Damp & mould glossary

Plain-English definitions of the damp, mould and timber surveying terms you will meet in a report, an inspection or an Awaab's Law case. Written for surveyors, housing teams, landlords and tenants.

Awaab's Law: Measures under the Social Housing (Regulation) Act 2023 setting statutory deadlines for social landlords to investigate and fix damp, mould and other hazards. See our Awaab's Law deadline calculator.
Black mould: Common name for dark mould growth, often Aspergillus or Stachybotrys species, that thrives on damp surfaces. A category of HHSRS hazard and the focus of Awaab's Law.
Carbide test (TMC): A calcium-carbide moisture test that gives a quantitative measurement of moisture content from a drilled sample. More definitive than a surface meter for confirming true damp.
Category 1 hazard: Under HHSRS, the most serious band of hazard. Local authorities have a duty to act on Category 1 hazards, and damp and mould can score at this level.
Category 2 hazard: A less severe HHSRS hazard band where the authority has a power, but not a duty, to take enforcement action.
Condensation: Moisture that forms when warm, humid air meets a surface colder than its dew point. The most common cause of household damp and mould in UK homes.
Damp-proof course (DPC): A horizontal barrier built into a wall near ground level to stop moisture rising. A failed, bridged or absent DPC is a common source of rising damp.
Damp-proof membrane (DPM): A sheet or coating that prevents moisture passing through a floor or wall, often linked to the DPC to form a continuous barrier.
Decent Homes Standard: The minimum standard social homes must meet, covering disrepair, hazards, modern facilities and thermal comfort. Damp and mould can breach it.
Dew point: The temperature at which air becomes saturated and water vapour condenses. Surfaces colder than the room's dew point will attract condensation.
Dry rot: Serpula lacrymans, the most serious timber-decay fungus. It can spread through masonry to find fresh timber, so it demands prompt, thorough treatment.
Efflorescence: A white, powdery salt deposit left on masonry as water evaporates. A sign of moisture movement, not itself harmful.
Equilibrium moisture content: The moisture level a material settles at for a given temperature and humidity. Comparing a reading to it helps judge whether timber or masonry is genuinely wet.
HHSRS: The Housing Health and Safety Rating System, the risk-based framework used to assess hazards in residential property in England and Wales, including damp and mould growth.
Hygrometer: An instrument that measures relative humidity and, with temperature, lets a surveyor calculate dew point and assess condensation risk.
Hygroscopic salts: Salts that attract and hold moisture from the air, left behind by evaporating ground water. They keep a wall feeling damp even after the original source is fixed.
Interstitial condensation: Condensation that forms inside a wall or roof structure when warm moist air passes into the fabric and reaches its dew point within, rather than on the visible surface.
Moisture meter: A handheld device that estimates moisture in a material. Electrical-resistance meters read surface moisture; readings are indicative and can be skewed by salts, so they support rather than replace a diagnosis.
PCA: The Property Care Association, the UK trade body for damp, timber, structural waterproofing and invasive-weed specialists, whose standards underpin defensible reports.
Penetrating damp: Water entering a building horizontally through a defect such as failed pointing, a cracked render, a leaking gutter or a bridged cavity, rather than rising from the ground.
Relative humidity (RH): The amount of water vapour in the air as a percentage of the maximum the air can hold at that temperature. Sustained high RH promotes mould growth.
RICS: The Royal Institution of Chartered Surveyors, which sets professional standards for surveying, including the Home Survey Standard used for property condition reports.
Rising damp: Ground moisture rising up through a wall by capillary action, typically to about a metre, leaving a tide mark and hygroscopic salts. Genuine rising damp is far rarer than it is diagnosed.
Salt analysis: Laboratory or field testing for nitrate and chloride salts in a sample. Ground-borne salts help confirm or rule out rising damp versus other moisture sources.
Thermal bridging: A path of higher heat loss, such as a lintel or wall junction, that creates a locally cold surface where condensation and mould readily form. Also called cold bridging.
U-value: A measure of how readily heat passes through a building element. A higher U-value means a colder internal surface and greater condensation risk.
Ventilation rate: How quickly indoor air is replaced with outdoor air, often expressed as air changes per hour. Adequate ventilation is central to controlling condensation.
Wet rot: Timber decay caused by fungi in consistently wet wood. It stays localised to the damp area, unlike dry rot, and is resolved by removing the moisture source.
WME: Wood moisture equivalent, the scale a resistance meter reports when used on masonry. It indicates relative dampness rather than a true percentage moisture content.
Woodworm: Common name for wood-boring beetle larvae that tunnel through timber. Active infestation is identified by fresh bore dust (frass) and clean exit holes.

New to diagnosing damp? Start with the complete damp and mould guide, try the free damp & mould tools, or see how the damp & mould survey software builds these classifications into every report.