Why is there damp coming through my wall?

What this usually means

Penetrating damp is the passage of rainwater through the external wall to the inside, and it happens where the wall's defence against water has failed somewhere above or around the damp patch. Water does not always enter directly behind the visible mark — it can run down inside the structure from a defect higher up, such as a leaking gutter, a cracked render panel, failed pointing, a defective sill, or a gap around a window — and emerge where it meets a path to the inside surface. This is why penetrating damp is essentially a detective problem: the internal patch tells you where the water is coming out, and the task is to follow the water back to where it is getting in.

It behaves differently from the other forms of damp, which is how it is identified. Penetrating damp usually correlates with weather — it appears or worsens during and after heavy or driving rain, and dries back in prolonged dry spells — and it is often higher up a wall or unrelated to ground level, unlike rising damp which is confined to the base of the wall. It is also distinct from condensation, which forms on the coldest surfaces from indoor humidity regardless of rain and concentrates in corners and behind furniture. Solid-wall and older properties are more prone to penetrating damp because they have no cavity to interrupt the water's path, and exposed elevations facing the prevailing wind and rain are the most affected.

Because the cause is an external defect, treating the inside achieves nothing lasting: replastering, sealing or painting over a penetrating-damp patch simply hides it until the next rain pushes water through again. The durable fix is to locate the specific defect letting water in — which may take inspection of the external fabric, the rainwater goods and the junctions, sometimes under wet conditions to see the water moving — repair it, and then allow the wall to dry out before making good inside. An investigation confirms it is penetrating damp rather than condensation or rising damp, finds the actual entry point rather than assuming, and ensures the repair addresses the source, so the patch does not return.

Common causes

Signs and symptoms

Penetrating damp is driven by liquid water under the action of rain and wind. Driving rain deposits water on the external face of a wall, and where the wall's water-shedding defences — render, pointing, flashings, sills, rainwater goods — are intact, that water runs off or is held in the outer fabric and dries back. Where a defect breaches those defences, water enters the structure and, under capillary and gravitational action, travels through and down the masonry until it reaches a path to the inner surface, where it evaporates and leaves the visible damp. Because the water can travel laterally and downward within the wall, the internal patch marks the exit, not necessarily the entry, which is why tracing is required rather than treating the patch's location.

The mechanism gives penetrating damp a distinct signature that separates it from the other forms. Its correlation with rainfall events, its tendency to occur away from ground level and on the most weather-exposed elevations, and its association with an identifiable external defect distinguish it from rising damp — capillary movement of ground water limited to the base of the wall with characteristic salts — and from condensation, which is governed by surface temperature and indoor humidity and occurs independently of rain. A handheld damp meter cannot make this distinction reliably, so diagnosis depends on the pattern of evidence: the timing relative to weather, the location, the external fabric inspection, and moisture measurement through the wall's depth where needed.

Solid and older walls are more vulnerable because they lack the cavity that, in modern construction, interrupts the water's path to the inner leaf; in a solid wall the masonry is a continuous bridge, so any breach can deliver water to the inside, and exposure to driving rain raises the load. The remedy follows from the physics: locate and repair the specific external defect so water can no longer enter, then allow the wetted fabric to dry — internal treatments such as waterproof renders or sealing the inside surface do not stop water entering and can trap it, worsening the fabric. An investigation that confirms penetrating damp, finds the actual entry point, and specifies the external repair is therefore what produces a lasting fix, in contrast to the recurring failure of treating the symptom on the inside.

How to stop damp coming through a wall

Confirm it is penetrating damp, trace the water back to the external defect letting it in, repair that defect, and allow the wall to dry before making good inside.

1. 01

   Confirm the cause

   Establish it is penetrating damp rather than condensation or rising damp, from the pattern and the weather correlation.

2. 02

   Trace the water to its entry

   Inspect the external fabric, junctions and rainwater goods above the patch to find where water gets in.

3. 03

   Repair the external defect

   Fix the failed pointing, render, flashing, gutter or sill responsible for the ingress.

4. 04

   Let the wall dry out

   Allow the wetted fabric to dry before redecorating, rather than sealing moisture in.

5. 05

   Avoid trapping moisture

   Do not apply waterproof internal coatings that stop the wall drying and can worsen it.

6. 06

   Make good inside

   Repair the internal finish once the source is fixed and the wall has dried.

How to prevent it coming back

• Maintain pointing, render, flashings, sills and guttering on exposed walls.
• Keep rainwater goods clear so they do not saturate the wall.
• Fix the external source rather than sealing the inside.
• Let the fabric dry rather than trapping moisture behind coatings.