Damp patch on the ceiling: leak or condensation?

Read the edge of the stain

A leak leaves a mark with a defined boundary, usually tea-coloured, often with a darker tide line where the water stopped and dried. It grows outward in stages, each rain event adding a new ring.

Condensation is vaguer. It spreads without a hard edge, tends towards grey or black rather than brown, and often carries mould speckling because it is a surface staying damp rather than a soaking event.

Read the position

Position tells you as much as appearance. Leaks cluster where the roof has a detail: below a chimney breast, near a valley, alongside a dormer cheek, under a bathroom soil vent, at the eaves where a gutter overflows.

Condensation gathers at the cold parts of the building fabric: external corners, the perimeter of an upstairs ceiling, around a loft hatch, on the north side, and in rooms with no extract such as a bathroom with a painted-over vent.

Read the timing

Keep a note for a fortnight. This is the single most useful thing you can do before anyone climbs a ladder.

• Worse within hours of heavy rain: almost certainly a leak
• Worse only in wind-driven rain from one direction: a leak at a verge, flashing or dormer
• Worse on cold mornings with no rain: condensation
• Worse when drying washing indoors or after showers: condensation
• Constant regardless of weather: possible plumbing leak rather than roof

What we look at in the loft

The loft usually settles it. A leak leaves clean water tracks down a rafter, dark staining on the underside of the sarking or felt, damp insulation in a defined patch, and often daylight at the fault.

Condensation coats the underside of the felt evenly with droplets, frosts on cold nights, wets the top surface of insulation across a wide area, and comes with a stale, still atmosphere because the eaves ventilation has been blocked, often by insulation pushed tight into the eaves.

What each one costs to put right

A leak is a repair: a tile, a flashing, a valley, a chimney. Condensation is a ventilation problem and is usually cured by clearing eaves ventilation, adding tile or ridge vents, fitting proper extract in bathrooms and kitchens, and pulling insulation back off the eaves so air can move.

Painting over either of them hides it for a few weeks. We would rather find the cause, tell you which of the two you have, and price only the work that fixes it.

When water reaches fittings or a ceiling starts to sag

Active water entry calls for a different response from a dry, old tide mark. Keep people away from a bulging or sagging ceiling and from wet light fittings, switches and appliances. Do not stand underneath to investigate, and do not touch electrical equipment affected by water. If you cannot isolate the affected electrical circuit safely from a dry location, seek urgent help from a qualified electrician.

From a safe position, note which room is affected and whether rain, wind or use of a bathroom preceded the leak. A ceiling below another occupied floor can receive water from plumbing above it; a top-floor stain can still have more than one cause. Give the roofer that distinction when calling rather than assuming every wet ceiling proves the covering has failed.

Roof repair and ceiling reinstatement are separate decisions

Finding the entry route comes before making the room look finished again. Water may travel along timber or the underside of the roof before arriving at the ceiling, so the visible mark is a starting point for investigation rather than the place to apply sealant. Record the room and the direction in which the stain spreads, without entering an unsafe loft or stepping on ceiling boards.

Once the source is resolved, damaged plasterboard, finishes and electrical fittings may need assessment by the appropriate trades. Replacing a ceiling alone does not stop a roof leak, while repairing the roof does not establish that soaked ceiling material is sound. Keep those scopes distinct in any quotation and allow the affected construction to be assessed before redecorating.

Common questions