BRANZ guidelines timber decking glass units cladding and product substitution

Issue 24 – February 2011

Summarised from BRANZ Guideline December 2010

Installing timber decking

Gun nailing and tight edge-butting of timber decking is not recommended by BRANZ. Tight edge-butting of timber with an unknown moisture content may mean the timber will swell (increase in size – both length and width) and buckle off the deck and/or close the 12 mm drainage gap required between any decking timber and the adjacent wall cladding.

The use of gun nails in dry decking may cause the decking to split and splinter around the nails. Also, the gun nails may not be sufficiently durable when used externally and also with treated timbers. If gun nails are used, they should be stainless steel or a verified quality hot-dipped galvanised – see BRANZ Appraisals for fixings that have been appraised.

BRANZ recommends that decking is laid as described in BRANZ Bulletin 489 and NZS 3604:1999 with gaps between boards. It is also considered good practice that all fixings (not just at the board end joints) are pre-drilled beforehand nailing to minimise the risk of splitting.

Insulating glass units

Insulating glass units (IGUs), commonly called double glazing, come on site with labelling that describes the type of glass used and the filling used within the unit, such as argon gas. If the labels are removed on site and it is possible to remove them intact, do so and keep them to give to the owner or specifier for their records.

Special care needs to be taken where some of the IGUs have different specifications. If there is a mixture of IGU types used on the project, be sure to also note the window designation from the plans for each of the units. If labels can't be successfully removed intact, photograph and identify them as a record. This is a lot easier than trying to prove later what was supplied and installed, particularly where higher performance units have been supplied (for example, low-E glass or argon filled). Note that, if all else fails, there is a test that can be carried out after installation to identify if a unit is argon filled.


When fixing cladding to timber or steel framing, make sure you are aware of and follow the cladding supplier's specific requirements. Recently BRANZ were informed of timber weatherboards being gun nailed to steel framing, which is not the way it should be done. Manufacturers or suppliers should have their fixing requirements in their technical data sheets – if not make sure you obtain them.

Fixing cladding or cavity battens to steel frames is usually done using Wingtek type screws. Thermal breaks between the cladding and the frame are typically secured in place when the cladding is fixed.

Building product substitution – it can affect performance

The unauthorised substitution of a specified item (in one case, the specified wall underlay was replaced without approval with a foil that was then installed to the outside of the framing) is a breach of the contractual requirements. To make matters worse, the builder in this case didn't understand the performance issues that were generated by his unauthorised change.

Foil must never be installed:

  • as a substitute for a specified wall underlay
  • on the outer (cold) face of the framing.

If a vapour barrier is placed on the cold side of a wall, condensation may form on its surface because it is a vapour barrier and not permeable to moisture vapour (it can't breathe), and this moisture can cause mould, rot, and structural damage. BRANZ research has also shown that vapour barriers are not necessary in New Zealand domestic buildings.

The installation of a vapour barrier must be subject to a specific design regime, and it is generally only required as a component of a moisture control strategy in spaces with excessive moisture levels (such as an indoor swimming pool or a wet processing area) or in a building in extremely cold conditions (such as a mountain ski lodge). Such installations are usually in buildings where mechanical ventilation to control humidity is required.

Published in building.