Roof Drain Pipe Size Calculator

How the Roof Drain Pipe Size Calculator Works

Three inputs drive the result:

1. Adjusted roof area - horizontal projected area draining to the pipe, plus 50% of any adjacent vertical wall area per IPC 1106.4.
2. Design flow rate - Q (GPM) = 0.0104 x Area x rainfall intensity (in/hr).
3. Pipe capacity lookup - IPC Table 1106.2 for horizontal storm drains, Table 1106.3 for vertical leaders. Both tables are rated at 4 in/hr rainfall and are scaled linearly for your local rainfall.

Horizontal Storm Drain Sizing (IPC Table 1106.2)

Maximum allowable roof area in square feet at 4 in/hr rainfall, at common pipe slopes:

Pipe Diameter	1/16 in/ft slope	1/8 in/ft slope	1/4 in/ft slope	1/2 in/ft slope
3 in	580	822	1,160	1,644
4 in	1,320	1,880	2,650	3,760
5 in	2,350	3,340	4,720	6,680
6 in	3,760	5,350	7,550	10,700
8 in	8,100	11,500	16,300	23,000
10 in	14,500	20,700	29,200	41,400
12 in	23,400	33,300	47,000	66,600

For locations with higher design rainfall, scale the allowed area down: allowed_area = table_value x (4 / local_rainfall_rate). A 6 in pipe at 1/8 slope handles 5,350 sq ft at 4 in/hr but only 2,675 sq ft at 8 in/hr.

Vertical Leader Sizing (IPC Table 1106.3)

Vertical downspouts move water faster than horizontal pipes and carry more area per diameter:

Leader Diameter	Max Roof Area (sq ft) at 4 in/hr
2 in	2,880
3 in	8,800
4 in	18,400
5 in	34,600
6 in	54,000
8 in	116,000

Adjacent Wall Adjustment (IPC 1106.4)

When a vertical wall drains onto the roof, you must add wall area to the roof area used for pipe sizing. The code multiplier is 50% of wall height times wall length for a single wall. For two walls that meet at a corner, use 35% because only the taller adds useful area. Three walls around a courtyard use 35% per wall. This roof drain pipe size calculator handles the adjustment automatically when you specify wall orientation, height, and length.

Typical Design Rainfall Rates (100-year, 1-hour)

• Pacific Northwest: 1.5-2.5 in/hr
• Northeast, Great Lakes: 2.5-3.5 in/hr
• Mid-Atlantic, Midwest: 3-4 in/hr
• Southeast, Florida: 4-6 in/hr
• Gulf Coast, South Texas: 5-7 in/hr
• Western mountain and desert: 1-3 in/hr

Use NOAA Atlas 14 precipitation frequency data for your exact location. Your local code may require a longer return period (500-year for critical facilities).

Worked Example: 10,000 sq ft Flat Roof

A 10,000 sq ft flat commercial roof in Atlanta (5 in/hr design) with 1/8 in per foot horizontal slope:

1. Adjusted roof area: 10,000 sq ft (no walls)
2. Design flow: 0.0104 x 10,000 x 5 = 520 GPM
3. Scaled capacity at 5 in/hr = table_value x (4/5) = 0.8x
4. 8 in pipe: 11,500 x 0.8 = 9,200 sq ft (undersized)
5. 10 in pipe: 20,700 x 0.8 = 16,560 sq ft (66% margin, passes)

Choose a 10 in horizontal storm drain. For the vertical leader, a 4 in leader handles 18,400 x 0.8 = 14,720 sq ft (47% margin).

When to Oversize

Increase pipe size by one nominal step when:

• Roof has pitched valleys or scuppers that concentrate flow
• Drain catches debris (pine needles, leaves)
• Commercial roof where a blocked primary drain triggers the secondary overflow
• Local code requires a 500-year design storm

Oversizing one pipe size typically adds 15-25% to material cost but doubles capacity margin.

Secondary (Emergency Overflow) Drains

IPC 1107 requires secondary roof drains at an elevation 2 inches above the primary. Size the secondary for the same design storm as if the primary were fully blocked. Use this roof drain pipe size calculator twice: once for primary, once for secondary overflow.