Roofing (C-39) · Lesson 4 of 6

Low-Slope Roofing: Minimum Slope and Drainage

"Low-Slope Roofing: Minimum Slope and Drainage" is lesson 4 of 6 in the Roofing (C-39) Learning Hub. It opens with "Steep Roofs Shed Water, Low-Slope Roofs Hold It" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Steep Roofs Shed Water, Low-Slope Roofs Hold It
  2. Minimum Design Slope Depends on the System
  3. Slope Math: Fall Equals Slope Times Run
  4. Drains at Every Low Point, Plus an Overflow Path
  5. Walk the Water Path Before You Roof

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Find the minimum slope" through "Locate the overflow".

What you'll be able to do

  • Explain the main idea of "Low-Slope Roofing: Minimum Slope and Drainage" in plain language you can use on the exam.
  • Apply this step with confidence: "Slope Math: Fall Equals Slope Times Run".
  • Apply this step with confidence: "Drains at Every Low Point, Plus an Overflow Path".
  • Apply this step with confidence: "Walk the Water Path Before You Roof".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Low-Slope Roofing: Minimum Slope and DrainageTAPERED INSULATION TO A DRAINRUN 36 ftSCUPPER4 in2 inHIGH END10 in1 in thin edgeDRAINThe deck is flat; the taper makes the slope.Scupper inlet 2 in above low point, 4 in min high.SLOPE MATH (MOD-BIT ROOF)Min slope 1/4 in per ft (1/4:12)Fall = slope x run1/4 in x 36 ft = 9 in of fallFar end: 1 in + 9 in = 10 in thickCheck: 9 / (36 x 12) = 2.08 percentCoal-tar BUR may go down to 1/8:121/8 in x 36 ft = 4.5 in of fallOVERFLOW (CRC R903.4.1):Inlet 2 in above the low pointScupper 3x drain size, 4 in highor overflow drain, same size36 ft run x 1/4 in per ft = 9 in of fallTapered insulation: 1 in at the drain, 10 in at the far endCBC 1507.10.1 (BUR), 1507.11.1 (mod-bit), 1507.12.1 (single-ply): minimum 1/4:12; coal-tar BUR 1/8:12.CRC R903.4 and R903.4.1; CBC 1502.2; CPC 1101.12.2: drains at low points, secondary overflow if water can be trapped.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Steep Roofs Shed Water, Low-Slope Roofs Hold It

A shingle roof sheds water because gravity moves it off quickly. A low-slope roof is a continuous waterproof membrane that has to carry water to a drain, so the slope, the drains, and the membrane all have to work together. Asphalt shingles are not allowed below 2:12 (2025 California Residential Code R905.2.2; 2025 California Building Code 1507.2.2). Below that line the roofer is installing a membrane or other low-slope system, and each system has its own minimum slope for drainage.

Step 2: Minimum Design Slope Depends on the System

Built-up roofs, modified bitumen, single-ply (TPO, EPDM, PVC), sprayed polyurethane foam, and liquid-applied roofs all need a design slope of at least 1/4 unit vertical in 12 horizontal (2 percent) for drainage (CBC 1507.10.1, 1507.11.1, 1507.12.1, 1507.13.1, 1507.14.1; CRC R905.9.1, R905.11.1, R905.12.1, R905.13.1, R905.14.1). The one exception is coal-tar built-up roofing, which may go down to 1/8:12 (1 percent). Metal roof panels are different: lapped, nonsoldered-seam panels need 3:12 without lap sealant or 1/2:12 with applied lap sealant, and standing-seam panels need 1/4:12 (CBC 1507.4.2; CRC R905.10.2).

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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