What does roof pitch (e.g., 6:12) actually mean?

Roof pitch expresses how many inches the roof rises vertically for every 12 inches of horizontal run. A 6:12 pitch means the roof ascends 6 vertical inches for every 12 horizontal inches inward. In degrees, a 6:12 pitch corresponds to a 26.57° roof slope.

How do you find rafter length from roof pitch and span?

First, find the run (which is half the total building span, e.g., 24 ft span has a 12 ft run). Next, calculate the pitch multiplier using the Pythagorean theorem: Multiplier = √(1 + (Pitch/12)²). Multiply the run by this pitch multiplier to find the true rafter hypotenuse length before adding the eave overhang.

What is a roof pitch multiplier and how is it used?

The pitch multiplier (slope factor) is the ratio between the actual sloped roof area and the flat horizontal footprint. For instance, a 6:12 roof has a multiplier of 1.118. A 1,500 sq ft flat house footprint under a 6:12 gable roof will require 1,500 × 1.118 = 1,677 sq ft of roofing surface.

How many shingle bundles make up a roofing square?

In North America, one "square" of roofing equals 100 square feet of roof surface. Standard asphalt architectural shingles are packaged at 3 bundles per square. Thus, each bundle covers roughly 33.3 square feet. Always include a 10% waste buffer for hips, valleys, starter strips, and ridge caps.

What is considered a walkable roof pitch vs steep slope?

Roofs with pitches from 4:12 to 7:12 are generally considered safely walkable by experienced roofers wearing soft rubber-soled boots in dry weather. Anything 8:12 and above is classified as steep slope, requiring OSHA-compliant safety harnesses, roof jacks, and slide guards. Slopes under 3:12 are low slope and require specialized waterproof membrane (EPDM/TPO) rather than standard shingles.

Roof Pitch Calculator (2026) – Slope Angle, Rafter Length & Shingles

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