Why damp deserves your full attention

Damp and mould are among the most common problems in British homes, and they are rarely just a cosmetic inconvenience. Left alone, mould can damage plaster, rot timber, stain soft furnishings and aggravate asthma and allergies, particularly in children and older people. For buyers, it can also mean expensive remedial work, and for renters it can mean a long and frustrating battle with a landlord.

The good news is that damp usually announces itself long before it becomes a serious structural problem. If you know what to look for during a viewing, you can spot the warning signs in minutes and ask the right questions before you commit to anything. Here is how to do it properly.

The tell-tale signs to look for

Start with your eyes, then use your nose. Damp has a distinctive set of clues, and they tend to cluster in the same places: corners, external walls, ceilings below bathrooms, and anywhere two walls meet.

• Peeling or bubbling wallpaper and paint. If paper lifts away from the wall or paint flakes in patches, there is moisture behind it. Run your hand along the skirting board — if it feels soft, crumbly or cold and damp, that is a red flag.
• Dark patches and tide marks. Look for brownish staining around windows, along the top of walls beneath the ceiling, and behind radiators. A visible "tidemark" often shows how high water has crept.
• Black speckling in corners. Small dark dots in the corners of a room, behind furniture or along the sealant in a bathroom usually mean condensation mould rather than something structural — but it still needs fixing.
• A musty, earthy smell. Sometimes this is the only clue. If a room smells fusty, ask yourself whether it has just been aired, or whether something is being covered up.
• White fluffy salts on plaster. This is efflorescence, a sign that moisture is pushing through the wall and drying on the surface.
• Warped skirting boards, window frames or floorboards. Timber swells when it takes on water, so gaps, bulges and springy floors are worth investigating.
• Fresh paint in one small area. A newly painted patch in an otherwise tired room can be a quick cover-up. Look closely for a difference in shade or texture.

Understanding which type of damp you are dealing with

Not all damp is the same, and the fix — and the cost — depends entirely on the cause. In practice you will come across three main types.

• Condensation damp is by far the most common. It happens when warm, moist air from cooking, showering and drying laundry meets a cold surface such as a single-glazed window or an uninsulated external wall. It leaves black mould, misted windows and damp patches in corners. It is usually solved with better ventilation, insulation and heating habits rather than builders.
• Penetrating damp comes from outside — a cracked render, a missing roof tile, a blocked gutter or a leaking downpipe. It tends to appear as a localised patch on an external wall or ceiling, and it often gets worse after heavy rain. It needs work on the building fabric.
• Rising damp travels up from the ground through a failed or bridged damp-proof course. It usually shows itself as a low-level stain up to about a metre high, often with white salts, and it is the least common of the three. Treat any claim that a property has "rising damp" with polite scepticism until a proper survey confirms it.

Check the ventilation and heating

Once you have spotted signs of damp, look for the reasons behind it. Poor ventilation is the single biggest cause of mould in modern, well-sealed homes.

Open the windows — do they actually open? Check that trickle vents above the frames are present and not painted shut. In the kitchen and bathroom, look for a working extractor fan, and ask when it was last serviced. A fan that wheezes or does nothing at all is a problem waiting to happen. If the property has a tumble dryer, check whether it vents outside or is a condensing model.

Heating matters too. A home that has been left unheated for months, or one where the heating is only switched on for short bursts, is far more prone to condensation. Ask how the property has been occupied — an empty house, or one lived in by several people who rarely open windows, tells you a great deal.

Look outside as well as in

Step outside and walk around the property, ideally in the rain if you can. Gutters overflowing, cracked render, damaged pointing, missing roof tiles and leaking downpipes all push water into walls. Check that the ground outside is lower than the internal floor level, since a raised patio or driveway can bridge a damp-proof course. Blocked air bricks are another common culprit — they are designed to ventilate the void beneath a suspended floor, and painting them over traps moisture inside.

Questions to ask and steps to take before you commit

Ask the seller or letting agent directly whether the property has ever had damp or mould, and whether any treatment has been carried out. Request receipts or guarantees if a damp-proof course or timber treatment has been done. If the answer is vague or defensive, take note.

For a purchase, commission a full building survey rather than relying on a mortgage valuation, which is for the lender's benefit, not yours. A qualified surveyor can distinguish between surface condensation and something more serious, and give you a realistic figure for putting it right. For renters, report any mould in writing as soon as you spot it, keep dated photographs, and remember that your landlord is legally responsible for repairing damp caused by defects in the property.

Damp is not automatically a reason to walk away. Many cases are simple to fix with a new extractor fan, a repaired gutter or a bit of discipline around ventilation. But you deserve to know what you are taking on before you sign, so look carefully, ask the awkward questions and get expert eyes on anything that worries you.