Black mould on outer walls and window reveals is one of the most common and contentious problems in German flats in winter. Landlords blame tenants for not airing enough, and tenants point to structural defects.

How to air properly (Stoßlüften)

Courts expect tenants to ventilate properly: open the windows wide for five to ten minutes two to four times a day with the radiator thermostat turned down. Tilted windows in winter mainly cool the walls and encourage condensation.

Documenting the defect

When you first notice the black spots, photograph them with a ruler for scale and keep a ventilation log (Lüftungsprotokoll), recording temperature and humidity with a hygrometer three times a day. Send the landlord a written notice of the defect (Mängelanzeige).

The right to reduce the rent (Mietminderung, Section 536 BGB)

If the mould is caused by structural defects such as thermal bridges (Kältebrücken) or a leaking roof, you may reduce the rent in proportion to the impairment (often 5 to 20%, depending on the affected area) until the defect is fixed.

Expert reports and the risks of going to court

If the landlord refuses to remove the mould and blames you, an independent building surveyor (Bausachverständiger) can determine the cause. Using a thermal imaging camera, they establish whether the damp is due to structural defects or insufficient heating.

What to check

  • Buy a digital thermo-hygrometer and monitor humidity (ideally around 40 to 60%).
  • Air in bursts (Stoßlüften) at least three times a day during the heating season.
  • Send the landlord a written notice of the defect with photos and a deadline for repairs.
  • Do not reduce the rent on your own initiative without first getting legal advice.

Example situation

A family documents mould in the bedroom caused by damp joints in a prefabricated building, brings in an expert from the tenants' association and obtains free professional remediation and a 15% rent reduction.

Next step

Mould is harmful to your respiratory system; document damp from the start and, if there is a dispute, ask for an expert assessment of the building.

Official information on this topic

Checklists and examples are organisational recommendations. Requirements depend on the procedure, the city and your situation; check them with the relevant office before you act. Please discuss legal and tax questions with an authorised professional.

This article was prepared with the help of AI. The example illustrates the approach and does not describe a client case.