Damp and water ingress: find the cause before paying for repairs

Repainting a damp patch costs a little. Repainting it every year for eight years costs a lot, and the wall is worse than when you started.

Damp is the subject buildings get wrong most often, because three very different phenomena leave marks that look alike. Until the cause is identified, every repair is wasted money: the stain comes back.

The board is not there to diagnose — that is a trade. But telling the broad families apart lets you ask the right questions, steer the investigation, and avoid voting works that will fix nothing.

Three families, three logics

• Condensation: moisture produced inside, settling on cold surfaces. It hits corners, window reveals and wet rooms, worsens in winter, and often comes with black mould.
• Ingress: water entering from outside. It follows rain and wind, appears in episodes, and sits under a window, a balcony, a downpipe or at the foot of a façade.
• Rising damp: water climbing from the ground into the walls. It is low, steady, present all year, and often leaves a whitish salt deposit.

A fourth cause is possible and mimics the others: a leaking pipe. It is constant, indifferent to weather, and often visible in consumption before it is visible on the wall — see spotting excess water use.

What to record at the first sign

The quality of the diagnosis depends almost entirely on what was observed before the professional arrived. Four pieces of information change everything:

• When the mark appeared and how it evolved, with dated photographs.
• Correlation with weather: after rain, in a west wind, in winter only, all year.
• Exact position: height above floor, wall orientation, proximity to a downpipe or balcony.
• What has already been done, by whom, and with what result.

That is exactly what an incident log is for: with no dated history, each new contractor starts from zero and proposes the solution they sell.

Private or common: the question that stalls everything

Ingress from the façade, roof or a common pipe falls to the association. Condensation linked to a blocked vent inside a flat falls to the occupant. A leak on a private pipe falls to that owner.

Until that is settled, nobody pays and nothing moves — while the damage progresses. The boundary is detailed in private works and common areas, and liability in water damage: who pays.

The most frequent causes in apartment blocks

• Mechanical ventilation switched off, blocked or never serviced: the leading cause of widespread condensation.
• Façade joints or window sills at the end of their life.
• A blocked rainwater drain, usually with leaves.
• Cracked balcony or terrace waterproofing.
• A roof defect, sometimes far from where the water appears.

That last point explains many errors: water travels along structures before it drops. Where it shows is almost never where it enters.

Treat the cause, then the damage

Repainting before removing the cause throws the association’s money away. The order is always the same: identify, treat the cause, let it dry, then redecorate.

Where the cause is structural, it joins the multi-year works plan and often the façade renovation, where waterproofing is dealt with as a whole.

Where water appears is almost never where it enters: that is why so many damp repairs fail.

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Two structural causes keep coming back: stopped ventilation and end-of-life waterproofing.

The basement concentrates several of these causes: see cellars, damp and ventilation.