penetrating damp on exterior brickwork

What are the Damp Patches on my Walls After It Rains?

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Key Takeaways:

  • Identifying Causes of Damp Patches: Damp patches on walls after heavy rain are a result of moisture caused by water ingress. Common causes include leaky pipes, damaged roofs, overflowing gutters, and poor building structure.
  • Effects of Rain on Buildings: Rain can impact buildings in various ways, contributing to damp patches. Factors such as cracks in walls, broken or leaky roof tiles, compromised foundation damp-proofing, poor insulation, and porous brickwork can allow rainwater to penetrate internal surfaces.
  • Causes and Consequences of Penetrating Damp: Penetrating damp occurs after heavy downpours, facilitated by winds that drive moisture through masonry or brickwork. Incorrectly filled cavities, leaky gutters, broken brickwork, and improper window installations contribute to this issue.

Have you ever noticed yellow streaks appearing or damp patches on the wall after heavy rain? It is a result of moisture present in your walls caused by water ingress. Damp can attack both your external and internal walls rendering them wet for several reasons.

Many factors cause enough moisture in the property, such as leaky pipes, damaged roofs, and overflowing gutters. The major damp problems in the properties occur due to rainwater penetration, rising damp, and condensation. 

>> Related Reading: Condensation in Loft – Damp Timbers | The Ultimate Guide

A great number of homeowners notice damp patches on walls after rain. It leads to the accumulation of water on the roofs of your building, which can penetrate your walls due to poor building structure.

Besides, a crack in your wall can cause damp or wet walls allowing water or moisture to get into the building from an external environment. 

>> Related Reading: Can Internal Wall Insulation Cause Damp? Know the Fact

How Does Rain Affect Buildings?

You observe damp patches shaping up if it has been raining. It may be due to one of the following causes:

  • Cracks in the walls
  • Broken or leaky tiles on the roof
  • The damp-proofing of the foundation has been affected due to rain. Puddles may also get formed by the foundation
  • Non-existent or poor-quality of the insulation material
  • Porous exterior brickwork
inside house with damp walls
an apartment with wet walls on which mildew is forming

What is penetrating damp?

Summers and winters in the UK bring in heavy downpours, sometimes along with winds that plunge moisture and rain through the masonry or brickwork. When the water from outside or a defective roof moves to internal surfaces, penetrating damp occurs.

It occurs specifically after heavy downpours in properties that lack an insulating cavity or the same have been filled incorrectly. When the cavity is filled with unsuitable material, it soaks and holds a huge amount of excess moisture which slowly seeps into the internal walls, causing damp patches on the wall after rain. 

Moreover, penetrating damp can also occur due to leaky gutters being wrongly installed, broken, or wrongly sized for the building, leading water towards the wall.

Similar issues can be caused due to broken or cracked brickwork when the windows are poorly installed or the sill is damaged, allowing the water to infiltrate into the building materials. 

Penetrating damp commonly occurs on the external walls where the façade ground levels are above the internal floors. It can also be caused when there is a wrong fall to the other side of the buildings.

The rainwater cannot avoid the walls efficiently due to these defects and ending, causing moisture to permeate. Such damp patches appear as silhouettes on the walls caused due to penetrating damp.

Damp patches on the wall after heavy rain also occurred in the basement wall that was earthed as they were subjected to water table level changes. 

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Where does penetrating damp come from?

Often, penetrating damp is identified after a heavy downpour, which is common during the winter season in the UK.

Also, heavy snowfall can have a similar result if the freezes on the roofs and outer wall of your building and then melts. Usually, the water permeates into the property from areas such as:

  • Burst pipes
  • Cracks in the brickwork
  • Defective plumbing
  • Leaking or broken gutters
  • Broken or slipped roof tiles
  • damaged house rendering

Commonly, penetrating damp occurs on outer walls where the ground level is above the interior walls. It creates an effect of bridging for moisture to keep away from the damp-proof course and travels easily from the external to internal surfaces.

Poor or defective brickwork could also be the reason for any type of walls that are above the ground level to be damp. The defects in the building process/construction or damages caused by extreme weather conditions also result in wet walls. 

Even a minute crevice or crack in a porous wall can introduce huge quantities of water and causes extensive damage.

Unfortunately, it can be exceptionally difficult to find the reason behind penetrating damp to the inexperienced eye.

That’s why it is always advised to consult a professional to survey your property if you have doubted your house has a damp problem of any type. 

Where does penetrating damp come from?

How to identify penetrating damp from rain?

Penetrating damp can attack from anywhere in the building, including roofs and inside walls. Rain penetration leaves signs like other damp that are spotted by property owners, which warn them of the damp issues. 

The main sign is the damp patches on the walls or ceilings that increase in size and become darker after it has rained heavily.

It is due to the amount of water that has penetrated the house. Damp often spreads in a circular pattern of watermarks. Moreover, when the upper plaster layer is wet or crumbling, penetrating damp is behind it. 

Other symptoms of penetrating damp are:

  • Rotting skirting boards
  • Peeling of wallpapers
  • Blistered plastering
  • Droplets trickling on the surface
  • Dark mould on inside walls

Such symptoms also reflect the signs of other types of damp, making it difficult to differentiate between them. Whatever kind of damp you have in your building, be it rising damp or penetrating damp, the dangers they invite to your house and the health of residents are the same.

They need to be addressed immediately to avoid further damage. 

inside of house with damp patches after rain

What if penetrating damp is left untreated?

The damp problem will exist without resolving the causes of penetrating damp, whether it occurs due to broken brickwork, leaking gutters, damaged cavity insulation, warm air, or cracked walls. As a result, it may lead to serious problems around your building. 

If penetrating damp is left untreated, it can cause deterioration to the fabrics along with damage to ceilings, walls, floors, and even timber. The penetrating damp can get worse after the rain. Besides, it causes:

  • Structural problems
  • Dry rot
  • Wet rot
  • Salt deposits
  • Mould growth
  • Timber decay

These issues can cause severe damage to the foundational integrity of the building, especially if it is on the exterior walls, and not to forget the health.

For example, mould or mildew growth can affect patients with respiratory illness, irritating asthma patients and people with other lung conditions. In addition, mould growth can also cause result in flaring up skin allergies like eczema. 

If left untreated for a longer period, penetrating damp can make your home quite unsafe to live in. That is why it is essential to call in damp specialists at the first indication of wet or damp walls to diagnose and treat the issue and its cause. 

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Get a no obligation FREE quote now. Our team is here to help. Take the first step towards a guaranteed solution by filling out our form below. 

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What if penetrating damp is left untreated

How to treat penetrating damp?

Penetrating damp is usually the outcome of structural defects. You need first to identify the source of damp if you want to treat the issue. With the following, you can treat moisture-causing damp problems on your property:

  • If the penetrating damp is caused due to failing door/window frames, roofing or guttering, these must be repaired and replaced with damaged timber and plasterwork.
  • If the water is infiltrating through the wall to the floor point, then lasting and effective remediation while treating penetrating damp is to inject the joint with hydrophilic polyurethane along with the length of the joint. 
How to treat penetrating damp

How to prevent penetrating damp?

If you drag solving the penetrating damp issue, then it gets quite harmful to your property. Here are some of the common ways to prevent penetrating damp:

  • Unclog downpipes
  • Clearing blocked guttering
  • Repair leaking roof
  • Replaced old aged bricks 
  • Repair walls regularly
  • Apply the waterproof solution to the masonry or bricks
  • Keeping the area clean

Besides, some remedial measures you need to take care of, especially for rainwater drainage. The damp patches on the wall after heavy rain increase in size, and so is the problem of penetrating damp. Good maintenance of your building or property is essential.

Simple tasks such as clearing rainwater pipes and clearing gutters can save you from expensive and extensive work later. Snow should be cleared from valley gutters and parapets to prevent the seeping of moisture through joints.

Improving the drainage system at the ground level helps in the reduction of penetration of the wall base. 

damp patches on walls


Penetration of rainwater into the properties is no surprise, given the climate of London and other areas of Southeast England.

However, property owners often tend to misdiagnose the penetration damp with other types of damp and go for DIY repairs, which often lead to inefficient and costly in the end. It adds to the damage as well. 

If you are looking for damp experts in London and the home counties, then don’t hesitate to call our experts for a quick survey of the property, followed by a recommendation for the treatment. 

Picture of Jake Fitzgerald - Damp Surveyor

Jake Fitzgerald - Damp Surveyor

Surveyor of Timber & Dampness in Buildings (CSTDB) & ( MRICS ) Chartered Quantity Surveyor

Jake is a qualified chartered quantity surveyor and experienced damp specialist with a unique skill set in thermal imaging. With a focus on effective solutions, Jake combines his expertise in quantity surveying and thermal imaging to accurately assess and address damp-related issues. He utilises advanced technology to identify hidden moisture sources and develop targeted strategies, ensuring comprehensive and cost-effective solutions for his clients. With a commitment to open communication and client satisfaction, Jake delivers cutting-edge solutions that tackle damp challenges head-on.

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