Rafter Lengths for Roof Calculator

Calculator Inputs

Unit

Rafter Type

Full Roof Span

Custom Run Optional

Pitch Input Type

Pitch Value

Horizontal Overhang

Ridge Thickness

Roof Length

Rafter Spacing

Jack Rafter Spacing

Birdsmouth Depth

Waste Percent

Price Per Unit Length

Quantity Override Optional

Example Data Table

Span	Pitch	Overhang	Run	Common Length	Hip Length
24 ft	6 per 12	1.5 ft	11.94 ft	15.03 ft	20.77 ft
30 ft	8 per 12	2 ft	14.94 ft	20.39 ft	27.45 ft
18 ft	4 per 12	1 ft	8.94 ft	10.48 ft	14.62 ft

Formula Used

Run: Run = Span / 2 - Ridge thickness / 2

Slope: Slope = Rise per 12 / 12

Common rafter: Length = Horizontal run × √(1 + Slope²)

Total common rafter: Length = (Run + Overhang) × √(1 + Slope²)

Rise: Rise = Run × Slope

Hip or valley rafter: Length = Horizontal run × √(2 + Slope²)

Roof angle: Angle = arctan(Slope)

Jack reduction: Reduction = Jack spacing × √(1 + Slope²)

How to Use This Calculator

Select the unit used for your measurements.
Choose common, hip, or valley rafter type.
Enter the full roof span from wall edge to wall edge.
Use custom run only when you already know the run.
Enter pitch as rise per 12, angle, or percent slope.
Add horizontal overhang, ridge thickness, spacing, and cost values.
Press the calculate button to view the result above the form.
Use CSV or PDF download for records and sharing.

Rafter Length Planning

A rafter length calculator helps turn roof ideas into clear framing numbers. It uses the roof span, pitch, and overhang to find the sloped board length. This saves time before cutting timber. It also reduces waste during layout.

Why Rafter Length Matters

Rafters carry roof sheathing, covering, snow, wind, and maintenance loads. A wrong length can shift the ridge line. It can also create uneven eaves. Good measurements make the roof cleaner and safer. The calculator gives common rafter, tail, hip, valley, and jack information. These values help builders compare several choices.

Key Inputs

Start with the outside span. Use the full building width. The calculator divides it by two for the basic run. Add the ridge thickness when needed. Enter pitch as rise per twelve, angle, or percent slope. Add the horizontal overhang. Add rafter spacing, roof length, waste, and price when estimating material.

Using the Result

The common rafter length shows the board from plate to ridge. The tail length shows the extra sloped part beyond the wall. Total rafter length combines both values. Rise gives the vertical height above the wall plate. The roof angle helps with saw settings. Hip and valley lengths are longer because they travel diagonally across the plan.

Better Field Planning

Use the result as a layout guide, not as final structural approval. Real roofs may include fascia thickness, ridge beams, birdsmouth limits, wall irregularities, and local framing rules. Check actual site dimensions before cutting. Mark one test rafter first. Fit it in place. Then use it as a pattern for matching rafters.

Material Control

The estimate section converts rafter lengths into board quantity. It includes waste because offcuts and defects are common. A small waste allowance is useful for simple roofs. Complex roofs need more. Download the CSV for records. Download the PDF for sharing. Keep both with your framing notes.

Accuracy Tips

Always measure from the same reference faces. Do not mix inside and outside wall dimensions. Keep units consistent. Record whether the ridge is structural or a board. Check code span tables before buying lumber. Longer rafters may need larger sections. Steeper roofs may need safer access. Complex intersections should be checked by an experienced framer first.

FAQs

What is a rafter run?

Rafter run is the horizontal distance from the wall plate to the ridge center or ridge face. This calculator uses half the span, then adjusts for ridge thickness unless a custom run is entered.

What does rise per 12 mean?

Rise per 12 means the roof rises a set amount for every 12 horizontal units. A 6 per 12 pitch rises 6 units over 12 units of run.

Does the calculator include overhang?

Yes. It calculates the body length to the ridge and the sloped tail length from the horizontal overhang. The total common rafter includes both parts.

Can I calculate hip rafters?

Yes. Select hip rafter as the rafter type. The tool uses a diagonal factor because hip rafters travel across the roof plan at an angle.

Can I calculate valley rafters?

Yes. Select valley rafter. The same diagonal length method is used for a standard right-angle roof intersection with equal slopes.

What is jack rafter reduction?

Jack rafter reduction is the repeated length difference between adjacent jack rafters. It depends on jack spacing and the common roof slope factor.

Is this enough for structural design?

No. This calculator gives geometric lengths and estimates. Structural sizing needs span tables, species data, loads, local code checks, and professional review when required.

Why add waste percent?

Waste covers trimming, knots, defects, mistakes, and unusable offcuts. Simple roofs may need less waste. Complex roofs usually need more extra material.