10.3 Roofing Systems (steep-slope and low-slope) and Underlayment

Slope Classification

IBC Chapter 15 governs roofing. Slope is expressed as a ratio of vertical rise to 12 in. of horizontal run. Steep-slope roofs have a slope of 3:12 (25%) or greater and shed water by overlap (shingles, tile, metal). Low-slope roofs are below 3:12 and require a continuous membrane because standing water is likely.

Asphalt shingles require a minimum slope of 2:12 (IBC 1507.2.2). Between 2:12 and 4:12, a double underlayment is required.

Steep-Slope Minimum Slopes (IBC 1507)

Underlayment: ASTM D226 Type I (No. 15) or Type II (No. 30) felt, or ASTM D1970 self-adhered for ice-dam zones.

Ice Barrier & Underlayment Lap

Where the average January temperature is 25 deg F or less, IBC 1507.1.1 requires an ice barrier (two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet) extending from the eave edge to 24 in. inside the exterior wall line.

Underlayment laps: single-layer No. 15 felt is applied with 2 in. side laps and 4 in. end laps at 4:12 and steeper. Below that, double coverage with 19 in. side laps is used.

Low-Slope Membrane Systems

Low-slope (commercial) systems include:

• Built-Up Roofing (BUR): alternating plies of felt and hot bitumen, topped with gravel/cap sheet.
• Modified Bitumen (Mod-Bit): SBS or APP-modified asphalt rolls, torch- or cold-applied.
• Single-ply: EPDM (rubber), TPO, and PVC sheets, mechanically fastened, ballasted, or fully adhered.

IBC 1507.10-1507.13 covers these. Minimum slope for drainage is 1/4 in. per foot (2%) per IBC 1507.10.1 to avoid ponding.

Roofing Takeoff: The Square

Roofing is sold by the square = 100 ft2. To estimate, you must convert plan (footprint) area to actual roof area using the slope factor.

Worked example: A gable roof footprint is 40 ft x 30 ft = 1,200 ft2 at 6:12. Roof area = 1,200 x 1.118 = 1,342 ft2 = 13.42 squares. Add 10% waste = 14.76 → order 15 squares.

OSHA Fall Protection on Roofs

Per OSHA 29 CFR 1926 Subpart M (1926.501), fall protection is required at 6 ft in construction. On low-slope roofs, work within 6 ft of an edge requires guardrails, safety nets, or personal fall arrest; between 6-15 ft a designated-area + warning-line system may be used. On steep roofs (above 4:12), fall arrest, guardrails, or nets are mandatory. Use the 6-foot threshold here as an editorial reference-navigation check; PSI does not publish its appearance frequency.

Steep-Slope vs. Low-Slope and Slope Notation

Roof slope is rise-over-run, written as x:12 (e.g., 4:12 = 4 in rise per 12 in run). Steep-slope (≥3:12) roofs shed water and use overlapping units — asphalt shingles, tile, metal, slate. Low-slope (<3:12, often called "flat") roofs must be watertight membranes because water ponds — built-up (BUR), modified bitumen, EPDM, TPO, PVC. Asphalt shingles are not permitted below their minimum slope (typically 2:12 with special underlayment, 4:12 standard).

Underlayment, Ice Dams, and Flashing

Under shingles goes underlayment (felt or synthetic). In cold climates, ice-and-water shield (self-adhering membrane) is required from the eave to at least 24 in inside the warm wall line to stop ice-dam leaks. Drip edge at eaves/rakes, step flashing at walls, and valley flashing direct water. Shingles install from the eave up, overlapping so laps face downslope.

Low-Slope Membranes and Drainage

Low-slope roofs need positive drainage (min 1/4 in per ft slope to drains/scuppers) to avoid ponding, which voids warranties and overloads the deck. Single-ply (TPO/PVC/EPDM) is mechanically fastened, ballasted, or fully adhered; BUR is multiple felt plies in bitumen with a gravel/cap surface. Tapered insulation builds slope on a flat deck.

Editorial Practice Checks

• Trap: Asphalt shingles on a 1:12 roof. They need ≥ 2–4:12; use a membrane below that.
• Trap: Ice-and-water shield only at the very edge. Extend 24 in past the warm wall.
• Trap: Flat roofs need no slope. Provide ≥1/4 in/ft to drains.
• Trap: Installing shingles top-down. Start at the eave and lap downslope.

Ventilation, Fastening, and Wind Uplift

Steep-slope attics need balanced ventilation — intake at the soffit/eave and exhaust at the ridge — so heat and moisture escape and ice dams are reduced; the code rule of thumb is 1 sq ft of net free vent area per 150 sq ft of attic (1:300 with a vapor retarder). Shingles are nailed with the specified number of fasteners (commonly 4, or 6 in high-wind zones) in the nail line; high-nailing or under-driving causes blow-offs. Low-slope membranes are rated for wind uplift, with denser fastening at corners and perimeters where suction is highest.

Roofing Estimating in Squares

Roofing is quantified in squares (1 square = 100 SF). Worked example: a gable roof with two planes each 40 ft x 25 ft = 1,000 SF each = 2,000 SF total = 20 squares; add ~10-15% waste for hips, valleys, and starter/ridge → order about 23 squares. Account separately for underlayment, ice-and-water shield, drip edge (linear feet), ridge/hip cap, and fasteners. Pitch increases the actual surface area over the building footprint, so apply the slope (roof-area) factor before ordering.