Damp in or under a roof has three common parents, and treating the wrong one wastes money. A leak lets rainwater through a gap in the covering or at a junction. Condensation forms when warm, moist indoor air meets a cold surface and drops its water as droplets. A plumbing or tank fault releases water from inside. They can leave similar stains, smells and mould, and a homeowner staring at a brown patch on a bedroom ceiling cannot tell by eye which it is. A damp and condensation survey is the process of finding out. It combines careful looking, instrument readings and, where needed, testing, and the aim is a single clear answer: where is the water coming from and what stops it?
Why the cause is easy to misjudge
Water on a roof travels. It enters at a crack, runs along the underlay or down a rafter and appears wherever it finally drips or soaks into plaster, which may be a metre or two from the entry point. A ceiling stain directly under a ridge often turns out to come from a flashing on a chimney further along. Condensation misleads in the opposite direction, appearing over a wide area with no single source and often in the colder months, which tempts people to blame the weather on the roof when the cause is the way the house is used.
Seasonal timing helps as a first clue. Damp that appears within hours of heavy or wind-driven rain, and fades in dry weeks, points at a leak. Damp that builds through the winter, peaks on cold mornings and eases in spring points at condensation. Damp that shows regardless of weather, or after a bath or shower, may be a plumbing or extraction fault. These are guides, not rules, and some roofs have more than one problem, which is why a survey checks all three.
What a survey starts with
The first step is a conversation and a walk around. We ask when the marks appeared, in what weather, whether the roof has been worked on lately and whether anything has changed indoors, such as new insulation, a loft conversion, a new boiler, a tumble dryer or a bathroom without a fan. Then we look at the outside of the roof for the usual suspects: slipped or broken tiles, failed mortar at ridges and verges, worn flashings at chimneys and abutments, blocked valleys and overflowing gutters. Our companion page on inspecting your loft describes the inside half of that check.
In the loft, the focus is on the underside of the roof and the insulation. Wet patches that darken timber are recorded and photographed. The felt is checked for tears and condensation droplets. The eaves are checked for insulation that blocks ventilation. Roof penetrations such as soil pipes, extractor ducts and flues are looked at closely. Where the roof allows, a visit during or just after rain is best, since the evidence is fresh; our answer on whether rain is a good time to inspect a roof explains the trade-offs.
Instruments and tests
A moisture meter is the everyday tool. A pin-type meter measures the moisture content of timber and plaster by pressing probes into the surface, while a pinless meter reads through the surface with a flat sensor. Readings tell you how wet a material is compared with a dry reference, and a series of readings across a stain maps the damp patch and shows where it is wettest, which is usually closest to the source. The numbers are guides, not diagnoses; wood that carries a high reading in January and a low one in July suggests condensation, while wood that stays wet after weeks of dry weather suggests a persistent leak.
A thermal imaging camera adds a picture of surface temperature. Wet insulation and wet plaster hold a different temperature from dry, and missing or compressed insulation shows as a cold stripe. It is a screening aid, not a verdict, and it works best when there is a temperature difference between inside and outside. We cover where it helps and where it does not in our guide to thermal imaging roof surveys.
Water testing is the decisive step when a leak is suspected but not found. It means wetting the roof in stages with a hose, starting at the lowest point and working up, while someone watches the loft. Each section is wetted for several minutes, so that when water appears we know which section let it in. It takes patience, and it should be done carefully so that the test does not cause the damage it is looking for, but it is the most reliable way to find a leak that does not leave a visible hole.
Condensation: how it happens and what to look for
An ordinary household produces a surprising amount of moisture through cooking, bathing, drying clothes and breathing. Warm air holds more moisture than cold air, so that vapour drifts upward through gaps in the ceiling into the loft, where it meets cold timber and felt and condenses. In a well-ventilated loft the moist air leaves through eaves and ridge vents before it does harm. In a poorly ventilated one it lingers.
The signs are consistent: droplets or a film of moisture under the felt, black speckled mould on rafters and on the north-facing slope's timbers, damp or matted insulation, and rusty nail points where they protrude through the boards. Damp that appears in winter, worsens with cold snaps and clears in summer fits the pattern. On flat roofs the equivalent is interstitial condensation inside the build-up, which is harder to see and needs a core sample or moisture survey.
The causes are rarely mysterious. Insulation stuffed into the eaves blocks the air path. Recessed lights and loft hatches that do not seal leak warm air into the space. Bathroom and kitchen extractors that vent into the loft instead of outdoors dump moisture where it does the most harm. Roofs refitted with modern, low-permeability membranes without adding ventilation can trap moisture that old, leaky felt used to let through. And a foam sprayed onto the underside of the roof turns a cold, ventilated loft into a sealed one; the RICS consumer guide notes that it can trap moisture against timbers, and our spray foam insulation pages cover what to do if that is your situation.
Leaks: how to tell and where they hide
True leaks leave their own signs. Staining that tracks down a rafter from a particular point, a tide-line on the chimney breast, drips that start during heavy rain and stop a few hours after it ends, and damp that clusters around a junction rather than spreading evenly. The usual entry points are well known to anyone who repairs roofs: worn lead or mortar flashings at chimneys and walls, cracked or slipped tiles, failed ridge or hip bedding, blocked or broken valleys, split felt at the eaves, leaking rooflights and the places where cables, vents and pipes pass through.
Finding a leak can be harder than fixing one. Wind-driven rain pushes water upslope under tiles, so the entry can sit higher than the stain. Gutters that overflow behind the fascia can soak the wall plate and look like a roof leak from inside. And an old stain that was never dried can seem fresh when the humidity rises. This is why a survey does not stop at the first plausible culprit: it checks that the suspected cause explains the pattern of stains and the timing, and it uses a water test to confirm where needed. Our short piece on hidden roof damage deals with defects that do not announce themselves.
What the repair looks like for each result
If the answer is a leak, the repair is usually local: replace the broken tiles, re-form the flashing, clear and re-lay the valley, reseal the penetration or re-bed the ridge. The wet timber and insulation are then dried or replaced, and a follow-up check after the next heavy rain confirms the fix. If the roof's covering is old and failing in several places, we say so, and price the alternatives honestly, because repeated patching is a poor use of money on a covering past its useful life.
If the answer is condensation, the work is mostly about air. Restore the ventilation at the eaves by clearing insulation back and fitting baffles, add ridge or tile vents where needed, seal ceiling penetrations and the hatch, redirect extractor ducts to outside, and check that insulation is the right depth and not compressed. Dehumidifiers and opening windows help indoors, but the roof must breathe. Where mould has taken hold, timbers are cleaned and checked for decay. Spray foam is a special case, and the fix generally involves removal and a ventilation check.
If the answer is a plumbing fault, such as a leaking tank, a loose pipe joint or an overflowing cistern, we report it and suggest the right trade. A roofer who patches a roof to cure a leaking pipe has done the owner no favours.
When the survey is worth doing
Book one if stains recur after a repair, if you cannot tell leak from condensation, if mould is spreading, if you are buying a house with damp signs, or if you plan to insulate or convert the loft and want a baseline. The findings are especially useful after storm damage, where claims depend on showing a clear cause; see our page on roof inspections after a storm. Our free survey includes a damp check as part of the roof inspection, with photos of the actual roof and a written finding. Where work is needed you receive a fixed quote, and where the roof is fine and the cause lies elsewhere, we say that too. Start from our roof inspections in Basingstoke page or call 01256 213956.
