16.5 Sizing Vertical Conductors, Leaders, and Storm Drains
Key Takeaways
- IPC Table 1106.2 rates a 4-inch vertical storm drain at 180 gpm and a 4-inch horizontal storm drain at 81 gpm at 1/16 inch per foot, 115 gpm at 1/8, 163 gpm at 1/4 and 231 gpm at 1/2.
- IPC Table 1106.2 rates a 6-inch vertical storm drain at 538 gpm and a 6-inch horizontal drain at 344 gpm at 1/8 inch per foot.
- IPC Table 1106.3 rates a 2-inch vertical leader at 30 gpm, 3-inch at 92 gpm, 4-inch at 192 gpm, 5-inch at 360 gpm, 6-inch at 563 gpm and 8-inch at 1,208 gpm.
- IPC Table 1106.3 lists rectangular leader shapes with the same capacity as their round equivalents, such as 2 by 4 inches at 92 gpm, matching a 3-inch round leader.
- IPC 1101.5 prohibits reducing the size of a storm drainage pipe in the direction of flow.
Two sizing tables
| Table | Sizes | Variable |
|---|---|---|
| 1106.2 Storm drain pipe sizing | Vertical and horizontal storm drain piping | Vertical capacity, plus horizontal capacity at four slopes |
| 1106.3 Vertical leader sizing | Vertical leaders — the exterior downspouts from gutters or roof drains | Round and rectangular shapes |
Both are read in gallons per minute, the flow you computed from Equation 11-1.
Table 1106.2 — storm drain pipe sizing
| Pipe size (in) | Vertical drain (gpm) | Horizontal at 1/16 in/ft | at 1/8 in/ft | at 1/4 in/ft | at 1/2 in/ft |
|---|---|---|---|---|---|
| 2 | 34 | 15 | 22 | 31 | 44 |
| 3 | 87 | 39 | 55 | 79 | 111 |
| 4 | 180 | 81 | 115 | 163 | 231 |
| 5 | 311 | 117 | 165 | 234 | 331 |
| 6 | 538 | 243 | 344 | 487 | 689 |
| 8 | 1,117 | 505 | 714 | 1,010 | 1,429 |
| 10 | 2,050 | 927 | 1,311 | 1,855 | 2,623 |
| 12 | 3,272 | 1,480 | 2,093 | 2,960 | 4,187 |
| 15 | 5,543 | 2,508 | 3,546 | 5,016 | 7,093 |
Three observations that resolve most sizing items:
- A vertical pipe carries far more than the same horizontal pipe. A 4-inch vertical drain passes 180 gpm; the same pipe at 1/8 inch per foot passes 115 gpm — 36 percent less.
- Doubling the slope multiplies horizontal capacity by about 1.41 (the square root of 2). Check: 4-inch at 1/8 is 115 gpm, at 1/4 is 163 gpm. 115 x 1.414 = 162.6. That relationship holds down every row.
- Capacity scales steeply with diameter. From 4 inches to 6 inches at 1/8 inch per foot: 115 to 344 gpm, nearly triple, for a 50 percent increase in diameter.
Unlike sanitary Table 704.1, Table 1106.2 tabulates the 1/16 inch per foot column for every size, including small pipe. That does not override Table 704.1 where it applies; it reflects that storm piping is often engineered at flatter slopes.
Table 1106.3 — vertical leader sizing
| Size of leader | Capacity (gpm) |
|---|---|
| 2 in round; 2 x 2; 1-1/2 x 2-1/2 | 30 |
| 2-1/2 in round; 2-1/2 x 2-1/2 | 54 |
| 3 in round; 2 x 4; 2-1/2 x 3 | 92 |
| 4 in round; 3 x 4-1/4; 3-1/2 x 4 | 192 |
| 5 in round; 4 x 5; 4-1/2 x 4-1/2 | 360 |
| 6 in round; 5 x 6; 5-1/2 x 5-1/2 | 563 |
| 8 in round; 6 x 8 | 1,208 |
IPC 1106.3: vertical leaders shall be sized based on the flow rate from horizontal gutters or the maximum flow rate through roof drains, and the flow rate through vertical leaders shall not exceed Table 1106.3.
The rectangular shapes are the practical value of this table: a 2 x 4 inch rectangular downspout has the same 92 gpm capacity as a 3-inch round leader, and a 3 x 4-1/4 inch downspout matches a 4-inch round at 192 gpm. That lets you substitute a commercially available rectangular profile without a separate calculation.
Compare the vertical leader values against the vertical storm drain values in Table 1106.2 — 4-inch leader 192 gpm versus 4-inch vertical drain 180 gpm — and note they are close but not identical. Use the table that matches the pipe.
Full worked sizing: a Denver warehouse
Given: a 20,000 square foot roof in Denver, four roof drains with equal 5,000 square foot tributary areas, interior conductors dropping to a horizontal collector under the slab at 1/8 inch per foot, then out to a building storm sewer.
Step 1 — design rainfall rate
Denver: R = 2.4 inches per hour (Appendix B / Figure 1106.1(1)).
Step 2 — flow per drain
Q = 0.0104 x 5,000 x 2.4 = 124.8 gpm per drain. Total = 4 x 124.8 = 499.2 gpm.
Step 3 — verify the roof drains
Check each drain's published flow rate at the anticipated ponding head (1105.2, 1106.2). A drain rated for at least 125 gpm at the design head is required.
Step 4 — size each vertical conductor
Table 1106.2, vertical drain column, for 124.8 gpm:
- 3-inch = 87 gpm — too small.
- 4-inch = 180 gpm — sufficient.
Each conductor is 4 inches.
Step 5 — size the horizontal collector, segment by segment
The collector picks up the four conductors in series at 1/8 inch per foot.
| Segment | Accumulated flow | Table 1106.2 at 1/8 in/ft | Size |
|---|---|---|---|
| After drain 1 | 124.8 gpm | 4-inch = 115 (too small); 6-inch = 344 | 6 inch |
| After drain 2 | 249.6 gpm | 5-inch = 165 (too small); 6-inch = 344 | 6 inch |
| After drain 3 | 374.4 gpm | 6-inch = 344 (too small); 8-inch = 714 | 8 inch |
| After drain 4 | 499.2 gpm | 8-inch = 714 | 8 inch |
Note that at 124.8 gpm a 5-inch horizontal at 1/8 inch per foot carries 165 gpm and would also work for the first segment. Because 1101.5 prohibits reducing size in the direction of flow, and because the run will reach 8 inches anyway, most designers would start at 6 inches.
Step 6 — consider slope
If the collector could be run at 1/4 inch per foot instead:
| Accumulated flow | Size at 1/4 in/ft |
|---|---|
| 124.8 gpm | 4-inch = 163 gpm — 4 inch |
| 249.6 gpm | 5-inch = 234 (too small); 6-inch = 487 |
| 374.4 gpm | 6-inch = 487 |
| 499.2 gpm | 6-inch = 487 (too small); 8-inch = 1,010 |
Doubling the slope drops the first segment from 6 inches to 4 inches and the third from 8 inches to 6 inches — a substantial saving where the fall is available.
Step 7 — secondary system
Required because of the parapet (1108.1). Sized for the same 499.2 gpm with no credit for the primary system (1108.3), discharging separately, above grade, in an observed location (1108.2).
Step 8 — the rest of the checklist
| Item | Requirement |
|---|---|
| Materials | Conductors above ground per Table 702.1; underground storm drain per Table 702.2; storm sewer per Table 1102.4 |
| Fittings | Table 706.3 patterns (1101.6) |
| No size reduction in the direction of flow | 1101.5 |
| Cleanouts | Per Section 708 (1101.8) |
| Backwater valve | Per Section 714 where subject to backflow (1101.9) |
| Trap | Only if connected to a combined sewer (1103.1) |
| Test | Water or air per 312.2 or 312.3 (312.8) |
| Storm sewer connection | Independent connection where the public sewer is combined (1109.1) |
A leader example
A gutter collects 3,000 square feet of roof in Pueblo (R = 2.5).
- Q = 0.0104 x 3,000 x 2.5 = 78 gpm.
- Table 1106.3: 2-1/2 inch round = 54 gpm — too small; 3-inch round = 92 gpm — sufficient.
- Rectangular alternatives at the same 92 gpm: 2 x 4 inches or 2-1/2 x 3 inches.
Move the same building to Lamar (R = 3.0): Q = 0.0104 x 3,000 x 3.0 = 93.6 gpm, which exceeds the 3-inch leader's 92 gpm. The leader goes to 4 inches (192 gpm) or a 3 x 4-1/4 inch rectangular profile.
What is the maximum flow rate for a 4-inch vertical storm drain under IPC Table 1106.2?
A horizontal storm drain must carry 300 gpm at a slope of 1/8 inch per foot. What is the minimum size?
Under IPC Table 1106.3, which rectangular leader has the same capacity as a 3-inch round leader?
Doubling the slope of a horizontal storm drain from 1/8 inch per foot to 1/4 inch per foot changes a 6-inch pipe's capacity from 344 gpm to what value?