This paper describes a full-scale experimental investigation into the effects of box gutter geometry on the open channel flow conditions above siphonic roofwater outlets. In particular, the effects of channel width (300, 400, 480 and 600 mm) and length (14.86 and 32.00 m) were investigated through measurements of flow rate, water depth and longitudinal velocity in the box gutter. The experimental results showed that for the same outlet flow rate, the depth of water in the gutter varied by up to 211% for the two different gutter lengths tested. Generally, the greatest water depths for the different flow rates were recorded in the 400-mm wide gutter and the lowest water depths were recorded in the 300-mm wide gutter. It was also found that the maximum flow rate through the single 110-mm diameter outlet varied depending on the width of the gutter. Practical application: The depth of water in the open channel box gutters above the outlets of siphonic drainage systems is an important design variable and it is imperative to be able to accurately estimate these water depths during all phases of operation to reduce flood damage risk. This research study has found that for the same flow rate, varying the length of gutter on either side of a siphonic outlet strongly influences the depth of water along the gutter and above the outlet. The results suggest that there may be an optimum gutter width and length for which different siphonic outlets may perform more efficiently.