Flat Roof Rainwater Drainage Calculations

The one challenge with flat roofing is ensuring proper drainage.

Flat roof rainwater drainage calculations.

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 number of roof drains shown should be verified with local code requirements. 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. In sizing roof drains and storm drainage piping one half of the area of any vertical wall that diverts rainwater to the roof shall be added to the projected roof area for inclusion in calculating the required size of vertical conductors leaders and horizontal storm drainage piping.

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

Poor drainage can lead to serious issues with flat roofing from leaks to structural damage. 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. Take the roof s total square footage and divide by the total square footage handled by one drain.

Calculate drains needed. This reveals that the minimum size for the leader pipe is 5. For flat roof drainage where you place your drains scuppers and the final material of the roofing is more important. 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 system outlined below will walk you through the steps of determining your rain barrel sizes based on average roof runoff. 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. The result is the number of drains needed.

Anyone can begin the process of turning a home or business green by making and installing a basic rain barrel. The angle is not significant enough to prevent water from pooling on the surface.