Exterior Envelope, Cladding and Weathertightness
Domain 4 covers Building Code clause E2 and E2/AS1: how walls, windows and roofs are detailed so water is deflected, drained and allowed to dry. The exam favours questions about cavity construction, flashing sequence and where a leak will actually originate.
The four principles
- E2 is performance-based; E2/AS1 is the acceptable solution, with a risk matrix scored on wind zone, number of storeys, eaves width, envelope complexity and cladding type.
- Deflection: flashings and laps keep water out.
- Drainage: a 20 mm nominal drained and ventilated cavity behind the cladding.
- Drying: a ventilated cavity and a vapour-permeable wall underlay.
- Durability: materials and flashings that satisfy the B2 period.
Flashings and clearances
- Wall underlay laps upper sheets over lower and laps over flashing upstands. Sealant is not a substitute for a flashing.
- Head, sill, jamb, corner and inter-storey flashings need the slopes, laps and end dams specified in E2/AS1, and fixings must not pierce a flashing in the water path.
- Keep the cladding base at least 100 mm above finished ground level, and more where ground is unpaved, plus the specified clearances above decks and roofs.
Worked example
A damp patch at the head of a ground-floor window, directly under a first-floor window. The defect is usually at the junction above: check the first-floor sill and jamb flashings, whether the underlay was lapped the wrong way, and whether the cladding was fixed hard down onto the flashing with no drainage gap.
Traps that cost marks
- Smearing sealant where a flashing is required.
- Underlay lapped lower sheet over upper.
- Cavity blocked with mortar or offcuts.
- Penetrations and meter boxes without backflashings or seals.