Quick checks
•
Check run, rise, and overhang before cutting lumber
Roof framing
Roof Rafter Calculator for Length, Size, Span & Birdsmouth Cuts
Calculate common rafter length and size planning from roof span, run, pitch, and rafter spacing, plus plumb and seat-cut angles, birdsmouth depth, overhang, and lumber count for common roof layouts.
Rafter Calculator
Scenario presets
Pick the framing layout
Use a garage, shed, hip-roof, or barn-style preset.
Quick checks
•
Separate geometry from final structural sizing
Quick checks
•
Use roof type and spacing to narrow the framing plan
Framing setup
Check the framing plan before you cut lumber
Use the calculator to solve length and angles. Then verify member size with span tables.
Before you cut or order rafters
1
Confirm span, run, and overhang
A small span mistake compounds into wrong rafter length, birdsmouth placement, and sheathing layout.
2
Separate geometry from structural sizing
The calculator can solve length and angles quickly, but final member size still depends on span tables, load, and code.
3
Know the roof system you are framing
Gable, shed, hip, and ridge-beam layouts create different cut logic and different follow-up checks for ties or supports.
What usually breaks the first estimate
Spacing assumptions
Moving from 16-inch to 24-inch spacing changes sheathing, load path, and the acceptable member range.
Load and snow exposure
Snow country or heavy roofing systems can invalidate a member size that looks fine on a generic table.
Ridge and seat-cut constraints
Birdsmouth depth, bearing, and ridge thickness all need room in the board before the geometry is truly buildable.
Lock the geometry first. Then verify the structural call with span tables or engineering.
Fast planning rules
Start with the rafter geometry that changes the cut list
Check span, pitch, overhang, spacing, and birdsmouth layout before you cut or order lumber.
Roof rafter length calculator
Enter roof span, pitch, overhang, and spacing to estimate common rafter length, cut angles, and the rough lumber count.
Use the output for planning, then verify the final span, species, load, and code requirements before cutting.
What size rafter do I need?
Rafter sizing depends on the span or run, rafter spacing, roof and other loads, lumber species, grade, and local building code.
This is planning guidance only. Confirm the final size with local span tables, code requirements, or a structural engineer before ordering or cutting lumber.
How do I calculate rafter length?
For a common gable roof, start with half the building span as the run, convert pitch into rise, then find the diagonal rafter length before adding overhang and ridge adjustments.
A 24-ft span at 6/12 pitch is about 13.4 feet before overhang.
Birdsmouth and cut angles
Roof pitch sets the plumb cut, seat cut, and birdsmouth layout, while overhang and wall thickness change the final mark.
Keep birdsmouth cuts within accepted framing limits and check local code before cutting.
How to Use This Rafter Calculator
Use this calculator to estimate rafter length, cut angles, and framing quantity. It works best after you confirm span, pitch, and overhang.
Understanding Roof Framing Basics
Start with these key terms:
- Span - The total width of the building from outside wall to outside wall
- Run - Half the span (for standard gable roofs) - the horizontal distance from wall to ridge
- Rise - The vertical height from the top plate to the ridge
- Pitch - The roof slope expressed as rise per 12 inches of run (e.g., 6:12)
- Rafter Length - The actual length of lumber needed, measured along the rafter
- Overhang - How far the rafter extends past the wall (the eave)
Step 1: Select Your Roof Type
Different roof styles require different framing approaches:
- Gable (Two-Slope) - The most common residential roof. Two sloped surfaces meeting at a central ridge. Simple to frame with straight common rafters.
- Hip (Four-Slope) - All four sides slope toward the walls. More complex framing with hip rafters, jack rafters, and common rafters. Better wind resistance.
- Shed (Single-Slope) - One sloped surface, often used for additions, porches, and outbuildings. Simplest framing with rafters running in one direction.
- Gambrel (Barn Style) - Two different slopes on each side, creating more headroom in the attic. Common on barns and Dutch colonial homes.
Step 2: Enter Building Dimensions
Measure carefully - accurate dimensions are critical for proper fit:
- Building Span - Measure from the outside of one wall to the outside of the opposite wall
- Building Length - The length of the ridge (parallel to the rafters)
- Overhang - Typical residential overhangs are 12-24 inches. Longer overhangs provide better weather protection.
Step 3: Determine Roof Pitch
Roof pitch is expressed as a ratio of rise to run. For example, a 6:12 pitch rises 6 inches for every 12 inches of horizontal run.
| Pitch | Angle | Description |
|---|---|---|
| 2:12 - 3:12 | 9° - 14° | Low slope - requires special roofing (membrane or built-up) |
| 4:12 - 6:12 | 18° - 26° | Standard residential - most common, works with shingles |
| 7:12 - 9:12 | 30° - 37° | Medium steep - good water runoff, harder to walk on |
| 10:12 - 12:12 | 40° - 45° | Steep - excellent for snow, requires safety equipment |
| 13:12+ | 47°+ | Very steep - specialty construction, dramatic appearance |
Understanding Rafter Cuts
Every rafter requires three basic cuts to fit properly:
Plumb Cut (Ridge Cut)
The plumb cut is made at the top of the rafter where it meets the ridge board. This cut is vertical (plumb) when the rafter is in position. The angle is determined by the roof pitch.
How to mark: Set your framing square with the pitch (e.g., 6 on the tongue, 12 on the blade) and mark along the tongue for the plumb cut.
Birdsmouth Cut
The birdsmouth is a notch cut where the rafter sits on the wall's top plate. It consists of:
- Seat Cut (Level Cut) - Horizontal cut that rests on the top plate
- Heel Cut (Plumb Cut) - Vertical cut against the outside of the wall
Rule of thumb: The seat cut depth should not exceed 1/3 of the rafter depth to maintain structural integrity. For a 2x6 rafter (5.5" actual), maximum seat cut is about 1.8".
Tail Cut (Fascia Cut)
The tail cut is made at the end of the overhang where the fascia board attaches. This is typically a plumb cut, but can be modified for different fascia styles.
Rafter Size and Span Planning
On a standard gable roof, the common-rafter run is usually about half the total building span. Rafter size and span planning also depend on spacing, snow and wind loads, roof covering, species, grade, and local building code. For example, a 20-foot building span is about a 10-foot common-rafter run per side before overhang. Use the directions below for early planning only:
| Planning situation | What it changes | Next check |
|---|---|---|
| Short, light runs | Usually leaves more conventional framing options to compare. | Check the applicable local span table and design loads. |
| Typical residential runs | Spacing, roof covering, and design loads can change the lumber choice. | Confirm the combination against local span tables and code. |
| Longer or heavier-load runs | May require larger members, shorter spacing, or additional support. | Have the framing system checked for the actual loads and run. |
| Structural ridge or open ceiling | Changes the load path through the ridge beam, posts, connections, and rafters. | Use local code and a structural engineer for the system review. |
For a full walkthrough of how span, pitch, and spacing change the lumber choice, see our rafter size and span guide. Confirm the final rafter size with local span tables or a structural engineer before you order lumber or cut.
What's the minimum roof pitch for shingles?
Most asphalt shingles require a minimum 4:12 pitch. Lower pitches (2:12 to 4:12) may require ice and water shield underlayment. Below 2:12, you'll need low-slope roofing like modified bitumen, EPDM, or TPO.
How we checked this page
Written by: TheSiteMath Editorial Team
Reviewed by: TheSiteMath editors (formula, source, and update review)
Last reviewed: 2026-03-20
Publisher:
TheSiteMath
Scope: U.S. construction material estimating, calculator workflows, and project planning guidance for contractors and homeowners.
What we checked:
- • Formulas checked against trade and source material
- • Verified against: American Wood Council span and framing guidance, Roof-framing layout and measurement references, Current U.S. framing material pricing benchmarks
- • Price ranges used for planning, not as fixed quotes
- • Examples checked in the live calculator
Methodology:
- • Example quantities and explanations on this page are cross-checked against the matching live calculator on TheSiteMath.
- • This framing content is scoped for U.S. planning and estimating workflows, not for stamped engineering or permit approval.
- • We review formulas, material assumptions, and practical steps against category-appropriate references before publishing updates.
- • We refresh pages when calculator logic, supplier assumptions, or pricing guidance materially changes.
- • Readers should confirm final dimensions, structural requirements, and local code obligations with qualified local professionals.
Editorial standards: We review pages before publication and update them when formulas or pricing need a fix. If you spot an issue, please
contact us
.
For our review process, corrections policy, and monetization disclosure, see the Editorial Standards page.
Framing FAQ
Rafter questions that change cuts, spacing, and lumber choice
These questions help visitors separate framing geometry from structural sizing so the calculator does not get mistaken for a generic span-table shortcut.
How do I calculate rafter length?
Rafter length is calculated using the Pythagorean theorem: Length = √(Run² + Rise²). For a 24-foot building with 6:12 pitch: Run = 12 feet, Rise = 6 feet, Length = √(144 + 36) = 13.4 feet. Add overhang for total length.
What size rafter do I need for a 20 foot span?
A 20-foot building span on a standard gable roof is usually split into two common-rafter runs of about 10 feet before overhang and other adjustments. The rafter size still depends on spacing, loads, species, grade, roof system, and local requirements; confirm it with local span tables, building code, or an engineer rather than treating the span alone as a structural prescription.
What spacing should I use for rafters?
16 inches on center is most common and provides good strength with standard sheathing. 24 inches on center is acceptable for light loads with proper sheathing. 12 inches on center is used for heavy snow areas or tile roofs.
Do I need a ridge board or ridge beam?
A ridge board is non-structural and just keeps rafters aligned - requires collar ties or ceiling joists. A ridge beam is structural and carries the roof load - required for cathedral ceilings without ceiling joists, needs posts at each end.
How do I handle hip rafters?
Hip rafters run at 45° from corners to the ridge. They're longer than common rafters and require compound cuts. Hip length = √(Run² + Rise² + Run²) × 1.414. Hip roofs also require jack rafters running from wall to hip rafter.
What's the minimum roof pitch for shingles?
Most asphalt shingles require minimum 4:12 pitch. Lower pitches (2:12 to 4:12) may require ice and water shield underlayment. Below 2:12, you'll need low-slope roofing like modified bitumen, EPDM, or TPO.