Principles of jointing

In any study of cladding it is important that the principles of weather protection are fully understood, as the joints are normally the most critical parts of the assembly.

Forty years ago the Norwegians initiated a scientific investigation into the mechanism of water leakage. Initially they were concerned only with the performance of casement windows, but their investigations subsequently led to a more thorough understanding of the behaviour of the wall itself. Birkeland (1962) published a treatise in which the principles of what was then referred to as the 'rain barrier' and its implications were discussed.

Following the Norwegian studies, scientists at the Canadian National Research Council's Division of Building research institute began similar investigations, and a year after the Norwegian publication a small pamphlet entitled Rain Penetration and its Control by Garden (1963) appeared. This publication, in which the terms 'rain screen' and 'rain screen principle' were clearly defined, is still considered to be a prime reference source.

A publication by the Architectural Aluminium Manufacturers Association in the USA (AAMA, 1971) summarizes the principles applied to curtain walling, and makes the point that the terms 'rain screen principle'and'pressure-equalized design', though closely related, are not strictly synonymous.The rain screen is the outer skin or surface of a wall or wall elements backed by an airspace, and so designed that it shields the wall joints from wetting. These principles are described in Anderson and Gill (1988), Rainscreen Cladding - A Guide to Design Principles and Practice. The rain screen principle is a principle of design that describes how the penetration of this screen by rainwater may be prevented. The use of pressure-equalized design is an essential part of this principle. Before discussing the application of this principle in detail, it is important to describe how rainwater acts on the surface of a wall.

2 East Croydon Station. (Architects: Brookes Stacey Randall)

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