How to trace and repair a leak in a metal roof

Water can travel along underlay, a batten or a structural member before it drips. Keep a record of the weather when the stain appears, including wind, rain or melting snow. This can narrow down the search and spare you the cost of sealing joints all over the roof without evidence.

Look for tracks inside the loft if you can reach it safely. Do not start stripping out wet layers without a repair plan. Leave the external inspection to a roofer with fall protection. During a downpour, protect the room and its contents; do not step onto wet sheet metal to look for the leak.

Condensation can leave droplets across several areas on the underside of cold metal. Possible causes include humid indoor air, leaks in the inner lining or insufficient drying. This pattern differs from water entering at a single hole. Covering the nearest visible joint may make no difference.

An anti-condensation backing can hold some moisture and release it later, provided it can dry. It does not replace insulation or a design that accounts for moisture movement. In a heated loft, assess all the layers, including joints in the vapour control layer at walls and windows.

A fixing may be loose, seated at an angle or fitted with a damaged washer. Overtightening can also cause leaks, so do not simply tighten every screw. For a replacement, check the hole, the supporting material and the approved fixing. The repair must restore both the attachment and the seal.

Assess abandoned holes from old equipment separately. A small local repair may be possible; a larger opening can require a designed repair or a replacement sheet. Sealant poured into a hole may not withstand thermal movement, and any repair material must be compatible with the coating.

Check the geometry of the joint and the direction of runoff first. A reversed lap, inadequate pitch or incorrectly shaped detail may require part of the covering to be lifted. Neither tape nor sealant changes the direction water flows. Reinstate the detail specified for the roofing system.

Give the chimney, valley, ridge and wall abutments particular attention. Position seals where the system drawings show them. Keep drainage paths and ventilation openings clear. Smearing a thick layer over the surface can hide ongoing damp and make the next inspection more difficult.

Check compatibility with the sheet finish, suitability for outdoor exposure and the required surface preparation. Sunlight, temperature and movement at the joint all affect selection. A product intended for indoor use or a different substrate may fail. Use a sealing method approved for the detail in question.

Prepare the surface as the manufacturer directs. Dirt, loose paint and moisture can reduce adhesion. Some products need a primer or a specified application pressure. The repair’s function and exposure should determine the choice, rather than tape width or sealant colour alone.

Replacement can make sense where corrosion has perforated a sheet, damage is extensive or a section is too distorted to form a reliable joint. The same applies to a flashing folded to the wrong shape. Repeated patching can cost more than replacement. Repair any damaged substrate before fitting new metal.

Check the replacement profile drawing, effective cover width and coating. A label such as T18 or “modular tile-effect roofing” alone does not establish a match. New sheets may also differ in shade from the weathered roof. Plan removal so the adjacent laps and underlay remain intact.

Clear away swarf, waste and surplus sealing material when the work is finished. Photograph the repaired areas and record what was done. Let the contractor choose a suitable method of checking for leaks. A jet of water aimed up beneath the laps does not reproduce normal rainfall.

Check that wet layers are drying and assess insulation that became soaked. Stopping new water from entering does not dry a loft immediately. If damp returns, reopen the investigation before adding more sealant. A lasting repair removes the cause and leaves the detail accessible for future checks.