Roof Pitch Rise Over Run: How to Calculate Roof Slope

The concept of rise over run is the standard way to describe roof pitch for builders, roofers, and homeowners evaluating drainage, materials, or code compliance. This article explains how to calculate, convert, and apply rise over run for roof pitch with practical examples, safety considerations, and common pitfalls.

Common Roof Pitch	Rise Over Run	Slope Fraction	Approximate Angle
Low	2 in 12	1:6	9.46°
Moderate	4 in 12	1:3	18.43°
Standard	6 in 12	1:2	26.57°
Steep	9 in 12	3:4	36.87°

What Rise Over Run Means For Roof Pitch

“Rise over run” describes the vertical rise for every horizontal 12 units of run; commonly expressed as “X in 12” in the U.S. For example, a 6 in 12 roof rises 6 inches vertically for each 12 inches horizontally. This measurement standardizes roof slope and simplifies material calculations.

Why Rise Over Run Matters

Roof pitch affects water drainage, material selection, building codes, and safety. Low slopes require specialized membranes and flashing while steep slopes need different shingle choices and fall protection during installation. Knowing the rise over run helps contractors estimate materials and ensure compliance with local regulations.

How To Measure Rise Over Run On An Existing Roof

Measure from a stable point at the top plate or ridge down to the eave to determine rise, and measure the horizontal span for run. For safety, most measurements can be taken from the attic whenever accessible, avoiding rooftop work when possible.

Step-By-Step Measurement From The Attic

• Locate the ridge board and top of the wall plate. Measure the vertical distance between these two points to get the rise.
• Measure the horizontal distance from the wall plate to the point directly under the ridge to get half the building width or the run for a symmetrical roof.
• Convert the run to a 12-inch basis: If the measured run is not 12 inches, use proportion: (Measured Rise / Measured Run) × 12 = Rise Per 12.

Simple Calculations And Conversions

Key formulas: Rise per 12 = (Measured Rise / Measured Run) × 12. Slope Fraction = Rise / Run. Angle (degrees) = arctan(Rise/Run). These conversions let users go between the common “X in 12”, fractional slope, and angle metrics used in architecture or engineering.

Example: Convert A 5-Inch Rise Over A 10-Inch Run

Measured Rise = 5 in. Measured Run = 10 in. Rise per 12 = (5 / 10) × 12 = 6 in 12. Fractional slope = 5/10 = 1/2. Angle = arctan(5/10) ≈ arctan(0.5) ≈ 26.57°.

From Rise/Run To Roof Area And Material Estimates

Roof area depends on pitch because a steeper roof has a larger surface area for the same building footprint. To estimate materials, first calculate the roof slope factor: Roof Slope Factor = sqrt(1 + (Rise/Run)^2). Multiply the building footprint by this factor to get the actual roof surface area.

Example: Calculating Roof Surface Area

House footprint for one slope: 20 ft wide × 30 ft long = 600 sq ft. For a 6 in 12 roof, Rise/Run = 6/12 = 0.5. Slope factor = sqrt(1 + 0.5^2) = sqrt(1.25) ≈ 1.118. Rooftop area for one plane = 600 × 1.118 ≈ 671 sq ft. For both sides of a gable, multiply by 2.

Common Roof Pitches And Typical Uses

Low-pitch roofs (under 3 in 12) are common on modern flat or shed roofs and require membrane roofing. Standard asphalt shingles typically require at least 4 in 12 or greater. Steeper roofs (over 9 in 12) are common in snowy climates to shed snow quickly and may use metal or slate for durability.

Pitch (In 12)	Use Case	Material Considerations
2 in 12	Low-slope sheds, modern homes	Membrane roofing, careful flashing
4 in 12	Common residential	Asphalt shingles acceptable
6 in 12	Traditional homes	Good drainage, wide shingle options
9 in 12+	Steep alpine roofs	Metal, slate, higher labor costs

Converting Rise Over Run To Degrees And Back

To convert to degrees use the arctangent function: Angle = arctan(Rise/Run). To convert an angle back to rise over run, use Rise/Run = tan(Angle). Calculators or smartphone apps simplify these conversions during planning or inspections.

Quick Conversion Reference

• 3 in 12 ≈ 14.04°
• 4 in 12 ≈ 18.43°
• 6 in 12 ≈ 26.57°
• 12 in 12 = 45°

Building Code, Safety, And Practical Limits

Local building codes may restrict minimum slopes for specific roof coverings and require underlayment or special flashing for low slopes. Steep roofs increase fall risk and often require fall protection and different installation methods.

Safety And Compliance Tips

• Check Local Codes: Verify minimum pitch for chosen roofing material in municipal codes or manufacturer installation guides.
• Use Fall Protection: For slopes above certain angles (often above 6/12 or by OSHA rules), implement harnesses, guardrails, or brackets.
• Prefer Attic Measurements: When possible, measure from inside to avoid rooftop hazards.

Common Mistakes And How To Avoid Them

Errors often arise from measuring the wrong run, not accounting for rafters or overhangs, or failing to convert to the 12-inch basis. Ensure measurements are taken perpendicular to the ridge, confirm the run is horizontal, and double-check calculations.

Checklist To Prevent Errors

1. Measure perpendicular to the ridge to avoid angled run measurements.
2. Exclude overhangs from run unless intentionally included for a design calculation.
3. Always convert to a 12-inch basis when reporting rise over run for industry clarity.

Tools And Apps To Simplify Calculations

Smartphone apps, roofing calculators, and digital inclinometers streamline measuring and converting rise over run to angles or material estimates. Digital levels and laser distance meters increase accuracy when working from the ground or attic.

Recommended Tools

• Digital Inclinometer or Angle Finder: Measures roof angle directly.
• Laser Distance Meter: Provides accurate run and rise distances from safe locations.
• Roofing Calculators/Apps: Convert rise/run to area and material counts quickly.

Practical Examples For Remodelers And Homeowners

When replacing shingles, knowing the rise over run helps determine waste, underlayment length, and drip edge needs. Architects use pitch to manage aesthetics, drainage, and attic space. Contractors present rise over run as a simple metric during bids.

Example: Estimating Shingle Squares

One roofing square covers 100 sq ft. If a gable roof has two planes each 671 sq ft (from earlier example), total roof area = 1342 sq ft. Squares = 1342 / 100 = 13.42 squares. Add waste factor (typically 10-15%) for starter shingles and valleys.

When To Call A Professional

If structural changes, moisture issues, or uncertain measurements arise, consult a licensed roofer or structural engineer. Professionals assess rafters, load paths, and code compliance that rise over run alone cannot determine.

Signs To Get Expert Help

• Visible sagging or rot in roof framing.
• Major remodeling or conversion that changes roof load or pitch.
• Unclear or inconsistent measurements that impact bids or permits.

Mastering rise over run enables better decision-making for material selection, cost estimation, and safety planning. Accurate measurements and correct conversions reduce waste, ensure compliance, and help homeowners communicate effectively with contractors.