Choosing claddings

This Issue This is a part of the Claddings feature

By - , Build 100

When choosing a cladding, there are many things to consider to ensure it performs as expected.

Since the problems with leaking buildings came to prominence there has been a shift in the market share of some domestic claddings. What is very obvious is the major resurgence in the use of bevelback weatherboard profile claddings (both timber and fibre-cement), the continued increase in the use of profiled metal and the use of multiple claddings on a single building. There has also been a small but significant move to cladding materials such as copper, glass and precast concrete.

Making the right choice

Design features and the owner’s aesthetic and performance expectations usually have a major influence on cladding choices. However, for each particular building location and style it is also important to consider the factors that impact on the cladding’s performance.

First, cladding materials need to be appropriate for their location on the building, and compatible with adjacent wall or roof cladding. Attention should also be given to the interface between claddings.

Allowances may need to be made for environmental factors such as wind, corrosion potential, rainfall intensity and sunshine. The level of weathertightness risk needs to be taken into account, as well as the product manufacturer’s recommendations.

The way in which window and door joinery is integrated into the cladding design should also be considered. Both bevelback weatherboards and profiled metal have a pronounced profile, which needs to be accommodated when fitting joinery.

Allowances for thermal movement should be made – this is particularly important for longer runs of metal cladding. The availability of sufficient skills to correctly install unfamiliar claddings, and ease of access to the cladding for maintenance, are other considerations. Finally, the modifications that will be made to the cladding by other trades such as electricians need to be thought through.

If the design and detailing of the building is not within the scope of the New Zealand Building Code Acceptable Solution E2/AS1, it will need to be consented as an Alternative Solution. Even some buildings that do fall within the scope of E2/AS1 may in fact be an Alternative Solution. For an Alternative Solution, the performance of the details needs to be proven to show compliance with the requirements of Clause E2.

Profiled metal claddings

Profiled metal claddings have increased in popularity, but there are specific factors that designers need to check off before choosing to use them.

First, the corrosion zone should be considered, as well as the compatibility of the profiled metal with adjacent materials. In some cases, there may be a risk of the performance of the metal being compromised, for example, by an acidic timber such as cedar or a timber containing copper-based treatment solutions.

Think about the suitability of the factory coatings selected for each building’s environment. Not all factory coated metal is suitable for all situations; there are a range of finish options to suit different levels of corrosion risk.

Sometimes, there may be areas of cladding created by the design that will not be washed by rain, such as the area of wall immediately under the eaves. This may negate the product warranty, and the owner should be made aware of the washing down and maintenance requirements for each cladding.

The way in which the details for window joinery and parapet wall junctions deal with the sinusoidal (corrugate) or trapezoidal profile of the metal also needs to be considered, as well as whether flashings are able to be pre-ordered, or must instead be ordered to suit the profile after the sheets are installed.

For direct-fixed vertical profiled metal, the correct wall underlay also needs to be specified and installed.

Bevelback weatherboard

There are several considerations when choosing bevelback weatherboard claddings. For timber, the level of treatment needs to be appropriate for the finish – for radiata pine boards that are clear finished or unpainted the minimum treatment is H3.2, and if painted they can be H3.1. If radiata pine is not being used, the selected timber must meet the minimum durability requirements (NZS 3602).

Wall heights and window head heights need to be determined based on the weatherboard module, and the effect of board width should be taken into account, as movement in timber weatherboards will be more pronounced where wider boards have been used.

Builders need to know how to correctly construct an internal corner and fix the boards. Back flashings should be specified for internal corners, and external corners should be either mitred with back flashing, mitred with soakers, or have coverboards with scribers.

Junctions with other claddings should be robustly detailed, with backup to intercept and drain water should it enter the junction. Accessory items, such as tilting fillets and weather-heads (where specified), should be correctly shown and installed.

Other considerations include how window heads are flashed, whether the flashing location is integrated with the board module, the expected life of the primer when exposed to the weather, and whether a standard profile has been specified or is instead proposed by the designer as a special run.

Multiple claddings

Finally, where multiple claddings are used, the detailing of the junction between the weatherboard and other claddings must make allowances for the different characteristics and requirements of each material.

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