Sheet Metal (C-43) · Lesson 1 of 8

Sizing Gutters and Leaders: CPC Tables 1103.3 and 1103.1

"Sizing Gutters and Leaders: CPC Tables 1103.3 and 1103.1" is lesson 1 of 8 in the Sheet Metal (C-43) Learning Hub. It opens with "Size by Storm and Roof Area, Not by Habit" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Size by Storm and Roof Area, Not by Habit
  2. Measure the Projected Roof Area
  3. Pick the Gutter from Table 1103.3
  4. Pick the Leader from Table 1103.1
  5. Metal, Protection, and Cleanouts

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Find the design rainfall" through "Compare another city".

What you'll be able to do

  • Explain the main idea of "Sizing Gutters and Leaders: CPC Tables 1103.3 and 1103.1" in plain language you can use on the exam.
  • Apply this step with confidence: "Pick the Gutter from Table 1103.3".
  • Apply this step with confidence: "Pick the Leader from Table 1103.1".
  • Apply this step with confidence: "Metal, Protection, and Cleanouts".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Gutter and Leader Sizing from Roof AreaShed roof, plan view, Palmdale60 ft1,500 sq ft60 x 25, horizontal projection25 ft6-in gutter, 1/8 in/ft slope3-in leaderDesign storm: 3.0 in/h (Table D 101.1)Look-ups, 2025 CPCGutter, Table 1103.3 (1/8 in/ft, 3 in/h)5 in: 1,172 sq ft FAIL6 in: 1,815 sq ft PASSLeader, Table 1103.1 (3 in/h)2 in: 960 sq ft FAIL3 in: 2,930 sq ft PASSSame roof in Los Angeles (2.0 in/h):gutter 5 in (1,760), leader 3 in (4,400)Roof area must not exceed the table value.6-in gutter + 3-in leader carry 1,500 sq ft at 3.0 in/h1,500 sq ft ≤ 1,815 (gutter) and 1,500 sq ft ≤ 2,930 (leader)2025 CPC Sec. 1101.12.1, Table 1103.3, Table 1103.1, Table D 101.1. Areas are horizontal projections.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Size by Storm and Roof Area, Not by Habit

The 2025 California Plumbing Code (CPC) Section 1101.12.1 says roof drains, gutters, and leaders (downspouts) are sized for a storm of 60 minutes duration and a 100-year return period, unless the Authority Having Jurisdiction requires otherwise. CPC Appendix D, Table D 101.1, prints that rainfall for cities: Los Angeles 2.0 in/h, San Diego 1.5, San Francisco 1.5, and Palmdale 3.0. Appendix D says these rates may be used unless higher values are established locally. A gutter that is too small overflows at the eave in the very storm it was sized for.

Step 2: Measure the Projected Roof Area

Tables 1103.3 and 1103.1 are written for the maximum horizontal projected roof area in square feet, which is the plan-view area, not the sloped surface. Multiply the eave length by the horizontal distance from the eave to the ridge for the roof plane that drains to that gutter. If a vertical wall drains onto the roof, CPC Section 1103.4 adds area: for one wall, add 50 percent of the wall area. Add every plane that drains to the same gutter and leader.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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