Roof Rafter Framing Calculator

Units

Imperial: ft/in. Metric: m/mm.

Ridge deduction

Waste allowance (%)

Lumber size (actual)

× in

Used for board foot estimate.

Building span

ft

in

Full width wall to wall.

Building length (eave)

ft

in

Used for counts and area.

Pitch (rise per 12)

/12

Leave angle empty to use pitch.

Or angle

°

Overrides pitch when provided.

Ridge thickness

in

Horizontal overhang

in

Tail (extra)

in

Seat depth

in

Birdsmouth seat cut depth.

Rafter spacing

in

On-center spacing.

Lumber depth

in

Plate width

in

Include hip/valley and jack table

Max jacks listed

Formula Used

Run = span ÷ 2. Effective run = run − ridge deduction. Angle = arctangent(pitch ÷ 12) or given angle. Rise = effective_run × tan(angle). Slope length = √(effective_run² + rise²). Overhang slope = overhang ÷ cos(angle).

Total rafter length = slope length + overhang slope + tail slope. Plumb cut = angle. Seat cut = 90° − angle. Rafter count = ⌊(length ÷ spacing)⌋ + 1. Area = 2 × (run × length ÷ cos(angle)).

Hip/valley approximation: plan factor 1/ cos 45°. Jack step-down ≈ spacing × tan(angle) ÷ cos 45°. Approximations assume square plan at corners.

How to Use This Calculator

Select units and ridge deduction mode.
Enter span and eave length using long and short fields.
Provide pitch or angle. Add overhang, tail, spacing, and seat depth.
Optionally enable hip/valley to get jack tables.
Click Calculate, then export CSV, PDF, or JSON.

Always verify results and site geometry before cutting materials.

Example Data Table

Span	Pitch (/12)	Overhang	Ridge	Length	Rafter Count
7.32 m	6	305 mm	38 mm	4.81 m	25
8.53 m	4	400 mm	38 mm	5.01 m	29
9.75 m	8	450 mm	38 mm	5.85 m	33

Additional Notes

Metric inputs: long fields are meters; short fields are millimeters.
Imperial inputs: long fields are feet; short fields are inches.
Hip/valley and jack outputs are approximations for quick planning.
Use waste allowance to round counts upward for ordering.

Span-to-Run Quick Reference

Run equals half the span, before ridge deduction. Handy guide for layout.

Span (ft)	Run (ft)	Run (in)
16	8	96
20	10	120
24	12	144
28	14	168
32	16	192
36	18	216

Pitch to Angle Lookup

Approximate angles for common pitches. Use angle if pitch is unknown.

Pitch (/12)	Angle (°)
3/12	14.04
4/12	18.43
5/12	22.62
6/12	26.57
7/12	30.26
8/12	33.69
9/12	36.87
10/12	39.81
11/12	42.51
12/12	45

Material & Fastener Checklist

Total common rafters (with waste): —
Ridge member length: —
Roof sheathing area (both sides): —
Recommended fasteners: 16d nails at connections; metal hangers as required.
Safety: verify bearing, seat depth, and code requirements on site.

Framing Glossary

Run: Horizontal distance from wall plate to ridge centerline.
Rise: Vertical height corresponding to the run at the given pitch.
Plumb Cut: Vertical end cut at the ridge; matches roof angle.
Seat Cut: Horizontal cut forming the birdsmouth bearing on the plate.
HAP: Height above plate: remaining rafter depth above the seat.

FAQs

Should I enter pitch or angle?

Use pitch when you know rise per 12. Enter angle when measured with a bevel gauge. If both are entered, angle overrides pitch automatically for calculations.

Which ridge deduction mode is correct?

Half is typical for centerline layout against a ridge board. Use none for structural ridge beams with hangers. Use full when plans specify measuring to the far face.

Does the birdsmouth check meet code?

The check is a sanity guide only. Local code and engineered details govern. Always confirm allowable seat depth, bearing length, and HAP with plans or a qualified professional before cutting.

Can I use metric inputs?

Yes. Switch units to Metric. Long fields accept meters; short fields accept millimeters. Results display in inches for clarity while maintaining accurate internal conversions.

How accurate are hip/valley and jack tables?

They are approximate for 45° plan corners. Irregular plans, unequal pitches, and off-center ridges require layout on site. Verify lengths with stringlines or full-scale templates before cutting.

What is HAP and why does it matter?

HAP means height above plate. It’s the remaining rafter depth above the seat cut. Maintaining adequate HAP preserves strength and helps align fascia and ceiling planes cleanly.

How do exports work?

After calculating, use CSV for cut lists, PDF for printable summaries, and JSON for structured data. Files download directly in your browser without storing project data online.