Condensation or a Leak: How to Tell the Difference

The short answer: watch the weather, not the stain. Water arriving during and just after rain is coming from outside and the roof is at fault. Water appearing on cold still nights, in dry spells, is condensation forming inside the construction. The mark on the ceiling looks the same either way, which is why so many people pay to have the wrong thing fixed.

What does the weather test actually involve?

Keeping a note for a month, and nothing more technical than that. Write down the date every time a mark appears, darkens or spreads, and beside it what the weather was doing. A pattern emerges quickly, and it usually settles the argument before anybody climbs a ladder.

• Arrives with rain, worse in heavy or wind-driven rain, dries back afterwards. That is water from outside. The severity tracking the downpour is the giveaway.
• Arrives on clear cold nights, worst in the morning, absent through a mild wet week. That is condensation. Cold air outside chills a surface inside, and the moisture already in the room settles on it.
• Arrives after a thaw, having been invisible during the freeze. Ambiguous, and the one case worth taking seriously either way: ice can block an outlet and back water up under a detail, and frost inside a construction melts too.

Where does each one show up?

Position is the second clue, and it points in the opposite direction to what most people assume. A leak tends to appear where the water finally ran out of somewhere to go, which is rarely above where it got in: a fault at a wall junction can drain along a joist and drip at the far side of a room.

Condensation is not travelling anywhere. It forms exactly where the surface is coldest, so it appears in the same places every time: round the edges of the ceiling, in corners, in a line where a joist crosses, and on and around the frame of anything glazed. Mould rather than a brown stain is another tell. Water that has come through a roof carries dust and bitumen with it and stains; water condensing out of clean indoor air does not, it just feeds whatever grows there.

Why does sealing the outside make it worse?

Because condensation is a ventilation problem wearing a leak's costume, and the usual response to a leak is to close things up. Where the roof is a cold construction, with insulation between the joists and a ventilated gap above it, sealing the outside blocks the very path that was keeping the space dry. The moisture keeps arriving from the room and now has nowhere to leave.

That is the sequence behind a lot of ruined decks. Somebody paints or patches over vents, the roof stops breathing, the boarding above the insulation spends every winter damp, and years later the real bill arrives. On a roof where that has already happened, moving the insulation above the deck through a warm roof conversion removes the cold surface entirely rather than trying to ventilate around it.

What does a survey check?

Both possibilities, in the same visit, because ruling one out is half the answer. From outside it is the covering, every detail and penetration, the upstands and the outlets. From inside it is where the marks are, what shape they take and whether the pattern matches a path water could physically follow. Moisture readings through the build-up say whether anything is wet inside rather than on the surface, and the ventilation arrangement is checked where the construction has one.

The output is a written answer with photographs behind it, which matters most when a landlord, an agent or a neighbour is going to be told what the conclusion is. A condition report that says plainly which of the two it is, and why, ends an argument that can otherwise run for years.

When is it the roof after all?

Often enough that nobody should assume condensation and stop looking. Marks that follow rain reliably, water that arrives in volume rather than as damp, staining rather than mould, and anything appearing near a rooflight during a downpour all point outside. Glazed openings are the awkward case, because a low kerb and a cold frame produce almost identical symptoms in opposite weather, which is why rooflight work starts with establishing which is happening before anything is bought.

There is one more case worth naming, because it catches out people who have read this far and concluded condensation. A roof that has been condensing for years eventually rots its own deck, and a rotted deck lets real water through. By that point both answers are true at once: the origin was internal, the leak is now genuine, and repairing the covering alone will buy a season at most.

Where it is the roof, it is usually a detail rather than the open field, and a repair costs a fraction of what gets spent chasing the wrong diagnosis.