Late winter is when damp shows itself. Tide marks appear at the base of walls, wallpaper lifts, and paint flakes. The usual verdict is “rising damp”, followed by a quote for injecting a chemical damp proof course and replastering. For landlords, owners and managing agents, that can be an expensive answer to the wrong question. True rising damp does occur, but it is much less common than often claimed.
The usual suspects
Before accepting a diagnosis of rising damp, rule out the more common causes:
- Condensation. Warm, moist air from cooking, bathing and drying clothes condenses on cold surfaces, especially in corners, behind furniture and at the base of external walls.
- Bridged damp proof courses. Raised ground levels, paths, flower beds or rendered plinths can let moisture bypass a perfectly good damp proof course.
- Leaks. Defective gutters, downpipes, plumbing and drains can soak walls at low level.
- Penetrating damp. Rain driving through cracked render, failed pointing or defective window sills.
- Hygroscopic salts. Old plaster contaminated with salts draws moisture from the air, so a wall can stay damp long after the original cause has been removed.
Why a meter is not enough
Electrical moisture meters are designed for timber. On masonry and plaster they can give high readings because of salts or other conductive materials, even when the wall is not especially wet. A meter is useful for mapping a pattern, but a reliable diagnosis needs more: careful inspection inside and out, a look at ground levels and drainage, and where necessary samples tested to measure actual moisture content.
Old buildings need different thinking
Many older buildings were built with solid walls, lime mortars and no damp proof course at all. They manage moisture by letting it evaporate. Cement renders, impermeable paints and hard plasters can trap moisture and make matters worse. In such buildings, the answer is often to restore breathability, lower external ground levels and improve drainage, rather than inject chemicals.
Our advice
- Treat the cause, not the symptom.
- Be cautious of diagnoses made by those who will profit from the treatment.
- Ask for an independent inspection before committing to remedial work.
See our Building Surveys and Defect Diagnosis service, and a later damp and moisture investigation in a London block.