8.2 Sizing Roof Drains, Leaders & Vertical Rainwater Piping (UPC Table 1103.1)

Sizing Roof Drains, Leaders & Vertical Rainwater Piping

The one sentence that drives this section

UPC Section 1103.1: vertical conductors and leaders shall be sized by the maximum projected roof area and Table 1103.1.

Two inputs, one table, one answer. The whole difficulty is in reading the table correctly and in getting the projected area right.

---

1. Projected roof area, not surface area

"Maximum projected horizontal roof area" means the area of the roof as it would appear looking straight down — its footprint, not the sloped surface. A gable roof measuring 40 feet by 60 feet in plan drains 2,400 square feet no matter how steep the pitch, because rain falls vertically and a steeper roof simply catches the same column of water over a smaller sloped area.

This is the first place candidates lose the question. If a problem gives you rafter length and building width, convert to plan dimensions before you touch the table.

---

2. Table 1103.1 — Sizing Roof Drains, Leaders, and Vertical Rainwater Piping

Maximum allowable horizontal projected roof area, in square feet:

(The published table also carries 7, 9, and 11 inch-per-hour columns; the rows above are the ones worth committing to memory as anchors.)

The three notes that turn into questions

1. The gpm column is a capacity, measured under a stated condition. It is the maximum discharge capacity in gallons per minute with approximately 1-3/4 inches (44 mm) of head of water at the drain. That head figure is the reason a 4-inch roof drain body is rated at 192 gpm here and a 4-inch horizontal storm drain at 1/8 inch per foot is rated at only 78 gpm in the next table — vertical flow with head behind it moves far more water than a partly full horizontal pipe.
2. Unlisted rainfall rates are a division problem. For rainfall rates other than those listed, determine the allowable roof area by dividing the area given in the 1 inch per hour column by the desired rainfall rate. At 2.5 inches per hour, a 4-inch leader handles 18,400 ÷ 2.5 = 7,360 square feet.
3. Shape is flexible; area is not. Vertical piping may be round, square, or rectangular. A square leader is sized to enclose its equivalent round pipe. A rectangular leader must have not less than the same cross-sectional area as its equivalent round pipe, and the ratio of its side dimensions shall not exceed 3 to 1 — so a 2-inch by 6-inch rectangular leader is at the limit, and a 2-inch by 8-inch is not permitted.

---

3. Worked example: a warehouse roof

Given: a warehouse measuring 180 feet by 240 feet in plan, drained by six roof drains that split the roof evenly. Design rainfall rate 3 inches per hour. Size the leaders.

1. Total projected area: 180 × 240 = 43,200 square feet.
2. Area per drain: 43,200 ÷ 6 = 7,200 square feet.
3. Read the 3 in/h column for a value at or above 7,200:
   • 4-inch handles 6,130 — too small.
   • 5-inch handles 11,530 — adequate.
4. Answer: 5-inch leaders, and each roof drain body is sized to match.

Now change one variable. If the design rate is 2.5 inches per hour instead, use note 2: a 4-inch leader handles 18,400 ÷ 2.5 = 7,360 square feet, which now covers 7,200. 4-inch leaders suffice. A half-inch per hour of design rainfall changed the pipe size for the whole building — which is why the rainfall rate is never a detail to skim past.

---

4. Choosing the rainfall rate in Iowa

Storm systems are designed to a 100-year, 1-hour rainfall rate for the building's location, and the design rate is set by the AHJ or the design documents. Iowa's design rates generally fall in the 3 to 4 inch per hour range, and a local ordinance may specify a value or a return period different from the state default.

Two exam habits follow. First, if the problem states a rate, use the stated rate — never substitute a number you remember from another jurisdiction. Second, if the problem states a city and no rate, the intended answer is almost always the rate printed in whatever rainfall table the exam expects you to consult, so look for it rather than guessing.

---

5. A checklist for any vertical storm sizing question

1. Convert to projected horizontal area.
2. Divide by the number of drains if the roof is split.
3. Add anything Section 8.4 requires for vertical walls draining onto the roof.
4. Find the rainfall rate column, or divide the 1 in/h value by the rate.
5. Read down the size column to the first row that equals or exceeds your area.
6. Confirm the drain body, the leader, and everything downstream never decreases in the direction of flow.