Wellington Wind, Driven Rain and the E2 External Moisture Clause

Rain in Wellington rarely falls straight down. It arrives sideways on a northerly, curls around corners, runs up the face of a window and gets pushed into gaps that would stay bone dry in a calmer city. For anyone looking after a building near the harbour, wind and water are best thought of as a single problem. New Zealand’s Building Code treats them that way too.

The E2 clause in brief

Clause E2 External Moisture is the part of the Building Code that requires buildings to keep out rain and other outside moisture. MBIE’s E2 External Moisture guidance is the starting point. For most houses and small buildings, the usual route to compliance is the Acceptable Solution E2/AS1, which sets out cladding, flashing and detailing options that are deemed to comply.

E2/AS1 rests on a simple idea, often summarised as the 4Ds.

The 4Ds

Deflection. The cladding, flashings and roof are the first line of defence. They should shed water away from the building, and in a good design they handle almost all of it.

Drainage. Some water will always get past the cladding, especially when wind is pushing it. A drained cavity behind the cladding gives that water a path down and out at the bottom of the wall.

Drying. Moisture that does not drain away needs to evaporate. A ventilated cavity lets air move behind the cladding and carry dampness off before it can cause harm.

Durability. The framing and other hidden parts of the wall have to stay sound for a very long time. The Building Code’s B2 Durability clause asks for at least 50 years for structure and elements that are hard to reach. The other three Ds exist mainly to protect that.

The cavity is a backup, not a licence for sloppy detailing. If battens are badly installed or the cavity is blocked, it can trap moisture instead of draining it.

Where wind comes into it

Under NZS 3604, every site is assigned a wind zone based on the region, the local topography and how sheltered or exposed the site is. The zones run from Low through Medium, High and Very High to Extra High, with design wind speeds rising at each step. Sites beyond Extra High need specific engineering design. Wellington sits in the windier of the standard’s two wind regions, and exposed hillside or waterfront sites can reach the upper zones.

The wind zone then feeds into the E2/AS1 risk matrix. That matrix scores each face of the building on six factors:

• wind zone
• number of storeys
• roof and wall junctions
• eaves width
• envelope complexity
• deck design

The total score for each face decides which cladding options are acceptable and whether a drained cavity is needed. Higher wind zones add to the score. In the Extra High zone, E2/AS1 requires a drained cavity regardless of the total.

Because the zone depends on the exact site, there is no single answer for “Wellington”. The city council can advise on the wind zone for a particular property, and a designer can work it through using the standard.

Where water gets in

Wind pressure is what makes Wellington weather so good at finding weaknesses. Typical problem spots include:

• Window and door heads and sills, especially where flashings are short or sealant has failed
• Junctions between roof and wall, such as where a lower roof meets a higher wall
• Balconies and decks over living space, where membranes, thresholds and balustrade fixings all meet
• Penetrations for pipes, vents, lights and cables
• Parapets and flat tops of walls, where water can sit and be driven into joins

A building that sheds water well on a calm day can leak on a stormy one simply because the wind changes the direction of flow. Water can even be pushed uphill under laps and flashings when the pressure difference across the wall is strong enough.

Living with it

For existing buildings, most of the work is observation and upkeep:

• Look after sealants. Sealant joints around windows and between panels have a limited life, and Wellington’s sun, wind and salt shorten it.
• Keep drainage paths clear. Blocked gutters, outlets and cavity vents turn small amounts of water into large ones.
• Watch for the signs. Staining on interior linings, swollen skirting boards, musty smells or paint bubbling near windows all deserve investigation.
• Check after big storms. A walk around the building after a severe weather event can catch damage early. MetService severe weather warnings are a useful prompt.
• Get proper advice for changes. Adding a window, a screen or a balcony fixing alters the envelope, and may need a building consent.

On the coast, wind and salt work together. Wind carries spray onto the building, and salt left on surfaces holds moisture and speeds up corrosion of flashings and fixings. Our note on salt air and painted exteriors covers washing and coatings.

The waterfront test

Buildings on the Wellington waterfront are some of the most exposed in the country, sitting over open water with nothing to break a southerly. They show clearly why the Code asks for layers of protection rather than a single perfect seal. When deflection, drainage, drying and durability all do their part, the building can take a Wellington winter in its stride.