Why does a metal roof leak? Where to start investigating

Record when the patch appeared and the weather conditions. Leakage after heavy rain suggests water entering from outside. Drops following a cold night, even without rain, may indicate condensation. These are clues rather than a definitive test. Water can travel along a membrane or structural member, so the drip may not be directly beneath the defect.

Photographs from several days and a roof sketch are useful. Mark chimneys, windows, valleys and service penetrations. Do not climb onto a wet roof to look for evidence. Observations from the attic and weather records can help the roofer start in the right area.

First, screws: an angled fastener may compress its washer unevenly, while overtightening can deform the seal or sheet. Second, laps, whose length and sealing depend on the product and pitch. Third, chimneys and wall junctions. Fourth, roof valleys, which receive runoff from a larger area.

Include holes left by removed accessories. Inspecting the large sheets alone is not enough: a pipe penetration, aerial bracket or later installation may cause the leak. The roofer should trace drainage from above the suspected damage down to the eaves.

Some faults appear only during rain driven from a particular direction, so wind information matters. If symptoms began after aerial or photovoltaic installation work, inspect those areas first. Timing alone does not prove the cause, but identifies a useful starting point.

A fifth cause is a roof pitch outside the system requirements. A sixth is damaged or incorrectly terminated underlay. A seventh is blocked drainage in a valley or beside flashing. An eighth is moisture trapped by inadequate ventilation or a leaking vapour-control layer. Not every problem can be repaired from above.

During renovation, check that insulation has not blocked the ventilation gap. Leaves and nests can restrict airflow at the eaves. A vapour-permeable membrane does not replace the gap: it permits vapour diffusion through the material but does not create an airflow route below the sheets.

A chimney-side stain after wind-driven rain warrants checking the flashing and masonry above it. Dripping along a line of sheets points towards a lap or fastener row. Scattered drops beneath cold metal call for assessment of humidity and ventilation. Staining below a valley requires checking drainage and the covering edges.

None of these symptoms gives a diagnosis on its own. A leaking chimney, for example, can release water far from the flashing. Inspect step by step rather than dismantling the entire roof initially. Water tests should cover small areas; wetting the whole slope at once makes the entry point harder to locate.

Sealant belongs in a detail designed to use it. It cannot replace correct flashing geometry or laps arranged with the water flow. A thick deposit in a recess may even hold water back. At chimneys, allow for movement between roof and masonry. One approach uses separate counterflashing to cover the lower flashing.

Remove the cause first, then reinstate the seal. Repairing the coating will not solve a torn underlay. When a fastener fails, assess its hole and substrate too. Installing a larger screw without diagnosing the problem should not be the default response.

Obtain a before photograph, an explanation of the cause and a photograph of the completed detail. Record the materials used and check the area after the next rainfall. Wet insulation needs separate assessment: immediately enclosing it may trap moisture within the roof.

Inspect the roof even when it is not leaking. Removing debris and checking the coating are basic maintenance tasks. Clear valleys and sound flashings are less costly than repairing a ceiling. Pay particular attention beneath trees and around earlier installation work.

Photograph the exposed layers before closing the lining. Note what was replaced and when the test took place. This record helps distinguish a recurring defect from a new problem later.

No. Vapour can condense on a cold surface. Check the conditions in which the moisture appears.

No. Unchecked tightening can damage seals. Locate the cause and assess the holes first.