Are damp surveys from damp-proofing companies reliable?
Frequently not, and this is one of the clearest conflicts of interest in home maintenance: the company diagnosing the problem sells the treatment, and the treatment they sell is rarely what old buildings actually need.
The core issue. A very large proportion of damp diagnosed as "rising damp" is something else — condensation, a failed gutter or downpipe, raised external ground levels bridging the damp-proof course, a leaking pipe, a blocked cavity, defective pointing, or thermal bridging producing cold surfaces where moisture condenses. These have completely different fixes, most of them cheaper.
Why the instrument is misleading. The hand-held meters used in most surveys are electrical conductance meters designed for timber. On plaster and masonry they measure conductivity, which responds strongly to salts in the wall. Hygroscopic salts left behind by historic moisture give high readings in walls that are entirely dry. A meter reading alone is not a diagnosis, and treating it as one is the source of a great deal of unnecessary work.
What a proper investigation involves: looking outside first at ground levels, gutters, downpipes, pointing and render; checking internal humidity, ventilation and heating patterns; measuring surface temperatures; taking drilled samples analysed for moisture content and salt profile where the diagnosis is genuinely unclear; and considering the building's construction, since solid-walled buildings behave differently from cavity ones.
Who to ask instead. An independent damp and timber surveyor who sells no remedial work, or a building surveyor with conservation experience. Paying separately for diagnosis is the whole point.
Why chemical injection is contentious. Injected damp-proof courses are widely used and their effectiveness in solid masonry is disputed among building conservation specialists. The associated replastering with dense waterproof render frequently traps moisture and moves the problem upward.
Where the incentive comes from. Lenders and buyers commonly require a "damp guarantee", which pushes people toward companies offering one — and such guarantees are frequently narrow and worthless if the firm ceases trading.
Start with the cheap fixes: clear the gutters, lower the ground level, ventilate, and heat evenly.