Why are My Walls Wet After it Rains?

If walls get wet after rain, it’s usually penetrating damp. This usually means that water may be entering via gutters, render, pointing or window seals. If patches appear regardless of the weather, check for condensation or rising damp. Start with quick external checks (gutters/downpipes, cracks, sills), then book a survey if patches persist.

Damp in the home or business premises can make living and working conditions uncomfortable, as well as potentially unsafe if the root cause is left untreated. There are many reasons why you might be experiencing damp in your property. For example, rising damp in your London home can occur from a missing or damaged DPC (Damp Proof Course). However, if you have noticed that your walls are wet on the inside after a bout of heavy rain, you may well be suffering from an issue known as penetrating damp. Start outside by looking for overflowing gutters/downpipes, hairline cracks in render, gaps in pointing, and tired window seals/sill drip grooves nearest the patch. These small defects commonly drive wet patches after rain.

If the damp patch appears or worsens within 24-48 hours of rainfall, penetrating damp is the likely cause. If it appears even in dry spells, compare signs of condensation (black mould in cold corners) and rising damp (low ‘tide mark’ and salts near skirtings).

What is Penetrating Damp?

Penetrating damp is the name given to the movement of water from exterior facades or roof defects to interior surfaces. It can occur after particularly heavy rainfall in properties with solid brick construction, properties without an insulating cavity, or if the cavity has been incorrectly filled. If a cavity has been filled with inappropriate material, it can soak up and hold a lot of moisture which slowly permeates into interior walls, causing wet patches on the wall after rain.

This type of damp can also be caused by leaky gutters that are broken, wrongly installed or incorrectly sized for the property, which causes water to be directed to the wall instead of away from your home. Similar problems can occur from cracked or broken rendering and brickwork, or if windows have been installed badly or if the sill has been damaged, directing water onto the wall below instead of funnelling it away from the house, resulting in a wet wall.

Basement walls that are earth retaining commonly suffer from penetrating damp as they are subjected to changes in water table levels that occasionally occur.

When is Penetrating Damp Most Common?

Penetrating damp is likely to occur on exterior walls where the ground level is above the interior walls, creating a “bridging” effect for moisture to avoid the DPC and easily travel from exterior to interior surfaces. Faulty brickwork could be the underlying reason for any walls above ground level to be wet with damp as a result of defects in the construction or damage caused by natural causes such as extreme weather (such as from frost and/or storms).

It is worth noting that even the tiniest crack or crevice in a porous wall can invite large quantities of moisture/water and cause extensive damage. This can make it exceptionally hard to identify the cause of penetrating damp to the untrained eye which is why it is always recommended you hire a professional to take a look around if you suspect you have a damp issue of any kind.

What are the Signs of Penetrating Damp?

If you suspect that your property may be suffering from penetrating damp, you are likely to see the following signs around your property:

• Wet patches on walls that grow in size after rain
• Damp patches that occur on any part of the wall (high or low)
• Peeling or blistering of wallpaper or paint
• Discoloured paintwork or wallpaper
• Crumbling or blistering of plaster
• Black mould from condensation appearing on walls cooled by water ingress

These symptoms may also be reminiscent of other types of damp, and that can make it hard to distinguish between them. Regardless of what type of damp you have, be it penetrating damp or rising damp, the risks to your home and your health are much the same.

Get a Survey today

What Happens if Left Untreated?

Without solving the source of penetrating damp, whether caused by defective cavity insulation, leaking gutters or broken brickwork, the problem will continue to persist and can lead to some severe problems around your property, including:

• Wet rot
• Dry rot
• Structural problems
• Mould growth
• Salt deposits (which cause increased moisture ingress)
• Timber decay

All of these issues can be costly to fix and can cause real damage to the foundation and structural integrity of your home, not to mention your health. Mould growth can greatly irritate asthma sufferers and those with lung conditions, as well as those with skin conditions such as eczema.

If left long enough, damp could make your house unsafe to live in, which is why it’s essential at the first sign of damp walls you employ professionals to identify and treat the problem at its cause.

Current UK guidance advises acting quickly on damp/mould for health and safety. Don’t ignore recurring patches!

Preventing Damp

Whilst penetrating damp is almost always the result of a defect outside of your control, it’s still important to minimise the risk of any other form of damp impeding on your home. There are several easy ways you can reduce the risk of excess moisture causing issues for you inside your home, including:

• Regularly opening windows in all rooms in the house. This will allow clean air to circulate, damp air to dissipate and improve the overall quality of the air in your home. At least an hour a day is ideal, even in winter.
• Installing an extractor fan in the bathroom and kitchen. In addition to opening windows in these rooms, it is wise to install an extractor fan to aid the eradication of excess steam and moisture in the air. Doing so could greatly reduce the risk of condensation which is common in these rooms.
• Placing furniture away from radiators. By leaving a gap (where possible) warm air will be able to move freely around the room. Not only will these benefit you in terms of heating up the room better, but it will prevent hot air from getting trapped between the furniture and radiator which could and often does cause damp.
• Buying a dehumidifier. If your home or rooms within it have poor ventilation, buying a dehumidifier will absorb moisture molecules in the air and prevent them from settling on the wall or ceiling (which causes damp). This is especially common in basements and wet rooms that don’t have a window.

Cleaning and maintaining gutters and drains to prevent spilling and subsequent water ingress.

Related guides to help you

Still unsure what’s behind the damp, or what to do next? The articles below cover the most common situations and the steps we recommend before and after a survey:

• Wet walls: causes, prevention & solutions
• Damp walls: causes, solutions & prevention
• Wet walls in bedrooms: causes & fixes
• How to stop water from penetrating walls

FAQs

Why is my wall wet inside after rain?

Usually, because rainwater is getting in, most often via gutters, render, pointing, window seals or sills. A few small maintenance jobs can stop it; if patches keep returning, book a survey with Garratts Damp & Timber so that we can trace the source.

How do I tell condensation from penetrating damp?

Condensation shows up in cold corners or behind furniture and isn’t tied to weather, often with small black spots. Penetrating damp flares after rain and sits on outside walls near defects. Track patch size against rainfall for a week.

Is it harmful to leave wet walls?

Moisture encourages mould and can damage plaster and timber. Treat small areas safely, improve ventilation/extraction, and, most importantly, fix the moisture source so it doesn’t come back.

What should I try first?

Clear and realign gutters/downpipes, reseal frames, repoint gaps, repair render cracks, and check sill drip grooves. These quick fixes often solve the issue before redecorating.

When should I call a damp surveyor?

If patches persist after maintenance, spread quickly or keep returning, call us for a professional damp survey where our experienced team will test and confirm the exact cause (including hidden plumbing) and provide you with a cost-effective, written plan that is tailored for you.

Contact Us

Unless there is an obvious source of moisture, property owners can often misdiagnose the type of damp that is affecting their structure, which can lead to costly and inefficient DIY repairs. If you are concerned about penetrating or rising damp in your London-based property, please don't hesitate to give our expert and friendly team a call on 0208 535 7536 for a survey.