Structural damp: when the water is coming from the building, not the air
If a damp patch has a defined edge, sits at a consistent height, worsens after rain, or never dries no matter what you do to the air, stop buying appliances. Water is entering the structure.
The short version. No amount of dehumidification, ventilation or insulation fixes liquid water coming through a wall or floor.
Condensation is by far the most common household damp, which is why it gets the most attention — and why the other three causes get misdiagnosed as it. The distinction matters because the repairs are completely different and range from free to five figures.
The good news is that telling them apart is mostly observation, and you can do it yourself in an afternoon.
- Rising damp height
- < 3 ft
- Ground grading
- 6 in / 10 ft
- Downspout discharge
- 4–6 ft
- Crawl space vapor barrier
- 6–20 mil
Consistent height with a tide mark; higher means something else
Fall away from the foundation over the first 10 feet
Minimum distance from the foundation wall
Sealed and lapped up the walls
Telling the four apart
Condensation appears on cold surfaces, is worst in the morning and in winter, spreads as diffuse speckling rather than a defined patch, and concentrates in corners, on window reveals and behind furniture. It responds to ventilation and heat.
Rising damp comes up from the ground through masonry. It stops at a consistent height — typically under three feet — often leaves a horizontal tide mark with salt deposits above it, and appears on ground-floor walls only. It is genuinely uncommon and frequently over-diagnosed by companies selling the cure.
Penetrating damp comes through the wall from outside. It is a defined patch, it gets visibly worse during and after rain, and it appears at any height — often traceable to a specific external defect: a cracked gutter, a failed sealant joint, damaged render, a blocked downspout, a missing flashing.
Leaks are constant regardless of weather, often warm to the touch if from a heating pipe, and get worse over time rather than seasonally. A water meter that moves with every tap closed confirms one within an hour.
When it does not work
The limits matter more than the benefits — this is where most money gets wasted.
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Chemical DPC injection is oversold
Genuine rising damp is far rarer than the industry that treats it. Before agreeing to injection, rule out a bridged damp course — raised garden beds, added render, a new patio above the DPC line — which causes the same symptoms and costs a fraction to fix.
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Tanking a basement moves water, it does not remove it
Waterproofing the inside face holds water back under pressure. If external drainage is the real problem, tanking can push water to find another path. Address drainage outside first where you can.
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Sealing the outside of a wall can trap moisture inside it
Waterproof coatings on masonry stop water getting in and also stop it getting out. On a wall that is already wet, this makes things worse.
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These repairs need a survey, not a diagnosis from a website
This page helps you tell the categories apart. Specification of a DPC, tanking or drainage works is a job for an independent surveyor with no stake in selling the treatment.
Doing it properly
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Photograph the patch and date it
Then photograph it again after the next heavy rain and after a dry week. The pattern over time identifies the cause more reliably than any single look.
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Check the height and edge
Diffuse and high in a corner is condensation. Sharp-edged and rain-linked is penetrating. Low, level and salt-marked is rising damp.
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Read your water meter
Turn off every tap and appliance, read the meter, wait an hour, read again. Movement means a leak, and that changes everything.
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Walk the outside
Gutters, downspouts, ground level against the wall, cracked render, failed sealant, the height of a patio or flower bed against the damp course. Most penetrating damp is found here in ten minutes.
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Fix the water management first
Clear gutters, extend downspouts 4–6 feet from the wall, regrade so ground falls away from the house. This is the cheapest work and it solves a surprising share of cases.
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Deal with the crawl space or basement floor
A sealed vapor barrier over bare earth in a crawl space is one of the highest-value moisture repairs in a US home.
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Get an independent survey before major work
Pay a surveyor who does not also sell the treatment. It is the cheapest insurance against a five-figure repair for the wrong problem.
Common mistakes
Accepting a free damp survey from a company that sells damp proofing
The incentive is obvious. Pay for independent advice.
Treating rain-linked patches with a dehumidifier
You are drying the air while the wall keeps refilling with water.
Painting over a damp patch
The paint blisters, and you have lost your best early indicator of whether the repair worked.
Venting a crawl space in a humid climate
Current practice is to seal and encapsulate. Venting warm humid air into a cool crawl space condenses it onto the joists.
Common questions
How do I know if it is rising damp or condensation?
Rising damp stops at a consistent height, usually under three feet, often with a horizontal tide mark and salt deposits, and only affects ground-floor walls. Condensation is diffuse, worst in corners and on cold surfaces, worse in the morning, and improves with ventilation and heat.
Does a damp patch that gets worse when it rains mean penetrating damp?
Almost always. Look outside at that exact spot for a cracked gutter, blocked downspout, failed sealant, damaged render or missing flashing. The external defect is usually visible from the ground.
Is rising damp real?
It exists but it is far less common than the number of treatments sold for it. A bridged damp course — from a raised patio, added render or a flower bed above the DPC — produces identical symptoms and is much cheaper to fix.
Does a crawl space need a vapor barrier?
Yes. Bare earth under a house transmits water vapor continuously — often several gallons a day — and no ventilation strategy competes with that. A lapped and sealed barrier across the whole ground surface is one of the highest-value moisture repairs available.