What does roof pitch mean?

Roof pitch describes how steeply a roof slopes, expressed as rise per 12 inches of run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. It can also be expressed in degrees or as a percentage slope.

How do I measure roof pitch?

Place a level horizontally on the roof surface and measure the rise (vertical distance) over a 12-inch run. You can also use a speed square, a phone inclinometer app, or measure from inside the attic from a rafter.

What is rafter length?

Rafter length is the distance from the ridge board to the outer edge of the wall plate (and overhang). It is calculated using the Pythagorean theorem: rafter = √(run² + rise²), scaled to the actual half-span of the building.

What is a steep pitch?

Pitches above 12/12 (45°) are considered steep. They improve drainage and shed snow effectively but require special steep-slope roofing practices and safety equipment.

Roof Pitch Calculator - Rise/Run, Degrees & Rafter Length

Understanding roof pitch

Roof pitch (slope) is the measure of how steeply a roof rises. In the US it is expressed as a ratio of rise to run: x/12, meaning for every 12 inches of horizontal distance (run), the roof rises x inches. A 6/12 pitch is the most common residential slope.

Pitch conversion formulas

Degrees: angle = atan(rise ÷ run)
Percent slope: (rise ÷ run) × 100
Rafter length: √(run² + rise²) scaled to the actual run

Rafter length calculation

The rafter runs from the ridge board to the outer wall plate, plus any overhang. The actual horizontal run equals half the building span plus the overhang distance. Use the Pythagorean theorem to find the rafter length from the run and the corresponding rise.

Roof surface area

For a simple gable roof, surface area = rafter length × building length × 2 (for both sides). Add 10–15% waste when ordering roofing materials to account for overlaps and cuts.

Pitch categories

Flat / Low slope: 0/12 – 3/12 (0°–14°)
Conventional: 4/12 – 9/12 (18°–37°)
Steep: 10/12 – 12/12 (40°–45°)
Very steep: above 12/12 (>45°)

Roofing material compatibility

Material	Minimum pitch	Notes
Asphalt shingles	2/12	Special low-slope underlayment required below 4/12
Metal (standing seam)	1/12 (1²⁄₂°)	Concealed fasteners allow very low pitch
Metal (exposed fastener)	3/12	Requires foam closure strips at low pitches
Wood shakes / shingles	4/12	Requires proper spacing for drainage and ventilation
Clay or concrete tile	4/12	Heavy; verify structural load capacity
EPDM rubber membrane	0/12 (flat)	Designed for low-slope and flat roofs
TPO / PVC membrane	0/12 (flat)	Commercial flat roofing; heat-welded seams

Roof drainage calculation

Gutter and downspout sizing depends on three factors: rainfall intensity, roof plan area, and runoff coefficient:

Flow (gal/min) = Rainfall intensity (in/hr) × Roof area (ft²) × 0.623 × Runoff coefficient

For a typical residential shingle roof, use a runoff coefficient of 0.95. A 1,500 ft² roof in a region with a 4-inch/hour design storm produces approximately 3,500 gal/min of peak flow — informing gutter width (K-style 6-inch can handle ~2,000 gal/min) and downspout count (one 3×4-inch downspout per 600 ft² of roof is a common rule).