Can a Roof Leak Cause Mould?

How a leak turns into mould

Mould spores are in the air of every house. They do nothing until they land on a damp surface with a food source, such as paper facing on plasterboard, dust on timber, or the backing on insulation. A leak provides the damp. If the area stays wet or humid for a few days, the spores start to grow, and a visible patch can appear within a week or two. In a loft, which is cool, still and often poorly ventilated, mould can spread quietly for months before anyone looks.

The leak does not have to be a drip. A damp patch from a poor flashing, a slipped tile or a condensing cold bridge may only add small amounts of water, but it is enough to keep the timber and the board damp. That is why mould sometimes shows on a ceiling near an outside wall long before there is any sign of a drip.

Where it hides

The most obvious signs are dark spots on a ceiling, a musty smell in a bedroom or landing, and peeling paint or wallpaper. Behind those signs, the mould tends to be in these places:

• Rafters and roof timber near the leak, where dark patches and sometimes a white or grey fluff appear.
• Loft insulation, which goes dark and flat when wet and holds water against the timber.
• The back of plasterboard, where it does not show until the stain comes through.
• Corners and chimney breasts, which are cold and collect damp from a failed flashing or a dried-out mortar joint.

If you want to look for a source, do it from the loft hatch with a torch. Do not step off the joists. Our guide on finding and fixing a roof leak covers how leaks are traced.

Mould from a leak or from condensation?

Not all roof-space mould comes from a hole. Warm, moist air from the house rises into a cold loft and condenses on the underside of the roof, which leads to a similar growth over a larger area. A true leak tends to be localised, with a stain that is wetter and darker in one spot and often worse after rain. Condensation is spread more evenly and is often worse in cold weather. Many roofs have both, so we check the ventilation as well as the covering.

What to do first

Stop the water. Until the leak is fixed, nothing else will last. If it is raining and water is coming in, we can arrange a temporary cover, and emergency calls come first. Once the roof is watertight, the area has to dry. Open windows, use a dehumidifier in the room below, and lift or remove wet insulation in the loft where you can reach it safely. Wet insulation cannot be left to dry in place and is usually replaced.

If the mould is on the ceiling, avoid scrubbing it dry or spreading it around. For a small patch, wearing gloves and a mask, a damp cloth and a mild detergent is the usual approach, but stained or soft plasterboard should be cut out. If the growth is extensive, or someone in the house has asthma or another health condition, take advice from your GP or the environmental health team at your council about how to deal with it.

Preventing a repeat

Once the roof is sound, keep the loft dry. Leave the insulation unpacked at the eaves so air can flow in through the soffit vents, keep stored boxes off the floor, and make sure bathroom and kitchen extractor fans vent outside, not into the roof space. Clear gutters before autumn so water does not back up under the lower tiles. A look at the loft after every major storm costs nothing and catches wet patches early.

What we do on the roof

We survey the roof, photograph the fault, and tell you what the repair involves. Common fixes include replacing slipped tiles, renewing a flashing, resealing a valley, re-bedding or replacing ridge tiles, and clearing a gutter that has been overflowing into the wall. We look at the timbers in the loft too, and tell you what we see, so you know whether the damp has gone as far as the structure. Where the leak is part of a bigger problem, such as an old flat roof, we say so and give you the options. How we approach these jobs is set out under emergency and general roof repairs.

Insurers usually want evidence that damp is recent, so photograph stains and the loft before anything is removed.