Rafter Length Calculator

Calculate rafter length, roof angle, plumb cut, and seat cut for any roof type.

Quickly determine the correct rafter length, roof angle, plumb cut, and seat cut for your construction project. Supports both metric and imperial units. Perfect for gable, hip, and shed roofs.

Examples

Click on any example to load it into the calculator.

Gable Roof (Metric)

gable

Standard gable roof with metric units, no overhang.

Run: 4.5 unit

Rise: 2.5 unit

Overhang: 0 unit

Thickness: 0.05 unit

Pitch: 29.05 °

Unit: Meters (m)

Shed Roof (Imperial)

shed

Simple shed roof with imperial units and overhang.

Run: 12 unit

Rise: 4 unit

Overhang: 1 unit

Thickness: 2 unit

Pitch: 18.43 °

Unit: Feet (ft)

Hip Roof (Metric, Overhang)

hip

Hip roof with overhang and rafter thickness included.

Run: 3.2 unit

Rise: 1.8 unit

Overhang: 0.4 unit

Thickness: 0.06 unit

Pitch: 29.74 °

Unit: Meters (m)

Steep Roof (Imperial)

steep

Steep roof with high pitch and no overhang.

Run: 10 unit

Rise: 8 unit

Overhang: 0 unit

Thickness: 1.5 unit

Pitch: 38.66 °

Unit: Feet (ft)

Other Titles
Understanding Rafter Length Calculator: A Comprehensive Guide
Master the calculation of rafter length, roof angle, and more for any construction project. Learn the formulas, best practices, and real-world applications.

What is the Rafter Length Calculator?

  • Core Concepts and Definitions
  • Why Rafter Length Matters
  • Types of Roofs and Rafters
The Rafter Length Calculator is a specialized construction tool designed to help builders, carpenters, and DIY enthusiasts accurately determine the length of roof rafters for any type of roof. By inputting the run, rise, overhang, and other parameters, users can quickly obtain precise measurements needed for cutting and installing rafters.
Why Accurate Rafter Length is Critical
Accurate rafter length calculations ensure structural integrity, proper load distribution, and a professional finish. Incorrect measurements can lead to wasted materials, structural issues, and costly mistakes.
Types of Roofs and Rafters
This calculator supports gable, hip, shed, and other common roof types. It can be used for both metric and imperial units, making it versatile for any project.

Key Concepts:

  • Rafter: The sloped structural member supporting the roof deck.
  • Run: The horizontal distance from wall to roof peak.
  • Rise: The vertical height from wall to roof peak.
  • Overhang: The extension of the rafter beyond the wall.

Step-by-Step Guide to Using the Rafter Length Calculator

  • Input Preparation
  • Calculation Process
  • Result Interpretation
To use the Rafter Length Calculator, gather all necessary measurements: run, rise, overhang, and rafter thickness. Select the appropriate unit (meters, centimeters, feet, or inches). Enter the values into the calculator and click 'Calculate'. The tool will instantly provide the rafter length, roof angle, plumb cut, and seat cut.
Input Preparation
Measure the run and rise accurately using a tape measure or laser tool. Include overhang and thickness if needed. Double-check all values before calculation.
Calculation Process
The calculator uses the Pythagorean theorem to determine rafter length: rafter length = sqrt((run + overhang - thickness/2)^2 + rise^2). Roof angle and plumb cut are calculated using trigonometric functions.
Result Interpretation
Review the results and use them to mark and cut your rafters. The seat cut is especially important for ensuring a proper fit on the wall plate.

Example Steps:

  • Measure run and rise.
  • Add overhang if needed.
  • Select unit and calculate.
  • Use results for cutting rafters.

Real-World Applications of Rafter Length Calculation

  • Construction Projects
  • DIY Home Improvement
  • Architectural Design
Rafter length calculation is essential for building new roofs, repairing existing structures, and designing custom roof shapes. It is used by professionals and DIYers alike for gable, hip, and shed roofs.
Construction Projects
Builders use rafter length calculations to ensure proper material ordering and minimize waste. Accurate measurements lead to efficient construction and cost savings.
DIY Home Improvement
Homeowners and hobbyists use the calculator for small projects like sheds, pergolas, and carports. It simplifies complex math and helps avoid mistakes.
Architectural Design
Architects and designers use rafter length calculations to create detailed plans and ensure compliance with building codes.

Application Scenarios:

  • Building a gable roof for a house.
  • Designing a shed with a custom roof angle.
  • Adding an overhang to a pergola.

Common Misconceptions and Correct Methods

  • Myths About Rafter Calculation
  • Avoiding Common Errors
  • Best Practices
Many believe rafter length can be estimated by eye or with simple rules of thumb. In reality, precise calculation is necessary for safety and durability.
Myth: All Roofs Use the Same Formula
Different roof types and designs may require adjustments to the basic formula. Always consider overhang, thickness, and pitch.
Avoiding Common Errors
Double-check all measurements and units. Use the calculator to verify manual calculations.
Best Practices
Always use accurate tools, measure twice, and cut once. Consult building codes and professionals when in doubt.

Best Practice Tips:

  • Always measure twice before cutting.
  • Include overhang and thickness in calculations.
  • Use the correct unit for all measurements.

Mathematical Derivation and Examples

  • Pythagorean Theorem
  • Trigonometric Functions
  • Worked Examples
The rafter length is calculated using the Pythagorean theorem: rafter length = sqrt((run + overhang - thickness/2)^2 + rise^2). Roof angle = arctan(rise / (run + overhang - thickness/2)). Plumb cut is the same as the roof angle. Seat cut is calculated based on rafter thickness and angle.
Worked Example 1
Given: run = 4.5m, rise = 2.5m, overhang = 0.3m, thickness = 0.05m. Rafter length = sqrt((4.5+0.3-0.025)^2 + 2.5^2) = 5.29m (rounded). Roof angle = arctan(2.5/4.775) = 27.5°. Plumb cut = 27.5°. Seat cut = thickness / cos(angle).
Worked Example 2
Given: run = 12ft, rise = 4ft, overhang = 1ft, thickness = 2in (converted to ft = 0.167ft). Rafter length = sqrt((12+1-0.0835)^2 + 4^2) = 13.66ft (rounded). Roof angle = arctan(4/12.9165) = 17.4°. Plumb cut = 17.4°. Seat cut = thickness / cos(angle).

Mathematical Examples:

  • Gable roof with overhang and thickness adjustment.
  • Shed roof with imperial units and high pitch.