There are three kinds and they have almost nothing in common. Condensation is produced inside and is a heating and ventilation problem. Penetrating damp comes through the fabric from outside and is fixed by repairing the fabric. Rising damp is far rarer than the industry implies and nearly always means something is bridging the damp-proof course.
- Diagnose before treating. The three look similar inside and need completely different work.
- Most damp we attend traces back to a gutter, raised ground levels or failed pointing.
- A chemical damp-proof course does nothing for penetrating damp or condensation.
- Let the wall dry before replastering, or the new plaster fails too.
Which of the three is it?
| Condensation | Penetrating | Rising | |
|---|---|---|---|
| Where | Cold surfaces, corners, behind furniture | Localised, one elevation | Low level, up to about a metre |
| When | Worst in winter | Follows rain, 1–2 days after | Constant, varies little |
| Pattern | Widespread, diffuse | Defined patch with a tide line | Horizontal tide mark, salts |
| Mould | Black spot mould typical | Sometimes | Rare |
| Matching defect outside | No | Yes, usually higher up | Bridging at the base |
| Cause | Moisture produced inside | Water entering the fabric | Ground moisture bypassing the DPC |
| Fix | Heating and ventilation | Repair the fabric | Remove the bridge |
| How common | Very | Very | Genuinely rare |
If you take one thing from this page: establish which column you are in before anyone quotes for treatment. The wrong treatment on the right symptom is the single most expensive mistake in this area.
Condensation: the most common and the least understood
A household produces a surprising volume of moisture every day — cooking, washing, drying clothes, showering, and simply breathing. That moisture stays in the air until it meets a surface cold enough for it to condense on.
So condensation is the product of three things together: moisture being generated, insufficient ventilation to remove it, and cold surfaces for it to land on. Change any one and it improves.
It has become more common in older houses, not less, and the reason is counter-intuitive. Draughty old houses ventilated themselves accidentally. Fit double glazing, draughtproof the doors and insulate the loft, and that accidental ventilation disappears while the moisture generation does not.
The fixes are unglamorous and they work: extract fans in the kitchen and bathroom that actually run and discharge outside, drying washing outside or in a ventilated space rather than on radiators, background ventilation such as trickle vents left open, and keeping heating steady rather than in short bursts. Where cold surfaces are the issue, insulation addresses it directly.
No damp-proofing treatment affects condensation at all.
Penetrating damp: find the defect outside
Water is getting through the fabric from outside, and there is almost always a matching defect to find — usually higher up than the damp inside, because water travels down behind the surface before it emerges.
The usual sources, roughly in order of how often we find them:
- A leaking or overflowing gutter keeping one strip of wall permanently wet. Cheapest fix on the list, and the most common cause.
- Failed pointing, letting water into the brickwork directly.
- Cracked or blown render, admitting water behind it.
- Defective window sills or failed sealant at frames.
- Roof defects — slipped tiles, failed flashings, perished eaves underlay.
- A blocked or broken downpipe or gully, discharging against the wall.
- A chimney stack with failed flaunching or pointing, where water travels down inside the brickwork and emerges well below.
Solid walls — most Coventry housing before roughly the 1930s — are more vulnerable because there is no cavity to drain. They also need to dry back out through their surface, which is why an impermeable render or paint on a solid wall makes penetrating damp worse rather than better.
Rising damp: why we are sceptical
Rising damp genuinely exists. It is also diagnosed far more often than it occurs, and that matters because the treatment is expensive and invasive.
True rising damp is groundwater drawn up through masonry by capillary action. It is limited in height — typically under a metre — shows a horizontal tide mark, leaves salt deposits, and does not vary much with the weather.
Most houses built after the mid-nineteenth century have a damp-proof course, and an original slate or bitumen DPC very rarely fails along its length. What happens instead is that something bridges it, giving moisture a path around the barrier:
- Raised external ground levels — a new path, patio or flower bed built up above the DPC. By far the most common.
- Render carried down across the DPC, making the render itself the bridge.
- Debris in the cavity at low level on a cavity wall.
- An internal screed or plaster carried across the DPC line.
- A patio or driveway laid against the wall with no gap.
In each of those cases the fix is removing the bridge — lowering the ground, cutting the render back above the DPC — not injecting chemicals into a wall whose damp-proof course is working perfectly well.
What to do, in order
- Observe before acting. When does it appear? Does it follow rain, or the seasons? Where exactly, and what shape?
- Walk outside and look up. Gutters, downpipes, pointing, render, sills, the roofline above the damp.
- Check ground levels against the damp-proof course, and look for anything bridging it.
- Deal with the cheap external causes first. A gutter repair and lowering a flower bed costs very little and resolves a large proportion of cases.
- Improve ventilation if the pattern suggests condensation.
- Allow drying time. A wall that has been wet for years takes months to dry, and this cannot be rushed.
- Replaster only once dry. New plaster on a wet wall will fail, usually within a year.
- Be sceptical of a free survey from a company that sells one treatment. An independent damp surveyor with no product to sell is a different proposition.
What it costs
These are the building figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Repointing | £20–£50 per m² | Brick condition dependent |
| Builder, hourly | £20–£40/hr (avg ~£24) | Skilled trade |
| Builder, day rate | £200–£350/day | Varies with firm size |
| General building project | £4,600–£14,700 (avg ~£8,593) | Scope dependent |
Most damp we are called to is resolved by work at the cheaper end — a gutter, some pointing, lowering a path. The expensive outcomes are usually the ones where a treatment was applied before the cause was found.
We diagnose which of the three it is before quoting anything, and we will tell you when the answer is a gutter rather than a damp-proof course.
Frequently asked questions
How do I tell condensation from damp?
Condensation is widespread, worst in winter, concentrated on cold surfaces and in corners or behind furniture, and usually comes with black spot mould. Penetrating damp is localised, follows rain by a day or two, leaves a defined patch with a tide line, and has a matching defect outside — usually higher up than the stain.
Is rising damp real?
Yes, but it is far rarer than the industry implies and it is diagnosed much more often than it occurs. Most houses have a damp-proof course that works. What usually happens is that something bridges it — raised ground levels, render carried down past it, cavity debris — and the fix is removing the bridge.
Do I need a chemical damp-proof course?
Only where the damp-proof course has genuinely failed, which is uncommon. Injecting chemicals into a wall suffering penetrating damp or condensation achieves nothing except expense. Establish which kind of damp you have, and whether something is simply bridging the existing DPC, before considering injection.
Why has damp appeared since I had new windows?
Almost certainly condensation. Replacing draughty windows and draughtproofing removes the accidental ventilation an older house relied on, while the moisture produced inside stays the same. The answer is deliberate ventilation — working extract fans, trickle vents left open, drying washing outside — rather than anything to do with the walls.
How long does a damp wall take to dry?
Months rather than weeks, and longer for a thick solid wall that has been wet for years. A rough guide used in the trade is about a month per 25mm of thickness, once the source has been removed. Replastering before it is dry means the new plaster fails, usually within a year.
Should I get a free damp survey?
Be cautious if it is offered by a company that sells one particular treatment, because the survey and the sale are the same visit. An independent damp surveyor with no product to sell will charge a fee and give you a diagnosis you can act on — including, often, the finding that the answer is a gutter.
Diagnosis before treatment
We establish which of the three you have and find the cause, including when the honest answer is a cheap external repair rather than anything structural. Free survey.