Diagnosing damp in a basement wall
Applies to: Assumes a masonry basement or cellar wall partly below ground, in a house without a modern tanked structure.
Last worked on 2015-04-21
Damp below ground has several possible sources and they leave different patterns. Reading the pattern first avoids treating the wrong one.
Where the water can come from
There are only four realistic routes: water arriving from outside through the wall, water rising through the structure from below, water condensing out of the internal air onto a cold surface, and water from a leak inside the building.
They are distinguishable. Penetrating damp follows the ground level outside and is worst after rain. Rising damp has a fairly level upper edge and does not respond to weather quickly. Condensation appears on the surface, is worst in cold weather and in unventilated corners, and is often accompanied by mould rather than by salt. A plumbing leak is localised and indifferent to weather.
The outside comes first
Before anything is done inside, walk the outside of the wall. Ground that has built up above the original level, a blocked or missing gulley, a downpipe discharging at the wall, a path falling toward the building, and a light well with no drainage account for a very large share of below-ground damp.
These are cheap to correct and they change the load on the wall immediately. Internal work carried out while the outside is still delivering water treats a symptom.
Ventilation and the cold surface
A basement is cold, often unheated, and frequently used for the appliances that generate the most moisture. That combination produces surface condensation reliably without any water entering from outside at all.
The test is simple: tape a piece of impermeable sheet tightly to the wall surface and leave it. Moisture appearing on the room side of the sheet is condensation from the air. Moisture on the wall side is coming through the wall.
Salts and what they tell you
A white crystalline bloom on the surface is salt carried out of the masonry by water that has since evaporated. It indicates that liquid water has been moving through the wall and reaching that surface, which rules condensation out as the sole cause.
Salts are also hygroscopic: once deposited, they draw moisture from the air and keep a patch damp long after the original source is fixed. This is why an area can stay dark for a long time after a genuine repair, and why the repair gets wrongly declared a failure.
| Pattern | Responds to rain | Usual source |
|---|---|---|
| Patch tracking external ground level | Yes, within hours to days | Penetrating damp from outside |
| Level band with a defined upper edge | Slowly or not at all | Moisture rising through the structure |
| Surface wetting, worst in corners and behind furniture | No | Condensation on a cold surface |
| Localised patch with a defined centre | No | A leak within the building |
| White bloom on the surface | Follows the underlying source | Salts left by evaporating liquid water |
Scroll the table sideways for the remaining columns.
Diagnosis before treatment is not a formality here: the four sources need four unrelated remedies, and applying the wrong one usually makes the wall worse by reducing its ability to dry.