A basement or garage has soil on the other side of its walls, and soil holds water. So damp here is not fixed by paint or by a machine: it is fixed in order, first what is outside, then drainage, waterproofing and ventilation. This guide explains that order and what you can do yourself.
Updated on 4 November 2026. General information, not a substitute for a technician’s assessment.
The essentials
- LNEC (Portugal’s national civil engineering laboratory, reports for the environment agency APA, 2020) writes that water in basement walls comes from the ground and that, often, watertightness is only achieved with proper drainage, for example vertical drains of coarse granular material against the outer face.
- A conservation guide from Ohio (2002, about old US buildings) says to look for the cause outside the building: gutters, downpipes, ground and paving sloping towards the wall, plants against the foundation.
- Waterproofing from the inside alone may not be enough. The same guide warns that a waterproof coating on the inside will not prevent damage and may make it worse, by stopping evaporation.
- Basement mortars have data sheets with clear limits, for example that they do not resist cracking.
- In basements radon can be higher than in the rest of the building. Ventilation helps with both damp and radon.
Signs and where the water comes from
The Ohio guide lists signs: puddles, mould, rotten wood, rusted metal, condensation on windows, musty smell, efflorescence (white powder), a high-water mark on the wall and damaged plaster or paint. Standing water is not needed for the problem to be serious.
Sources of water it names: missing or broken gutters and downpipes, low spots in soil or paving that send water to the foundation, plants that keep the wall wet, a high water table, wells, springs or streams near the building. The difference between ground water, capillary rise and condensation is in the types of damp and rising damp guides. For salts on the surface, see salt on walls.
Step by step, from simplest to costliest
The Ohio guide recommends removing one source of moisture at a time, so you know what worked. If several measures are tried together, the real cause may never be known. Start with the simplest:
- Clean gutters, downpipes and buried pipes at least twice a year, and extend downpipes to discharge farther away.
- Make sure ground and paving slope away from the wall, with no low or settled spots.
- Remove plants near the foundation and shield basement window wells from water.
- Repair pipe leaks at once, including the air-conditioner drain hose.
- Keep window and door seals in good order and do not stack things against walls: they need air to dry.
Drainage: when the water does not stop
LNEC says that watertightness against ground water is often achieved by drainage alone. For chronic problems that other measures do not solve, the Ohio guide suggests an exterior perimeter drain, or a sump pump with interior perimeter drains. For very wet conditions it mentions excavating and waterproofing the wall from outside, or installing a waterproof course or membrane in the wall.
We found no source we read with prices or timescales in Portugal.
Waterproofing: which side
Positive pressure is when water pushes the coating against the wall (applied outside, before backfilling). Negative pressure is when water passes through the wall before reaching the coating (applied inside). A commercial supplier (Casimper, 2026) says treating only the inside is “insufficient in most cases”.
Data sheets give concrete limits. Examples we read:
| Product | Stated use | Limits and conditions on the sheet |
|---|---|---|
| weber.dry KG (Saint-Gobain, 06-09-2024) | Waterproofing render for buried structures, positive and negative pressure | Does not resist cracking of the substrate; minimum 10 mm; 5 to 30 ºC; not on soaked substrates; in basements ensure ventilation to avoid condensation; wait 4 days before finishing |
| weber.dry KF (same) | Fine mortar applied by brush, pressure and counter-pressure | Also does not resist cracking; final thickness 2 mm; wet the substrate; treat singular points |
| Mapelastic Foundation (Mapei, undated sheet) | Foundation walls, basements, positive or negative pressure | Negative up to 1.5 atm; maximum 2 mm per coat; not below 5 ºC or on water-saturated substrates |
We pick no brand. With a cracking substrate or a soaked wall, these sheets do not apply. Before painting, see does anti-damp and anti-mould paint work? and cracks and plaster before painting.
Ventilation, damp and radon
The Ohio guide advises opening or closing vents and windows as appropriate, and using a fan or a dehumidifier, not both, in very damp periods. See dehumidifiers: when they help.
The LNEC radon reports for APA add that basements are where the highest radon concentrations in a building are likely, and that better natural or mechanical ventilation lowers them, and may be enough when levels are low. The reference level in Portugal quoted in the reports is 300 Bq/m³. Energy-efficiency works that cut natural ventilation can raise radon. LNEC also says damp speeds up cracking of walls, which eases the passage of the gas.
What you can do, and when to call a professional
| You can do | Ask a professional (civil engineer or waterproofing firm) |
|---|---|
| Gutters, slopes, plants, leaks, vents, photographing and recording | Water still entering after heavy rain, even after the simple measures |
| Measure humidity with a hygrometer and track it through the year | Excavation, perimeter drainage, a pump, waterproofing from outside |
| Clear the basement away from the walls | Cracks in the substrate (the sheets above do not cover them) |
The right-hand column is our reading of the sources: the Ohio guide and the supplier Casimper treat excavation, drainage and the water table as specialist cases. In apartment buildings, to see what is a common part, read condominium common parts and damp.
What we could not confirm
- LNEC, ITeCons and FEUP papers on buried walls: the repositories blocked automated access; we did not read them. We used the LNEC radon reports.
- Prices, timescales and durations of drainage and waterproofing in Portugal.
- Legal rules for habitable basements: not confirmed.
- The Ohio guide concerns old US buildings; climate and materials differ.
Sources (read on 4 November 2026)
- LNEC for APA, Guia para mitigação da admissão de radão para o interior de edifícios existentes, Report 479/2020, December 2020 (sections on buried walls, ventilation and reference level).
- LNEC for APA, Guia para prevenção da admissão de radão em edifícios novos, Report 480/2020, December 2020 (paragraphs on ground water in basement walls).
- Ohio History Connection, Judith L. Kitchen, FastFacts: About Wet Basements, PDF, 2002.
- Saint-Gobain Portugal, data sheets weber.dry KG and weber.dry KF, 06-09-2024.
- Mapei, Mapelastic Foundation data sheet, undated, hosted by a reseller.
- Casimper, Como impermeabilizar uma cave ou garagem húmida, updated 8 October 2026 (commercial source, used only for the limit of interior treatment).