There's a patch on a wall in the second bedroom, and by the time you've finished searching you've read that it's rising damp, that rising damp doesn't exist, and that it needs a chemical damp-proof course costing rather more than you'd hoped. Most of what you found was published by companies that sell damp treatment.

Three genuinely different problems get called damp. They look similar on a wall and they have almost nothing else in common.

The three types, and how they present

Condensation is moisture generated inside the building meeting a cold surface. It's by far the most common of the three. It shows up as black mould in corners, behind furniture, on cold external walls and around windows, and it's a product of how the building is heated and ventilated rather than a fault in its fabric.

Penetrating damp is water getting in from outside. Failed pointing, cracked render, blocked or leaking gutters, defective flashing, a bridged cavity. It's usually localised, and it's usually worse after rain. If a patch has a season, this is your first suspect.

Rising damp is groundwater moving up through masonry by capillary action. It's typically limited to about a metre above ground level, and it often leaves a tide mark of hygroscopic salts.

The order matters, because it's roughly the reverse of the order in which the three get diagnosed.

Why rising damp is the contested one

Both extremes here are wrong, and it's worth being precise.

Rising damp is a real phenomenon. The claim that it doesn't exist at all isn't the mainstream professional position, whatever you read on a forum.

But it's diagnosed considerably more often than it occurs. Guidance from the BRE and the Property Care Association acknowledges that a meaningful proportion of damp diagnosed as rising damp turns out to be condensation, penetrating damp, a plumbing leak, or hygroscopic salts sitting in an old wall. You'll find pages quoting a specific percentage for how often it's misdiagnosed. Every one we could trace led back to a treatment firm's own sample rather than a published study, so we're not going to give you a number.

Some senior figures have gone a good deal further. Stephen Boniface, a former chairman of the construction arm of RICS, described true rising damp as a myth and chemically injected damp-proof courses as "a complete waste of money". That was in the Architects' Journal in June 2009 and it's been argued about ever since. Treat it as a stated professional view at the strong end of the range, not as settled fact.

What follows from all this is practical rather than academic. Where damp has been misdiagnosed, injecting a chemical damp-proof course achieves nothing except temporarily masking the symptom. You pay, the wall is replastered, the patch comes back.

The meter reading proves less than you think

This is the single most useful thing to know, and almost nobody buying a house knows it.

A contractor arrives, presses a pinned meter against your skirting board, and reads out a number. That number is not a measurement of how much moisture is in your wall.

Resistance meters measure electrical resistance, and they aren't designed for testing masonry. Historic England's position is that they give no indication of the actual moisture content of a wall. Instruments for accurate, non-destructive measurement of moisture in brick and stone have proved genuinely difficult to devise, every available method has drawbacks, and there's no professional agreement on how they should be used.

Salts in a wall conduct. So does foil-backed plasterboard. A high reading tells you the probe found something conductive, which is not the same as finding water.

Historic England published updated guidance on exactly this in July 2026, Assessing Damp in Historic Buildings, which sets out to answer what these devices actually tell us and how accurate they are. If someone hands you a printout of meter readings as the evidence for a four-figure quote, that's the thing to be sceptical about.

Who's telling you, and what they sell

Damp is the clearest conflict of interest in the whole buying process, and it's usually hiding in plain sight in the phrase "free damp survey".

A free survey from a company that also sells the treatment is a sales visit. That doesn't make the company dishonest or the diagnosis wrong. It does mean the person deciding what's wrong with your wall is paid only if something is wrong with your wall, and only in one particular way.

The industry knows this. Historic England, RICS and the Property Care Association have jointly published a set of competencies and principles for investigating and treating damp, explicitly so that remedial work treats the cause rather than the symptoms. An industry doesn't co-author that document unless the misdiagnosis problem is real.

Historic England's own summary of the situation is blunter than anything we'd write: determining the causes of damp may not be easy, time and money can be wasted if the cause isn't properly diagnosed, and in those cases remedial work can make the problem much worse.

What to do about it

In this order.

Look at where the patch is. Ground floor, under a metre, with a tide mark points one way. Ceiling-height, in a corner, behind a wardrobe points at condensation. High on a wall or around a window points at something getting in.
Look at when it's worst. Worse after heavy rain suggests penetrating damp. Worse in winter, better with the windows open, suggests condensation.
Go outside and look up. Blocked gutters, a cracked downpipe, failed pointing and vegetation against the wall cause a lot of what gets diagnosed as rising damp, and you can see all of them from the pavement for free.
Check the ground level. Soil, a raised patio or a new driveway sitting above the damp-proof course will bridge it. This is common and it's fixable without injecting anything.
Get your own surveyor, not theirs. An independent RICS surveyor or a conservation professional with no remedial work to sell. If damp is the reason you're worried about the property, this is what a Level 3 survey is for.
Ask what the diagnosis rests on. If the answer is a meter reading and nothing else, ask what else was checked and what was ruled out. A good answer covers the gutters, the ground level, the ventilation and the plumbing.
Get the quote from someone who didn't diagnose it. Separating the diagnosis from the sale is the whole game.

The honest summary

Most damp is condensation, some is water getting in from outside, and rising damp is real but diagnosed far more often than it happens. The three have different fixes, so the diagnosis is worth more than the remedy. A meter reading isn't a diagnosis, a free survey from a treatment company isn't independent, and a chemical damp-proof course fixes nothing at all if the water was never rising in the first place.

The patch on the wall is information. What you want to know is what put it there, and the person best placed to tell you is the one who doesn't stand to sell you the cure.

This is general information, not legal, financial or structural advice. It describes practice in England and Wales. Guidance and professional positions as at August 2026. Instruct an RICS member or an independent conservation professional and take advice on your specific property before commissioning remedial work.