Roof Leaks, Damp and Mould: How One Leads to the Other

How does a roof leak cause damp?

The government guidance calls this penetrating damp: "water that gets into the building from outside due to defects in the walls, roofs, windows or floors." On a roof, the usual routes are slipped or missing tiles and slates, cracked or lifted flashing around chimneys and walls, failed valleys, and worn flat roof coverings.

Its list of outside checks for the source of damp includes "damaged or blocked pipes, gutters or downpipes" and "missing or broken roof finishes". Overflowing gutters soak the top of the wall below them, so a gutter problem can look like a roof leak from inside.

Water that gets past the roof covering soaks into the underlay, battens, rafters and loft insulation before it shows on a ceiling. By the time you see a stain, the timber and insulation above it may have been wet for some time.

Why does damp lead to mould?

The guidance is direct: "Water-damaged or damp materials are prone to the development of mould." Plasterboard, timber, wallpaper and insulation that stay wet give mould what it needs to grow.

It also warns that a smell of mould without anything visible "may indicate that there is mould behind a surface", such as the back of wallpaper, panelling or ceiling tiles. A musty smell in a top-floor room after a wet spell is worth investigating even if the ceiling looks clean.

National Mould Experts has a plain guide to the different causes of mould in a home, which is useful if you are not sure whether water from outside is the cause.

Is it a leak or condensation?

Condensation is the other common cause. The guidance describes it as "one of the most common causes of damp and mould in homes", caused by moisture from everyday things like showering, drying laundry and cooking building up in the air. It also says there may be more than one cause at once: water getting in from outside can make condensation worse.

Some practical clues:

• Points to a leak: a patch that appears or grows after rain or wind-driven rain, sits below a chimney, valley, roof window or junction with a wall, or has a tide-mark edge; wet timber or insulation directly above it in the loft.
• Points to condensation: mould on cold surfaces such as window reveals, the corners of outside walls and behind furniture, worse in winter and in kitchens, bathrooms and bedrooms, whatever the weather outside.
• In the loft: droplets on the underside of the roof or rusting nail tips across a wide area can be condensation from poor ventilation rather than a single leak. The guidance asks whether "roof and under floor spaces [are] properly ventilated to ensure timber remains dry".

The guidance also warns against assuming you can diagnose it yourself in every case. If you can't find an obvious source, a roof inspection can rule the roof in or out.

What should you fix first?

1. Stop the water. The guidance says repairing the source of water getting in, including "defective roofs" and "blocked gutters", reduces the risk of damp and mould and avoids "more expensive repairs later (such as rot treatment)". For active leaks, see emergency roof repairs; for the rest, roof repairs and guttering.
2. Dry it out, or replace it. Wet materials that "cannot be cleaned or completely dried" should be removed, and the surrounding area dried. Soaked loft insulation and stained ceiling plasterboard often fall into this category.
3. Then treat the mould. On hard surfaces, the guidance suggests a mould and mildew product rather than bleach, and a mask, gloves and goggles whoever does the cleaning. Soft materials such as carpets may need throwing away. It says larger areas should be tackled by professionals.
4. Check it stays dry. Look again after the next heavy rain. If the patch returns, the repair hasn't found the source.

For small patches you plan to clean yourself, National Mould Experts covers safe DIY mould removal. Our roof repair cost guide gives typical prices for the roof side of the job.

Is mould from a roof leak harmful?

The government guidance says damp and mould mainly affect the airways and lungs, with effects including cough, wheeze and shortness of breath, and the development or worsening of conditions such as asthma. It says damp and mould "pose a risk to anyone's health" and should be dealt with urgently where someone has a lung or heart condition or a weakened immune system, and for pregnant women and children.

If you are worried about symptoms, speak to your GP or a pharmacist. National Mould Experts summarises who is most at risk from mould and why.

What if you rent your home?

The same guidance says that whatever the type of damp, "landlords are legally responsible for addressing damp and mould". Report a leak to your landlord or managing agent in writing, with dates and photos, and keep a copy.

Frequently asked questions