How do I convert between degrees and x/12 pitch?

To convert x/12 to degrees: degrees = arctan(x/12). To convert degrees to x/12: x = 12 × tan(degrees). Common pitches: 4/12 = 18.43°, 6/12 = 26.57°, 8/12 = 33.69°, 10/12 = 39.81°, 12/12 = 45°.

🔨Professional Carpentry Tool

Rafter Calculator Pro

Q: How far apart should I space my rafters or trusses?

Standard rafter/truss spacing is 16 inches on-center for heavy loads (snow, tile) and 24 inches on-center for light loads (shingles, mild climate). 12-inch spacing is for very long spans or heavy timber roofs. Confirm with local building codes and span tables based on lumber grade, span, size, and load ratio.

Q: Should I use rafters or pre-fabricated trusses?

Choose rafters for usable attic space, custom design flexibility, dormers, no crane access. Choose trusses for faster one-day installation, longer spans (60 ft+), lower labor cost, and engineered consistency. Trusses block attic space; rafters need skilled carpenters and more on-site time.

Q: What size rafter do I need for my span?

General #2 SPF rules at 24" spacing: 2×6 up to 9 ft, 2×8 up to 12 ft, 2×10 up to 15 ft, 2×12 up to 18 ft. Reduce spans 1–2 ft in high snow zones or use 16" spacing. Always check 2021 IRC span tables or a structural engineer.

Q: What roof pitch should I choose for my climate?

Warm low-snow climates (FL, TX, AZ): 4:12–6:12. Moderate snow (Mid-Atlantic, Midwest): 6:12–8:12. Heavy snow (Rockies, Canada, New England): 9:12–12:12. Hurricane zones: 4:12–6:12 with hip roofs preferred. Steeper roofs shed water/snow better but use more material.

Q: What is the best roofing material for my pitch?

Below 2:12: use TPO/EPDM/PVC membrane only (no shingles). 2:12–4:12: metal standing seam or low-slope shingles with double underlayment. 4:12–9:12: asphalt shingles, metal, tile, slate all work. Above 9:12: use longer nails, extra fasteners, or dry-fasten steel.

Q: Can I use this calculator for metric and imperial units?

Yes. Supports both metric (mm/cm/m) and imperial (ft-in-fractions) with automatic conversion. Imperial mode supports smart input parsing like "8 11 3/4", "8' 11 3/4\"", or decimal formats, with results in standard carpentry fractions (1/16, 1/8, 1/4, etc.).

Calculate rafter lengths, angles, birdsmouth cuts, ridge reduction, and full roof structure. Interactive SVG diagrams with real-time results for professional carpenters and DIY enthusiasts.

📏 Metric & Imperial🎯 Real-time SVG📄 PDF Export🔍 3 View Modes

Input Parameters

All measurements update calculations in real-time.

Units

Rafter Run to Outer Wall

ft/in

Rafter Depth

Eave Overhang

Birds-mouth Seat

?Horizontal seat length (default)

Wall Height

Wall Thickness

Rafter Thickness

Ridge Thickness

Calculated Results

Results update automatically as you change inputs.

TOTAL RAFTER LENGTH✂️ Actual Cut Length

3,821 mm (12' 6-7/16")

Rafter Line Length3,746 mm (12' 3-1/2")

↳ Includes Ridge Reduction (21 mm (0-13/16")) and Overhang (762 mm (2' 6"))

✂️ Cutting & Marking

📐Plumb Cut Angle

68.0°

(4.85/12 Pitch) · Top vertical cut at ridge

🪚Tail / Fascia Cut

68.0°

(4.85/12) · Eave fascia trim angle

📐Seat Cut Angle

22.0°

(4.85/12 Pitch) · Birdsmouth seat line

📏Plumb Setback

2-15/16"

Tape setback from ridge end

✂️Ridge Reduction

0-13/16"

Bevel cut off ridge end

📍Rafter Line Length

12' 3-1/2"

Diagonal measure ridge → eave

🪵 Birds-mouth & Dimensions

🛡️ Birdsmouth Safety Indicators

⭐ HAP

6-3/4"

Heel Area · Critical Safety

Seat Cut

0-1/8"

Seat length

Birdsmouth Depth

Notch depth

🏠 Roof Geometry Reference

Rise4' 1-7/8"

Total Fall4' 7-1/4"

Total Run11' 4-3/4"

Eave Height11-1/4"

Ridge Reduction & Plumb Setback

These adjustments ensure your rafters meet perfectly at the ridge beam and the plumb cut is correctly positioned. The Plumb Setback tells you how far back from the ridge to measure to form the plumb cut.

Ridge Reduction

0-13/16"

Based on 1.50 ft ridge board

Plumb Setback

2-15/16"

Distance to measure back for plumb cut

Cut Length per Rafter

12' 6-7/16"

Actual length to cut on site

Formula Derivation

Ridge Reduction = (Ridge Thickness ÷ 2) ÷ cos(Pitch)

R_deduction = (1.50 ÷ 2) ÷ cos(22.0°)

R_deduction = 0.75 ÷ 0.9272

R_deduction = 0-13/16"

Plumb Setback = Rafter Depth × tan(Pitch)

PS = 7.25 × tan(22.0°) = 2-15/16"

Full Roof Structure Profile

Visualize your complete roof and wall structure. See the ridge height, eave clearance, and total span in one comprehensive diagram.

Rise to High End

12' 1-7/8"

Vertical distance from wall top to rafter ridge end

Total Span

0-11/16"

Full width between wall outer edges

Pitch Angle

22.0°

4.85/12 pitch ratio

Step-by-Step Rafter Cutting Guide

Follow these three precision cuts to transform a board into a perfect rafter.

🔺

Ridge Cut (Top)

Plumb cut at ridge end

Set your circular saw to

68.0°

(4.85/12 pitch · plumb/vertical angle)

①Measure Total Rafter Length from the tail end
②Mark the plumb line at the ridge point
③Cut along the plumb line at 68.0°
④Subtract Ridge Reduction: 0-13/16"

🦅

Birdsmouth Cut

Seat + plumb notch at wall

Seat Cut

22.0°

Seat Length

0-1/8"

⭐ HAP

6-3/4"

Notch Depth

①Measure 12' 3-1/2" from ridge cut along rafter edge
②This point marks the birdsmouth location
③Mark seat line (horizontal, 22.0°) and plumb line
④Verify HAP ≥ 1/3 rafter depth (critical!)

🏠

Tail / Fascia Cut

Eave end for fascia board

Fascia bevel angle

68.0°

Or plumb (90°) for traditional look

①The tail cut forms the eave end of the rafter
②Overhang: 0-1/8" beyond wall outer face
③Option A: Cut plumb (90°) for standard fascia
④Option B: Cut 68.0° for angled look

⚠️

Professional Structural Engineer Required

This cutting guide is for educational reference only. Always verify measurements on-site before cutting. Local building codes, snow load, wind load, and seismic requirements vary — consult a licensed structural engineer or architect before any construction. Never cut more than 1/3 of the rafter depth for the birdsmouth notch (HAP ≥ 2/3 of depth).

Common Roof Pitches Quick Reference

Standard roof pitch classifications with typical applications and geographic recommendations.

Angle	Class	Typical Uses & Notes	Snow/Wind
9.46°	Flat-Low	Porch overhangs, shed roofs, modern low-slope designs. Requires proper membrane drainage system. NOT recommended for heavy snow areas.	❌ Poor
14.04°	Very Low	Modern residential, metal roofing, shed-style homes. Minimum slope for asphalt shingles (some manufacturers require 4:12).	⚠️ Moderate
18.43°	Low	Most common minimum for asphalt shingles. Ranch-style homes, warm climates with low snowfall. Economical material use.	⚠️ Mild
22.62°	Medium-Low	Southern US residential, moderate climate. Balances material cost with water shedding capability.	✅ OK-Mild
26.57°	Standard	⭐ Most popular pitch in North America. Works with all roofing materials. Classic gable roof look. Safe for DIY projects.	✅ Moderate
30.26°	Medium	Transitional Northeast/Midwest homes. Better snow shedding than 6:12. Still walkable with care.	✅ Good
33.69°	Medium-High	Traditional Northeast, Midwest, mountain homes. Excellent snow shedding. Requires roofing boots for safe walking.	✅ Very Good
36.87°	Steep	Heavy snow regions (Rocky Mountains, Canada). Colonial, Cape Cod style. Walkable with extreme caution, use safety harness.	⭐ Excellent
39.81°	Very Steep	Alpine, ski chalets, historic European designs. Maximum snow shedding. Safety ropes required for any roof work.	⭐ Excellent
45.00°	Extremely Steep	Gothic Revival, dramatic architectural statement, premium custom homes. Roof work requires scaffolding. High material cost.	⭐ Max Snow

🏠 Walkable Pitches (Easy)

4:12 to 6:12 can be walked without extensive safety equipment. Wear slip-resistant boots.

⛰️ Steep Pitches (Caution)

7:12 to 9:12 require toe boards or safety harness. Roof brackets recommended.

🪨 Very Steep (Dangerous)

10:12 and above require scaffolding. Consider professional installation only.

Rafters vs. Roof Trusses

Understand the tradeoffs between traditional stick framing and modern pre-fabricated truss systems.

🪵

Stick-Framed Rafters

Built on-site, piece by piece

✓

Attic / Storage Space

Open web design leaves full attic usable for storage, bonus rooms, or HVAC runs.

✓

Flexibility & Customization

Easily adapt to irregular floor plans, dormers, skylights, or architectural details.

✓

No Heavy Equipment

Framers carry individual boards. No crane needed for delivery or installation.

✗

Slower Installation

Cutting and installing rafters takes 2–4× longer than trusses for comparable roof.

✗

More Skill Required

Precision layout and math (what our calculator helps with!). Quality depends on carpenter.

✗

Span Limitations

Beyond 16–20 ft span requires collar ties, rafter ties, or heavy dimensional lumber.

🏡 Best for: Custom homes, attic living space, renovations, small projects

🔺

Pre-Fabricated Trusses

Engineered, assembled in factory

✓

Faster Installation

Entire roof framed in a single day. Dramatic labor savings on site.

✓

Engineered & Consistent

Professionally engineered for exact loads. Every truss identical, no human error.

✓

Longer Spans (60 ft+)

Triangulated design spans garages, great rooms without intermediate support.

✗

No Attic Access

Internal webbing blocks usable attic space. "Attic trusses" cost more and reduce span.

✗

Transport & Crane

Require truck delivery and often a crane to lift into place on the foundation.

✗

Hard to Modify On-Site

Cutting a truss web voids the engineering. Must order custom for complex shapes.

🏗️ Best for: Production homes, warehouses, churches, commercial roofs, great rooms

⏱️ Quick Side-by-Side Comparison

Installation Time	🪵 2–4 days on site	🔺 1 day with crew + crane
Labor Cost (Typical US)	🪵 High ($15–35/sqft)	🔺 Low ($5–15/sqft)
Material Cost	🪵 2×6 to 2×12 dimensional	🔺 2×4 chords, metal plate gussets
Max Span	🪵 16–20 ft (simple)	🔺 60–80 ft (engineered)
Usable Attic	🪵 ✅ Full height available	🔺 ❌ Webs block (specialty only)
Design Changes	🪵 ✅ Easy on-site	🔺 ❌ Requires re-order
Dormers & Complexity	🪵 ✅ Natural fit	🔺 ⚠️ Custom trusses $$
Weight on Walls	🪵 Heavier (solid timber)	🔺 Lighter (distributed loads)

Popular Roof Type Styles

The gable roof our calculator handles is just the start. Here are 7 common roof styles and when to use each.

🔺

Gable (Peaked)

⭐ Most Common · 40% of US homes

Two sloped sides meeting at a central ridge. Simple, classic, excellent water shedding. This is what our calculator is designed for.

⛰️

Hip Roof

High wind / hurricane zones

All four sides slope downward from the ridge. More complex, stronger against wind, no gable ends for siding.

📐

Shed / Lean-To

Porches, additions, modern design

Single flat slope, easiest to frame. Common for outbuildings, carports, and contemporary architecture.

🏚️

Gambrel (Barn)

Barns, Dutch Colonial, sheds

Two pitches per side: shallow upper, steep lower. Maximizes attic/loft space with the Dutch bend.

🏛️

Mansard

French Second Empire style

Four gambrel-style sides, often with dormer windows. Creates full upper floor. Classic Parisian look.

🪟

Dormer (Add-on)

Attic conversions, natural light

Structure projecting from existing slope to add window, headroom, or architectural detail. Adds complexity.

🦋

Butterfly / V-Roof

Modern eco homes, rain collection

Inverted gable — two slopes meeting in a central valley. Modernist, great for solar panels, rainwater harvesting.

🟦

Flat / Low-Slope

Commercial, modern residential

Very low pitch (0–2:12). Requires TPO, EPDM, or built-up roofing membrane. Drainage is the #1 concern.

🎨

Combination

Custom homes, unique lots

Mix of gables, hips, shed, and dormers. Most interesting rooflines require multiple framing systems.

Rafter Math: Complete Formula Guide

Master the geometry behind every rafter calculation with our comprehensive formula reference.

📐

Rafter Line Length

L_line = (Run + Overhang) ÷ cos(Pitch)

The diagonal length of the rafter from the wall plate to the ridge, including the overhang. Uses right triangle trigonometry where the total horizontal distance (run + overhang) is the adjacent side and the rafter is the hypotenuse.

📏

Plumb Setback

PS = Rafter Depth × tan(Pitch)

The horizontal distance to measure back from the ridge plumb cut line to correctly position the plumb cut. This accounts for the rafter's depth and roof pitch, ensuring the plumb cut forms a perfect vertical line when installed.

✂️

Total Rafter Length

L_total = L_line + Plumb Setback

The actual length of material to cut. Adds the plumb setback to the line length so you have enough stock to form the plumb cut at the ridge end of the rafter.

📐

Cut Angles

Plumb = 90° − Pitch
Seat = Pitch

The plumb cut angle (top of rafter) is always complementary to the pitch. The seat cut angle (bottom notch) equals the pitch angle. Set your circular saw or table saw to these angles for precise cuts.

Free wall stud calculator and estimator: stud count with openings, corners, and blocking, 16 vs 24 OC spacing, and the 2x4 vs 2x6 comparison matrix.

Open calculator →

View All Calculators →

Rafter Guides & Tools

📐

How to Calculate Rafter Length and Cuts

Hands-on how-to manual for calculating rafter length and marking cuts on lumber.

📏

Rafter Span and Size Guide

Building code reference for rafter span limits and size selection.

🏠

Hip, Valley & Jack Rafter Guide

Complete guide to hip rafter, valley rafter, and jack rafter calculations for hip roofs and intersecting roof geometries.

🛖

Shed Roof Rafter Calculator

Step-by-step guide to calculating rafter length, angles, and cuts for shed roofs and lean-to roofs.

🧮

Rafter Length with Construction Calculator

Pro guide to using construction calculators for rafter length, angles, and cut calculations on the job site.

🏡

A-Frame Rafter Calculator

Complete guide to A-frame rafter calculation, floor framing, and structural geometry for A-frame house construction.

📐

Rafter Square & Framing Square Guide

Complete guide to using rafter squares and framing squares for manual rafter layout, including step-off methods and angle tables.

🪚

Rafter Birdsmouth & Seat Cut Guide

Step-by-step guide to calculating the rafter birdsmouth notch: seat cut length, HAP, heel and plumb cut angles, and notch layout.

🌍

Metric Rafter Calculator Guide

Complete guide to calculating rafter length, angles, and sizes in metric units for UK, Australia, and European builders.

🪓

What Is a Rafter? Definition, Anatomy & Types

Definition and anatomy of a roof rafter — plumb cut, seat cut, birdsmouth, ridge cut, and tail — plus Common, Hip, Valley, Jack, and Lean-to rafter types and sizing notation.

🛰️

How to Measure a Roof

Complete roof measurement methodology: tape and pitch multiplier, ground estimates, satellite and app tools, metal and shingle specifics, and common errors.

View All Guides →

Frequently Asked Questions

How do I calculate rafter length?▼

Rafter length is calculated using the formula: L = (Run + Overhang) / cos(Pitch) + Plumb Setback. The Run is the horizontal distance from the wall's outer edge to the ridge center. Plumb Setback = Rafter Depth × tan(Pitch) accounts for the extra material needed to form the plumb cut at the ridge.

What is the difference between plumb cut and seat cut?▼

The plumb cut is the vertical angle at the top end of the rafter that meets the ridge. It equals 90° minus the pitch. The seat cut is the horizontal notch at the bottom that sits on the wall plate. It equals the pitch angle. Combined, they form the birdsmouth joint that transfers roof loads to the walls.

Why is HAP important?▼

HAP (Height Above Plate) measures the remaining wood above the birdsmouth cut. Insufficient HAP (typically below 1/3 of rafter depth) weakens the connection to the wall plate and can cause structural failure in high wind or heavy snow. Most building codes require minimum 2/3 of rafter depth remaining after the notch, or about 1.5" minimum for a 2×6 rafter.

How far apart should I space my rafters or trusses?▼

Standard rafter and truss spacing is 16 inches on-center for heavy loads (snow, tile roof) and 24 inches on-center for light loads (shingles, mild climate). 12-inch spacing is used for very long spans or heavy timber roofs. Always confirm with your local building code and engineering tables, as the required spacing also depends on lumber grade, span, rafter size, and roof live/dead load ratio.

Should I use rafters or pre-fabricated trusses?▼

Choose rafters (stick framing) if you need usable attic space, custom design flexibility, dormers, or have no crane access. Choose trusses for faster one-day installation, longer spans (60 ft+), lower labor cost, and factory-engineered consistency. Trusses block attic space and require crane delivery, while rafters require skilled carpenters and 2–4× more on-site time.

What size rafter do I need for my span?▼

Rafter sizing depends on span, spacing, pitch, lumber grade, and snow load. General US rules for #2 SPF (Spruce-Pine-Fir) with 24" spacing: 2×6 → up to 9 ft span, 2×8 → up to 12 ft, 2×10 → up to 15 ft, 2×12 → up to 18 ft. Reduce spans by 1–2 ft in high snow zones or switch to 16" spacing. Always check the 2021 IRC span tables or get a structural engineer.

What roof pitch should I choose for my climate?▼

Warm / low-snow climates (Florida, Texas, AZ): 4:12 to 6:12 is standard. Moderate snow (Mid-Atlantic, Midwest): 6:12 to 8:12. Heavy snow (Rocky Mountains, Canada, New England): 9:12 to 12:12 for natural snow shedding. Hurricane zones: 4:12 to 6:12 with hip roofs preferred over gables for aerodynamic shape. Steeper roofs always shed water and snow better but use more material.

What is the best roofing material for my pitch?▼

Below 2:12 (flat/low slope): you MUST use TPO, EPDM, PVC, or built-up BUR membrane (no shingles). 2:12–4:12: only metal standing seam or special low-slope shingle applications with double underlayment. 4:12–9:12: asphalt shingles, metal, tile, slate — all standard materials work. Above 9:12: use longer nails, extra fasteners, or dry-fasten stone-coated steel since gravity pulls everything downward faster.

How does the PDF export work?▼

Our PDF export generates a dimensioned drawing with all calculated values. It includes calibration marks (50mm reference line) and registration marks at the corners for accurate printing. Always verify the printed scale before using as a cutting template.

Can I use this calculator for metric and imperial units?▼

Yes! The calculator supports both metric (mm/cm/m) and imperial (feet-inches-fractions) systems. All inputs and outputs automatically convert when you toggle the unit system. The imperial format supports smart input parsing like "8 11 3/4", "8' 11 3/4\"", or decimal, and displays results as feet + inches with standard carpentry fractions (1/16, 1/8, 1/4, etc.).

Can this tool be used as a lean to rafter calculator?▼

Yes! Our lean to rafter calculator handles single-slope shed roofs with ease. Simply enter the run (horizontal span), pitch, and overhang — the calculator computes the rafter line length, plumb cut angle, and birdsmouth dimensions for lean-to style roofs used on porches, sheds, carports, and home additions.

Is this a common rafter calculator?▼

Yes, this is a common rafter calculator designed for the most frequently used rafter type in gable roofs. Common rafters run perpendicular from the ridge board down to the wall plate, forming the main structural members of a pitched roof. The tool handles plumb cut, seat cut, birdsmouth, ridge reduction, and overhang — everything needed for common rafter framing.

Does this rafter calculator with birdsmouth output the notch dimensions?▼

Yes! As a rafter calculator with birdsmouth output, it computes three critical notch dimensions: the seat cut length (horizontal), the birdsmouth depth (vertical notch), and HAP (Height Above Plate) — the remaining wood above the notch that keeps the rafter structurally sound on top of the wall plate.

Can I use this roof rafter calculator with birdsmouth and overhang together?▼

Absolutely. This roof rafter calculator with birdsmouth and overhang support lets you enter both the eave overhang distance and the seat cut length, then outputs the total rafter length including the tail, the birdsmouth notch location along the rafter, and the fascia/tail cut angle — all in a single calculation.

How does the rafter calculator with overhang compute the tail length?▼

The rafter calculator with overhang adds the horizontal eave extension to the run before computing the diagonal rafter line length. Enter your overhang value in the Input Parameters panel, and the Total Rafter Length will automatically include the tail portion that extends beyond the outside face of the wall to form the eave.

Is this an online rafter calculator or do I need to install an app?▼

This is a free online rafter calculator — no download or installation required. It runs entirely in your web browser on desktop, tablet, or phone. You can bookmark the page for quick access on the job site, and the PDF export feature lets you print dimensioned cutting templates for field reference.

Is this a free gable roof calculator?▼

Yes. This free gable roof calculator sizes the common rafters that frame a gable roof. Enter the building span, roof pitch, and eave overhang, and the tool returns the total rafter length, plumb cut and seat cut angles, birdsmouth dimensions, and ridge reduction for a symmetric two-sided gable. As a rafter calculator free of charge with no sign-up, it's built for fast gable roof takeoffs on desktop or mobile.

How does this roof rafter length calculator work?▼

This roof rafter length calculator applies right-triangle geometry to your run and pitch. It adds the eave overhang to the run, divides by the cosine of the pitch angle for the line length, then adds the plumb setback (Rafter Depth × tan(Pitch)) for the total rafter length. Just enter your run, pitch, and overhang — the rafter length updates instantly in feet-inches-fractions (imperial) or millimetres/meters (metric), making this rafter calc tool the fastest route to cut-ready results without a dedicated construction calculator.

About Rafter Calculator Pro

Rafter Calculator Pro is a professional-grade roofing calculator designed for carpenters, construction contractors, and DIY home improvement enthusiasts. Built with modern web technology and validated against proven carpentry formulas, it provides instant, accurate calculations for all common rafter dimensions.

As a free online rafter calculator, there is nothing to download or install — the tool runs entirely in your browser and works on desktop, tablet, and mobile devices. Whether you are framing a gable roof, sizing a lean-to shed, or laying out a birdsmouth notch, every result updates in real time as you adjust the input parameters.

Key Features

✓ Real-time calculation as you type — no "calculate" button needed
✓ Interactive SVG diagrams with three view modes: profile, birdsmouth detail, and full structure
✓ Metric (mm) and Imperial (ft-in-fraction) unit systems with automatic conversion
✓ Degree and x/12 pitch input with quick presets for common pitches
✓ PDF export with calibration marks for field reference
✓ Birdsmouth cut details including HAP measurement
✓ Ridge reduction calculation for perfect rafter joints

Accuracy & Safety

All calculations use standard trigonometric formulas accepted in professional carpentry. However, this tool is for educational and planning purposes only. Always verify all measurements, angles, and structural decisions on site before making any cuts. Building codes and structural requirements vary by location — consult a licensed professional engineer for structural projects.