where there is a gradient to the sewer, the rooms are of subordinate use, the group of users is small and has a WC above the backflow level, and the drain can be left unused during backflow (DIN 1986-100, clause 13.1.2). Regular maintenance of the valve is mandatory.
- 3Drains above the backflow level: drain by gravity and generally need no protection against backflow from the sewer. The first planning step is therefore the height of every drain relative to the backflow level.
5. What we do — and what we do not
We assess your situation and design the appropriate backflow protection per DIN 1986-100 — with dimensioning, a site plan and a maintenance concept that documents the requirements. So the protection is not only present but verifiable.
We plan and sign off under professional responsibility — the installation is done by your plumbing or civil-works firm. Responsibility for the design stays with us, execution with your trades.
Whether your basement is at risk at all, we clarify beforehand — in the free initial consultation. Sometimes a simple valve is enough, sometimes a lifting station is needed; the honest answer depends on elevation and use.
Note: general information, not legal advice — please have your specific case reviewed by a lawyer.