Roof Drain & Rainwater Leader Sizing Calculator — IPC 1106

Size roof drains, vertical leaders, and horizontal rainwater piping by code-tabulated roof area and rainfall intensity per IPC Table 1106.1/1106.2, UPC 1105–1108, and ASME A112.6.4.

By TradeCalc, Plumber Calculators — Code-Referenced — IPC Table 1106.1, IPC Table 1106.2, IPC 1105, IPC 1108, UPC 1105–1108, ASME A112.6.4

Related Calculators

How to Size Roof Drains & Rainwater Leaders (IPC 1106)

Why Roof Drain Sizing Matters

An undersized roof drain system allows water to pond on the roof membrane. As ponding depth grows, the weight of accumulated water (62.4 lb/ft³) can deflect the structural deck, which deepens the pond, which adds more weight — a runaway failure mode that has collapsed flat-roofed buildings across the US. IPC 1105–1108 and ASME A112.6.4 require every roof drain system to carry the 100-year, 1-hour rainfall without overflow, plus a secondary (emergency) drainage system at a higher elevation that activates if the primary system clogs.

Rainfall Intensity — The Driving Input

IPC 1105.1 requires sizing for the locally published 100-year, 1-hour rainfall intensity (in/hr) from NOAA Atlas 14 or the local code. Typical values:

1–2 in/hr: arid West (Phoenix, Los Angeles, Denver)
2–4 in/hr: temperate (Seattle, San Francisco, Minneapolis)
3–5 in/hr: humid continental (Chicago, Boston, New York)
4–6+ in/hr: Gulf Coast / Southeast (Houston, Miami, New Orleans)

Always confirm the value with your local jurisdiction — most cities amend IPC with stricter local rainfall tables.

IPC Table 1106.2 — Vertical Leader Capacity

Leader size	1 in/hr	2	3	4	5	6
2"	2,880	1,440	960	720	576	480
3"	8,800	4,400	2,933	2,200	1,760	1,467
4"	19,000	9,500	6,333	4,750	3,800	3,167
5"	34,000	17,000	11,333	8,500	6,800	5,667
6"	56,000	28,000	18,667	14,000	11,200	9,333
8"	125,000	62,500	41,667	31,250	25,000	20,833

Values = max allowable horizontal projected roof area (sq ft) per vertical leader at each rainfall intensity.

Find the smallest leader size whose tabulated capacity at your rainfall intensity is ≥ your roof area.

Horizontal Drain Piping (IPC Table 1106.1)

Horizontal storm drain capacity scales with pipe size and slope. At 1/8 in/ft slope the table gives a fixed sq-ft-per-size at 1 in/hr, which is then divided by your rainfall intensity. Steeper slope (1/4 or 1/2 in/ft) increases capacity by roughly √(slope ratio).

Worked Example

Scenario: A flat commercial roof with 2,500 sq ft of horizontal projected area, located where the 100-year 1-hour storm is 4 in/hr. The horizontal storm drain runs at the minimum 1/8 in/ft slope. Size the leader and the horizontal drain.

Peak runoff: 2,500 sq ft × 4 in/hr ÷ 96 = 104.2 gpm
Leader — IPC Table 1106.2 at 4 in/hr: 2"=720 (✗), 3"=2,200 (✗), 4"=4,750 (✓)
Minimum leader = 4 in (handles 4,750 sq ft at 4 in/hr, 90% spare)
Horizontal drain at 1/8 slope, 4 in/hr: 4"=940 (✗), 5"=1,670 (✗), 6"=2,700 (✓)
Minimum horizontal drain = 6 in at 1/8 in/ft slope
Drain count: 1 drain for ≤10,000 sq ft (typical max spacing 200 ft per UPC 1105.2)

A 6" horizontal drain is required even though a 4" leader works, because horizontal flow at minimum slope is far less efficient than vertical downflow. Steepening the horizontal to 1/4 in/ft would let a 5" pipe handle 2,358 sq ft (still tight); only at 1/2 in/ft does a 5" pipe clearly work (3,335 sq ft).

Practical Tips

Always provide a secondary (overflow) system. IPC 1108 / UPC 1105.3 require an independent emergency drainage path (scuppers or a second set of drains) at a higher elevation, in case leaves or debris clog the primary drains. Without it, water can pond to failure depth.
Horizontal drains need bigger pipe than vertical leaders. Vertical downflow carries water by gravity with no friction loss from walls; horizontal flow at minimum slope needs a much larger pipe. In the worked example, the leader is 4" but the horizontal run is 6".
Space drains ≤ 200 ft apart and provide at least 1 drain per 10,000 sq ft of roof area (UPC 1105.2, SMACNA guidance). Roofs over 10,000 sq ft should use multiple drains to limit tributary area per drain.
Confirm the rainfall intensity locally. Most jurisdictions amend IPC with newer NOAA Atlas 14 values. A roof designed for 3 in/hr that should have been 5 in/hr is effectively 40% undersized.
Specify deck drains, not just pipe size. ASME A112.6.4 / ASME A112.3.1 cover the drain body, clamping ring, and dome strainer. The dome open area must be ≥ 2× the leader area to prevent leaf-clogging.

Code References

IPC Table 1106.1, IPC Table 1106.2, IPC 1105, IPC 1108, UPC 1105–1108, ASME A112.6.4

Frequently Asked Questions

How do I find the rainfall intensity for my project?

IPC 1105.1 requires using the locally published 100-year, 1-hour rainfall intensity from NOAA Atlas 14 (hdsc.nws.noaa.gov) or your local code amendment. Values range from ~1 in/hr in the arid West to 4–6+ in/hr along the Gulf Coast. High-rainfall jurisdictions (e.g., Houston, Miami, Phoenix) amend IPC with stricter local rainfall tables, so always confirm with the Authority Having Jurisdiction before sizing.

Why does my horizontal drain need a bigger pipe than the vertical leader?

Vertical leaders carry water by free-fall — gravity does the work and pipe walls contribute little friction. Horizontal drains at minimum slope (1/8 in/ft) must push water along with a very shallow hydraulic gradient, so the same flow needs a much larger pipe. In the worked example, a 4" leader handles 2,500 sq ft at 4 in/hr but the horizontal drain requires 6" at 1/8 slope. Steepening the horizontal to 1/4 or 1/2 in/ft reduces the required size.

How many roof drains do I need?

UPC 1105.2 and SMACNA guidance recommend at least one roof drain per 10,000 sq ft of roof area, with drains spaced no more than ~200 ft apart to limit the tributary area per drain. On a 50,000 sq ft roof you would typically provide 5 drains, each sized for its 10,000 sq ft tributary area. More drains means smaller individual leaders and better redundancy if one clogs.

Is a secondary (overflow) drainage system really required?

Yes. IPC 1108 and UPC 1105.3 require an independent secondary (emergency) roof drainage system — either scuppers at a higher elevation or a second set of drains — that activates only if the primary system is blocked. The secondary system must be sized for the same rainfall intensity as the primary. The most common code violation on flat roofs is omitting or undersizing overflow scuppers, which has caused documented roof collapses during intense storms.