Water Stain On The Ceiling But No Obvious Leak

What's happening?

A ceiling stain is the end of a journey, not the start of one. Water entering a roof runs along the top of a rafter, across a wall plate or down a cable until it meets something that stops it, and only then does it soak the plasterboard. That means the stain is frequently metres from the actual defect and often on the opposite side of the room. A dry stain tells you water has been there and stopped — either the defect only leaks in specific conditions such as wind from one direction, or it is not a roof leak at all. Condensation in a poorly ventilated loft produces almost identical marks, as does a slow plumbing weep from a bathroom above.

What usually causes it?

• Slipped or cracked tile letting water onto sagging underlay that channels it sideways before it drips
• Failed flashing at a chimney, abutment wall or dormer, with water tracking along a rafter
• Blocked or overtopping valley discharging under the tiles during heavy rain only
• Loft condensation — warm moist air reaching cold roof timber where eaves and ridge ventilation is blocked, often by insulation pushed into the eaves
• Overflowing gutter or a blocked downpipe throwing water back under the eaves and onto the wall plate
• Plumbing rather than roofing: a weeping joint, a leaking shower tray or a hot water cylinder overflow in the floor void above
• A one-off event already passed — driven snow melting into the loft, or a storm that lifted tiles which have since settled back

When should you call a roofer?

Call a roofer once you have established that the stain is live, or straight away if it is spreading, if plasterboard is sagging, or if water is anywhere near lighting. If the room above is a bathroom, en-suite or airing cupboard, get a plumber to rule out plumbing first — it is quicker and cheaper than scaffolding a roof for a leak that turns out to be a shower waste. If the loft shows droplets on the underlay and mould on the timber rather than a run of water, the answer is ventilation, and a roofer should address that rather than hunting a hole.

What will the roofer actually do?

• Start in the loft and work back from the stain along the drip path, following tide marks on timber rather than guessing
• Distinguish ingress from condensation by looking for droplets on the underlay, mould on rafters and blocked eaves ventilation
• Inspect the outside of the roof above the suspected entry point, checking tiles, flashing, valley, verge and any penetration
• Water-test where the cause is not obvious, running water over specific areas in sequence with someone watching in the loft
• Repair the defect found — re-fix or replace tiles, renew flashing, clear or re-line a valley
• Restore eaves and ridge ventilation where condensation is the cause, including fitting eaves trays and keeping insulation clear of the airway, then advise when the ceiling is dry enough to be made good and whether the plasterboard needs replacing rather than repainting