Committees responsible for this British Standard

The preparation of this British Standard was entrusted by the Civil Engineering and Building Structures Standards Policy Committee (CSB/-) to Technical Committee CSB/33, upon which the following bodies were represented:

Association of British Roofing Felt Manufacturers
Association of Consulting Engineers
Autoclaved Aerated Concrete Products Association
Brick Development Association
British Cement Association
British Ceramic Research Ltd.
British Precast Concrete Federation Ltd.
Building Employers’ Confederation
Calcium Silicate Brick Association Limited
Department of the Environment
Department of the Environment (Building Research Establishment)
Department of the Environment (Construction Directorate)
Incorporated Association of Architects and Surveyors
Institution of Civil Engineers
Institution of Structural Engineers
National Cavity Insulation Association
National House-building Council
Royal Institute of British Architects
Society of Chief Architects of Local Authorities

The following bodies were also represented in the drafting of the standard, through subcommittees and panels:

Aggregate Concrete Block Association
Concrete Brick Manufacturers’ Association
Eurisol (UK Mineral Wool Association)
Institution of Clerks of Works of Great Britain Inc.
Loss Prevention Council
Mortar Producers’ Association Limited
National Association of Teachers in Further and Higher Education
National Federation of Clay Industries Ltd.
Refractories Association of Great Britain
Royal Institution of Chartered Surveyors

This British Standard, having been prepared under the direction of the Civil Engineering and Building Structures Standards Policy Committee, was published under the authority of the Standards Board and comes into effect on 15 August 1992

The following BSI references relate to the work on this standard:

Committee reference CSB/33
Draft for comment DD 93

Foreword

This British Standard has been prepared under the direction of the Civil Engineering and Building Structures Standards Policy Committee. It supersedes DD 93 : 1984 which is withdrawn.

During the revision of CP 121 : Part 1, which resulted in BS 5628 : Part 3, the technical committee recognized the need to develop a method of assessing exposure to wind-driven rain which could be applied to many aspects of the design of masonry.

New data made available by the Meteorological Office made it possible to develop more accurate methods of assessment of local wind-driven rain than were available before. A comparison with the earlier method described in BS 5618 has revealed some significant regional variations.

It became apparent that there are two distinct aspects of exposure to wind-driven rain, as follows:

average moisture content of masonry, for which the local annual index is the most significant factor;
rain penetration through the masonry, for which the local spell index is the most significant factor.

It should be emphasized that neither index is precise enough to enable fine distinctions between degrees of exposure to be made. Hence the user should always take account of local knowledge and experience.

Since the publication of DD 93, further work has included:

research into the relation between local building environments and instances of rain penetration;
research into the effect of local environment and of building size and design features on rainfall actually falling on walls in buildings.

This further work has led to suggestions for a number of modifications to DD 93. Basic meteorological data remain largely unchanged except that the Channel Islands have now been added to the maps (appendix B). Factors to modify rainfall according to local conditions are now based on empirically determined data from experimental sites rather than solely on wind speed data as previously. In addition, factors to allow for building shape and size have been introduced, also based on data from experimental sites.

Future work will include:

regular reviews of climatic data to take into account long term climatic changes;
research into the relation between exposure and the properties of individual materials, such as absorptive capacity, and the effects of evaporation and moisture content.

This code of practice represents a standard of good practice and therefore takes the form of recommendations.

Compliance with a British Standard does not of itself confer immunity from legal obligations.

0 Introduction

The quantity of rain falling on a vertical surface, such as a wall, at any point depends on both the intensity of the rainfall and the wind speed. The Building Research Establishment Report Driving Rain Index (1976) postulated that the quantity of rain falling on a vertical surface was proportional to the rate falling on a horizontal surface and to the local wind speed, and incorporated maps of an annual wind-driven rain index, which is the product of the annual average ‘airfield’ wind speed and annual average rainfall on the horizontal. Appropriate correction factors were given to compute the local value. (One version of this method, which expresses the index as a logarithmic value, is described in BS 5618.)

More recently the Meteorological Office has produced improved values, based on the fact that prolonged rainfall is usually associated with stronger than average winds. New maps for the ‘airfield’ annual wind-driven rain index have been prepared by summing the hourly products of wind speed and rainfall during the 33 year period 1959 to 1991. These data form the basis for the method of prediction of quantities of rainfall on vertical surfaces, which is described in this code of practice.

Features of this code of practice are that it allows calculations of driving rainfall for different orientations, it allows annual average values to be calculated as well as quantities for the worst likely spell in any 3 year period and it allows corrections to be made for ground terrain, topography, local shelter, and the constructional characteristics of the building concerned.

Site investigations have given information not only on the appropriate choice of modifying factors but also on the reproducibility and accuracy of rainfall data calculated for a given building. This is reflected in the degree of precision with which factors R, T, O and IF are given in this code. Any extra precision which would be achieved by a closer definition of the building/site description would not be sufficient to justify the much increased complexity that this would lend to the code.

Appendix A gives worked examples using the wind-driven rain maps in appendix B and the topography factors in appendix C. Further details of the background to this code are given in appendix D.

Code of practice

1 Scope

This British Standard gives recommendations for two methods for assessing exposure of walls in buildings to wind-driven rain, namely the local spell index method and the local annual index method.

It should be noted that this code is not appropriate in respect of the weathertightness of components such as windows. However, the performance of such components can be influenced by the design of the surrounding structure and appendix E may be helpful in this respect.

NOTE. The titles of the publications referred to in this standard are listed on the inside back cover. Other references will be found throughout the text, indicated by bracketed numbers thus [1], and these are listed in the bibliography in appendix F.

2 Definitions

3 Choice of methods

4 General

5 Procedure

5.1 Basic data

5.2 Step by step procedure

BS 8104:1992

Code of practice for assessing exposure of walls to wind-driven rain

Current, Under Review

Published:

15 Aug 1992