Lines of defence

The design and construction of a building should incorporate lines of defence that manage water, following 4 Ds (deflection, drainage, drying and durability) principles. The elements that make up the exterior of a building are the primary line of defence against water penetration, deflecting and draining water off the external surface. Where water gets past this primary line of defence and leaks into the wall assembly, there needs to be a second line of defence that will allow this water to drain and dry.

Another way of considering building details is to determine whether or not the surface or element will be wet or dry in service. The face of cladding will generally be wet, the back of cladding dry but occasionally wet (except for masonry veneer), and the face of wall underlay will be dry (but in direct-fixed cladding systems may occasionally be wet).

Primary line of defence

The primary line of defence should create a barrier to water penetration and allow water to drain from it. This primary line of defence includes:

  • the cladding system and its coating
  • components (such as eaves and verandas, and windows and doors that incorporate specific deflection devices such as flashings and facings) that deflect water or allow it to drain over critical junctions.

The primary line of defence will generally be wet in service.

Ensure that all aspects of the building exterior create an effective primary defence.

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Secondary lines of defence

Secondary lines of defence are specifically designed and constructed to manage water that may penetrate the wall assembly when the primary line of defence fails. Secondary lines of defence allow water to drain or dry before it can reach more sensitive or less durable components of the wall assembly such as bulk insulation, plasterboard lining and timber wall framing.

Secondary lines of defence will generally be dry in service but may occasionally be wet.

Secondary lines of defences in direct-fixed claddings

In direct-fixed cladding systems, the secondary line of defence is the drainage plane formed on the back of the cladding, in the space between it and the face of the wall underlay. Water will drain down drainage paths to the outside of the building.

The potential for gravity drainage to occur in direct-fixed claddings is directly related to the profile and absorbency of the cladding and the absorbency of the wall underlay:

  • Profiled claddings will provide more drainage capacity than flat claddings, which have a greater contact area with the wall underlay and less void.
  • Absorbent claddings and wall underlays will absorb some water.

As water will be in contact with the wall underlay, it is fundamental that the builder installs the wall underlay specified by the designer to ensure that it forms a continuous drainage plane. This means that:

  • laps must have sufficient cover (and should be taped)
  • any penetration or tear must be taped and sealed
  • the underlay must exit the wall assembly in a manner that will allow water to drain out.

Some air will circulate in direct-fixed assemblies, and this air will dry water absorbed by components, but air drying is less effective than drainage at removing water.

The secondary line of defence in direct-fixed cladding systems is the only back-up – water that penetrates this secondary line of defence may be absorbed by vulnerable components.

Drained and vented cavity systems allow for more drainage and drying than a direct-fixed system.

Secondary lines of defences in drained and vented cavities

With claddings that are installed on a drained and vented cavity, the assembly needs to be designed and built so that water that has leaked in will drain down a drainage plane formed by the back of the cladding. Water will drain down drainage paths to the outside of the building.

The back of the cladding and components within the cavity – such as cavity battens and the wall underlay – will absorb some moisture, but the cavity offers good potential for air entry and circulation that will dry this moisture out. 

In extreme failures, water may bridge the cavity from the back of the cladding and reach the wall underlay, where it must be able to drain down a drainage plane formed by the face of the wall underlay. This forms a third line of defence. It is fundamental that the builder installs the wall underlay to ensure that it forms a continuous drainage plane

Updated: 9 September 2014