10.2 Sizing Leaders, Conductors, Horizontal Storm Drains & Gutters (Virginia Tables)

Two versions of the same tables

Your permitted exam reference is the model 2021 IPC. In it, storm piping is sized by flow rate: Equation 11-1 converts roof area to gpm, and Table 1106.2 (storm drain pipe), Table 1106.3 (leaders) and Table 1106.6 (gutters) give capacities in gpm. Virginia's VCC 2901.1.1, items 29 to 34, replaces those sections with the older projected-roof-area tables, and adds 1114.1. PSI's bulletin says a state code overrides the national code where they conflict, so for Virginia work, use the Virginia tables below. Be able to read both, because a question may be written from either.

Virginia Table 1106.2(1): circular vertical conductors and leaders

Maximum horizontally projected roof area (square feet):

The table continues to 12 inches per hour. Each column is the 1-inch-per-hour area divided by the rate, so a rate between columns, such as 3.3, can be handled by dividing the 1-inch value by the rate. The 2-inch row prints 2,280 square feet in the 1-inch column, while its other columns follow 2,880 divided by the rate. Use the printed column for the rate you have. Sizes in the table are circular diameters. Other shapes are allowed if they fully enclose that circle, and interpolation between pipe sizes is permitted.

Virginia Table 1106.2(2): rectangular leaders

For rectangular shapes not in the table, compute an equivalent circular diameter, then use Table 1106.2(1):

$$D_e = \sqrt{\text{width} \times \text{length}}$$

Example: a 3 × 5-inch leader has $D_e = \sqrt{15} \approx 3.87$ inches, which falls between the 3-inch and 4-inch rows. Interpolation is permitted.

Virginia Table 1106.3: building storm drains and sewers

Under Virginia 1106.3, building storm drains, building storm sewers and their horizontal branches with slopes of 1/2 inch per foot or less are sized by projected roof area. The slope of horizontal branches must be at least 1/8 inch per foot unless otherwise approved.

The table continues to 15-inch pipe and 6 inches per hour. As with sanitary drains, a steeper slope raises capacity: a 4-inch drain at 1 inch per hour goes from 7,520 square feet at 1/8 inch per foot to 15,040 at 1/2 inch per foot.

Virginia Table 1106.6: semicircular gutters

Gutters are sized by projected roof area. Examples at a 1/16-inch-per-foot slope:

Steeper gutter slopes (1/8, 1/4 and 1/2 inch per foot) have larger values.

Worked examples (Virginia tables)

1. Leader. An 8,000 sq ft roof in Richmond at 3.3 inches per hour. Equivalent area at 1 inch per hour: 8,000 × 3.3 = 26,400 sq ft. A 4-inch leader allows 18,400, which is too small. A 5-inch leader allows 34,600, which works.
2. Leader near the limit. A 5,500 sq ft roof. In Richmond: 5,500 × 3.3 = 18,150, so a 4-inch leader (18,400) works. In Norfolk: 5,500 × 3.4 = 18,700, more than 18,400, so a 5-inch leader is needed.
3. Horizontal storm drain. A 13,500 sq ft roof in Lynchburg at 3.2 inches per hour, drained at 1/4 inch per foot. Equivalent area: 13,500 × 3.2 = 43,200 sq ft. A 6-inch drain allows 30,200, which is too small. An 8-inch drain allows 65,200, which works.
4. Rectangular leader. A 3 × 4-inch leader in a 3-inch-per-hour area serves up to 4,400 sq ft (Table 1106.2(2)).

The model 2021 IPC tables (gpm)

Model 2021 IPC 1106.2 sizes storm drain piping from the flow rate through the roof drain, checked against the roof drain manufacturer's published rating. The model and Virginia methods can produce different answers. In example 3, the flow is 0.0104 × 13,500 × 3.2 ≈ 449 gpm, and model Table 1106.2 would allow a 6-inch drain at 1/4 inch per foot (487 gpm), while Virginia's Table 1106.3 requires 8 inches. On a Virginia job, follow the Virginia table.