Rising damp is groundwater moving upward through a porous wall by capillary action, in a building where nothing is stopping it. It needs three things at once: a wall in contact with wet ground, pores fine enough to draw water up against gravity, and no working barrier in between. Remove any one of the three and it stops, which is why the useful question is rarely whether damp exists but which of the three is missing.
How rising damp walls behave: the mechanism, briefly
Water climbs the fine pores of brick and mortar in the same way it climbs a paper towel, and it stops where evaporation from the wall face balances the rate of supply. That is why the tide mark sits at a fairly consistent height and why it is higher on a wall that cannot breathe, behind a cement render or a plastic paint. The salts dissolved in the groundwater travel with it and are left behind at the top of the rise, which is what damages the plaster.
The four things it is usually confused with
A raised external ground level that bridges an otherwise sound damp course. A blocked or buried air brick under a suspended floor. A leaking gutter, downpipe or gulley soaking one patch of wall from outside. And condensation, which forms on cold surfaces from inside and is the commonest damp complaint in British housing by a wide margin. All four produce staining low on a wall and none of them is cured by injecting a course.
Why the misdiagnosis is so common
The instrument most often used is an electrical resistance meter designed for timber, and on a salted plaster wall it reads high whether or not there is liquid water present. Read alone it will confirm rising damp almost anywhere. A surveyor who has ruled out the four alternatives above, checked ground levels and taken the trouble to explain the pattern has done something a meter cannot do on its own.
What is DPC, and how a damp proof course fails
A damp proof course is a horizontal barrier built into a wall near ground level to stop groundwater rising through the masonry by capillary action. In Victorian houses it is often slate or engineering brick; in modern ones a plastic sheet. It rarely fails on its own. What normally happens is that it is bridged: a path, a patio, a raised flower bed or render carried down past it gives the water a route around the barrier, or debris in the cavity does the same. That is why the first thing a competent surveyor looks at is external ground levels, not the wall.
Chemical injection DPC: when an injected DPC is justified
A chemical damp proof course is a water repellent cream or fluid injected into a line of holes drilled along a mortar course, forming a new barrier as it spreads. It is a real technique with real products and it is also the most over sold remedy in British housing, routinely specified for walls whose actual problem is a bridged course, a leaking gutter or condensation. The cost covers drilling and injection, usually the removal and replacement of contaminated plaster with a salt resistant system, which is often the larger half, and a guarantee. Before accepting one, ask what ruled out bridging and the other damps, and get an independent opinion from someone who is not quoting for the work.
Other questions people ask
Is rising damp real?
Yes, and it is also over-diagnosed. Both statements are true at once. The physics is uncontroversial and demonstrable; the frequency with which it is identified in houses that have a leaking downpipe or a bridged course is what the argument is about.
How high does rising damp go?
Usually about a metre, because that is roughly where evaporation catches up with supply in a typical wall. Staining well above that on an internal wall is a strong hint that something else is going on, most often a plumbing leak or condensation.
Can a DPC repair be done without a chemical damp course?
Often, if the cause is a bridge rather than an absent barrier. Lowering a path below the course, clearing air bricks, mending a gutter and letting the wall dry are the cheapest and most durable fixes there are, and they are the first things a good survey looks for.
DPC meaning: what does DPC stand for?
Damp proof course: the horizontal barrier in a wall that stops groundwater rising through the masonry. A damp proof membrane does the same job in a floor.
DPC injection cost: is an injected damp proof course always necessary?
Frequently not. Bridged ground levels, defective rainwater goods and condensation are all diagnosed as rising damp and none is cured by injection. Ask what was ruled out before agreeing to it.