Buying a home with damp: what to check before you sign

Is it worth buying a home with damp?

It can be, as long as you know the cause and what it costs to fix before you sign. With about 70% of Portugal's housing stock built before thermal insulation rules and 32.1% of residents living in homes with damp or leaks (INE 2025), finding signs of damp at a viewing is the rule rather than the exception.

The difference between a good and a bad deal is the cause. Condensation in a flat with single-glazed windows is often fixed with ventilation and habits, for €20–300. Rising damp in an old house, or a building roof that needs work, can cost thousands — in the second case, shared among all the owners.

This guide gives you a viewing checklist, the documents to ask for and price ranges for negotiating. To understand the three types of damp, see condensation, rising damp and water ingress.

Viewing checklist: what to look for, room by room

Take this list printed or on your phone. Open wardrobes, ask to move a sofa, look up and down. A rushed viewing shows the home the seller wants you to see.

You can do the meter test during the viewing: with every tap off, watch the meter for a few minutes. If the smallest dial turns with nothing running, water is being used — which points to a leak. A slow leak may take 30–60 minutes to show, so a still meter doesn't rule everything out.

In a house, check the outside too: soil banked against the walls above the indoor floor level, blocked or broken gutters and downpipes, cement skirting outside and the state of the roof seen from the street. Many of these causes are fixed from the outside, before any indoor treatment.

And ask the seller direct questions: how long has the home been empty? Have there been leaks or insurance claims? What work has been done, and are there invoices? The answers, and how they are given, count too.

Bring a hygrometer and view after rain

A hygrometer at €10 gives you a number instead of an impression. Measure in the living room and a bedroom, away from windows. In a closed-up home, readings above 70% are a sign to investigate; 40–60% is typical. An infrared thermometer (€15–20) shows cold walls, where condensation starts. See which hygrometer to buy.

If you can, book a second viewing after a few days of rain and, ideally, early on a winter morning. Water ingress shows after rain; condensation shows at 7 am on a January morning, with wet window panes. A viewing at 3 pm on a sunny July day shows the home at its best.

With the living room's temperature and humidity, the dew point calculator tells you at what temperature walls start getting wet. Example: at 18 °C and 75% relative humidity, any surface below about 13.5 °C condenses.

In a flat: ask for the condominium minutes

In a flat, part of the risk is not in the unit — it is in the building. A cracked façade or a worn roof is a common part, and the work is paid by all the owners in proportion to their permilagem (share of the building, in thousandths). If the work has already been discussed, it is in the minutes (atas).

• Ask for the minutes of the owners' meetings for the last 2–3 years and look for "fachada" (façade), "cobertura" (roof), "terraço" (terrace), "infiltração" (water ingress) and "colunas" (stacks).
• Ask whether there is approved work still to be paid, or extraordinary levies under way — and who pays them after completion (escritura).
• Ask whether there is a common reserve fund (fundo comum de reserva) and its balance.
• If you get the chance, talk to a neighbour on the top or ground floor: they are the first to feel the building's problems.

Who pays for work approved before the sale should be written into the promissory contract (contrato-promessa). Check the wording with your lawyer or solicitador. See also the guide on common parts of the condominium.

What it costs to fix: ranges for negotiating

If you have spotted damp, the next step is to turn it into a figure. These ranges were observed on Google.pt, cume.pt, Zaask, Habitissimo and in customer reports (October 2026). They give orders of magnitude and do not replace a quote.

Marketplaces contradict each other — one shows €4–50/m², another an average of €157.50/m² for similar work. Use the ranges to back up a discount or to ask for the repair before completion, not to fix an exact figure. More detail in how much rising damp treatment costs and how much a VMI costs.

Pre-purchase screening: an independent opinion before you sign

If the home has signs you can't interpret, run the free diagnosis with what you saw at the viewing: 12 questions, a probability for each cause and, when confidence is low, a recommendation for a technical visit. It doesn't replace seeing the home in person, but it helps you know what to ask.

humidade.pt is preparing a pre-purchase screening service (triagem pré-compra), which will appear under services. A screening is an assessment focused on damp — signs, readings, likely cause and next steps — and not an engineering inspection of the building. For structure, installations or compliance you need an engineer or an inspection company.

What to do before the promissory contract

1. Do the viewing with the checklist and the hygrometer and, if possible, a second one after rain.
2. Ask for the condominium minutes and the energy certificate (certificado energético), which gives clues about insulation and windows.
3. If there are signs, get a quote or an independent assessment.
4. Negotiate with numbers: a discount, a repair by the seller before completion, or both.
5. Put what was agreed in writing in the promissory contract — with help from your lawyer or solicitador.

After you buy, the damp is your problem. Before, it is a negotiating argument.