Simple Domestic Arrangement CI/SfB52.5 ROOF DATA SOLUTIONS
Pages 3

Rule of thumb for sizing rainwater pipes (useful for estimating) Allow 650 mm2 cross-sectional area of rainwater pipe per 9 m2 of roof area*, therefore

As the cross-sectional areas of 50 mm and 63.0 mm rainwater pipes are 2000 mm2 and 3100 mm2 (approximately) respectively, the designer would probably select the 63.0 mm size which is ‘oversizing’ but allows for a large safety factor.