Rainwater Drainage Calculator for Flat Roofs
The roof drainage outlets and rainwater downpipes are calculated based on the design principles for gravity drainage systems established in
SS525:2006 Code of Practice for Drainage of Roofs.
The calculations below are meant for drainage from flat roofs where water are expected to spread over the entire flat roof during rain.
The flat roof must have sufficient slope to allow water to flow towards the roof outlets.
Project Information
Reference No:
Project Name
Roof Description
Reference No:
Project Name
Roof Description
Catchment Area and Rainfall Intensity
Roof Area (Ah)
m.sq.
Elevational Area (up to height of 10m) (Av)
m.sq.
Effective Elevational Area (50% of Av)
0 m.sq.
Total Effective Catchment Area (Ae)
0.00 m.sq.
Rainfall Intensity (I)
mm/hr.
Total Flow Rate (Qroof)
0.00 l/s.
Roof Area (Ah)
m.sq.
Elevational Area (up to height of 10m) (Av)
m.sq.
Effective Elevational Area (50% of Av)
0 m.sq.
Total Effective Catchment Area (Ae)
0.00 m.sq.
Rainfall Intensity (I)
mm/hr.
Total Flow Rate (Qroof)
0.00 l/s.
Roof Drainage Outlets
Number of roof outlets
Percentage of opening on strainer (leaf guard)
%
Effective diameter of roof outlet (D)
mm
Notes: roof outlet diameter should be similar to diameter of rwdp
Design water depth at roof outlet (h)
mm
Rainwater Down Pipes
Required Flow Rate (Q)
l/s
Pipe inner diameter (Di)
mm
Note: pipe inner diameter should not be smaller than the diameter at the throat of roof outlet
Roof outlet level
mm
Horizontal offset below roof outlet
Horizontal offset below roof outlet
Pipe gradient:
1 TO
( 0.5729 degree)
Offset pipe length
m
Offset upstream level
Offset downstream level
Intermediate floor horizontal offsets
Intermediate horizotal offset No.1
Pipe gradient:
1 TO
( 0.5729 degree)
Offset pipe length
m
Offset upstream level
Offset downstream level
Intermediate horizotal offset No.2
Pipe gradient:
1 TO
( 0.5729 degree)
Offset pipe length
m
Offset upstream level
Offset downstream level
Horizontal discharge pipe
Horizontal discharge pipe
Pipe gradient:
1 TO
( 0.5729 degree)
Offset pipe length
m
Offset upstream level
Offset downstream level
Pipe levels calculator
