cavity wall

A cavity wall is a wall assembly made of two separate layers of masonry, usually brick or blockwork, separated by an air gap typically 50 to 100 millimetres wide. The two layers, called skins or leaves, are connected at intervals by metal ties to share structural load. The air space itself is the defining feature: it acts as a thermal break and as a path for moisture management, allowing water that penetrates the outer skin to drain downward and outward rather than traveling into the building interior.

The outer leaf, exposed to weather, bears the visual finish and the first impact of rain and wind. It is often a single brick thickness, 102.5 millimetres. The inner leaf is usually blockwork, ranging from 100 to 150 millimetres depending on structural and thermal demands. Mortar is applied to both leaves independently; the cavity itself remains largely clear, though it may contain batts of mineral wool or other insulation material if thermal performance is required.

Tie systems and cavity management

Wall ties are mild steel or stainless steel straps bedded into mortar joints at regular intervals, typically every 450 millimetres horizontally and every 600 millimetres vertically. They transfer lateral wind loads from the outer leaf to the inner leaf and prevent outward bulging. Cavity trays, usually of slate, tile, or plastic, are installed above windows and doors to catch water that enters the cavity and direct it outward through weep holes rather than allowing it to pool. Without effective cavity tray design and installation, water ingress becomes chronic.

The cavity wall was developed as a response to failures of solid masonry in wet climates. Traditional single-thickness stone or brick walls allowed capillary action to carry water deep into the structure, causing internal dampness, frost damage, and decay. The air gap prevents this wicking action. The system became standard in British construction from the early twentieth century onward and is now normal practice in temperate maritime climates.

Common problems include cavity ties corroding in older work, particularly in coastal locations and where mortar quality was poor; insulation material slumping or absorbing moisture, reducing performance; and poor detailing at junctions, allowing moisture to bypass the system entirely. Successful cavity wall performance depends as much on correct detailing at openings and terminations as it does on the basic principle of separation. Inspection of cavity fill and tie condition may be needed in walls more than 30 years old.