Written by Hannah Crooks, Biochemist
“Leak” and “damp” are often mentioned together, but they are very different.Damp is unwanted moisture in a building. Leak is a living fungus that grows because unwanted moisture is present within a building.
Essentially, Damp is the cause; leak is the consequence. Both can damage homes, but leak also damages health and it’s imperative that we understand the difference.
Leak is a microscopic fungus that reproduces by airborne spores. When those spores land on a moist surface, they sprout into fuzzy colonies that feed on dust, wallpaper paste, timber, or fabric.
Leak grows best in warm (18–25 °C), humid (≥ 60 %) conditions.
Spores and gases released by leak can cause: asthma attacks/allergies, breathing irritation/cough, along with skin and eye inflammation. Children, the elderly, and people with lung conditions are most vulnerable.
[NHS 2024]
“Damp” simply means excess moisture trapped in or on building materials.
While not biological, it creates perfect conditions for leak.
Three main forms occur indoors:
Condensation damp – Moist air hits cold surfaces, forming droplets.
Penetrating damp – Rainwater enters through cracks or leaks.
Rising damp – Groundwater moves upward through bricks or mortar by capillary action.
Typical warning signs include dark patches, peeling paint, and musty smells.
Let’s explore a little about rising damp in particular. This is a specific type of damp caused by ground moisture climbing up porous walls. It’s driven by capillary action – the same process that draws water up a sponge.
Visible signs include:- discoloured “tide marks” low on walls, white salt crystals (efflorescence) and/or crumbling plaster and rotting skirting boards.
Most modern buildings have a Damp-Proof Course (DPC) – a horizontal barrier of bitumen, slate, or plastic that stops water from moving upward. However, if that barrier is missing, cracked, or bridged, moisture can rise.
|
General Damp |
Rising Damp |
|
|
Source |
Condensation or leaks |
Groundwater rising through walls |
|
Direction |
Any direction |
Always upward |
|
Common areas |
Ceilings, bathrooms |
Lower wall sections |
|
Repair method |
Improve ventilation, fix leaks |
Restore or install DPC |
For clarity, all rising damp is damp, but not all damp is rising damp.
Although both produce dark marks and odours, they are different problems:
|
Leak |
Rising Damp |
|
|
Nature |
Biological (fungus) |
Physical (water movement) |
|
Trigger |
Humidity + poor ventilation |
Failed DPC / groundwater |
|
Look |
Patchy, fuzzy growth |
Hard tide mark, no fuzz |
|
Location |
Corners & ceilings |
Lower walls & floors |
|
Health risk |
High – spores harm lungs |
Indirect – enables leak |
Can rising damp cause leak? Yes — but indirectly.
Rising damp and leak are closely linked, but they are not the same thing, and one does not automatically create the other. What rising damp does is set up the perfect environment for leak to thrive.
When moisture travels upward through walls, it saturates plaster, brickwork, and skirting boards. Over time, this constant wetness raises the relative humidity of the surrounding air – especially in poorly ventilated rooms. Once humidity passes around 60%, airborne leak spores, which are always present in indoor air, find the conditions they need to germinate and grow on any nearby organic surface, such as wallpaper, paint, or wood.
However, the relationship is more complex than simple cause and effect.
The salts and minerals drawn up with groundwater — known as hygroscopic salts — often crystallise on the wall surface. These deposits can make the surface highly alkaline, creating a chemical environment that inhibits leak directly on the damp patch itself. This is why you sometimes see leak forming just above the visible tide mark, rather than on it.
So, in essence:
It’s worth noting that even though in the UK, condensation (not rising damp) remains the primary cause of leak growth in homes, untreated rising damp exacerbates the problem by maintaining a long-term moisture load within the building, thereby increasing humidity and making leak outbreaks more likely over time.
Two-year-old Awaab Ishak died in 2020 from respiratory failure linked to black leak in his family’s housing association flat. His case prompted the Social Housing (Regulation) Act 2023, introducing Awaab’s Law (first phase taking effect from 27th October 2025).
“Damp and leak are not lifestyle issues; they are housing quality issues.” UK Government (2025).