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Roof rainfall demand
Flow demand only
About Roof Drain Design
Roof drains are essential components of flat and low-slope roof drainage systems. Proper sizing prevents excessive ponding that can cause structural damage, leaks, and premature roof failure.
From rainfall demand to a selected drain
This calculator supplies runoff demand only. Obtain the published flow for the selected drain model at the allowable water head before assessing capacity or choosing a size. Generic diameter tables cannot replace this product-specific rating.
2021 IPC Sections 1105.2 and 1106.2 require published roof-drain flow data. Section 1106.4 addresses wall contribution. Secondary drainage must be evaluated without credit for the primary system flow under Section 1108.3. Ponding limits, overflow elevations, scupper geometry, and siphonic design need separate evaluation.
Flow Calculation
The design flow rate is calculated from drainage area and rainfall intensity:
Q = (A x i) / 96.23
Where:
- Q = Flow rate (gpm)
- A = Drainage area (sf)
- i = Rainfall intensity (in/hr)
Reference & standards
What This Solves
Calculates total roof rainfall flow and the demand per drain for a chosen drain count. Drain selection and capacity require manufacturer flow-versus-head data.
Best Used When
- You need total rainfall flow for a defined contributing roof area
- You want to compare per-drain demand for different drain counts
- You are preparing a required flow for comparison with a selected drain model
Do NOT Use When
- You are sizing edge-mounted gutters for a sloped residential roof — Use Gutter Flow Calculator
- You need to size the downspouts or leaders that carry water from the drains to grade — Use Downspout Sizing Calculator
Key Assumptions
- Rainfall intensity is uniform across the entire roof area
- The entered roof area is the contributing area for the selected drainage group
- Total demand is divided equally among the entered number of drains
- Drain capacity, ponding head and structural adequacy are not calculated
Input Quality Notes
Use the required design rainfall, contributing area and intended drain count. Compare the resulting per-drain demand with the selected manufacturer’s flow-versus-head data and project ponding limits.
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Last verified: February 2026