Roof Pitch, Slope & Grade

Slope shows up everywhere — roofs, drainage, ramps, and grading.

Roof pitch

Pitch is rise over run — how many inches it rises per 12 inches of horizontal run. A "6/12" roof rises 6 inches every 12 inches. Steeper pitch = bigger first number.

Slope and grade

• Slope is often a ratio (a wheelchair ramp is 1:12) or rise over run.
• Grade is slope as a percentage — a 2% grade drops 2 feet per 100 feet, common for site drainage.

Going Deeper (Intermediate)

Roof pitch is rise : run over 12 (e.g., 6:12 = 6″ of rise per 12″ of run). Slope/grade for drainage, ramps, and sites is rise/run as a % or ratio. Same idea, different notation by trade.

Advanced / Pro-Level

The numbers behind the cuts and grades:

• Rafter length per foot of run = √(rise² + 12²) / 12 — for 6:12 that's a 1.118 multiplier (also the roof-area slope factor).
• Grade % = rise ÷ run × 100. Key thresholds: ADA ramp max 8.33% (1:12), minimum site drainage ~1–2%, sanitary sewer often ¼″ per foot (~2%).
• Convert freely between pitch, ratio, %, and degrees (degrees = arctan(rise/run)).
• The framing square's rafter tables give length-per-run for common pitches so you don't recompute every time.

Practice Challenge

A walkway must rise 24″ over a 20-ft run to meet a door. Does it meet the ADA ramp maximum? (Answer: grade = 24″ ÷ 240″ = 10%, which exceeds the ADA max of 8.33% (1:12) — you'd need at least 24 ft of run (or a landing/longer ramp) to comply; the slope math directly drives accessibility compliance.)

In Practice

A site graded at 2% drops 1 foot over a 50-foot run. Get that slope wrong and water pools against the foundation. Slope math literally protects the building.

Common Mistakes to Avoid

• Confusing pitch (rise per 12) with grade (a percent)
• Building a ramp steeper than code allows
• Ignoring drainage slope on flatwork

Takeaway: Roof pitch is rise-over-12 (like 6/12); slope is a ratio or rise/run; grade is slope as a percent (2% for drainage).

Educational overview — confirm structural and layout specifics with the project plans and engineer.