A new wind zone category, extra high, has been added in the revised NZS 3604:2011 Timber-framed buildings, and the amended E2/AS1.
The extra high (EH) wind zone category allows design for a wind speed up to 55 m/s. This means more buildings come within the scope of NZS 3604:2011 and E2/AS1. Specific engineering design-classified buildings are beyond the scope of NZS 3604 and E2/AS1. To determine wind speed classification, see this Build pages 24–25.
Areas of change
Acceptable Solution E2/AS1, a companion to NZS 3604, was revised to align with the changes. The extra high wind zone has been added to E2/AS1 Table 1. Buildings designed to E2/AS1 in the extra high wind zone also require:
- rigid wall underlays under all claddings
- a cavity behind all claddings
- bigger upstands and covers to flashings
- seals under head flashings
- roof underlay to all masonry tiles
- changes to the detailing of profiled metal roofs.
Rigid wall underlays
E2/AS1 Table 3 (note 5) and paragraph 188.8.131.52 require that claddings in extra high wind zones must have a rigid wall underlay and be installed over a drained cavity. The rigid underlay ensures that the depth of the cavity is maintained and remains functional in high wind, and helps protect the internal linings from excessive air pressure changes.
Table 3 (note 6) also states that direct-fix corrugated metal profile cladding is regarded as a drained cavity system, which means it must also be installed over a rigid wall underlay in extra high wind zones.
Rigid wall underlays must be in accordance with Table 23, and paragraph 184.108.40.206 sets out other requirements including that they must:
- be either 7 mm thick, H3 plywood or 6 mm fibre-cement sheet
- have sheet edges fixed over solid framing
- have a flexible wall underlay fixed over the rigid sheet (note that proprietary rigid wall underlays may not require the additional flexible wall underlay but must be submitted for consent as an Alternative Solution)
- have the flexible wall underlay folded into opening reveals as shown in E2/AS1 Figure 72B
- have cavity battens at 600 mm centres maximum
- be finished flush with the underside of the bottom plate or bearer.
E2/AS1 paragraph 220.127.116.11 also requires that a rigid wall underlay is installed to the external walls of attached garages that are unlined, and it references E2/AS1 paragraph 18.104.22.168(c).
Flashing requirements differ for extra high wind zones. E2/AS1 paragraph 4.5.1 requires that hooks and hems as per E2/AS1 Figure 5 (see Figure 1) are used for the edges of flashings, and flashing upstand dimensions are increased by 25 mm over those shown in Table 7 or referenced anywhere else in E2/AS1.
E2/AS1 Table 7, which sets out general flashing dimensions, has been revised and now includes an extra column for wind zone situation, i.e. situation 3, which applies to all roof pitches in an extra high wind zone (see Table 1).
Window head treatment
Window heads in extra high wind zones require a sealant between the underside of the metal head flashing and the top edge of the window head flange in accordance with E2/AS1 Figure 71(c) (see Figure 2).
Roof underlay with masonry tiles
All masonry tiled roofs in very high (VH) and extra high (EH) wind zones require roof underlay regardless of pitch in accordance with E2/AS1 Table 10 (note 2).
Profiled metal roofing
E2/AS1 Tables 11–15, which set out maximum spans and fixing patterns for profiled metal roofs, have been changed to incorporate fixings for extra high wind zones.
Flashing requirements for eaves have changed so that eaves flashings are now required for all roofs in extra high wind zone or where the pitch is less than 10° and the soffit width is less than 100 mm in accordance with E2/AS1 paragraph 8.4.11 and Figure 45(a) (see Figure 3).
A change of roof slope is not permitted.
|Flashing type/location||Description||Minimum dimension for all roof pitches in extra high wind zone|
|Changes in roof pitch||Not permitted under E2/AS1|
|Barge flashing||Overlap to barge board and claddings||90 mm|
|Roof or deck to wall junction||Lap under cladding above||90 mm|
|Head flashing||Lap under cladding above||60 mm|
|Corner flashing||Vertical flashing at wall corners||75 × 75 mm|
|Inter-storey junction||Lap under cladding above and lap over cladding below||60 mm|
Articles are correct at the time of publication but may have since become outdated.