How do you calculate tributary area per drain?

How do you calculate tributary area per drain?

• A. The sum of all drain diameters.
• B. Roof slope multiplied by drain spacing.
• C. Total roof area divided by the number of drains.
• D. The area around the main entrance.

Answer: (C)

Tributary area per drain is the portion of roof area that drains to that specific drain. When the system is designed so each drain handles an equal share, you find this area by dividing the total roof area by the number of drains. This gives the catchment area for each outlet, which you use to estimate rainfall runoff and verify the drain can handle the flow. The other ideas don’t measure the area that contributes to a drain. Drain diameter relates to capacity, not how much area drains to it. Slope times drain spacing isn’t a direct method for determining catchment area. The area around the main entrance isn’t used in this calculation.

Tributary area per drain is the portion of roof area that drains to that specific drain. When the system is designed so each drain handles an equal share, you find this area by dividing the total roof area by the number of drains. This gives the catchment area for each outlet, which you use to estimate rainfall runoff and verify the drain can handle the flow.

The other ideas don’t measure the area that contributes to a drain. Drain diameter relates to capacity, not how much area drains to it. Slope times drain spacing isn’t a direct method for determining catchment area. The area around the main entrance isn’t used in this calculation.