Water beneath a metal roof after rain: tracing the leak

Photograph the same areas in different conditions. Mark the stain on a simple room plan with the locations of the chimney, roof window and ridge. A record of when and where the garage roof drips can save the contractor from opening the wrong part of the roof.

Inspecting an accessible loft often narrows the search. Water can follow an underlay, timber or a cable before it drips. A mark on a rafter therefore need not lie beneath the opening. Inspect service penetrations and the edges of the underlay as well.

Avoid cutting the underlay to release trapped water. A cut can interrupt drainage and obscure the cause. Any inspection opening needs a defined purpose and a way to close it properly. If wiring is nearby, restrict access to the wet area and have its electrical safety assessed.

Valleys, the upslope side of chimneys and large penetrations receive runoff from a wider area. Leaves and sediment can obstruct drainage. A defect may become visible only in heavy rain. Damaged flashing can also direct water beneath the covering.

At the eaves, inspect how the sheet, underlay and gutter connect. Each layer needs a drainage path. A watertight gutter cannot collect water discharged behind the fascia. Leave high-level inspections to a crew with suitable access equipment. Wet sheet metal is unsafe to walk on.

Inspect fasteners for damaged washers, angled screws and distorted sheets. Tightening every screw as a trial can worsen distortion or damage the seal. Fasteners must suit the substrate and thickness of the pieces they connect.

At an end lap, check overlap length, support and seal arrangement against the profile and roof pitch. At side laps, also inspect the edge geometry. Stretched or crushed sheets may no longer sit together correctly. An extra bead of sealant cannot restore the profile shape.

Warm, humid air can condense on a cold surface. In heated buildings, assess the insulation, the continuity and airtightness of the vapour control layer, and ventilation. Painting over a mark leaves those conditions unchanged.

In an unheated shelter, a fleece backing can temporarily hold condensation. The fleece must be able to dry. It provides neither thermal insulation nor a barrier to rain entering through a faulty lap. Establish whether rain penetration, indoor moisture or both require attention.

Ask the contractor to identify the cause, proposed repair and materials. The solution might involve replacing flashing, correcting a joint or rebuilding part of the roof assembly. Unplanned use of silicone can hinder later cleaning and may be incompatible with the coating.

After the repair, have the contractor remove metal swarf and inspect adjacent areas. Keep photographs and a record of what changed. Check the result in weather similar to that which originally caused the stain. A specialist should plan any controlled water test. Spraying the whole roof at once makes it difficult to isolate a defective detail.

Distinguish fresh ingress from moisture retained in the roof assembly. Mark the stain’s edge and take further photographs. Agree separately how to dry or replace wet material after the leak itself has been repaired.