Cavity Wall Damp: Problems, Causes, and Solutions

Damp patches appearing on your cavity walls. You’ve been told you need expensive damp proofing. But cavity walls were specifically designed to keep moisture out — so what’s gone wrong? As an independent damp surveyor across Derby, Nottingham, and Leicester, I investigate cavity wall problems regularly. Many cavity wall damp issues are fixable without expensive damp proofing — the challenge is identifying which of several possible causes is affecting your property.

How Cavity Walls Work

Cavity walls (standard from the 1920s) consist of an external brick leaf, a 50–100mm air gap, and an internal leaf, connected by wall ties. The cavity prevents rain reaching the internal wall — water that penetrates the outer leaf runs down the cavity face and escapes through weep holes at the base. When working properly, internal walls stay completely dry.

Common Cavity Wall Problems

Mortar bridging (most common): during construction, mortar droppings fall into the cavity and accumulate on wall ties, creating bridges for water to cross. Symptoms: damp patches at any height, random locations, worsening after rain. Fix: cavity clear-out via removed bricks (£400–£800).

Failed or missing cavity trays: cavity trays above openings direct water back outside. When missing or damaged, damp appears above windows or doors. Fix: install new stepped cavity trays (£300–£600 per window).

Blocked weep holes: painted over, filled with mortar during repointing, or rendered over. Damp appears at base of walls. Fix: clear blocked weep holes or drill new ones (£100–£200).

Cavity wall insulation problems: damp appearing after insulation is installed, especially on exposed walls after wind-driven rain. Fix: specialist insulation extraction (£2,000–£4,000). Not all insulation causes problems — proper assessment is essential before extraction.

Wall tie corrosion: common in 1950s–1970s properties with original mild steel ties. Symptoms: horizontal cracking in brickwork, bulging walls, damp patches. Fix: wall tie replacement (£80–£120 per m²) — structural engineer assessment often needed.

Common Mistakes to Avoid

Chemical DPC injection doesn’t fix bridging, missing cavity trays, or insulation problems — it wastes £1,000–£3,000 on the wrong solution. Rendering over damp walls traps moisture and makes things worse. Internal tanking doesn’t fix external cavity problems. Always get proper diagnosis before committing to any treatment.

Get Expert Diagnosis

If you’re in Derby, Nottingham, Leicester, or surrounding areas with cavity wall damp, Richard Bull provides borescope camera cavity inspection, thermal imaging moisture tracking, comprehensive diagnosis, and treatment specifications. Don’t waste money on damp proofing that won’t fix cavity problems.

📞 07983 550 662
📧 richard.bull@dampdetectives.co.uk
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Richard Bull MISSE, ACIEH — Independent & Unbiased — No Sales Pressure