What This Requirement Covers
Approved Document C2 requires that external walls adequately resist the penetration of rain and snow to the interior. The method of weather resistance depends on the wall construction type and the level of exposure to wind-driven rain.
Key Requirements
Exposure Zones
The UK is divided into four exposure zones based on the severity of wind-driven rain
- Zone 1 (Sheltered): Central and eastern England, sheltered urban locations
- Zone 2 (Moderate): Most of England and Wales
- Zone 3 (Severe): Western regions, coastal areas, elevated sites
- Zone 4 (Very severe): Western Scotland, exposed coastal and highland areas
Cavity Wall Construction
- The most common form of external wall in the UK
- The cavity provides a break that prevents rain penetrating to the inner leaf
- Minimum cavity width: 50 mm clear (for partial-fill insulation); no minimum for full-fill insulation, but the outer leaf must be properly pointed
- Weep holes: Required above all cavity trays at a maximum of 900 mm centres to allow moisture to drain from the cavity
- Cavity trays must be provided above lintels, at abutments, and at DPC level
Rendered Walls
- External render provides weather protection for solid walls and some lightweight construction systems
- Render must be applied to a suitable substrate and must be compatible with any external insulation
- Cracks in render allow water penetration; maintain render in good condition
Rainscreen Cladding
- A ventilated cavity behind the cladding panels allows moisture to drain and dry
- The cavity must be at least 25 mm and ventilated at the top and bottom
- A breather membrane behind the cladding provides a secondary line of defence
Timber Cladding
- Timber cladding must be treated or naturally durable (Use Class 3)
- A ventilated cavity behind the cladding is essential
- A breather membrane must be installed behind the cladding
Practical Compliance Tips
- Assess the exposure zone for the site and specify the external wall construction accordingly
- Ensure the cavity is kept clean during construction; mortar droppings bridging the cavity can transfer moisture to the inner leaf
- Check that all cavity trays are properly installed, lapped, and dressed into the weep holes
- Maintain rendered walls; cracks and defects allow moisture penetration
- Ensure weep holes are not blocked by mortar or debris
- For rainscreen cladding, verify that the ventilation gap is maintained at all locations
- Keep records of the external wall specification and construction for the building file