Damp or Leak? The Readings That Tell Them Apart, and the Ones That Do Not

The four things it could be

A damp patch is a symptom, not a diagnosis. In a London property there are four realistic explanations, and they demand entirely different remedies. Confusing them is expensive in both directions: opening a floor to chase a leak that is actually condensation, or improving ventilation in a flat where a heating pipe has been weeping into a screed for two years.

The four are condensation, penetrating damp, rising damp and a plumbing or drainage leak. They can coexist, and in older converted stock they often do, which is why the diagnostic question is rarely which one is it and more usually which ones are contributing and in what proportion.

Cause	Typical pattern	What it correlates with	What normally confirms it
Condensation	Diffuse, on cold surfaces, corners, reveals, behind furniture, often with surface mould	Cold weather, occupancy, poor ventilation, time of day	Logged temperature and humidity showing surface temperature at or below dew point
Penetrating damp	Localised, usually on an external wall, often at a defect	Rainfall, and wind direction in driven rain	External fabric inspection plus wetting that follows rain events
Rising damp	A band at low level, commonly with a salt tide mark at the upper limit	Nothing external; persistent and seasonally stable	Moisture profile by height, plus hygroscopic salt analysis
Plumbing or drainage leak	Localised, often with a defined wet edge that expands	Water use: flushes, showers, heating cycles, appliance runs	Meter or pressure test showing loss, then located by survey

That final column is the whole discipline. Each cause has a confirming measurement, and none of them is a moisture meter reading taken on a plaster surface.

Why a surface moisture reading proves almost nothing

The instrument most associated with damp investigation is the one most often misused. It is worth being precise about what each type measures, because the limitations are not incidental.

Instrument	What it measures	Depth it reads to	What it cannot tell you
Resistance meter, two pins	Electrical conductance between the pins	The depth the pins reach, typically a few millimetres	Whether the conductance is water or salts; the source of the water
Capacitance meter, non-invasive	Dielectric response of the material below the pad	Commonly around 10 to 20mm, depending on the instrument	Whether a reading comes from moisture, density change, foil backing or metal
Deep wall probes	Conditions in a drilled hole at a chosen depth	Whatever depth is drilled	Nothing about the surface layer unless read separately
Carbide or gravimetric test on a drilled sample	Actual moisture content of the sample by mass	The sample location	Anything outside the point sampled

The calibration problem

Resistance meters are calibrated for timber. On plaster, masonry or screed the number they display is not a moisture content but a relative value, often expressed as wood moisture equivalent, and it is affected by anything conductive in the material. Hygroscopic salts left behind by long-term evaporation will read high on a wall that is, in terms of free water, almost dry. This is the mechanism behind a large proportion of rising damp misdiagnoses.

Surface versus depth is the question that matters

The single most informative thing a moisture survey can do is read the same location at the surface and at depth. Condensation wets from the surface inwards, so surface readings exceed depth readings. A leak wets from behind or below, so depth readings exceed surface readings. That comparison, recorded properly, does more diagnostic work than any number of surface readings across a wall, and it is the reason a report should state whether each reading is a surface or a depth measurement. A report that does not say is not reporting a finding.

What temperature and humidity readings are actually for

Condensation is not a plumbing phenomenon, it is an air phenomenon, and it cannot be diagnosed or excluded without measuring the air. Water vapour condenses on a surface when that surface falls to or below the dew point of the air in contact with it. Dew point is a function of air temperature and relative humidity, so establishing whether condensation is occurring requires three quantities: air temperature, relative humidity, and surface temperature at the affected location.

BS 5250:2021, Management of moisture in buildings, is the code of practice governing this in the UK; it replaced the earlier BS 5250 code of practice for the control of condensation in buildings and takes a broader view of moisture risk in the fabric as a whole. The practical implication for a survey is that a spot reading of humidity on the day of the visit is close to useless. Internal humidity in an occupied flat swings enormously with cooking, bathing, drying laundry and occupancy, and a single reading catches an arbitrary point on that curve.

• Log rather than spot-read. A data logger recording temperature and relative humidity at intervals over several days captures the pattern: whether humidity spikes with occupancy, whether it recovers overnight, and whether ventilation is clearing it.
• Measure the surface, not just the air. An infrared surface thermometer or a thermal camera gives the surface temperature that has to be compared against the calculated dew point.
• Record the external conditions too. Penetrating damp is confirmed by correlation with rainfall, which means the rain has to be recorded alongside the readings.
• Note occupancy. A flat that was empty during the logging period tells you about the fabric; one that was heavily occupied tells you about the interaction between fabric and use. Both are useful, but only if which one you have is written down.

Sustained high humidity at a cold surface is what supports mould germination, and the relevant figure is the humidity at the surface rather than in the middle of the room. A room averaging around 70 per cent relative humidity will have considerably higher humidity at a cold external wall, which is why mould appears in corners and behind furniture while the room itself feels merely stuffy. The government's guidance on the health risks of damp and mould in the home is explicit that landlords should not assume damp and mould is caused by occupier lifestyle, and that the cause has to be identified.

Condensation: the most commonly misdiagnosed of the four

Condensation is the default explanation offered by landlords and the default explanation rejected by tenants, and both positions are usually asserted rather than evidenced. The distinguishing features are consistent.

It appears on the coldest surfaces first: external wall corners, window reveals, lintels, the wall behind a wardrobe on an external wall, and areas of poor insulation or thermal bridging. It is diffuse rather than defined. It correlates with the heating season and with the hours the property is occupied. Surface moisture readings exceed depth readings. Mould growth is usually superficial, and is often the first thing noticed.

What condensation does not look like

Condensation does not produce a wet patch with a sharply defined expanding edge. It does not produce a reading that is higher at 30mm depth than at the surface. It does not track along a single line across a ceiling. And it does not appear in the middle of a well-insulated internal wall with dry surroundings. Where any of those features is present, the condensation diagnosis is incomplete at best, and a damp and moisture detection survey should establish what else is going on before remediation is specified.

Penetrating damp and its relationship to rain

Penetrating damp is water entering through the external fabric: a failed pointing joint, a cracked render, a blocked or leaking gutter, a defective flashing, a slipped tile, a failed window seal, or a saturated solid wall in driven rain. It is often the easiest of the four to confirm, because it has an external cause that can be inspected and a timing signature that can be observed.

The confirming test is correlation with weather. Internal wetting that worsens during and after rainfall, and improves in a dry spell, is penetrating damp until shown otherwise. Wetting that is indifferent to the weather but tracks water use is a leak. Wetting that is indifferent to both, and follows the heating season instead, is condensation.

• Inspect the external face at the height and position corresponding to the internal patch, allowing for the fact that water tracks downwards and sideways inside a wall.
• Check rainwater goods under load rather than in dry weather. A gutter joint that is sound when empty can discharge heavily when the gutter is running full.
• Look above the patch, not only behind it. Flashings, copings, parapets and balcony thresholds are common sources for damp that manifests a storey lower.
• Where a flat roof or balcony is suspected, remember that the point of entry is frequently well away from the point of appearance.

Rising damp, and why it is diagnosed more often than it occurs

True rising damp is groundwater drawn up through porous masonry by capillary action. It is real, and it does occur, particularly where a damp proof course has been bridged by a raised external ground level, a solid floor, an added render coat or a flower bed. It is also, by a wide margin, the most over-diagnosed condition in British housing, because the instrument commonly used to identify it responds to salts as well as to water.

The genuine article has a characteristic profile. Moisture content is highest at the base of the wall and decreases with height, usually reaching background within about a metre. At the upper limit of evaporation there is frequently a band of hygroscopic salts, chlorides and nitrates carried up from the ground, which appears as a tide mark and which will itself read high on a resistance meter long after the wall has dried.

The test that actually distinguishes it

Distinguishing rising damp from a low-level leak or from a bridged damp proof course requires a moisture profile by height, taken at depth rather than at the surface, and where the diagnosis matters, analysis of the salts present. Groundwater salts are chemically distinguishable from the plain water of a plumbing leak, which deposits no such band. A low-level wet patch with no salt band, that correlates with water use and reads higher at depth on one side of a wall than the other, is a leak in a buried pipe and not rising damp, and treating it with a chemical damp proof course will not improve it.

The diagnostic sequence, in order

Order matters because each stage constrains the next, and because the cheap eliminations come first.

Stage	What is done	What it settles
1. History	When it appeared, whether it changes with weather, heating or a particular fixture, and what has been done already	Which correlations are worth testing; previous remedial work often explains an odd pattern
2. Is there a plumbing loss at all	Timed meter test with every outlet closed; heating circuit isolated and its pressure watched separately	Eliminates or confirms the most expensive of the four causes, usually within two hours
3. Environmental logging	Temperature and relative humidity over several days, plus surface temperatures at the affected locations	Whether condensation is occurring, and whether it is sufficient to explain what is visible
4. Moisture mapping	Readings across and beyond the affected area, at surface and at depth, recorded as such	The extent, and the gradient that indicates which direction the water comes from
5. External inspection	Fabric above and around the internal patch; rainwater goods observed under load	Whether a defect in the envelope can account for the pattern
6. Location	Thermal imaging, acoustic listening, correlation or tracer gas, chosen to suit the system and construction	The position of the defect, once a leak has actually been established

A falling boiler pressure gauge at stage two is as often a failed expansion vessel or a discharging pressure relief valve as it is a leak, which is why the heating circuit is watched separately rather than assumed. Five of the six stages are non-destructive and inexpensive, and together they usually settle the question. Our comparison of acoustic, thermal and tracer gas methods covers which applies where at stage six. Opening up is the last step, not the first.

Where the diagnosis goes wrong

Most misdiagnoses in this field come from a small number of recurring errors, and all of them are avoidable by recording more and concluding later.

• Surface readings treated as moisture content. On masonry and plaster a resistance meter gives a relative indication, not a percentage, and salts read as water.
• A single humidity reading standing in for a pattern. Internal humidity swings hourly. One reading proves nothing about the condensation risk over a week.
• A thermal anomaly reported as a leak. A cold patch on a thermogram can be a thermal bridge, a missing section of insulation, an emissivity difference or a reflection. Without a corroborating moisture reading it is an observation, not a finding. Our page on thermal imaging surveys sets out how those are excluded.
• Assuming one cause. A flat with a slow heating leak and inadequate extract ventilation has two problems, and fixing either alone leaves a visibly damp property and an unhappy occupier.
• Attributing it to lifestyle without measuring. This is both poor practice and, in a tenanted property, a weak legal position; the causes of damp and mould have to be identified rather than assumed.

Where the outcome will be argued about, with an insurer, a leaseholder, a tenant or a local authority, the readings and their timings are the entire value of the exercise. A survey that records the eliminations, states whether each moisture reading was taken at the surface or at depth, and logs the ambient conditions alongside is a document that settles the question. One that reports a conclusion without them invites the argument it was commissioned to end. Related reading: what a brown stain on a ceiling indicates, whether a wall stain is a leak or damp, and how a leak produces mould.