The 4Ds of weathertightness design

The Acceptable Solution E2/AS1 includes a section on the assessment of potential weathertightness risks and ways to manage them, or to reduce risk by incorporating other compensating features.

The development of a risk assessment approach was based on work undertaken by two Canadians, Don Hazelden and Paul Morris. They developed a simple concept, called ‘the 4Ds’, to describe the basic principles of water management in buildings.

The 4Ds as used in E2/AS1 are:

• Deflection – keeping water away from potential entry points.
• Drainage – providing means of removing water that does enter.
• Drying – allowing any remaining moisture to be removed by ventilation or diffusion.
• Durability – providing materials with appropriate durability.

The 4Ds are further described in the Department’s forthcoming guidance document External moisture – An introduction to weathertightness design principles.

External moisture – a guide to using the risk matrix is a guide to understanding and applying the risk principles, with a series of worked examples. Using the risk matrix, with nominated cladding types and the associated inclusion of a drained cavity in higher risk situations, is only one means of achieving weathertightness. However, building officials, designers and builders will find the information useful in identifying individual features that will require particular care and attention during design and construction.

Ideally, a design will incorporate and balance all 4Ds. Keeping these basic principles in mind will assist when assessing designs or preparing and evaluating proposed alternative solutions that are outside the scope of E2/AS1.