What Causes Damp in a House? A Room-by-Room Guide
Noticing damp is easy. Working out what’s causing it is harder. Most online advice gives you a list of damp types and leaves you to figure out the rest. But in our experience, the most useful starting point isn’t the type of damp, it’s where it’s appearing.
Location is one of the most reliable diagnostic clues there is. Damp in the corner of a bedroom ceiling tells a very different story to damp at the base of a ground floor wall. Damp in a bathroom is almost never the same problem as damp spreading across a living room’s external wall.
After 40 years and more than 20,000 properties surveyed annually across London and the South East, we’ve learned that where damp appears is usually the fastest route to understanding why. This guide works through the most common locations, what causes dampness in each, and what to do about it.
Damp on External Walls
What it looks like
Damp patches, tide marks, or dark staining on walls that face outside. Paint or wallpaper may be peeling. White powdery deposits, known as efflorescence, can appear on the surface as salts are carried out of the masonry by moisture. Externally, you may notice moss growth, spalling brickwork, or dark streaking below gutters and window sills.
What likely causes it
The two most common culprits are penetrating damp and rising damp.
Penetrating damp occurs when water enters through defects in the building fabric. Cracked render, failing mortar joints, damaged masonry, and blocked or leaking gutters are the most frequent entry points. Wind-driven rain is particularly effective at forcing water through even small defects in older brickwork.
Rising damp affects the lower section of external walls where a damp proof course has failed, is absent, or has been bridged. Common causes of bridging include raised ground levels, a patio or path built up against the wall, external render taken below the DPC line, or soil from a garden bed sitting against the masonry.
How to tell the difference
Rising damp produces a fairly consistent tidemark across the lower metre of a wall. Penetrating damp tends to be more localised, spreading outward from a specific point of entry, and worsens noticeably after heavy rain. If the damp patches grow and shrink with the weather, penetrating damp is the more likely cause.
What to do
Inspect the external envelope before considering any internal treatment. Gutters, downpipes, pointing, render condition, and ground levels should all be checked. If rising damp is suspected, a full damp survey including salt analysis of plaster samples will confirm whether a damp proof course is needed and whether the existing one has failed or been bypassed.
Damp on Internal Walls
What it looks like
Damp patches on walls that don’t face outside, often with no obvious external source. May be accompanied by a musty smell, peeling decoration, or black mould. The patches can appear seemingly out of nowhere and may not follow any obvious pattern.
What causes dampness on internal walls
Damp on internal walls most commonly comes from one of three sources: condensation, a plumbing leak, or lateral moisture movement from an adjacent wet area.
Condensation is particularly common on north-facing internal walls, where surface temperatures stay consistently lower than the rest of the room. Warm, moist air from cooking, bathing, and everyday activity condenses on these cooler surfaces and, over time, leads to mould growth and damp patches.
Plumbing leaks are frequently overlooked. A slow leak from a buried pipe, a poorly connected radiator valve, or a washing machine supply hose can produce persistent damp patches with no visible water source. These leaks can continue for months before causing visible damage.
Lateral moisture movement occurs when water migrates through a shared wall from an adjacent wet room, or when moisture tracks through a solid floor from a nearby area of rising damp.
What to do
Check for pipes running behind or beneath the affected wall. Consider whether there is a bathroom, kitchen, or utility room on the other side. If condensation is suspected, improving ventilation should come before any treatment. A professional survey using moisture meters, thermal imaging, and relative humidity readings will identify the source where it isn’t immediately clear.
Damp in the Corner of a Room
What it looks like
Dark staining or black mould concentrated in the upper corner of a room, often where two walls meet the ceiling. It frequently spreads outward in a fan shape. It’s one of the most commonly misidentified damp problems, often reported as penetrating damp or rising damp when the cause is something else entirely.
What causes dampness in the corner of a room
Corner damp is almost always condensation. Corners are the coldest parts of a room because two walls, and sometimes a ceiling, meet at that point, reducing the surface temperature further than elsewhere. Warm, moist indoor air condenses on these surfaces first.
A key contributing factor is cold bridging, where a gap or weakness in insulation at the wall or ceiling junction creates a particularly cold spot. This is common in properties built before modern insulation standards, which covers a large proportion of housing stock across London and the South East.
Corner damp is frequently worse in bedrooms, particularly in rooms that are kept cool or poorly heated, and in properties where curtains or furniture are positioned against external walls, restricting airflow across the surface.
What to do
Improving ventilation is the first step. Trickle vents, extractor fans, and positive input ventilation systems all help reduce indoor humidity. Improving insulation at the affected junction addresses the cold bridging directly. In more severe cases, an anti-condensation coating can help, but it is not a substitute for tackling the root cause. Treating mould without addressing the condensation that produced it is a short-term fix at best.
Damp in Bathrooms and Kitchens
What it looks like
Mould around window frames, on silicone seals, on ceilings, or behind furniture pushed against external walls. Peeling paint above windows, damp patches on walls adjacent to showers or baths, and surfaces that seem to stay permanently damp even in dry weather.
What causes dampness in bathrooms and kitchens
These are the highest moisture-generating rooms in any home. Steam from showers, baths, and cooking raises indoor humidity rapidly. If that moisture-laden air can’t escape efficiently, it condenses on the coldest available surfaces. Bathrooms without extractor fans, or with fans that vent into a roof void rather than outside, are particularly prone.
Failed tile grout and silicone seals are a specific and underappreciated cause of damp in these rooms. Water gets behind tiles and into the wall structure through gaps that may not be immediately visible. This can continue for a long time before the damage becomes apparent, and by the time it does, the plaster and sometimes the structural background behind it can be significantly affected.
Penetrating damp can also present in kitchens and bathrooms if there are defects in the external wall above or adjacent to the room, particularly around window frames, where the junction between frame and masonry can fail over time.
What to do
Ensure mechanical extraction is working properly and venting directly to outside. Check all silicone and grout seals annually and replace them when they show signs of cracking or shrinkage. If damp persists despite good ventilation and intact seals, investigate the external envelope for defects above and around the affected area.
Damp in Basements and Below Ground Level
What it looks like
Wet walls, pooling water on floors, white salt deposits on masonry, a persistently damp atmosphere, and a strong musty smell. In more severe cases, visible water tracking through wall joints or floor-to-wall junctions. Below-ground spaces in older properties often show a combination of symptoms from multiple sources simultaneously.
What causes it
Basements and below-ground spaces are subject to hydrostatic pressure — the weight of groundwater in the surrounding soil pushing against the structure. This is a fundamentally different problem to rising damp or condensation and requires different treatment.
The most common routes for water entry are failed or absent tanking, cracks in the structural walls or floor slab, and blocked or inadequate drainage channels around the perimeter. In London in particular, where clay soils retain water and ground conditions vary significantly across short distances, below-ground damp can be highly localised and unpredictable.
Condensation is also common in basements due to low surface temperatures and limited airflow. In many cases, both hydrostatic ingress and condensation are present at the same time, which makes accurate diagnosis essential. Treating one without identifying the other produces results that don’t last.
What to do
A basement damp survey should assess both the structural condition of the space and the relative humidity levels within it. Tanking systems, cavity drain membranes, and sump pump installations each suit different scenarios. There is no universal solution for below-ground damp, and the consequences of getting it wrong are costly. This is one area where cutting corners on the survey is a false economy.
A Note on Causes of Dampness in Walls Generally
Location is a powerful diagnostic tool, but it’s rarely the whole picture. The age of a property, its construction type, local ground conditions, how it’s heated, and how it’s used all affect where and why damp appears. A Victorian terrace in South London will present very differently to a 1970s semi in Kent, even with similar-looking symptoms.
The most common mistake is treating the symptom rather than the cause. Painting over mould, applying damp proof paint to a rising damp problem, or replastering without fixing the source of penetrating damp will produce results that last months, not years. An accurate diagnosis before any treatment is not optional — it’s the only thing that makes the treatment worthwhile.
Frequently Asked Questions
What causes dampness on internal walls?
The most common cause is condensation, particularly on north-facing walls or walls adjacent to high-humidity rooms. Plumbing leaks and lateral moisture movement from wet areas are also frequent causes. A professional survey is the most reliable way to identify the source.
What causes dampness in the corner of a room?
Corner damp is almost always condensation caused by cold bridging at wall junctions. Corners are the coldest points in a room and the first surfaces on which warm, moist air condenses. Improving ventilation and insulation at the affected junction is the correct course of action, not surface treatment alone.
Can damp on external walls be caused by something inside the house?
Rarely. Damp on external walls almost always has an external cause, whether that’s penetrating damp from a building defect or rising damp from the ground. Internal condensation occasionally affects external walls, particularly in cold weather, but is usually distinguishable by its pattern and behaviour across seasons.
If you’ve spotted damp somewhere in your home and want a straight answer about what’s causing it, book a survey with Kenwood. Our experienced surveyors cover London and the South East and will give you an honest diagnosis based on evidence, not assumptions.
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