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Roof Drain Size Calculator

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When designing or maintaining buildings, one crucial aspect often overlooked is the roof drainage system. It's not just about preventing puddles on your roof; it's about safeguarding the entire structure from water damage. This is where the Roof Drain Size Calculator comes into play. It's a tool designed to help you determine the optimal size for your roof drains, ensuring they can handle even the heaviest of rains.

Purpose and Functionality

The primary purpose of the Roof Drain Size Calculator is to calculate the required capacity for roof drains based on several factors: the size of the roof area that drains into them, the maximum expected rainfall intensity, and adjustments for the slope of the roof. This tool takes these complex calculations and simplifies them, making it easier for homeowners, builders, and architects to make informed decisions.

The functionality of the calculator is grounded in a straightforward formula that accounts for:

  1. Roof Area (A): The total surface area of the roof that contributes water to the drain.
  2. Rainfall Intensity (I): The highest rate of rainfall expected, usually given in inches per hour or millimeters per hour.
  3. Roof Slope (S): A correction factor based on the slope of the roof, with flat roofs generally having a factor of 1.
  4. Drain Coefficient (C): A factor that considers the drain's efficiency, affected by potential debris and the type of drainage system.

Step-by-Step Examples

Let's consider a practical example:

  • Roof Area (A): 1000 square feet
  • Rainfall Intensity (I): 5 inches per hour
  • Roof Slope (S): Flat roof, so we'll use 1
  • Drain Coefficient (C): 1 (assuming a clean, efficient drain)

Using our formula, the required drain size in gallons per minute (GPM) is calculated as follows:

Drain Size=Roof Area×Rainfall Intensity×Drain Coefficient96.23Drain Size=96.23Roof Area×Rainfall Intensity×Drain Coefficient​

Drain Size=1000×5×196.23≈52 GPMDrain Size=96.231000×5×1​≈52 GPM

This means you'll need a drain capable of handling 52 gallons of water per minute to effectively manage the water from a 1000 square foot section of roof during a rainfall intensity of 5 inches per hour.

Relevant Information Table

Here's a simple table that can help you with the common inputs for the calculator:

FactorDescriptionTypical Values
Roof Area (A)Size of the roof area draining into the drain500 - 2000 sq ft (or more)
Rainfall Intensity (I)Maximum expected rainfall intensity1 - 6 inches per hour
Roof Slope (S)Correction factor based on roof slope1 (for flat roofs)
Drain Coefficient (C)Factor considering drain's efficiency and potential blockages0.9 - 1.1


The Roof Drain Size Calculator is a vital tool for anyone involved in building design, construction, or maintenance. By ensuring that your roof drains are sized correctly, you can prevent water accumulation, reduce the risk of damage, and maintain the structural integrity of your building. This calculator simplifies the complex calculations required to determine the optimal drain size, making it an indispensable tool for making informed decisions about roof drainage systems. Whether you're a professional or a DIY enthusiast, understanding and using this calculator can save you time, money, and a lot of headaches in the long run.

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