The Gutters, Downpipes and Overflow (GDO) Calculator assists in developing a better understanding of the NCC Volume Two Part 3.5.3 Gutters and Downpipes Acceptable Construction Practice (ACP) provisions. To use the GDO Calculator enter information about the location, roof design and types of overflow measures. Based on the location, roof design and overflow types, the calculator provides the allowable gutter types, downpipe sizes and whether the selected overflow measures are adequate. It is a MS Excel file and uses data contained in NCC Volume Two for the calculations. This calculator has been created and designed for Excel 2013 for Windows. Compatibility with other versions and operating systems is not guaranteed.