Flat Roof Rainwater Drainage Calculations

The calculations below are meant for drainage from flat roofs where water are expected to spread over the entire flat roof during rain.

Flat roof rainwater drainage calculations. Take the roof s total square footage and divide by the total square footage handled by one drain. The above sizing data is offered as a guide only. Box gutter with rain water head at end of box gutter. The full flow capacity of a drain is not realized until the water reaches approximately 2 5 above the inlet in a four inch drain and 3 5 in a six inch drain.

Upon completion a full set of rainwater drainage design calculations are provided confirming the. Simplified suitable for flattish roofs with little or no vertical faces or steep roofs draining to one side of box gutter only or any roof if you are in a hurry and make the necessary allowances for all catchments. The tools guide the user step by step allowing fast and simple roof drainage design whilst ensuring full compliance with bs en12056 3 rainwater drainage design standard. To size the vertical roof drain for the same parameters 5 000 square feet of room area and 6 per hour of rain consult table 1106 2 figure 3.

Multiply the area of your roof by the total obtained in step 2 as outlined in the following equation. Sizing a roof water drain. Our online tools have been designed with simplicity in mind removing the need for any prior specialist industry knowledge. The next step is to determine each roof area that will be served by a separate leader drain pipe scupper or downspout.

Category 2 and 3 flat roof drainage linked to local rainfall data. The result is the number of drains needed. Calculate drains needed. I downloaded installed and tried out harmer drainage s free roof drainage calculator for a flat roof choosing a roof drain product line which has implications of drain pipe diameter and thus capacity geographic area the tool can plug in rainfall rates for the u k and i used an assumed a roof area of 10x10m 100 m2.

According to the chart for 1 8 slope per 12 of pipe for 5 000 square feet of roof area with a 6 per hour of rainfall the required pipe size is 8. This table is based on zurn s roof drain vertical requirement for horizontal roof areas at various rainfall rates table. Alumasc s leading drainage calculator allows architects specifiers and building engineer s to design and quantify all their rainwater drainage requirements using this dedicated software. 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.