Rising damp is one of the most misunderstood and frequently misdiagnosed forms of dampness in UK homes. Often confused with condensation or penetrating damp, rising damp is a specific issue that occurs when moisture from the ground rises up through the walls of a building.
In this guide, we explore the key causes of rising damp, how it manifests, and what you can do to prevent it. Whether you live in an older property with no damp proof course (DPC) or you’re seeing strange stains on your walls, understanding what leads to rising damp is the first step toward fixing it for good.
What is Rising Damp?
Rising damp is a form of moisture ingress that starts from the ground and rises upwards through the capillaries of porous building materials like brick, stone, and mortar. The process is similar to how a sponge absorbs water from a surface — it’s known as capillary action.
Unlike condensation (which comes from internal humidity) or penetrating damp (which comes from external leaks), rising damp comes from the ground up. It usually affects walls up to one metre above ground level, but can go higher in extreme cases.
What Causes Rising Damp?
Rising damp is caused when moisture from the ground travels up through porous building materials like brick, mortar, or stone due to capillary action. It usually occurs when there’s no damp-proof course (DPC) in place, the DPC has failed, or it has been bridged.
What does “bridging” mean?
An example of this – by raised garden soil or plaster overlapping the DPC line. Other contributing factors include high groundwater levels, poor drainage, and walls made from absorbent materials. Even in homes with a DPC, rising damp can still occur if external ground levels are too high or internal changes allow moisture to bypass the protective barrier.
Common Causes of Bridging
- Raised External Ground Levels: Soil, paving, or patios that sit higher than the DPC can allow moisture to bypass the barrier.
- Internal Plaster Extending Below DPC: Plaster that goes below the DPC can act as a conduit for moisture.
- Debris in Cavity Walls: Rubble or mortar droppings can create a bridge between the inner and outer walls, facilitating moisture transfer.
- Insulation or Render Over DPC Line: Applying insulation or render over the DPC can negate its effectiveness.
The image illustrates bridging causes for rising damp. The DPC is still physically there, but the moisture “bridges” around it, making it ineffective — that’s the key idea.
Why Is Rising Damp a Problem?
Rising damp can damage both the structure and health of your home:
- Causes plaster to crumble and walls to stain
- Encourages mould growth and fungal decay
- Leads to poor indoor air quality
- Can trigger respiratory problems and allergies
- Reduces the energy efficiency of a building by keeping walls cold and damp
How long does rising damp take to show?
Here’s a visual chart illustrating how rising damp may progress over time. In early months, it’s often mistaken for condensation, only becoming clearly visible after a year or more in some cases.
Now let’s dive into the core causes of rising damp in detail.
1. Missing or Faulty Damp-Proof Course (DPC)
A Damp-Proof Course (DPC) is the main line of defence against rising damp. It’s a horizontal waterproof barrier — typically made of plastic, bitumen, slate, or other impervious materials — that’s installed near the base of walls to stop moisture from rising.
Why It Fails:
- In older properties (pre-1875), there may be no DPC at all.
- In newer homes, DPCs can fail due to age, poor installation, or physical damage.
- Slated DPCs can crack or shift over time due to ground movement or settlement.
Signs of a Failed DPC:
- Damp patches appearing above skirting boards
- Peeling wallpaper or blistering paint
- Salt deposits (efflorescence) on internal walls
2. Bridged DPC: An Overlooked But Common Cause
Even when a DPC is present and intact, it can be rendered useless if it’s been “bridged” — meaning moisture bypasses it by finding an alternate route.
What Causes Bridging?
- Raised external ground levels (e.g. garden soil, paving, or patio that sits higher than the DPC)
- Internal plaster that extends below the DPC and acts as a moisture bridge
- Rubble or debris in cavity walls that connects inner and outer walls
- Insulation or render applied over the DPC line
Bridging often goes unnoticed because it doesn’t affect the DPC itself — just renders it ineffective. A damp surveyor can usually detect bridging issues using moisture meters and thermal imaging.
3. Porous Building Materials: The Role of Wall Composition
Not all walls are created equal. Some materials are naturally more prone to absorbing and transmitting moisture.
Materials That Contribute to Rising Damp:
- Common bricks with high porosity
- Lime mortar, often found in period properties
- Soft sandstone or chalk-based masonry
These materials act like straws, sucking up water from the ground. If the walls lack a barrier or are improperly sealed, they’ll continue absorbing moisture indefinitely.
4. High Water Table or Saturated Ground Conditions
In some areas — especially those close to rivers or flood plains — the groundwater level (also known as the water table) is naturally higher. This puts more pressure on the base of buildings and increases the likelihood of rising damp.
Contributing Factors:
- Heavy clay soils that retain water
- Poor drainage systems
- Frequent rainfall or seasonal flooding
Even a functioning DPC can struggle in such environments without additional protective measures like drainage channels, french drains, or sump pumps.
5. Poor Ventilation & Internal Conditions
Although rising damp originates from the ground, poor indoor conditions can amplify its effects.
Why Ventilation Matters:
- Poor airflow traps moisture inside the property
- Humidity from cooking, washing, and breathing builds up
- Damp conditions make it harder for walls to dry, especially at lower levels
When ventilation is poor, any existing moisture (whether from rising damp or another source) will persist longer, increasing the chances of mould, rot, and odours.
6. Damage or Alterations to External Walls
External walls act as the first line of defence. If they’ve been modified, rendered incorrectly, or poorly maintained, they can allow ground moisture to bypass existing barriers.
Examples Include:
- Exterior render applied over the DPC line
- Cracked render or pointing that lets water into the wall structure
- Non-breathable paints or sealants that trap moisture inside walls
- Retrofitted insulation that’s poorly installed
7. Salt Contamination: The Invisible Enemy
One lesser-known cause of persistent dampness — even after moisture is removed — is salt contamination. As groundwater travels up through masonry, it carries hygroscopic salts (salts that attract moisture from the air). When this moisture evaporates, the salts remain and continue to pull moisture from the atmosphere.
Common Salts Found:
- Chlorides
- Nitrates
- Sulfates
These salts can damage plaster, paint, and internal finishes. If not removed or neutralised properly, they will cause the damp problem to reappear, even after DPC treatments.
8. Lack of Damp-Proof Membrane (DPM) in Floors
Rising damp can also travel upward through the floor slab. In properties without a damp-proof membrane (DPM) beneath concrete or stone floors, moisture can rise and eventually affect skirting boards, walls, and floor coverings.
Tell-Tale Signs:
- Cold, damp floors
- Curling floor tiles or lifting carpets
- Dark patches around the floor perimeter
Can Rising Damp Be Prevented?
Yes — but prevention starts with early identification and targeted action. Here’s what you can do:
Best Practices for Prevention:
- Ensure external ground levels are 150mm below internal floors
- Keep wall vents and air bricks clear of obstruction
- Don’t stack soil or decking up against walls
- Use breathable finishes like lime plaster and mineral paints
- Regularly inspect pointing, render, and guttering for water ingress
FAQ
What is the most common cause of rising damp?
The most common cause is a failed or missing DPC, followed by bridged DPCs due to raised external ground levels or internal alterations.
Can you permanently fix rising damp?
Yes, with the right diagnosis and treatment. This often includes installing or repairing the DPC, removing contaminated plaster, and improving ventilation.
Is rising damp dangerous?
Rising damp can lead to timber decay, mould spores, and respiratory problems, especially in vulnerable individuals. It also damages structural elements over time.
How long does rising damp take to show?
Rising damp may take months or even years to become visible, depending on the rate of water uptake and environmental conditions. It’s often misdiagnosed as condensation in its early stages.
Final Thoughts: Catch It Early, Treat It Properly
Rising damp is not something to ignore. While it may start subtly, the long-term effects can be both financially and structurally devastating. The key lies in recognising the signs, understanding the root causes, and working with experienced professionals for long-term prevention.
If you’re unsure whether you’re dealing with rising damp or another issue like condensation or penetrating damp, always seek a professional damp survey before jumping into costly repairs.
Need Help with Rising Damp?
At Damp and Mould Solutions Ltd, we provide detailed damp surveys and rising damp treatments across London and surrounding boroughs. With over 15 years of experience, we’ll find the cause and help you fix it for good — no guesswork.
👉 Call us today for a free consultation or book your damp survey online.