Updated Carpentry & Roof Framing

Rafter Length Calculator

Compute common rafter length, hip and valley rafter length, roof rise and cut angles from span, pitch and overhang. Use simple mode for quick calculations, detailed geometry for hips and valleys, or the pro layout mode for framing-friendly outputs.

Common Rafters Hip & Valley Rafters Pitch & Span Cut Angles

Calculate Rafter Length from Span, Pitch & Overhang

This Rafter Length Calculator offers three levels of detail. The Simple Common Rafter tab uses span, pitch and overhang to give fast results. The Hip & Valley Geometry tab lets you approximate hip and valley rafter lengths plus rafter counts based on spacing. The Pro Framing Layout tab adjusts for ridge thickness and outputs framing-friendly dimensions and angles you can transfer to your layout tools.

All formulas use standard right-triangle roof geometry. Pitch is entered as rise per 12 units of horizontal run (for example, a 6:12 roof has 6" of rise for each 12" of run). Overhang is treated as horizontal distance from the wall line to the fascia.

Total width from outside wall to outside wall across the building.

Example: enter 6 for a 6:12 roof, 4 for 4:12, etc.

Horizontal distance from wall face to fascia (soffit overhang).

The calculator computes the half-span run, roof rise and common rafter length using the Pythagorean theorem. Overhang is converted from horizontal distance into a rafter tail length along the slope.

Width across the gable end (outside to outside).

Length along the eaves (outside to outside).

Typical values are 16" or 24" on center.

This mode treats the roof as a simple hip roof. The calculator approximates common, hip and valley rafter lengths from building dimensions and pitch, then estimates how many common and jack rafters you might need based on spacing. For complex rooflines, use the results as a starting point only.

Actual thickness (for example, 1.5" for a 2x ridge).

Horizontal cut depth sitting on the plate. Used here for heel height approximation.

The pro layout mode adjusts the rafter run for half the ridge thickness, then computes theoretical rafter length from plate to ridge, total length with overhang, plumb and level cut angles, plus an approximate heel height based on the birdsmouth seat depth. It is meant as a framing helper, not a substitute for plans.

Rafter Length Calculator – Common, Hip & Valley Rafters Explained

Cutting rafters accurately is one of the key skills in roof framing. The Rafter Length Calculator from MyTimeCalculator helps you turn building span, pitch and overhang into usable dimensions, so you can double-check layout math or visualize how roof geometry changes as you adjust the design.

The calculator uses right-triangle trigonometry, treating each rafter as the hypotenuse of a triangle with a horizontal run and vertical rise. By entering span and pitch, you can compute the sloping rafter length, roof rise and cut angles without digging through tables or reference books.

1. Mode A – Simple Common Rafter Length

The Simple Common Rafter tab focuses on the most common scenario: a symmetrical gable roof where the building span, pitch and overhang are known. Behind the scenes, the calculator:

  1. Computes the run as half of the building span.
  2. Uses pitch (rise per 12) to find the total rise over that run.
  3. Applies the Pythagorean theorem to get rafter length from wall to ridge.
  4. Converts the horizontal overhang into a rafter tail length along the slope.
  5. Adds core length and tail length to give the total rafter length.

The mode also reports the plumb cut angle at the ridge in degrees. This is the angle you would set on a miter saw or layout square for the vertical cut at the top of the rafter.

2. Mode B – Hip & Valley Rafter Geometry

Roofs with hips and valleys use rafters that run diagonally in plan view. These hip and valley rafters are longer than common rafters with the same pitch because their horizontal run is measured along a diagonal across the building footprint.

The Hip & Valley tab takes building width and length, plus pitch and overhang, and:

  • Computes the common rafter run as half the width.
  • Derives the hip plan run from corner to ridge using diagonal geometry.
  • Calculates common rafter length and hip/valley rafter length.
  • Estimates the number of common rafters along the length using your chosen spacing.
  • Approximates the number of jack rafters running into each hip.

For real-world roofs with dormers, intersecting hips or complex layouts, this mode gives a useful starting point, but you should always refine counts and lengths to match your plans and local code details.

3. Mode C – Pro Framing Layout

Professional framers often need to adjust rafter layout for ridge board thickness and birdsmouth cuts. The Pro Framing Layout tab introduces these details while keeping the math readable:

  1. The run is shortened by half the ridge board thickness so that rafters bear correctly against the ridge.
  2. The calculator finds the rise to the top of the ridge using the adjusted run and pitch.
  3. It computes rafter length from plate to ridge plus a sloping tail based on overhang.
  4. It reports the plumb cut angle and corresponding level (seat) cut angle.
  5. It uses birdsmouth seat depth as an input to approximate heel height.

This mode is intended as a framing helper and cross-check. It does not replace engineered drawings or the details shown on project plans, but it can help you sanity-check numbers or translate design pitch into practical layout angles.

4. How Rafter Length is Calculated from Span and Pitch

The core of all three modes is the right triangle formed by run, rise and rafter length. If the building span is S and the roof is symmetrical, the run is S/2. With pitch specified as R per 12 units of run (for example 6:12), the rise over that run is:

Rise = Run × (Pitch / 12)

The common rafter length from wall to ridge is then:

Rafter length = √(Run² + Rise²)

Overhang is converted to a sloping tail length using a rafter factor based on pitch. For each 12" of horizontal run, the rafter length increases by √(12² + Pitch²), so a given horizontal overhang is multiplied by the same factor per foot of run.

5. Understanding Plumb and Level Cut Angles

The plumb cut angle is the angle between the top edge of the rafter and a vertical line. In trigonometric terms, it is:

Plumb angle = arctan(Rise / Run) = arctan(Pitch / 12)

The seat (level) cut angle for the birdsmouth is simply 90° minus the plumb angle. Reporting both angles makes it easier to set saws or framing squares, especially if you are checking a pitch that is not listed on a printed framing square.

6. Practical Tips for Using the Rafter Length Calculator

  1. Use the Simple Common Rafter tab first to get a feel for how pitch and span affect rafter length and roof rise.
  2. Switch to Hip & Valley Geometry when you have a hip roof or want to estimate how many common and jack rafters you will need for a basic layout.
  3. Move to the Pro Framing Layout tab when you want framing-friendly outputs that incorporate ridge thickness, birdsmouth seat depth and cut angles.
  4. Always compare calculator outputs with your plans, building codes and manufacturer requirements before ordering lumber or cutting rafters in bulk.
  5. For structural questions such as rafter sizing, deflection limits or load capacity, consult a qualified engineer or your local building authority.

Related Tools from MyTimeCalculator

Rafter Length Calculator FAQs

Frequently Asked Questions

Quick answers to common questions about using span, pitch, overhang and ridge thickness to compute rafter lengths and cut angles.

For a simple gable roof you usually need the building span (outside to outside), the roof pitch (rise per 12 of run) and the overhang. From these, the calculator finds the half-span run, roof rise and rafter length using the Pythagorean theorem, and converts horizontal overhang into a tail length along the rafter.

Hip and valley rafters run diagonally in plan view. The calculator finds the diagonal run from a corner to the ridge using the building width and length, then applies the same pitch to compute a sloping length. The result is an approximation for simple hip roofs. Complex roofs with multiple hips, valleys or dormers should be checked against detailed drawings or layout methods used by experienced framers.

When you add a ridge board, the theoretical meeting point of two rafters shifts by half the ridge thickness. Adjusting the run for this thickness changes the rafter length slightly. Birdsmouth seat depth affects how high the rafter sits above the wall plate, which is reflected in the heel height. Including these values in the calculation produces numbers that better match the pieces you actually cut on site.

No. The Rafter Length Calculator focuses on geometry: runs, rises, lengths and angles. It does not check loads, allowable spans, deflection or fastening requirements. For code compliance and structural safety you should use span tables provided by your building authority, manufacturer documentation or a licensed engineer, then use the calculator to help visualize and verify geometric dimensions only.

For simple gable and basic hip roofs the geometry used is standard and matches common framing methods. For roofs with multiple hips, valleys, dormers or irregular shapes, the calculator still provides useful approximations, but it cannot see every detail of the design. Use it for planning and cross-checking, and rely on your plans, layout lines and local expertise to finalize lengths and counts for complex projects.

The current version assumes feet and inches for inputs and outputs. You can still use it with metric projects by converting dimensions to feet before entering them, then converting the results back to millimeters or meters as needed. A future version may include a dedicated unit switch between imperial and metric systems.

Related Calculators