Gwilym T. Still and Terry Thomas (2002)
Guttering, in a roofwater harvesting system, has the purpose of intercepting the roof run-off and conveying it to a downpipe (which in turn carries it to a store). The two phases of 'interception' and 'conveyance' make different and sometimes conflicting demands upon a guttering design. Their respective failures (overshoot and overflow) occur under similar circumstances, namely intense rain, and for most analytic purposes it is appropriate to consider as total water loss just the higher of the overshoot loss and the overflow loss, rather than their sum. A good gutter design must satisfy many criteria including durability, cheapness and ease of fixing. In this paper on gutter sizing, the primary approach is to find that gutter size and shape that maximises the ratio of water benefit to system cost. The work was motivated by field observation of evidently over-sized gutters and the absence of any published 'informed' guidance on sizing. The study entailed theoretical analysis, laboratory experimentation and some field studies. The findings are that:
- 'U' or trapezoidal-section gutters give the best economy,
- roof area is the primary determinant of gutter size and
- a 'U'-shaped or trapezoidal section gutter of width only 70 mm will be sufficient for most house roofs in the tropics.
The optimum gutter location and fixing trajectory are also explored to produce recommendations.