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Deluge Rainfall

• Introduction
• Guidance
• Further reading

Introduction

• During the latter months of 2004 there was considerable discussion, in both national newspapers and the specialist construction press, about whether rainfall intensity was increasing. This was generated, in part, by the devastating flash flood in Boscastle, Cornwall, and the inevitable speculation about the effects of 'global warning'.
• Several large practices suffered major flat roof leaks on recently completed projects and believed that the problems may have been exacerbated by unusually intense rainfall.
• The key source document to be used for calculating rainfall, and sizing drainage systems, is BS EN 12056-3. You should be aware that the data used to calculate the rainfall intensities given in this BS is now thirty years old. There is some speculation, but no hard evidence, that these figures may be out of date.

Guidance

• Ensure that you know the design rainfall intensity to be used in the drainage design. For larger projects this will be calculated by the Services Engineer.
• Ensure that you know the risk factor to be applied to the design rainfall intensity for the particular roof/gutter/drainage system under consideration. For larger projects this will be determined by the Services Engineer.
• Consider the implications of a significant build-up of water on the roof, due either to the blockage of one or more outlets or the inability of the drainage system to cope with rainfall intensities above the design intensity.
• Consider the implications of 'verges' - which assume water will overflow in extreme conditions; rather than 'parapets' - which assume that water will increase in depth.
• Consider providing for extreme conditions by:
• increasing the design rainfall intensity.
• providing weirs, overflows, and the like to clear temporary surges of water.
• Consider what will happen if the roof is flooded, for however short a time, to a depth of, say, 150mm, or 200mm, or even 250mm. Consider:
• the structural load on the roof.
• flashing heights and perimeter detailing.

Further reading

• NBS subscribers should read General Guidance Note 5, Design Rainfall and Overflow, in NBS Section R10.
• An excellent article on the subject was published in the September 2004 issue of Roofing, Cladding, & Insulation as RCI Technical Note No.134. An electronic copy is available as a PDF file from this link.

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