The short answer
Loft damp usually comes from a roof leak, from condensation, or from a mix of both. A leak leaves dark, localised patches that grow after rain, while condensation covers wider areas, shows as beads or mould on cold timber and felt, and gets worse in cold weather. Ventilation, insulation and the roof covering all need checking before anyone decides on a fix.
Leak or condensation?
The two look alike at first glance, and the fix for one makes no difference to the other, so working out which you have comes before anything else. A leak is local. It shows as a dark patch around a particular rafter, a chimney, a valley or a nail, and it grows or drips within a day or two of rain. Look for tide marks, stained insulation below one spot and wet timber that dries out after a few dry days.
Condensation is general. It appears across the felt or boards beneath the tiles as beads of water, a dewy sheen or black mould in patches, and it tends to be worst in cold, still weather and after a day of cooking, showering or drying washing indoors. It can drip from nail tips on a frosty morning and then disappear by afternoon.
Some lofts have both. A small leak at a chimney can sit alongside a general condensation problem, so a visit should look at each rather than stop at the first thing found.
A simple test helps. Note the weather for the three days before you spot the damp. Wet timber that follows rain by a day points to a leak, while wet timber that follows a cold, dry night points to condensation. A moisture meter in a roofer's hands gives a number to back up the observation, but your own diary of dates is a good start.
Why ventilation matters
Warm air leaving the living space carries water vapour. In a cold loft, which is the usual arrangement above an insulated ceiling, that vapour meets cold surfaces and condenses unless fresh air can move through. Marley's guidance on ventilating a cold roof, which cites BS 5250, describes a continuous eaves opening of 25 mm for pitches of 15 degrees or less with a non-breathable underlay, and 10 mm for steeper pitches, with an extra 5 mm at high level on roofs steeper than 35 degrees and on lean-to roofs.
Those figures depend on the underlay, the covering and the pitch, so they are a reference rather than a quick fix. What matters on a visit is whether the air can move. Insulation pushed hard into the eaves, a blocked soffit vent, a bathroom extractor discharging into the loft, or a roof with no vents at all can each leave moisture trapped.
Piling on insulation without thinking about airflow is a familiar way for a dry roof space to start sweating.
Simple fixes to try first
Some causes of loft damp cost little to cure. Check that every bathroom and kitchen extractor vents through a duct to the outside, not into the roof space. Make sure the loft hatch closes snugly, because a loose hatch lets warm, moist air up from the landing. Seal gaps round pipes and cables where they pass through the ceiling, and keep stored boxes clear of the eaves.
Those steps reduce the moisture reaching the roof space before any building work is considered. If the damp persists, the next step is a survey of the roof and the ventilation, so the cost goes on the cause rather than a guess. The main roof inspection service is described under roof inspections and surveys.
What a roofer checks from inside
Up in the roof space, the first thing to check is whether any light gets in, and the second is whether any water has. Pinpricks of daylight betray a displaced tile. Dark patches show where water has come in. We trace each stain back along the rafter to its highest point, because the source sits uphill of the wet. We note the condition of the felt, the felt laps, the battens and the timbers, press suspect areas for softness and look for fungal growth or old, dry-rot signs.
We check how the insulation is laid, whether it blocks the eaves, whether pipes, cables and light fittings leak warm air through the ceiling, and whether extractor fans vent outside. Our broader guide to what to check in your loft lists the same points for homeowners who want to look first.
If you are alone, stay on the joists or boards, take a torch, wear a mask in dusty or mouldy areas and avoid stepping between ceiling joists.
What a roofer checks from outside
Outside, the question is where water could get in. The covering comes first, whether tile or slate, and then the fixings along the ridge and hips, the gable edges, and the lead at each junction with a chimney, a wall or a window in the slope. Valleys and gutters are checked for blockage and overflow. For flat roofs the questions are different: ponding, upstand condition, laps and outlets.
A leak in the roof may be a small repair, and we price what the fault demands and nothing beyond it. Where the cause is condensation, a new covering will not help, and we tell you so up front. In that case the fix lies in ventilation, extractors and sometimes insulation, and a roofer may suggest a ventilation specialist or building professional for the wider design.
Pictures of your loft and roof reach you with the written quote, which means you have seen the evidence before agreeing to any work.
Spray foam and other special cases
Some lofts have spray foam sprayed directly onto the rafters. The RICS consumer guide links poor installation to dampness through uncontrolled condensation, and foam also hides the timbers, so you cannot easily see what is behind it. Lenders differed as of November 2024: a few refused to lend on such homes while the rest weighed each application separately, so speak to your own lender if you plan to sell or remortgage.
New-build lofts raise a different point: builders sometimes leave ventilation gaps blocked or insulation badly placed. Our page on new build roof snagging covers those faults. If you simply want someone to look, our free roof inspection page explains how a first look works, and rented properties are covered on landlord roof inspections.