Flat Roof Rainwater Drainage Calculations

Box gutter with rain water head at end of box gutter.

Flat roof rainwater drainage calculations.

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. Take the roof s total square footage and divide by the total square footage handled by one drain. 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. Sizing a roof water drain.

Category 2 and 3 flat roof drainage linked to local rainfall data. 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 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. The above sizing data is offered as a guide only.

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. 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. F x p a 4 estimate the rainfall per event in your area r by dividing the annual precipitation rate by the number of rain days per year. The calculations below are meant for drainage from flat roofs where water are expected to spread over the entire flat roof during rain.

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. Calculate drains needed. Drain calculator factors which affect proper roof drainage are deck slope drain size and placement of drains. The next step is to determine each roof area that will be served by a separate leader drain pipe scupper or downspout.

Multiply the area of your roof by the total obtained in step 2 as outlined in the following equation. 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. Upon completion a full set of rainwater drainage design calculations are provided confirming the. The result is the number of drains needed.