Damp Patches After Heavy Rain? Signs of Damp Coming Through and What to Do

When the wet weather sets in, it's common for homeowners to spot new damp patches on walls and ceilings. Sometimes it's harmless condensation, but often it's a sign that rain is finding its way into your home. Left alone, damp can ruin plaster and decorations, rot timber and lead to mould.

Here's how to spot the signs early, work out what type of damp you have, and know when to call in a professional.

Signs of Damp Coming Through

Keep an eye out for these warning signs, especially after heavy or prolonged rain:

• Dark or discoloured patches on walls or ceilings that appear or grow after rain
• Bubbling, flaking or blown plaster
• Peeling wallpaper or paint, particularly around windows, corners and skirting boards
• A musty, damp smell in a room
• Black mould spots in corners, behind furniture or around window frames
• Tide marks — brownish rings left as a damp patch dries out
• White, powdery salt deposits (efflorescence) on brick or plaster
• Cold, wet-feeling walls even when the heating is on
• Rotting or soft skirting boards, window frames or floorboards

Why Damp Appears in Wet Weather

There are three main types of damp, and wet weather can make all of them worse.

1. Penetrating Damp (the most common after rain)

Penetrating damp happens when water gets in from outside through a defect in the building. It usually shows up as patches that appear or spread after heavy rain and dry out in dry spells. Common causes include:

• Blocked or leaking gutters and downpipes overflowing down the wall
• Cracked or missing roof tiles and damaged flashing around chimneys
• Cracked render or crumbling pointing between bricks
• Failed seals around windows and doors
• Chimney problems — missing caps, damaged stacks or flaunching
• Porous brickwork, especially on walls facing the prevailing wind and rain

2. Rising Damp

Rising damp is groundwater soaking up through the walls from below. It usually appears as a tide mark up to around a metre above the floor, often with salt deposits. Wet weather raises ground moisture levels, so existing rising damp can get worse in winter. Common causes are a failed or missing damp-proof course (DPC), or garden soil and paving built up above the DPC level.

3. Condensation

Condensation isn't caused by rain getting in, but it's at its worst in cold, wet months when windows stay shut and washing is dried indoors. Warm, moist air hits cold walls and windows and turns to water, leading to black mould, steamed-up windows and damp corners, especially in bathrooms, kitchens and bedrooms.

Quick Checks You Can Do Yourself

Before calling anyone, a few simple checks can help pinpoint the problem:

1. Check your gutters and downpipes during or just after rain. Look for overflowing water, leaks at joints and blockages from leaves.
2. Look at the outside wall behind the damp patch. Check for cracked render, missing pointing or water staining.
3. Inspect the roof from the ground (binoculars help). Look for slipped or missing tiles and damaged flashing.
4. Check window and door seals for gaps or perished sealant.
5. Note when the patch appears. If it gets worse after rain, it's likely penetrating damp. If it's worst in the mornings and on cold days, it's more likely condensation.
6. Check ground levels outside. Soil, flower beds or paving should be at least 150mm below the damp-proof course.

Safety first: don't climb onto roofs or use ladders in wet or windy weather. Leave roof and high-level work to a professional.

How to Reduce Condensation in Wet Weather

If condensation is the culprit, small changes can make a big difference:

• Use extractor fans when cooking and showering, and leave them running for a while afterwards
• Open trickle vents on windows and air rooms daily, even briefly
• Dry washing outside or in a vented tumble dryer where possible
• Keep the heating at a steady, low level rather than short bursts
• Leave a gap between furniture and external walls
• Consider a dehumidifier for problem rooms

When to Call a Professional

Get a damp specialist or builder in if:

• Damp patches keep coming back or are spreading
• Plaster is crumbling or timber is soft or rotting
• You can see roof, chimney or brickwork damage
• Mould keeps returning despite good ventilation
• You suspect rising damp or a failed damp-proof course
• You're buying or selling a property and need a damp survey

A professional will find the actual source of the moisture, rather than just treating the symptoms. Be cautious of anyone recommending expensive chemical damp-proofing before they've checked simple causes like gutters, pointing and ground levels — many damp problems are fixed by sorting the leak at its source.

Don't Leave It Too Long

Damp rarely fixes itself, and wet weather will only make it worse. The sooner the cause is found, the cheaper and easier it is to put right. Catching a blocked gutter or cracked render early can save you from replastering, redecorating and timber repairs later on.

Need help with damp? Post your job for free on Compare My Builder and get quotes from verified local damp proofing specialists, roofers and builders. Compare profiles and reviews, and choose the right tradesperson for the job.