Flat Roof Rainwater Drainage Calculations

Unlike other roofing types that are slanted or sloped to allow water to drain automatically flat roofs need a built in drainage system to help remove moisture and debris.

Flat roof rainwater drainage calculations.

The first step in sizing a roof drain is to calculate the surface area of the roof that the drains will be covering as well as the average hourly rainfall rate at the building location. The calculations below are meant for drainage from flat roofs where water are expected to spread over the entire flat roof during rain. 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 next step is to determine each roof area that will be served by a separate leader drain pipe scupper or downspout.

The above sizing data is offered as a guide only. The one challenge with flat roofing is ensuring proper drainage. Poor drainage can lead to serious issues with flat roofing from leaks to structural damage. The system outlined below will walk you through the steps of determining your rain barrel sizes based on average roof runoff.

This table is based on zurn s roof drain vertical requirement for horizontal roof areas at various rainfall rates table. This reveals that the minimum size for the leader pipe is 5. The roof drain calculator below is based on the maximum amount of rainfall in inches per hour of the hypothetical worst storm in the past 100 years and which has a two percent chance of occurring in any given year. Sizing a roof water drain.

To keep damage like leaks from occurring water should not stand on a flat roof for more than 48 hours. A simple rain catchment system can provide water for outdoor use resulting in a lower water bill. The speed at which the rain collects at the drain. For flat roof drainage where you place your drains scuppers and the final material of the roofing is more important.

Calculate drains needed. The angle is not significant enough to prevent water from pooling on the surface. Anyone can begin the process of turning a home or business green by making and installing a basic rain barrel. The result is the number of 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. Take the roof s total square footage and divide by the total square footage handled by one drain.