Quick Answer

Rising damp is caused when groundwater travels upward through porous building materials via capillary action. It occurs most commonly because of a failed, missing, or bridged damp-proof course (DPC). Contributing causes include high groundwater levels, porous masonry, poor drainage, salt contamination, and the absence of a damp-proof membrane (DPM) beneath floors. It typically affects walls up to 1 metre above ground level.

What Is Rising Damp?

Rising damp occurs when moisture from the ground travels upward through the porous materials of a building — brick, mortar, stone — by a process called capillary action. Think of it like a sponge absorbing water from a surface: the tiny pores in masonry act as tubes, drawing moisture upward from saturated ground.

Unlike condensation (which comes from internal humidity and appears on cold surfaces) or penetrating damp (which enters horizontally through walls or roofs), rising damp comes strictly from the ground up. This distinction matters because the wrong diagnosis leads to the wrong treatment.

⚠️ Commonly misdiagnosed: Rising damp is frequently confused with condensation, especially in early stages. The key difference is that rising damp typically does not appear on ceilings or upper walls — it stays low, usually below 1 metre, and leaves a horizontal tide mark of salt deposits.

Signs of Rising Damp

Before diagnosing the cause, confirm you’re actually dealing with rising damp. Look for these indicators:

〰️Tide mark / stainingHorizontal line on wall, usually 0.5–1m above floor
🧂White salt depositsEfflorescence — salts left as moisture evaporates
📄Peeling wallpaperAt low level, often above skirting board
🎨Blistering paintOn plaster at low wall level
💨Musty odourEarthy smell, especially at floor level
🪵Damaged skirting boardsRotting or staining at the base

8 Causes of Rising Damp — At a Glance

# Cause Description Risk
1Failed or missing DPCMost common cause — no barrier to stop upward moisture movementHigh
2Bridged DPCMoisture bypasses intact DPC via soil, plaster, rubble or renderHigh
3Porous building materialsOlder brick, lime mortar and sandstone absorb water readilyMedium
4High water tableElevated groundwater puts more pressure on wall basesHigh
5Poor drainageWaterlogged ground increases moisture available for uptakeMedium
6External wall damage or alterationsCracked render, non-breathable paint, or retrofitted insulationMedium
7Salt contaminationHygroscopic salts continue attracting moisture even after damp is treatedHigh
8Missing DPM in floorsNo damp-proof membrane under concrete slab allows floor damp into wallsMedium

1 Failed or Missing Damp-Proof Course (DPC)

A Damp-Proof Course (DPC) is the primary defence against rising damp. It’s a horizontal waterproof barrier — plastic, bitumen, or slate — installed near the base of walls, typically 150mm above external ground level, to stop moisture from rising further.

Why DPCs fail:

  • In properties built before 1875, there may be no DPC at all — it wasn’t a requirement
  • Slate DPCs in Victorian homes can crack or shift due to ground movement
  • Bitumen-based DPCs degrade over time, especially in older housing stock
  • Poorly installed DPCs may have gaps or have been incorrectly positioned
🔍 How we detect it: A calibrated moisture meter shows high readings at low wall level. Thermal imaging can also reveal the moisture profile behind plaster — without any damage to your walls.

2 Bridged DPC — The Overlooked Cause

A bridged DPC is one of the most commonly missed causes of rising damp. The DPC may be physically intact — but moisture bypasses it by finding an alternative pathway, rendering the barrier useless.

Common causes of DPC bridging:

  • Raised external ground levels — garden soil, paving slabs, or patios sitting above the DPC line are the most frequent cause in London properties
  • Internal plaster extending below the DPC — the plaster acts as a conduit, wicking moisture upward
  • Rubble or mortar debris in cavity walls — connects inner and outer leaves, allowing moisture transfer
  • Render or insulation applied over the DPC line — especially common after external wall insulation (EWI) installation
⚠️ Important: If external ground has been raised by landscaping, a new driveway, or a patio extension, bridging is the first thing we check. It’s an easy fix — but only if correctly diagnosed.

3 Porous Building Materials

Not all walls absorb moisture equally. Older UK properties — particularly Victorian and Edwardian terraces — are built from materials that are naturally more porous:

  • Soft common bricks — high porosity, absorb ground moisture readily
  • Lime mortar — breathable but absorptive; common in pre-1920s properties
  • Soft sandstone or chalk-based masonry — particularly susceptible in older rural or period buildings

These materials act like straws — the smaller the pores, the higher water can rise through capillary action. If no barrier exists or has failed, absorption continues indefinitely.

4 High Water Table or Saturated Ground

In areas with naturally elevated groundwater — near rivers, flood plains, or on heavy clay soils — the water table puts sustained pressure on the base of buildings. This increases both the volume of moisture available and the upward force driving it through masonry.

This is particularly relevant in parts of London built on London Clay or near the Thames and its tributaries. Even a functioning DPC can be overwhelmed without additional protection such as drainage channels, French drains, or sump pumps.

💡 London context: Many North and West London properties on clay-heavy ground suffer from seasonal rising damp that worsens after heavy rainfall — even in homes with relatively intact DPCs.

5 Poor Drainage Around the Property

Even where groundwater levels are not inherently high, inadequate drainage can create saturated ground conditions at the base of walls:

  • Blocked or poorly positioned gutters that overflow at wall base
  • Driveways or patios that slope towards the house rather than away
  • Blocked air bricks preventing underfloor ventilation
  • Compacted soil with no drainage channel around the building perimeter

Resolving drainage issues is often the most cost-effective first intervention — and is always assessed during a professional damp survey.

6 Damage or Alterations to External Walls

External walls that have been modified, incorrectly rendered, or poorly maintained can allow moisture to enter or trap it inside the wall structure:

  • Render applied over the DPC line — creates a direct bridge from ground to wall
  • Cracked render or failed pointing — allows rainwater penetration that compounds rising damp
  • Non-breathable masonry paint or sealants — trap moisture inside the wall, preventing natural evaporation
  • Retrofitted cavity wall insulation below DPC level — can compress or bridge the DPC
⚠️ Common mistake: Applying a waterproof render or sealant to a damp wall without treating the cause is the single most counterproductive DIY intervention we encounter. It traps moisture inside the wall, accelerating plaster damage and masonry decay.

7 Salt Contamination — The Invisible Problem

One of the most misunderstood aspects of rising damp is salt contamination. As groundwater travels up through masonry, it carries soluble salts — chlorides, nitrates, and sulphates. When the moisture evaporates at the wall surface, these salts remain behind.

The critical issue: these salts are hygroscopic — they continue drawing moisture from the atmosphere indefinitely. This means that even after the DPC is repaired and the moisture source is cut off, contaminated plaster will continue appearing damp.

💡 Why this matters for treatment: Contaminated plaster must be physically removed and replaced with salt-resistant renovation plaster. No amount of painting or surface treatment will permanently resolve salt-contaminated walls.

8 Missing Damp-Proof Membrane (DPM) in Floors

Rising damp can also enter through the floor slab, particularly in older properties with solid concrete or stone floors that were laid without a damp-proof membrane beneath them.

Moisture rising through the floor then migrates laterally into the base of walls — creating patterns that can look identical to wall-based rising damp. Signs include:

  • Cold, damp patches on the floor surface
  • Lifting floor tiles or curling carpet edges, especially near external walls
  • Dark staining or damp patches around the floor perimeter

Why Rising Damp Is a Problem

Left untreated, rising damp causes both structural and health damage:

  • Plaster crumbling and walls staining permanently
  • Mould growth — triggering respiratory problems, asthma, and allergies
  • Timber decay — floor joists, skirting boards, and door frames are particularly vulnerable
  • Reduced thermal insulation — damp walls conduct heat away from the building more efficiently, increasing heating bills
  • Structural masonry damage in severe or long-term cases

How to Prevent Rising Damp

  1. Ensure external ground levels sit at least 150mm below the DPC — check after any landscaping work
  2. Keep air bricks and wall vents clear at all times
  3. Ensure gutters and downpipes are directing water away from the building
  4. Don’t stack soil, compost, or raised beds directly against external walls
  5. Use breathable finishes — lime plaster, mineral paint — rather than non-breathable sealants
  6. Inspect pointing and render annually for cracks that could allow water ingress
  7. Book a professional damp survey if you’re seeing any early signs — early intervention is far cheaper than remediation

Related Guides

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    FAQ

    The most common cause is a failed, missing, or bridged damp-proof course (DPC). In properties built before 1875 there may be no DPC at all. In newer homes, the DPC is often rendered ineffective when external ground levels — soil, paving, patios — are raised above the DPC line, allowing moisture to bypass it entirely. This bridging is the most frequent finding in our London damp surveys.
    Rising damp on internal walls is most commonly caused by DPC bridging — where internal plaster extends below the DPC line and acts as a moisture conduit, or where debris in cavity walls creates a bridge between inner and outer leaves. It can also occur when there is no DPC on the internal wall itself, or when a solid concrete floor without a damp-proof membrane allows moisture to travel laterally into adjacent walls.
    Rising damp typically affects walls up to one metre above ground level. In extreme cases — very porous masonry, no DPC, or exceptionally high groundwater — it can rise higher. The maximum height depends on the rate of evaporation from the wall surface versus the rate of water uptake from the ground. If the evaporation rate is higher than the uptake rate, moisture stabilises at a lower level.
    Rising damp can take months to years to become clearly visible, depending on wall porosity, groundwater levels, and how long the DPC has been compromised. In early stages it is frequently mistaken for condensation. Key early signs include damp patches just above skirting boards, white salt deposits (efflorescence), and peeling wallpaper at low level.
    Yes — rising damp poses both health and structural risks. It encourages mould growth which can trigger respiratory problems, asthma, and allergies. Structurally, it causes plaster to crumble, promotes timber decay in floor joists and skirting boards, reduces insulation effectiveness, and over time can weaken masonry. Children, elderly people, and those with respiratory conditions are most vulnerable to the health effects.
    A bridged DPC occurs when material creates a pathway for moisture to bypass the damp-proof course. The DPC may be physically intact, but moisture finds an alternative route around it. Common causes: raised garden soil or paving above the DPC line; internal plaster extending below the DPC; rubble in cavity walls connecting inner and outer leaves; and render or insulation applied over the DPC. Even a perfect DPC provides no protection if it is bridged.
    Yes. Rising damp creates persistently damp wall conditions that are ideal for mould growth. Additionally, the hygroscopic salts left behind in contaminated plaster continue attracting atmospheric moisture even after the initial water source is addressed — sustaining mould growth at low wall and skirting board level. If you have rising damp, mould treatment alone will not solve the problem; the root cause must be addressed first.
    Yes. The most common treatment is a chemical DPC injection — damp-proofing cream injected into the mortar course at DPC level — followed by removal of salt-contaminated plaster and replastering with renovation plaster. External remediation (lowering ground levels, improving drainage) is essential in bridging cases. A professional damp survey is always the recommended first step to confirm the cause before any treatment begins.

    References & Sources

    1. BS 8102:2022 — Code of Practice for Protection of Below Ground Structures Against Water from the Ground
    2. BS 5628 — Code of Practice for Use of Masonry (Damp-Proof Courses)
    3. Building Regulations Approved Document C — Site Preparation and Resistance to Contaminants and Moisture (2022)
    4. PCA (Property Care Association) — Rising Damp Guidance for Surveyors
    5. RICS — Damp in Buildings Guidance Note (2022)
    6. BRE Digest 245 — Rising Damp in Walls