CLIMATE CHANGE AND ROOFING: RAINFALL INTENSITY

Rainfall patterns are changing. There has been severe local flooding in parts of the UK, for example in East Sussex 150mm of rain fell overnight on the 11th and 12th October 2000. As our climate warms, the mean precipitation is expected to increase. The question is raised, how will the rate at which rain falls affect roofing and cladding?

For many roofing systems the increase in the rainfall intensity should not affect the performance of the individual roof tiles, sheets or membrane. Where there is concern is in how the water is collected in the rainwater goods system and safely channelled down to ground level. Sudden downpours of high volume can cause local flooding in the gutters, leading to overtopping and water entry into the rooms below. Thus, for buildings with valley or boundary wall gutters the rainfall intensity is an important design issue.

This Technical Note discusses the rate at which rain falls. The combined effect of rainfall and wind, causing driving rain, will be discussed in a separate Technical Note.

Rainfall intensity design data

The new British Standard for the design of roof drainage is BS EN 12056 Part 3:2000. This standard was approved by CEN in October 1999 and came into effect in September 2000. It supercedes BS6367:1983, which has now been withdrawn.

The statistical meteorological data for the UK are given in the national annex NB. The old rainfall intensity maps have been redrawn for different return periods, while expressing the rainfall intensity in units of litres /second per m². The rainfall intensities for London and Glasgow for the different return periods are shown in the Table.

Met Office Records

The Met Office recording stations measure rainfall hourly. In the past rainfall has not been recorded over shorter intervals. Thus the rain from short duration storms lasting only a few minutes has been measured after 60 minutes, giving an average hourly value and losing the peak rainfall intensity rates. It is understood that there are no plans at present to start recording rainfall over short intervals across the country.

The previous rainfall intensity data in BS 6367 was gathered during the 1970's as part of the work for the 'Flood Studies Reports'. The Met Office set up special recording equipment at selected sites to measure the rainfall over short intervals. The correlations between short interval and hourly records were then used on the hourly data available throughout the country to prepare the rainfall maps.

In drafting the new British Standard, no new data has been used for the two minute duration storms. Instead the original figures have been redrawn, albeit in a different format, such that there should not be any significant changes from the old BS 6367. Consequently the latest British Standard is based on data recorded more than 20 years ago, with no allowance made for predicted climate change. If we do continue to experience very intense short duration rainstorms, at present there isn't a nationwide data collection programme and only limited historic data for comparative purposes.

It is important for those designing rainwater goods systems to be aware of the imprecise nature of the basic data.

Gutters

Sloping valley : If the design rate of rainfall increases, then for tile and slate roofs with sloping valley gutters, there may be a need to increase the gutter width, in accordance with BS 5534.

As a minimum the width of the valley gutter should be 100mm. For valleys on shallow pitches draining more than 25m² in plan, the minimum gutter widths should be 150mm for a 0.02 l/s storm (75mm/h), 200mm wide for a 0.04 l/s storm (150mm/h) and 250mm wide for a 0.06 l/s storm (225mm/h).

Valley, parapet and boundary wall gutters : For short duration intense rainstorms, it may be that the outlets and downpipes can't drain away the surge of rainwater quickly enough. In such situations the gutters will store volumes of water, acting like a 'reservoir'.

The detailed hydraulic design of gutter outlets and downpipes is not a precise science. However, providing good sized gutters will improve the short term storage, making the overall system more robust and less likely to overtop.

Siphonic drainage systems

Because of their hydraulic characteristics, siphonic drainage system are more sensitive to increases in rainfall intensity when compared to conventional systems. Small increases in design rate require large increases in available head, or downpipe height, for the system to drain siphonically. This is not always possible. The Hydraulics Research Report published in September 1996 concluded that any error made in selecting an appropriate value of design rainfall intensity is more likely to become apparent with a siphonic system.

Conclusions

The rainfall intensity data used in the latest British Standard are based on limited data gathered more than twenty years ago. In the future it would be helpful if rainfall data could be recorded at selected sites over shorter intervals, such as 15 minute or 5 minute periods. This would give more reliable measurement of short duration rainstorms and could be used to determine how rainfall intensity changes in future years.

For the design of rainwater goods and especially siphonic drainage systems, it is worthwhile adopting a robust approach. If in doubt, increase the design rainfall intensity. This should be defined by the project design team, so that at the tender stage there is fair competition by those offering design and build packages.

References

© Keith Roberts, 2001

First published in Roofing Cladding Insulation, March 2001

RCI Technical Notes

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