Even more questions from the BRANZ ‘You Asked' seminars

BRANZ answers even more questions from the recent ‘You Asked’ seminars.

Which direct-fixed claddings are permitted by E2/AS1?

A limited number of claddings that are considered to have a lower weathertightness risk (due to the cladding itself or its jointing details) and proven historical performance are permitted to be direct-fixed to low-risk buildings under E2/AS1. Claddings that can be direct-fixed are:

  • bevel-back weatherboard – up to risk score 12
  • rusticated weatherboard – up to risk score 6
  • fibre-cement weatherboard – up to risk score 6
  • vertical profiled corrugate profile metal – up to risk score 20 as the voids behind the cladding and limited framing contact provide for drainage and drying
  • vertical profiled trapezoid profile metal – up to risk score 6
  • flat sheet fibre-cement or plywood with the sheet joints having either cover battens or a jointer and with all horizontal joints flashed – up to risk score 6
  • vertical timber board and batten – up to risk score 12.

However, E2/AS1 requires that all stucco, EIFS, flush-finished fibre-cement, and horizontal profiled metals are installed over a cavity.

How can an air seal be incorporated into traditional casement timber window details?

Incorporating an air seal into a casement window system should be possible by inserting the seal over a backing rod into the fitting gap between the window frame and the trimmed opening.

Do you now have to carry out geotechnical investigations for all sites to determine the site subsoil when using NZS 3604:2011 Timber-framed buildings to determine earthquake-bracing demand?

No, as the tables for earthquake-bracing default to the worst-case subsoil conditions (soils D and E and zone 3). Where subsoil conditions are identifiable or known, they can be used to moderate the level of earthquake bracing that needs to be provided. For large-scale projects, there may be some advantage in carrying out a geotechnical investigation to gain a cost saving by having to provide less earthquake bracing if the subsoil conditions allow it. What needs to be balanced against this is that, in most instances, the wind loads determine the amount of bracing required (apart from a building with heavy roof and wall construction in low and medium wind zones or a square building).

A designer must still be sure of what ground conditions exist below a site before proceeding with the above-ground design. This will involve proving that the ground is ‘good ground’ by one of the means that are listed in section 3.1.3 of NZS 3604:2011. This is all irrespective of the seismic and wind demands and is concerned with bearing capacity under gravity loads.

How do you calculate the loaded dimension for bearers?

Figure 1.3 of NZS 3604:2011 gives details for calculating the bearer loaded dimension. Where the bearer is supporting a loadbearing external wall, the calculation of the loaded dimension is described in Build 137 Loaded dimension for bearers (page 32).

Does a ceiling diaphragm have to be horizontal (flat)?

No. A ceiling diaphragm can be installed on slopes up to 45°. However, the design and installation parameters depend on the slope and are categorised as:

  • diaphragms with a roof slope of up to 15°
  • diaphragms with a roof slope of up to 25°
  • diaphragms with a roof slope of up to 45°.

Can you use an intermediate vertical batten within a cavity to restrain the flexible wall underlay rather than tape?

Yes.

Do you have to use stainless steel fixings for non-structural cavity battens?

No. Fixings for cavity battens are considered temporary in that the cladding fixings must go through the batten and into the framing timber – typically, 35 mm penetration into the stud is required.

Why can the wind zones determined using NZS 3604 be different from those determined using AS/NZS 1170 Structural design actions?

NZS 3604:2011 has coarser steps, which means that it is sometimes more conservative than AS/NZS 1170.

What level of H1 compliance is required for existing parts of a building during a renovation or addition?

Where an existing construction is altered, it is recommended that the construction be brought as close as possible to current standards. However, for H1 compliance, the thermal performance after the alteration is made must be no worse than the original building. Remember that a building consent is required when existing external walls have any form of insulation fitted.

Where parts of the existing building are unchanged, there is no statutory requirement to upgrade that portion of the building.

What is the effect of moisture on the back of an unpainted absorbent cladding?

Where the presence of the water is intermittent and the amount small and the cladding is installed over a ventilated cavity, there is probably little effect because of the drying that occurs. Problems that may arise are:

  • reduced durability of any coating that might get applied to the weather face of the cladding in the future
  • possibility of swelling causing distortion of a timber cladding.

Where there is a significant amount of water regularly wetting the cladding, there is a risk of biocontamination and possible health consequences.

Do internal walls that support beams resisting uplift need to be tied down at the base of the wall?

Yes, as the uplift loads need to be transferred to the ground via the wall framing and the slab/floor construction including connections.

Published in building.

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