How do you find a roof leak?

A roof leak discovered as a water stain or damp patch on an interior ceiling is rarely located directly below the point of entry. Water entering the roof space travels along the angle of the roof structure, following rafters, battens, sarking felt and the underside of tiles until it reaches a point where it drips downward. This lateral travel can be several metres from the actual entry point. As a result, looking directly above the interior stain on the ceiling is rarely the correct starting point for finding the source of a leak. A systematic approach that starts from the inside and works outward is significantly more reliable.

Inspecting the loft or roof space first

Access the loft or roof space during or shortly after heavy rain if at all possible, as active leaks are far easier to trace when they are running. Bring a torch and look upward along the roof structure. You are looking for wet timbers, damp patches on the sarking felt, water dripping from rafters or battens, dark staining or mould growth on timber and any visible daylight penetrating through the roof boards. Do not step directly onto the plasterboard ceiling between joists as it is not designed to support weight and you will fall through. Walk only on the joists or on boards laid across them.

If you find a wet area on the sarking felt or timber, trace it upward and toward the ridge, as water will have run down to the point where you see it from a higher entry point. Mark the approximate location on the timber with chalk or tape so you can correlate it to the external roof geometry from outside. If you cannot access the loft, or if the roof has no sarking felt, you will need to rely entirely on external inspection and a professional roofer's assessment.

What to look for from outside without going on the roof

Stand back from the property at a safe distance and scan the roof systematically with binoculars if available. Look above the location you identified inside. Specific features to check include missing or cracked tiles in that area, tiles that appear out of alignment or lifted at the lower edge, visible gaps in the ridge tile mortar, any flashing that appears to have lifted or pulled away from the abutment it is sealing, lead flashing around chimney stacks that appears cracked, corroded or has lost its sealant at the wall junction, condition of valley gutters where two roof slopes meet and whether any soil pipe flashings or skylight frames appear to have failed seals. Overflowing gutters should also be checked as they can stain external walls in a way that appears to be a roof leak but is actually a drainage problem.

Where residential roof leaks most often originate in London

What requires a professional roofer rather than DIY investigation

The internal inspection and ground-level external scan described above are tasks any homeowner can carry out safely. What should never be attempted without professional equipment and training is accessing a pitched roof surface to investigate or repair a leak. A wet pitched roof is among the most dangerous surfaces a non-professional can attempt to walk on. Professional roofers use appropriate safety equipment, access the roof from scaffolding or a properly set-up ladder system and have the experience to trace the source of a leak reliably. If your ground-level and loft inspection has not clearly identified the source, or if the repair required involves working on the roof surface, contact a qualified roofing contractor. Some contractors also offer thermal imaging drone surveys that locate moisture and entry points with exceptional accuracy without accessing the roof at all.