Comparing empirical water depth observations of a box gutter roof drainage system to three different international design guidelines

Effective roof drainage is necessary to protect buildings and their contents from rainfall damage. Regulatory documents exist to ensure that a building's roof drainage system is adequate for the anticipated rainfall events experienced during the life of the building. Empirical observations that inform international design guidelines are largely attributable to a small number of investigative studies with a limited range of roof drainage system configurations and flow conditions. This paper compares the observed hydraulic performance of a 600 mm wide, rectangular box gutter case study model under various flowrates to values predicted in the Australian, US and European standards. The study measured the depth of water at various locations in the box gutter over a range of flowrates from 3 L/s to 20 L/s to determine whether the gutter depths recommended by the various design guidelines were adequate. Four different gutter slopes (1:200; 1:150; 1:100; and 1:40) were tested in the study. The results of the study indicated that the depth of flow in the box gutter was highly dependent on the inflow rate and the gutter slope. The results showed that the observed gutter water levels may or may not be considered adequate and safe, depending on the individual guideline applied. As regulatory documents aim to provide effective guidance, a deficit of box gutter depth is concerning while a surplus represents wasted resources. The findings of this study demonstrate some of the challenges faced by designers of roof drainage systems and hydraulic engineers.