Roof leak or condensation? How to trace water beneath metal sheeting

A leak may occur only when rain comes from a particular direction. Condensation is more likely when the surface cools and the air is humid. These are clues rather than a diagnosis, and both processes can occur together. A record of several events gives the inspector more to work with than a single photograph of a ceiling that has already dried.

Water can run along the underside of the sheet, a flashing or a structural member before dripping onto the ceiling. Trace possible routes upslope from the patch. Dirt trails, staining on supports and differences in moisture between adjacent layers may indicate where the water travelled.

Mark the patch relative to the chimney, valley, roof window and sheet joints. Avoid cutting a large exploratory hole in a finished ceiling. Use the observations to decide whether an opening is necessary, arrange safe access and expose only the area that needs inspection.

Inspect sheet laps, fasteners, service penetrations and flashings. A sealing washer may be crushed, damaged or incorrectly seated. At a wall abutment, look for breaks in the flashing. At a valley, make sure debris has not obstructed drainage.

Tightening every screw without inspection can crush the washers or distort the sheet. Drilling another hole does not necessarily improve the fixing. Before repairing a joint, assess the fastener, the condition of the material it grips and the seal. The size of the visible drip tells you little about the work required.

Water vapour condenses when a surface is colder than the dew point of the air touching it. Both air temperature and humidity therefore matter. A wet vehicle introduces moisture into a garage, while work processes may supply moisture in a workshop or industrial hall. A watertight roof can still be wet underneath.

Check ventilation and the continuity of the air barrier. A leak on the warm side can carry moist air to a cold part of the roof. Adding insulation does not seal that air path. Blocking ventilation openings can in turn prevent drying. Assess the whole build-up and its junctions.

Fleece on the underside of the sheet holds a limited amount of water and must have a chance to dry. A constant supply of moisture from the room can saturate it. It cannot replace a weathertight covering or waterproofing. Wet fleece alone does not establish where the water came from.

A sealant belongs in a specified joint and must be compatible with the coating. Smearing it over a broad area can hide the defect, collect dirt and complicate the next repair. Take particular care to keep drainage paths open. The joint geometry should shed water, with sealant used where the detail calls for it.

Record what was repaired, the date and a photograph. Keep the specifications for any replacement fixings or flashings. Inspect the same point and the surrounding area after further rain. Wet insulation or timber takes time to dry and may need separate assessment and drying measures.

Have the contractor carry out any water test under controlled conditions. A strong jet aimed up a lap can drive water where rain would not reach. Do not climb onto a wet roof for a quick check. If water is present near electrical equipment, make the area safe before continuing the investigation.