Damp in a Leith flat: condensation, penetrating damp or something worse

Condensation: the most common, and the most misdiagnosed

Condensation shows up on cold surfaces — behind wardrobes on an external wall, in the corners of north-facing rooms, around single-glazed windows — and it usually brings black spot mould with it. It is caused by moist air with nowhere to go, which is why it is worse in flats with sealed windows, no extractor in the shower room and washing dried indoors.

The fixes are unglamorous and effective: working extraction in the kitchen and bathroom that vents outside, background ventilation, heating the flat steadily rather than in short bursts, and dealing with cold bridges. If someone proposes a chemical treatment for a problem that looks like condensation, ask them to explain why first.

Penetrating damp: water getting in from outside

Penetrating damp is localised and usually traceable. A blocked or split gutter running down a wall, failed pointing, a cracked cill, a leaking flat roof on a conversion, defective flashing at an abutment — the tell is that the patch is in one place, often worse after heavy rain, and it is on the wall that faces the defect.

In Leith, wind-driven rain off the water and exposed elevations near the Shore make this more common than people expect. The repair belongs on the outside of the building, not on the inside plaster.

• Check gutters and downpipes in heavy rain — that is when defects are visible.
• Look at pointing, cills and flashings on the elevation behind the damp patch.
• On a flat-roofed conversion, the roof itself is the first suspect.
• Redecorating before the source is fixed simply buys a few months.

Rising damp and the ground floor

True rising damp is far rarer than the number of people selling treatments for it would suggest. It affects ground floor and basement walls, shows a tide mark low on the wall, and is often really a bridged or blocked situation — raised external ground levels, a solid floor covering trapping moisture, cement render across the base of a breathable stone wall.

On a traditional stone building the fix is usually to remove what is trapping the moisture and let the wall breathe again, rather than to inject it. A survey that starts with the building's construction rather than with a moisture meter reading tends to reach a better answer.

What to ask before you accept a quote

Ask which of the three the contractor thinks it is and why. Ask what evidence points that way. Ask what happens on the outside of the building as well as the inside. A damp quote with no diagnosis is a decorating quote with extra steps.