Roof Drain & Storm Pipe Sizing Calculator
Enter the roof area, rainfall rate, drain count, tested drain capacity, and pipe slope to size a preliminary IPC storm-drain path. The result separates primary drain demand from the independent secondary drainage check.
Minimum horizontal storm-pipe size4 in
- IPC adjusted drainage area
- 10,000
- Total design runoff
- 415.67
- Primary flow per drain
- 103.92
- Minimum drains at rated capacity
- 4
- Installed drain-capacity margin
- 20.29%
- Minimum vertical leader size
- 4 in
- Secondary flow per outlet
- 103.92
- Minimum secondary leader size
- 4 in
Sizing checks
| Check | Design flow | Selected size | Table capacity |
|---|---|---|---|
| Horizontal storm pipe per primary drain | 103.92 | 4 in | 115 |
| Vertical leader per primary drain | 103.92 | 4 in | 192 |
| Secondary leader per outlet | 103.92 | 4 in | 192 |
Run a separate calculation for each roof section that cannot freely drain to the others.
Primary and secondary drainage are treated as independent systems carrying the full design storm where required.
How to use this calculator
- Enter the plan-view roof area served by one independent drainage section.
- Add any vertical wall area that sheds rain onto that roof section.
- Enter the adopted 100-year hourly rainfall rate and the proposed primary-drain layout.
- Select the actual horizontal storm-pipe slope and enter the number of secondary outlets.
- Review the drain count, pipe sizes, and notes before applying local code requirements.
How the roof drainage size is calculated
The calculator follows the ordinary 2024 IPC model-code path for roof runoff, drain capacity, and tabulated pipe sizing. It starts with the projected horizontal roof area. If a vertical wall drains rainwater onto the roof, IPC Section 1106.4 adds one-half of that wall area to the projected roof area:
Ad = Ar + 0.5Aw
Ar is the horizontal roof area, Aw is the contributing vertical wall area, and Ad is the adjusted drainage area. The rainfall conversion then estimates design runoff:
Q = Ad × R ÷ 96.23
R is the 100-year hourly rainfall rate in inches per hour. This is the same practical conversion as the IPC gallons-per-minute equation, rounded for the stated units.
The primary flow per drain is Qp = Q ÷ Np, where Np is the number of proposed primary drains. The minimum primary drain count is ceil(Q ÷ C), where C is the tested capacity of one drain at the expected water head. The installed capacity margin is 100 × (NpC - Q) ÷ Q; a negative value means the proposed primary-drain layout is short of the entered rated capacity.
How pipe and leader sizes are selected
The horizontal storm-pipe size is selected from IPC Table 1106.2 at the chosen slope. The calculator does not interpolate between nominal diameters. It picks the smallest listed pipe whose tabulated capacity is at least the primary flow per drain.
Vertical primary leaders and secondary leaders are selected from IPC Table 1106.3 in the same way. The secondary flow per outlet is Qs = Q ÷ Ns, where Ns is the number of separate secondary outlets. The secondary system is treated as carrying the full design storm independently, not only the overflow left after the primary drains operate.
What changes the result most
Rainfall rate and adjusted area move every result directly. Doubling either one doubles the design runoff. Increasing the primary drain count reduces flow per primary drain, which can reduce the pipe and leader sizes, but only when the reduced flow crosses a table threshold. A tested drain capacity below the calculated flow per drain can fail the drain-count check even when the pipe table produces a nominal size.
What this calculator leaves out
This is not a hydraulic model of a roof. It does not evaluate ponding depth, roof slope, structural deflection, drain blockage, unequal piping resistance, siphonic behavior, scupper geometry, underground storm sewer capacity, local amendments, or whether the roof structure can support the water depth before overflow occurs. Use a separate calculation for each roof section that cannot freely drain to the others.
Worked example
For a 10,000 ft2 roof with no contributing wall area, a 4 in/hr rainfall rate, four primary drains, 125 gpm tested capacity per drain, 1/8 in/ft horizontal pipe slope, and four secondary outlets, the adjusted drainage area is 10,000 ft2.
The design runoff is 10,000 × 4 ÷ 96.23 = 415.67 gpm. Dividing by four primary drains gives 103.92 gpm per drain. The minimum drain count is ceil(415.67 ÷ 125) = 4, and the installed drain-capacity margin is about 20.29%. At 1/8 in/ft, IPC Table 1106.2 selects a 4 in horizontal storm pipe because 3 in is below the flow and 4 in carries 115 gpm. IPC Table 1106.3 selects a 4 in vertical leader for the same 103.92 gpm. The secondary flow is also 103.92 gpm per outlet, so the secondary leader is 4 in.
Common questions
What rainfall rate should I use for roof-drain sizing?
Use the rainfall intensity required by the adopted plumbing code and the authority having jurisdiction. The IPC model path uses the 100-year hourly rainfall rate or other approved local weather data. NOAA precipitation data can help locate rainfall values, but the approved design value is a code and jurisdiction decision.
Why is vertical wall area added to roof area?
A wall that sheds rain onto a lower roof increases the water entering that roof drainage section. IPC Section 1106.4 adds one-half of the contributing vertical wall area to the projected roof area for sizing leaders and storm drainage piping. Zero wall area means there is no added wall contribution.
Why can the horizontal storm pipe be larger than the vertical leader?
Horizontal storm pipes have slope-dependent capacities in IPC Table 1106.2. A shallow horizontal run may carry less water than a vertical leader of the same nominal size, so the selected horizontal pipe can be larger. The calculator checks the horizontal pipe and vertical leader against their separate tables.
Does outlet diameter determine a roof drain flow capacity?
No. Roof drain capacity depends on the tested drain assembly, strainer, outlet configuration, and water head. Enter the manufacturer-published flow rate for the specific drain condition rather than assuming that outlet diameter alone is enough.
Must secondary roof drainage handle the full design storm?
Where secondary drainage is required, it is generally treated as an independent system rather than a supplement to the primary system. This calculator sizes secondary flow from the full design runoff divided by the number of proposed secondary outlets. Local amendments and the approved drainage design control the final requirement.
Can primary and overflow drains connect to the same pipe?
Primary and secondary drainage must remain independent where the adopted code requires independent secondary drainage. Combining them can defeat the purpose of overflow protection if the common downstream pipe is blocked or undersized. Confirm the allowed arrangement with the authority having jurisdiction.