Why is my roof leaking?

Most leaks begin where two things meet rather than in the middle of a slope. Chimney flashings, abutments against walls, valleys between slopes, roof lights, soil vent pipes and the ridge and verge details account for the majority of what we find.

Coverings matter too, but the failure is often a slipped, cracked or missing slate or tile combined with underlay that has aged, split or was never there. One without the other frequently produces no visible leak at all.

Once past the covering, water follows the easiest route. It runs down underlay laps, along a rafter, across a ceiling joist or down a cable before it drips, and it can travel several metres horizontally before it does. That is why the wet patch on a ceiling is evidence of where water left the structure, not of where it got in.

Working backwards from the stain, through the roof space, to the highest point where timber or underlay is wet is the part of diagnosis that gets skipped most often — and skipping it is the most common reason repairs fail.

When a leak appears is diagnostic in itself. Only in heavy, prolonged rain suggests overwhelmed drainage or a slow route with a large catchment. Only with wind suggests driven rain entering a junction that resists vertical rainfall. Appearing after rain has stopped suggests water held somewhere and released slowly.

A leak that appears in dry weather, or only in cold weather, may not be a leak at all — condensation in a roof space produces very similar staining.

Once the entry point is identified and the surrounding area assessed, the options are usually clear: repair the defect, renew the detail, or investigate further because the extent cannot be established from the surface. What should not happen is a replacement recommendation offered before anyone has traced the water.

Ask for the evidence — images of the defect, the internal findings, and a written statement of what was observed against what is inferred.