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Penetrating damp is a common but often misunderstood issue in homes and buildings across the UK. Unlike rising damp, which moves upwards through masonry from ground moisture, penetrating damp occurs when water travels horizontally through walls, roofs or other surfaces due to external defects. This form of damp can lead to structural deterioration, decorative damage and even health risks if left unchecked.

In this blog, we’ll explain how to identify the causes of penetrating damp, how to treat it effectively, and the common mistakes homeowners make when tackling the issue.

What causes penetrating damp?

Penetrating damp occurs when rainwater or moisture finds its way through the external fabric of a building. It typically affects walls, roofs and ceilings, and is more likely in older properties or those exposed to severe weather.

The most common causes include:

• Damaged or missing roof tiles: Broken, slipped or missing roof tiles allow rainwater to enter roof spaces, soak through ceilings, and spread through internal walls.
• Cracked render or brickwork: Holes, cracks, and gaps in the exterior finish give rainwater a direct path into the wall cavity or inner surface.
• Leaky or overflowing gutters and downpipes: Blocked or broken rainwater goods can cause water to overflow and run down the wall, penetrating the surface over time.
• Defective window or door seals: Poorly sealed frames allow water to seep through and collect around internal surfaces, especially during heavy rain.
• Porous masonry: Older bricks or stonework can absorb water, especially if the external wall has lost its protective render or pointing.
• Incorrect detailing or construction faults: Poorly designed balconies, flashing or roof junctions can trap or redirect water into the structure instead of away from it.

How to identify penetrating damp

Penetrating damp is often localised to specific areas and tends to get worse after rainfall. It differs from rising damp or condensation in terms of location and symptoms.

Look for these key signs:

• Damp patches on walls or ceilings, especially on external-facing walls
• Localised staining or discolouration that darkens after rain
• Dripping or damp around windows and doors, especially during storms
• Mould growth in specific spots, not necessarily near floors or ceilings
• Flaky or bubbling paint and plaster, especially mid-way up walls

One telltale clue: penetrating damp often appears higher than 1 metre from the ground and worsens during wet weather, while rising damp rarely exceeds that height.

How to treat penetrating damp

Treating penetrating damp involves stopping water from getting in and repairing the damage it caused. The specific treatment depends on the source, so an accurate diagnosis is essential before any repairs begin.

1. Repair external defects

This is the most important step—and often the one overlooked in favour of cosmetic fixes.

• Inspect and repair: Cracked render or bricks: Fill and seal any visible cracks with the correct materials (lime mortar for older properties, modern sealants for newer ones).
• Faulty pointing: Rake out and repoint damaged mortar joints to stop water ingress.
• Roof damage: Replace missing or broken tiles, repair flashing, and check underfelt for tears or displacement.
• Gutters and downpipes: Clear out blockages, replace broken sections, and realign gutters so water flows away properly.
• Window and door frames: Reseal or replace old or perished sealant around frames.

2. Improve external waterproofing

For persistent problems, especially in exposed locations, consider additional weatherproofing solutions:

• Apply breathable water-repellent coatings to external walls (e.g. silane/siloxane-based masonry creams).
• Re-render affected areas if the existing render is cracked or has come away from the wall.
• Install cavity trays or lead flashings where required (e.g. at junctions between roofs and walls).

These treatments must be breathable to avoid trapping moisture inside the wall.

3. Dry out and repair internal damage

Once the source of the water is eliminated, internal surfaces need time to dry before cosmetic repairs are made.
Steps to take:

• Allow several weeks to months for walls to dry naturally (speed this up with dehumidifiers and good ventilation).
• Remove damaged plaster and replace with breathable alternatives if required (e.g. lime plaster in period homes).
• Repaint only once the wall is fully dry using breathable paints.

What doesn’t work – common mistakes to avoid

Many people unknowingly make the issue worse by using the wrong methods or skipping critical steps.

1. Painting over damp areas: Applying a fresh coat of paint might seem like a quick fix, but it doesn’t treat the cause. Damp will return, often causing the paint to peel or bubble within weeks.
2. Using non-breathable materials: Rendering or repainting with waterproof but non-breathable products can trap moisture inside the wall, exacerbating the problem over time.
3. Ignoring the root cause: Fixing internal damage without addressing leaks, cracks, or external faults is pointless. The damp will return and likely spread further.
4. Relying on damp-proof paint alone: Damp-proof or anti-mould paints can help reduce surface issues but should only be used after the source of water ingress is fixed – not as a primary treatment.

Should you get professional help?

Yes – especially if the cause isn’t obvious, the damp has been present for some time, or your home is older or listed. A qualified surveyor or damp specialist will inspect both inside and outside the building, using moisture meters and visual checks to diagnose the issue correctly.

Conclusion: successful treatment starts with the source

Penetrating damp can be stubborn, but it’s usually straightforward to fix once the source of moisture is correctly identified and addressed. The golden rule? Always fix the cause first – whether that’s a cracked brick, leaking gutter, or damaged render. Internal repairs should only follow once the wall has dried and breathing space is restored.

By avoiding cosmetic quick fixes and focusing on the building’s fabric, you’ll not only stop damp in its tracks but also protect the health, comfort and long-term value of your property.