The short answer
Yes. A roof leak that is left soaks insulation, timbers and plaster, and damp materials give mould somewhere to grow. It often starts out of sight in the loft, and a musty smell or a dark patch on a ceiling can be the first sign. Mending the roof stops new water, but the wet materials still need drying or replacing, and the loft needs ventilation to stay dry afterwards.
A repaired roof is only half the job once water has soaked in, because the damp left behind has to go somewhere.
How a leak turns into mould
Mould needs moisture, and a leaking roof supplies it. Water gets in at a displaced tile or a worn flashing, the metal seal against a wall, wets the underlay, the layer beneath the tiles, and runs down the rafters onto the insulation across the loft floor. Mineral wool holds water and stays wet for a long time, and a ceiling beneath it can soak through to the plaster.
Where does it show first?
The loft is where mould tends to begin, and few people ever look there. Check the underside of the roof for dark spotting on the timber and the underlay, and for insulation that looks flattened, stained or heavy. Inside the house, look for a brown ring on the ceiling, paint that blisters or flakes, and wallpaper that lifts at a corner near the outside wall.
A musty smell in a bedroom or landing with no visible mark is worth following up. It can mean a slow leak, or it can point to condensation, which is the same problem with a different cause. Our answer on finding the source of a roof leak helps you separate the two.
What happens to damp insulation and timbers
Wet insulation does not work as it should, and it holds moisture against the timber around it. Where a leak was short and has dried, the material can sometimes stay. Where it has been soaked more than once, or smells and shows dark growth, it is usually better replaced than saved.
Can wet roof timbers be saved?
It depends on how long they were wet and how deep the damage goes. Battens, the narrow timbers the tiles sit on, are slim, and long soaking can soften them and loosen the nails. Rafters and ceiling joists are larger and often dry out without harm if the source is stopped. A roofer can see which is which on the survey, press the timber with a tool, and show you photographs.
If there are signs of wood rot, a soft, crumbling or cracked timber surface, bring in a timber and damp specialist before the area is closed up again. Do not paint over the stain or fit new insulation on top of wet timber, because the moisture is then sealed in and the problem can become hidden for years.
Drying out after the repair
Once the roof has been made watertight, drying out begins. Open the loft hatch for a spell on dry days, and let air move through. In a loft with eaves ventilation, check that insulation has not been pushed into the gap at the eaves, the lower edge of the roof, because that blocks the air flow the roof needs.
How long does a ceiling take to dry?
There is no safe rule of thumb, because it depends on how wet the plaster got, the season and the heating. A ceiling should feel dry to the touch and show no dark patches before it is repainted. A stain-blocking primer helps hide the old mark once the plaster is truly dry. Painting early traps moisture and invites the stain to come back through.
Dehumidifiers and fans can help in the room below, but they do not reach a soaked loft floor. If the damp was widespread, ask the roofer to look at ventilation while the roof is open. Under BS 5250, the width of the eaves gap depends on the pitch and the type of underlay, so ask the roofer to check it while the roof is open. Without the air flow, a repaired roof can still grow mould from condensation.
What to do while you wait for the repair
If water is still coming in, the first aim is to stop it feeding the damp. Where water is still coming in, a temporary cover or patch goes on first, and the permanent work is planned after.
Is it safe to stay in a room with mould?
Small patches on a painted surface can be wiped clean with a damp cloth and mild detergent, but do not scrub dry mould into the air. Keep children and anyone with breathing problems away from the area, and open the window for ventilation. If the growth is spread across a large part of a wall or ceiling, or someone in the house is unwell, get advice from a doctor and from a specialist before disturbing it.
Move furniture away from the wet ceiling, lift rugs, and photograph everything before cleaning, because the insurer will ask what the damage looked like before cleaning. Our answer on home insurance and a leaking roof covers what to keep and who to tell first.
A photo of any dark patch, plus one of the roof from the ground, is enough to start. A survey follows, and it comes with photographs of your slopes and one total in writing; the emergency roof repairs page explains the steps.
More on damp and mould
Is black mould on a ceiling always from the roof?
No. Mould on a ceiling can also come from condensation, a leaking pipe or poor ventilation. A roof leak usually follows the rain, while condensation follows cold, still weather and steamy rooms. A roofer who looks in the loft can usually tell which one you have from the pattern of staining.
Do I have to replace all the loft insulation after a leak?
Not always. Insulation that was wetted once, has dried fully and shows no staining or smell can sometimes stay. Insulation that is soaked, flattened, discoloured or smelly is usually replaced. The roofer can show you the condition with photographs from the loft, so the decision comes from what the loft shows.
The full picture: Emergency and general roof repairs in Taunton ยท More short answers