Roof Pitch, Rafter & Area
Pitch as rise-per-12, degrees, or percent; rafter length from span and overhang; sloped roof area, squares, and shingle bundles at your count per square.
The roof figures a gable job needs: the pitch in every form, the rafter line length, the actual sloped area, and how many squares or bundles to order.
Example: A 6:12 pitch is 26.6° with a slope factor of 1.118; on a 24 ft span with a 1 ft overhang the rafter line is 14.53 ft.
One slope factor
does all the work.
How pitch, rafter, and area connect, and what a simple gable model leaves out.
Pitch in every form
Pitch is rise ÷ run. Enter it as rise per 12 (6:12), as an angle, as a percent, or as a measured rise and run; the others are computed from the same ratio: angle = atan(rise ÷ run), 6:12 = 26.57° = 50 %. The slope factor √(1 + (rise ÷ run)²) is the length of the slope per unit of horizontal run — 1.118 at 6:12, 1.155 at 30°.
Rafter and area
On a gable the rafter run is half the span. Rafter line length = (run + horizontal overhang) × factor, measured from the outer wall line to the ridge centreline — before the ridge-board deduction, birdsmouth, or tail cut. Roof area = footprint (length × (span + 2 × overhang)) × factor; a square is 100 ft², and bundles are squares × the bundles per square printed on the product you buy — no count is assumed.
What the model leaves out
Hips, valleys, dormers, gable-end (rake) overhang, and the extra for ridge caps and starter courses are not modelled — add them to the waste allowance you set. Rafter size, spacing, and any structural question belong to the design and local rules. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.