8.3 Sizing Horizontal Storm Drains, Storm Sewers & Gutters (UPC Tables 1103.2 and 1103.3)
Key Takeaways
- UPC Section 1103.2 sizes building storm drains, storm sewers, and their horizontal branches from the maximum projected roof or paved area using Table 1103.2.
- Table 1103.2 is divided into three slope blocks — 1/8, 1/4, and 1/2 inch per foot — and the sizing data are based on the pipes flowing full.
- At 1 inch per hour and 1/8 inch per foot slope, Table 1103.2 allows 3,288 square feet on 3-inch, 7,520 on 4-inch, 13,360 on 5-inch, 21,400 on 6-inch, and 46,000 on 8-inch pipe.
- Doubling the slope from 1/8 to 1/4 inch per foot raises the capacity of a given horizontal storm pipe by roughly 40 percent.
- UPC Section 1103.3 sizes semi-circular gutters from Table 1103.3, which is organized by gutter diameter, slope, and rainfall rate.
Sizing Horizontal Storm Drains, Storm Sewers & Gutters
The horizontal table works differently from the vertical one
Vertical rainwater piping runs with head behind it and does not care about slope. Horizontal storm piping runs on grade, and UPC Table 1103.2 is therefore divided into three separate slope blocks: 1/8 inch per foot, 1/4 inch per foot, and 1/2 inch per foot. Note 1 to the table states the assumption behind the numbers — the sizing data for horizontal piping are based on the pipes flowing full, which is the opposite of the half-full assumption behind sanitary drainage sizing.
UPC Section 1103.2: the size of building storm drains, or building storm sewers or their horizontal branches shall be based on the maximum projected roof or paved area to be handled and Table 1103.2. Note that phrase — roof or paved area. Parking lots, plazas, drives, and courts feed the same table as roofs.
1. Table 1103.2 — Sizing of Horizontal Rainwater Piping
Maximum allowable horizontal projected roof area, in square feet:
At 1/8 inch per foot slope
| Size (in) | Flow (gpm) | 1 in/h | 2 in/h | 3 in/h | 4 in/h | 5 in/h | 6 in/h |
|---|---|---|---|---|---|---|---|
| 3 | 34 | 3,288 | 1,644 | 1,096 | 822 | 657 | 548 |
| 4 | 78 | 7,520 | 3,760 | 2,506 | 1,880 | 1,504 | 1,253 |
| 5 | 139 | 13,360 | 6,680 | 4,453 | 3,340 | 2,672 | 2,227 |
| 6 | 222 | 21,400 | 10,700 | 7,133 | 5,350 | 4,280 | 3,566 |
| 8 | 478 | 46,000 | 23,000 | 15,330 | 11,500 | 9,200 | 7,670 |
| 10 | 860 | 82,800 | 41,400 | 27,600 | 20,700 | 16,580 | 13,800 |
| 12 | 1,384 | 133,200 | 66,600 | 44,400 | 33,300 | 26,650 | 22,200 |
At 1/4 inch per foot slope
| Size (in) | Flow (gpm) | 1 in/h | 2 in/h | 3 in/h | 4 in/h | 5 in/h | 6 in/h |
|---|---|---|---|---|---|---|---|
| 3 | 48 | 4,640 | 2,320 | 1,546 | 1,160 | 928 | 773 |
| 4 | 110 | 10,600 | 5,300 | 3,533 | 2,650 | 2,120 | 1,766 |
| 5 | 196 | 18,880 | 9,440 | 6,293 | 4,720 | 3,776 | 3,146 |
| 6 | 314 | 30,200 | 15,100 | 10,066 | 7,550 | 6,040 | 5,033 |
| 8 | 677 | 65,200 | 32,600 | 21,733 | 16,300 | 13,040 | 10,866 |
| 10 | 1,214 | 116,800 | 58,400 | 38,950 | 29,200 | 23,350 | 19,450 |
| 12 | 1,953 | 188,000 | 94,000 | 62,600 | 47,000 | 37,600 | 31,350 |
At 1/2 inch per foot slope
| Size (in) | Flow (gpm) | 1 in/h | 2 in/h | 3 in/h | 4 in/h | 5 in/h | 6 in/h |
|---|---|---|---|---|---|---|---|
| 3 | 68 | 6,576 | 3,288 | 2,192 | 1,644 | 1,310 | 1,096 |
| 4 | 156 | 15,040 | 7,520 | 5,010 | 3,760 | 3,010 | 2,500 |
| 5 | 278 | 26,720 | 13,360 | 8,900 | 6,680 | 5,320 | 4,450 |
| 6 | 445 | 42,800 | 21,400 | 14,267 | 10,700 | 8,580 | 7,140 |
| 8 | 956 | 92,000 | 46,000 | 30,650 | 23,000 | 18,400 | 15,320 |
The pattern worth internalizing: going from 1/8 to 1/4 inch per foot buys about 40 percent more capacity; going from 1/8 to 1/2 buys about 100 percent. And notice the symmetry across the table — the 1/8 inch per foot 1 in/h value equals the 1/4 inch per foot 2 in/h value doubled, and so on. Note 2 gives the same division rule as the vertical table: for rainfall rates other than those listed, divide the area in the 1 inch per hour column by the desired rate.
2. Worked example: an office building storm drain
Given: a 22,000 square foot roof plus a 14,000 square foot paved parking area, both draining into one building storm drain. Design rainfall 3 inches per hour. The drain is run at 1/8 inch per foot because the storm sewer is shallow.
- Total area: 22,000 + 14,000 = 36,000 square feet (Section 1103.2 covers roof or paved area).
- Read the 1/8 inch per foot block, 3 in/h column:
- 8-inch handles 15,330 — too small.
- 10-inch handles 27,600 — still too small.
- 12-inch handles 44,400 — adequate.
- Answer at 1/8 inch per foot: 12 inches.
- Now re-grade it. If the run can be pitched at 1/4 inch per foot, the 10-inch line handles 38,950 square feet, which covers 36,000. 10 inches. Two extra inches of fall per 8 feet of run saved a pipe size across the whole building — the kind of trade-off a good estimator makes before the trench is dug.
3. Gutters — UPC Section 1103.3 and Table 1103.3
Section 1103.3: the size of semi-circular gutters shall be based on the maximum projected roof area and Table 1103.3. Gutters get their own table because an open channel behaves differently from a closed pipe, and because gutters are commonly run at very flat slopes.
Maximum roof area in square feet, semi-circular gutters:
| Gutter diameter (in) | 1/16 in/ft @ 4 in/h | 1/8 in/ft @ 4 in/h | 1/4 in/ft @ 4 in/h | 1/2 in/ft @ 4 in/h |
|---|---|---|---|---|
| 4 | 360 | 510 | 720 | 1,020 |
| 5 | 625 | 880 | 1,250 | 1,770 |
| 6 | 960 | 1,360 | 1,920 | 2,770 |
| 8 | 1,990 | 2,800 | 3,980 | 5,600 |
| 10 | 3,600 | 5,100 | 7,200 | 10,000 |
Read across a single row and the slope effect is obvious: a 6-inch gutter carries 960 square feet at 1/16 inch per foot and 2,770 square feet at 1/2 inch per foot for the same 4-inch-per-hour rain — nearly triple. Read down a column and the diameter effect is equally sharp. The full published table covers 3 through 10 inch gutters at 2, 3, 4, 5, and 6 inches per hour for each of four slopes; find it, tab it, and read the correct block.
4. Three rules that outrank the tables
- Never reduce size in the direction of flow. If a vertical leader computes to 5 inches and the horizontal drain it lands in computes to 4, the horizontal drain is still 5 inches.
- Combine before you size. A horizontal branch collecting three leaders carries the sum of their tributary areas, not the largest one.
- Use the stated slope. Half the wrong answers on horizontal storm questions come from reading the 1/4 inch per foot block when the problem said 1/8.
A building storm drain sloped at 1/8 inch per foot serves 20,000 square feet of projected roof area at a design rainfall rate of 3 inches per hour. What is the minimum pipe size from UPC Table 1103.2?
What assumption underlies the horizontal rainwater piping capacities in UPC Table 1103.2?
A 6-inch semi-circular gutter is run at 1/16 inch per foot and must handle a 4 inch per hour rainfall. Roughly how much projected roof area does Table 1103.3 permit, and what happens if the gutter is re-pitched to 1/2 inch per foot?