Designing claddings that comply

This Issue This is a part of the Claddings feature

By - , Build 94

There is enough information around to make sensible judgements about cladding details as long as you keep four key principles in mind. With these in place, whatever the design, your cladding should be weathertight.

Applying the ‘4Ds’ should help make your alternative acceptable.

When working drawings are being prepared and the claddings come to be detailed, certain decisions have probably already been made. The client has likely signed off on a design concept incorporating the cladding materials and how they are to be treated.

In preparing the working drawings, many more decisions, too numerous to recall, will be made. These will be made after considering many factors that ensure the initial design concept will be constructed to be durable and weathertight. These decisions will, however, follow a logical pattern.

First, is the design within the scope of the Acceptable Solution E2/AS1 for weathertightness? If the design does fall within this scope, one option is to just use the details provided.

However, there are a multitude of reasons why a design might be outside the scope:

• wind – site exposure or contours may cause a building to be exposed to wind speeds in excess of 50 m/s

• height in excess of three storeys or more than 10 m from ground to eaves

• complexity – where a combined score of roof/wall junctions, small or no eaves, general complexity, decks and wind take the risk score in the E2/AS1 risk matrix beyond 20

• materials not included in the Acceptable Solution

• choice – some details may differ to those in the Acceptable Solution.

Building officials prefer Acceptable Solutions because they are standard (see pages 48–49 for what they will look for). However, a prime objective of the 1991 and 2004 Building Acts was to allow for innovation. New and different approaches to design, new materials, new fixings, new ways to put them together. No building official has the information at hand to make a serious judgement about the fitness for purpose of a new material (they rely on other agencies for that) but there is enough information around to enable them to make sensible judgements about whether details will keep water out.

Following the alternative route

For many architects, following the code minimum – the Acceptable Solution – is to not design at all. Choosing the Alternative Solution route allows the freedom to follow a consistent theme through concept, planning, detailing and finish to give the client a well-designed and individualised product that meets their needs.

Whether the Alternative Solution route is being followed by choice or because the building is outside the scope of E2/AS1 shouldn’t matter. Architects worth their salt will wish to provide details that provide a higher level of weathertightness, that go beyond the minimum and will provide clear drawings of their intentions for the contractor and Building Consent Authority. Failure to do so would surely be a less than satisfactory design solution.

To provide a more robust design, the architect has many tools to choose from, which all fall conveniently into four easy-to-follow categories – the ‘4Ds’. By considering any of these four tools, robust weathertight design can be achieved and easily compared with the minimums contained in the Acceptable Solution.


Keep rain away from sensitive areas by using features such as eaves or flashings. The effectiveness of deflection can be increased at macro and micro levels as shown in Table 1.

Table 1: Increasing the effectiveness of deflection.
Macro levelMicro level
Increase eaves overhang Provide hoods over openings
Provide verandas Increase flashing cover
Overhang an upper floor Provide facings
  Increase window section/cladding cover


Provide paths for any water that gets behind cladding to drain away. The effectiveness of drainage can be improved by:

• restricting cavities to each floor height

• ensuring primary and secondary drainage paths are clearly defined

• providing flashing systems that drain to the exterior

• ensuring pressure equalisation occurs at drained joints

• ensuring drainage outlets are open.


Drying can best be achieved by allowing air circulation. Aside from not restricting airflow by means of planting etc., airflow can be assisted by limiting the height of cavities and providing additional ventilation (e.g. at the top of a cavity).


Make sure materials used to construct walls and roofs have an appropriate level of durability for the situation they will be used in. Durability is achieved first of all by keeping all water out at the face of a cladding. Second, if drainage paths or rain screen joints are provided, ensure the wet face is constructed of robust materials.

These are all tools that have been used for decades and are easily assessed. Trying to take short cuts with any of them may end in tragedy.

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