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PENETRATING DAMP

Damp on External Wall

Damp on an external wall often results from cracked render, eroded pointing, overflowing gutters, defective seals, poor drainage or high external ground levels.

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Trace and stop the water entry before repairing the inside surface

Do not rely on stain paint, sealant or internal waterproof coatings to hide an active external defect. Use safe access and call a roofer or builder where roof-level inspection is required.

MOST LIKELY CAUSES

Start with the matching external area

Cracked render or pointing Rain penetrates damaged external finishes.
Gutter or downpipe leak Repeated wetting saturates the wall.
Poor drainage or splashback Water remains against the base of the wall.
Failed penetration seal Pipes, cables and vents may have open joints.
◫ Symptom variations Match the timing, weather and stain pattern. ⌄
  • Patch worsens after rain
  • External wall visibly cracked
  • Green staining outside
  • Damp near downpipe
  • Lower wall remains wet
✓ Quick checks Simple observations before opening finishes. ⌄
  1. Record whether the damp follows rain, wind or bathroom use.
  2. Inspect the matching external area from ground level.
  3. Check gutters, downpipes, seals, render and pointing.
  4. Look above and beside the visible patch because water can travel.
  5. Do not redecorate until the source is repaired and the material is dry.
Important: A visible patch may be some distance from the actual point of entry.
1 Step 1 — Inspect the matching outside area Trace the external water path before repairing internal plaster or paint. ⌄

Check directly above and beside the internal damp patch.

2 Step 2 — Look for concentrated water flow Trace the external water path before repairing internal plaster or paint. ⌄

Inspect gutters, downpipes, paving falls and splashback.

3 Step 3 — Repair external defects and allow drying Trace the external water path before repairing internal plaster or paint. ⌄

Internal decorating should wait until the wall moisture falls.

⌁ Why the damp appears How this water-entry pattern develops. ⌄

Damp on an external wall often results from cracked render, eroded pointing, overflowing gutters, defective seals, poor drainage or high external ground levels.

⌘ Advanced checks Further diagnosis for recurring or hidden water entry. ⌄
  1. Check cavity bridging and debris where relevant.
  2. Inspect parapets, copings and wall tops.
  3. Assess ground level against the damp-proof course.
  4. Check render termination and movement cracks.
  5. Trace concealed downpipe or overflow leaks.
◇ Tools and information needed Useful items for accurate diagnosis. ⌄
  • Weather and moisture history
  • Ground-level external inspection
  • Moisture meter used with care
  • Roof, gutter and window details
  • Safe professional access where required
▣ Parts or repairs that may be involved Common items linked to this problem. ⌄
  • Pointing mortar
  • Render repair system
  • Gutter or downpipe components
  • External sealant
  • Drainage improvements
! When to call a professional Conditions that should end basic DIY investigation. ⌄
  • Roof, chimney or high-level access is required.
  • The ceiling is bulging, soft or actively dripping.
  • Water is near electrical fittings.
  • Structural timber, plaster or insulation is saturated.
  • The source cannot be traced from safe access.
  • The damp persists after an obvious repair.
? Frequently asked questions Common penetrating-damp questions. ⌄

Can damp paint stop penetrating damp?

No. It may hide staining temporarily, but the external water-entry defect still needs repair.

Why does the patch appear away from the leak?

Water can travel along roofing felt, joists, cavities, pipes and cracks before becoming visible.

How long should a wall dry before repainting?

Drying time depends on wall construction, saturation and ventilation. The surface should be demonstrably dry before refinishing.