How Much Roof Area Before I Need Roof Drainage

A large roof is going to move a lot of water which means the downspout locations may need to be adjust to compensate.

How much roof area before i need roof drainage. A good rule to follow is to provide two roof drains for a total roof area less than 10 000 square feet. Determine leader or pipe size. Sizing the drains for each system including the number of drains required is based on factors such as the roof area the drain location s the inflow rate and desired drainage rate and the expected rainfall volume. Refer to as nzs 3500 3.

According to the international building code the typical built up flat roof that uses tar or asphalt goes by the guideline that for every foot 12 inches of a flat roof a minimum of inch must step up or down. This is the basis for calculating the angle which is approximately a 1 19 degree slope. An additional roof drain should be added for each 10 000 square foot area. 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.

The more roof surface above the gutters the more rainwater will be caught and funneled down into those gutters. For hipped roofs with eaves gutters a simplified formula can be used to calculate catchment area but the slope of the roof must be known. Roof drainage systems design as nzs 3500 3. Calculate total roof area.

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. It can be argued that the minimum roof slope for drainage is 1 0 since this is sufficient to allow water to flow but there are other design considerations that must be taken into account when determining the final minimum slope. The drains should be placed at a maximum of 75 feet in each direction away from each other. Vertical walls abutting a roof must be included in the catchment area.

There are 5 easy steps to figure out how many drains your commercial roofing project will need. Dropping from the 5 drain in the table to a 3 drain cuts the serviced or drainable roof area to about 0 26 x the roof areas shown in the table.