Module 2 · Buildings and How They Handle Moisture
2.3 Early Cavity Walls Are Their Own Thing
9 min read
Cavity construction spread through British housing from the 1920s, and for fifty years it meant something quite different from what it means now: a narrow unfilled void, wall ties without drips, few or no cavity trays, and no insulation. Millions of these houses are still standing and performing adequately. Assessing one against current detailing generates a long list of non-compliances, none of which is the reason it is wet — and it usually leads to the one intervention most likely to make it worse.
Learning objectives
By the end of this lesson you will be able to:
- Describe interwar cavity construction and what its designers relied on.
- Identify cavity bridging by its characteristic pattern rather than by opening up at random.
- Assess whether an early cavity is a suitable candidate for retrofit fill.
- Distinguish a genuine defect from a difference in standard.
Key terms
- Cavity bridging
- Any continuous path across the cavity that carries water from the outer to the inner leaf — mortar droppings, a fouled tie, debris on a tray, or a solid element built through.
- Mortar snots
- Extruded mortar left on ties and cavity faces during construction. Ubiquitous in older work, and the commonest bridging mechanism.
- Cavity tray
- A damp-proof tray directing water out of the cavity above an interruption — a lintel, an abutment, an air brick. Frequently absent in pre-1970s work.
- Wall tie failure
- Corrosion of ferrous ties, which both loses the structural connection and expands, cracking bed joints in a characteristic horizontal pattern.
Read the rest of Buildings and How They Handle Moisture
Module 1 is open to everyone. The remaining six modules — building science, each damp mechanism in depth, meters and thermal imaging, the full inspection method, and report writing — are free too. We just ask for an email so you can pick up where you left off, and so we can send your certificate when you finish.