What is the required downspout size for a rainfall intensity of 100 mm on a 1200 m2 deck roof?

To determine the required downspout size for a specific rainfall intensity and roof area, it’s essential to apply the formula that takes into account the volume of water that must be drained. For a rainfall intensity of 100 mm on a 1200 m² roof, you first convert the rainfall intensity into an equivalent amount of water:

1. Calculate the volume of water over one hour: A rainfall intensity of 100 mm means that in one hour, the roof will collect 100 mm of water over the entire area. The total volume can be calculated in cubic meters (m³) as follows:

Volume (m³) = area (m²) x rainfall intensity (m)

Volume = 1200 m² x 0.1 m = 120 m³

2. Next, you need to analyze the drainage capacity. The downspout needs to accommodate this volume of water efficiently to prevent overflow, ponding, or other drainage issues.

Understanding that larger downspouts allow for a higher flow rate is essential. A downspout size of 150 mm can handle the expected flow from the roof in a rainfall event of this intensity, allowing water to drain quickly and reducing the risk of clogs and water damage