Roof Drain Sizing Calculator

Calculate roof drain size for flat and low-slope roofs. Uses IPC tables for primary drains, overflow drains, and scuppers. Free professional-grade drainage calculator.

Calculate Roof Drain Size

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Drainage Requirements

Drainage Area

Total roof area draining to these drains

Rainfall Intensity

in/hr

Design storm intensity for your location

Drain Properties

Drain Type

Type of roof drain being designed

Dome/Strainer Type

Affects flow capacity reduction factor

Drain Diameter (optional)

Specify to check capacity, or leave blank for sizing

Number of Drains

Drains serving this drainage area

Design Parameters

Head Over Drain

Water depth above drain inlet

Max Ponding Depth

Maximum allowable water depth on roof

Flat Roof

Yes, flat/low-slope roof

Determines if overflow sizing is needed

Roof Drain Sizing Overview

Roof drains collect and convey rainwater from flat or low-slope roofs to the storm drainage system. Proper sizing prevents ponding that can cause structural damage or leaks.

Primary Drains - Main drainage from roof surface
Overflow Drains - Secondary drains at higher elevation for redundancy
Scuppers - Wall openings that act as overflow relief
Siphonic Drains - High-capacity drains using siphon action

Roof Drain Capacity (IPC Table 1106.1)

Drain Size (in)	@ 1" Head	@ 2" Head	@ 4" Head	Max Area (sf)
2"	26 gpm	36 gpm	52 gpm	5,000
3"	63 gpm	90 gpm	127 gpm	12,200
4"	121 gpm	172 gpm	242 gpm	23,300
5"	196 gpm	278 gpm	392 gpm	37,700
6"	295 gpm	418 gpm	590 gpm	56,700
8"	563 gpm	796 gpm	1126 gpm	108,300

Flow rates in GPM at various head depths. Max area based on 1 in/hr at 4" head. Source: IPC 2021, Table 1106.1

Dome/Strainer Reduction Factors

Dome Type	Flow Factor	Reduction
Standard Dome	0.90	10%
Flat Grate	0.85	15%
Beehive	0.92	8%
Vandal-Proof	0.80	20%
None	1.00	0%

Dome/strainer reduces effective flow capacity. Select "None" for theoretical maximum capacity.