Roofing (C-39) · Lesson 2 of 6

Shingle Slope Limits and Underlayment Layers

"Shingle Slope Limits and Underlayment Layers" is lesson 2 of 6 in the Roofing (C-39) Learning Hub. It opens with "Slope Decides Whether Shingles Are Allowed" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Slope Decides Whether Shingles Are Allowed
  2. 4:12 and Steeper: One Layer, 2-Inch Lap
  3. 2:12 up to 4:12: Two Layers, Half-Width Strip Plus Full Sheets
  4. Nails and Shingle Attachment
  5. Count the Runs Before You Order

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Find the code requirement" through "Sanity check against the roof area".

What you'll be able to do

  • Explain the main idea of "Shingle Slope Limits and Underlayment Layers" in plain language you can use on the exam.
  • Apply this step with confidence: "2:12 up to 4:12: Two Layers, Half-Width Strip Plus Full Sheets".
  • Apply this step with confidence: "Nails and Shingle Attachment".
  • Apply this step with confidence: "Count the Runs Before You Order".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Double Underlayment on Low-Slope Shingle Roofs3:12 ROOF: 11 RUNS PER SIDEEach bar is one run, numbers are inches wide1836363636363636363624EAVE168 in up the slopeRIDGECOUNT THE RUNS (3:12)Strip 18 in + 36 in sheets (assumed roll)Each sheet adds 36 - (18 + 2) = 16 in(168 - 36) / 16 = 8.25, so 9 more10 sheets + 1 strip = 11 runs11 x 40 ft = 440 lf per sideWidth: 18 + 10 x 36 = 378 in378 / 12 x 40 ft = 1,260 sq ftRoof side: 14 x 40 = 560 sq ft1,260 / 560 = 2.25 layers laidAt 4:12 or more, one layer:(168 - 36) / 34 = 3.9, so 5 sheets3:12 roof: 11 runs of 40 ft per side = 440 lf (at least 2 layers)Same 14 ft slope at 4:12 or more: 5 runs = 200 lf (one layer, 2 in lap)CRC Table R905.1.1(2) and CBC Table 1507.1.1(2): 2:12 up to 4:12 = two layers (strip half a sheet wide, laps half a sheet + 2 in).4:12 or greater = one layer, 2 in lap. End laps 4 in, offset 6 ft. Shingles only on 2:12 or steeper (R905.2.2).The 36 in roll width is an assumed example; the code prints only fractions of a full sheet.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Slope Decides Whether Shingles Are Allowed

Asphalt shingles may be used only on roof slopes of 2:12 (17-percent slope) or steeper (2025 CRC R905.2.2; 2025 CBC 1507.2.2). Anything flatter needs a low-slope system instead. From 2:12 up to 4:12 (33-percent slope) the code also requires double underlayment (Table R905.1.1(2)), because a shallow roof sheds water slowly and wind-driven rain can back up under the laps.

Step 2: 4:12 and Steeper: One Layer, 2-Inch Lap

Table R905.1.1(2) (CBC Table 1507.1.1(2)) says that at 4:12 or greater the underlayment is one layer applied shingle fashion, parallel to and starting from the eave, lapped 2 inches. End laps are 4 inches and are offset by 6 feet. Distortions in the underlayment must not interfere with the shingles sealing. A single layer of self-adhering polymer modified bitumen underlayment (ASTM D1970), installed per the manufacturer, is also listed. Where wind design is not required, CBC Table 1507.1.1(1) accepts ASTM D226 Type I or II, D1970, D4869 Type I through IV, D6757, or D8257 for asphalt shingles, and the underlayment must bear a label showing the standard (R905.1.1).

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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