Mould on ceilings

Leak or condensation — how to tell

This is the first thing to establish, because the solutions are completely different. With a leak, treatment without a repair is pointless — the water will keep coming. With condensation, no repair is needed, but the ventilation or heating has to change.

If you are not sure, a measurement settles it immediately: with a leak, moisture in the material at the centre of the patch is sharply higher than around it. With condensation, the readings are evenly raised right around the ceiling perimeter, with no pronounced centre.

A leak is dealt with by a plumber or a roofer. Condensation is dealt with by air exchange. Treatment sits between those two steps.

A third case: an unheated space above

There is a third, rarer situation: the ceiling is damp because there is an unheated space above it. That is typical on a top floor beneath a cold loft, and on a ground floor above an unheated basement, where it is the floor slab rather than the ceiling that is damp. In these cases both the measurement and the solution are closer to the condensation scenario, except that the cold surface is inside the structure rather than in the room.

Top floor

On the top floor, mould on the ceiling is usually not the flat's problem. The reason is normally the roof covering, failed waterproofing or a poorly insulated loft floor. In that case treatment is a temporary fix — the problem itself has to be dealt with at building level, together with the property manager.

We say that plainly, even when it is not the answer the client wanted to hear. Treating a ceiling knowing that the roof leaks means taking money for work that will not hold.

Telling a roof leak from loft condensation

The clue that separates a roof leak from loft condensation on a top floor is timing. A roof leak is tied to specific events: heavy rain, a thaw, snow building up on the roof. The patch appears or darkens within a few days of such an event, and nothing changes in between. Condensation on a loft floor behaves in the opposite way — it grows slowly across the whole heating season, regardless of rain.

In pre-war buildings the top floor is a special case. There the floor structures are often timber with organic infill, and a long-running leak in them means not only mould but damage to the timber. In that situation, treating without opening up the structure is risky — first you have to understand how deep the water has gone.

Bathroom ceilings

In a bathroom it is almost always condensation, and the reason is almost always ventilation. After a shower the humidity in the room reaches a high level, and if the extract is not working the moisture settles on the cooler ceiling.

The check is simple: hold a thin sheet of paper against the ventilation grille. If it does not stick, the extract is not working, and that has to be dealt with before any treatment.

There are two more things to check in the same minute:

• Whether the grille has become clogged with dust or painted over during a refurbishment.
• Whether there is a gap under the bathroom door — without an air supply the extract will not work even with a clean duct.
• Whether there are neighbours above the bathroom who might have a slow leak — then the patch will be local rather than around the perimeter.
• Whether there are recessed lights in the ceiling — the finish is thinner around them and condensation appears there first.

In panel blocks the bathroom is often windowless and served by a shared ventilation shaft. If the airflow in that shaft is weak for the whole riser, it is hard to solve at the level of an individual flat — then the question is one for the property manager, or it has to be solved with a powered extract fan.

How we remove it

Ceilings differ from walls in one practical way: the work is done overhead, which means the room has to be prepared. Furniture and floors have to be covered, because stripping ceiling finishes brings debris down. If the area is large, that is a separate time cost, and we account for it in the quote.

What decides the scope of work

The price of ceiling work varies by a factor of several, and the reasons are concrete rather than a matter of judgement.

Suspended ceilings are a case of their own. If there has been a leak above them, the structure has to be opened up, because the moisture and the fungus are usually on the upper face of the board and inside the void, where nothing is visible from the room. Treating only the underside here means leaving untouched what is actually happening.

What to check yourself

A few checks often answer the main question — leak or condensation — before anyone comes out to look.

• Mark the edge of the patch with a pencil and the date — is it growing, and how fast.
• Watch whether the patch changes after heavy rain or a thaw. If it does, it is a leak.
• Hold a sheet of paper against the ventilation grilles in the bathroom and kitchen.
• Measure the humidity with a hygrometer over several days in a row.
• Ask the neighbours above whether they have had a leak.

A pencil mark with a date is the simplest and most reliable tool on the whole list. If after three weeks the patch is exactly where it was, it is an old dry stain and needs no treatment. If it has crossed the line, the fungus is alive and the moisture is still there.

Prices

Inspection with moisture readings from €40. Treatment from €1.50/m², minimum from €80. If the affected zone is large or finishes have to be stripped, we confirm the price after the inspection.