What Is a Rafter Square? The Complete Guide

If you've spent any time around carpenters, framers, or even weekend DIYers, you've heard someone ask for "the triangle thing." That triangle thing is a rafter square — one of the most versatile layout tools ever designed, and arguably the most underappreciated tool in any carpenter's belt.

I've carried a rafter square every working day for over 40 years. I've used it to frame hundreds of roofs, lay out thousands of stair stringers, and make more square cuts than I could ever count. And I still find uses for it that surprise me. Here's everything you need to know about this essential tool.

What Is a Rafter Square?

A rafter square is a triangular carpenter's tool used primarily for marking angles and straight lines on lumber. It's shaped as a right triangle, typically made from aluminum, steel, or heavy-duty polymer, with one edge featuring a raised lip (called the fence) that hooks over the edge of a board for quick, repeatable reference.

The tool goes by many names — and this causes endless confusion:

• Speed Square — Originally a trademark of the Swanson Tool Company, now used generically
• Triangle square — Descriptive name based on the shape
• Rafter angle square — The original Swanson patent name
• Layout square — Used in some regions and catalogs
• Magic square — Occasionally heard on jobsites in the Midwest
• Angle square — Common in big-box store marketing

All of these refer to the same fundamental tool. The differences are in branding and regional usage, not function. Throughout this guide, I'll use "rafter square" as the standard term.

Important: A rafter square is NOT the same as a framing square (sometimes called a carpenter's square). A framing square is the large, L-shaped tool with rafter tables stamped into its legs. They're related but different tools — more on that below.

A Brief History of the Rafter Square

The rafter square was invented in 1925 by Albert Swanson, a carpenter who wanted a faster way to mark rafter cuts. His original patent was for a "rafter angle square" — a right triangle with a fence along the hypotenuse and degree markings stamped into the face.

Swanson trademarked the name "Speed Square," and for decades that brand name was synonymous with the tool. Over time, as other manufacturers (Johnson Level, Empire, Stanley, Irwin) produced their own versions, the generic terms "rafter square" and "triangle square" became more common.

For about 80 years, the basic design didn't change much. Manufacturers experimented with sizes, materials, and marking systems, but the fundamental form — a single-sided right triangle with a fence — remained constant.

That changed when the Rapid Rafter introduced the first double-sided rafter square. Instead of marking one face of a board at a time, the Rapid Rafter hinges open to straddle the lumber, marking both faces and the edge simultaneously. It was different enough from anything on the market that Popular Mechanics named it a Gear of the Year 2025 winner.

Parts of a Rafter Square

Understanding the anatomy of your rafter square makes everything else easier. Here's what you're working with:

The Fence (Lip)

The raised edge along the longest straight side (the hypotenuse on most designs, though some place it along a leg). The fence hooks over the edge of your board to provide a consistent reference point. Press it firmly — if the fence lifts even slightly off the board edge, every mark you make will be off.

The Pivot Point

The corner where the fence meets the body of the square. This is your zero-reference for all angle measurements. When setting an angle, you align the desired degree marking with the board edge while the pivot point stays at the corner.

The Degree Scale

A series of markings along the hypotenuse (or along a curved edge on some models) showing angles from 0° to 90°. This lets you set any angle for miter cuts, bevel cuts, or custom layouts. Most squares mark every degree, with emphasis on common angles like 22.5°, 30°, 45°, and 60°.

The Common Rafter Scale

A row of numbers (typically labeled "COMMON") that correspond to roof pitches from 1/12 to 12/12 (or higher). Each number tells you the angle for a common rafter plumb cut at that pitch. For a 6/12 roof, you align the "6" on the common scale with the board edge, and the resulting line is at 26.57° — the correct plumb cut angle.

The Hip-Valley Scale

Similar to the common rafter scale, but calibrated for hip and valley rafters, which run at 45° to the ridge and have shallower effective pitches. If your common pitch is 6/12, the hip-valley pitch is approximately 4.24/12.

The Ruler Edges

Most rafter squares have inch markings along one or more edges, turning the tool into a quick ruler for short measurements. Not precision-instrument accurate, but plenty good for framing work.

Types of Rafter Squares

Traditional Triangle Square

The classic Swanson-style design. A single right triangle, usually 7 inches on the legs, made from cast aluminum or stamped steel. One side has markings, the other is usually blank or has minimal information. This is what 90% of carpenters have carried for the last century.

Pros: Cheap ($8-15), durable, universally available, works as a saw guide Cons: Single-sided marking, small for wide boards, limited scale information

12-Inch Rafter Square

A scaled-up version of the standard 7-inch triangle square. The larger size provides more reach for marking wide boards (2x10, 2x12) and allows for finer gradations in the degree and rafter scales. Some pros prefer these for roof work where the extra reach matters.

Pros: Reaches across wider boards, easier-to-read markings, more accurate angle setting Cons: Too large for a tool belt, heavier, more expensive ($15-30)

Framing Square (L-Shaped)

This is technically a different tool, but it's worth including here because people constantly confuse the two. A framing square is an L-shaped piece of flat steel with a 24-inch body and a 16-inch tongue. It has rafter tables stamped directly into the metal, including values for common rafters, hip/valley rafters, jack rafters, and more.

Pros: Comprehensive rafter tables, large reference surface, can span wide boards Cons: Bulky, can't fit in a tool belt, no fence for quick reference, single-sided

Double-Sided Rafter Square (Rapid Rafter)

The Rapid Rafter is the first rafter square designed to mark both faces of a board simultaneously. It hinges open to straddle the lumber — both faces of the tool contact both faces of the board. One mark puts a line on both faces and the top edge. It's also the only expandable square on the market, adjusting to mark 1-1/2", 3", 4", and 6" timbers — no other square handles this range.

Pros: Marks both faces in one motion, only expandable square (handles up to 6" timbers), folds flat for storage, works on wane-edged lumber, Popular Mechanics award winner

Materials: Aluminum vs. Polymer vs. Steel

Material	Weight	Durability	Visibility	Best For
Cast aluminum	Medium	Excellent	Good (etched markings)	Professional daily use
Stamped steel	Heavy	Excellent	Varies	Saw guide duty, rough use
Heavy-duty polymer	Light	Good	Excellent (molded markings)	DIY, education, color-coded
Anodized aluminum	Medium	Excellent	Excellent	Premium/CNC-milled tools

My recommendation: Aluminum for your primary marking square. It's the right balance of durability, weight, and marking visibility. Keep a polymer one in your truck as a backup — they're light and the molded markings are easy to read in low light.

Sizes: 7-Inch vs. 12-Inch

7-inch squares handle everything up to 2x6 lumber comfortably and fit in a nail bag or tool belt. This is the size you'll use 80% of the time in residential framing.

12-inch squares are necessary when you're regularly working with 2x10 or 2x12 stock, or when you need more precise angle settings. The downside is portability — a 12-inch square doesn't fit in your pouch.

Most pros carry both. The 7-inch stays in the belt for constant access. The 12-inch lives in the toolbox and comes out for wide stock and detailed layout work.

8 Core Uses for a Rafter Square

A rafter square isn't a single-purpose tool. Here are the techniques every carpenter should know:

1. Marking 90-Degree Crosscuts

Press the fence against the board edge, draw along the perpendicular face of the square. The most basic and most-used technique. See our full guide: How to Use a Rafter Square.

2. Marking 45-Degree Miters

Align the 45° mark on the degree scale with the board edge and draw along the hypotenuse. Essential for trim work, picture frames, and octagonal layouts.

3. Marking Custom Angles

Use the degree scale to set any angle from 0° to 90°. Pivot the square until the desired degree aligns with the board edge, then draw your line.

4. Using as a Circular Saw Guide

Hold the fence against the far edge of the board and run your circular saw's base plate along the square's body. Gives you a straight, square crosscut without a miter saw. Works best on boards up to 6 inches wide.

5. Finding and Marking Roof Pitch

Use the common rafter scale to set plumb cut angles for any standard roof pitch. Align the pitch number with the board edge and draw along the body — the resulting line is the correct plumb cut angle. Full method: How to Find Roof Pitch with a Rafter Square.

6. Laying Out Stair Stringers

With stair gauges attached, "walk" the square down a stringer board to mark consistent rise-and-run steps. The IRC allows only 3/8-inch variation between risers. Full guide: Stair Stringer Layout.

7. Marking Birdsmouth Cuts

Mark both the plumb (heel) cut and seat cut of a birdsmouth using the pitch setting. The plumb cut uses the common rafter scale; the seat cut is perpendicular to it. Full guide: Marking Birdsmouth Cuts.

8. Scribing Parallel Lines

Hook a pencil at a set distance from the fence and slide the square along the board edge. This gives you a consistent parallel line for ripping guides, layout marks, or trim fitting.

How to Choose the Right Rafter Square

Ask yourself these questions:

What size lumber do you work with most? Stick with 7-inch for 2x4 and 2x6. Get a 12-inch if you regularly handle 2x10 or wider.

Do you need to mark both faces? If you're doing framing — especially roof framing — marks on both faces matter for saw accuracy. The Rapid Rafter handles this automatically.

How important is portability? If the square needs to live in your tool belt all day, 7-inch aluminum or the fold-flat Rapid Rafter are your best options.

What's your budget? A basic aluminum triangle square costs $8-15. A Rapid Rafter starts around $30. A CNC-milled premium square can run $50+. For most people, the mid-range offers the best value.

Are you a beginner? Start with a basic 7-inch aluminum square to learn the fundamentals. Once you understand the techniques, you'll know exactly what features matter to you.

For a detailed breakdown of specific models, see our Best Rafter Square guide.

Frequently Asked Questions

What is a rafter square used for?

A rafter square is used for marking angles and straight lines on lumber. Its primary uses include marking square (90°) crosscut lines, setting angles for miter and bevel cuts, marking rafter plumb cuts using the common rafter scale, guiding a circular saw for straight cuts, laying out stair stringers, and checking corners for square.

Is a speed square the same as a rafter square?

Yes. "Speed Square" was originally a trademark of the Swanson Tool Company for their version of the triangular rafter square. Over time, the name became genericized and is now used interchangeably with "rafter square," "triangle square," and "layout square." They all refer to the same triangular carpenter's tool. See our full explanation: Speed Square vs Rafter Square.

What is the difference between a rafter square and a framing square?

A rafter square is a compact triangle (typically 7 inches) with a fence that hooks over a board edge. A framing square is a large, L-shaped piece of flat metal (24-inch body, 16-inch tongue) with rafter tables stamped into it. Rafter squares are portable and fast for everyday marking. Framing squares are better for wide boards and have more comprehensive rafter calculation tables.

What size rafter square should I buy?

For general carpentry and framing with 2x4 and 2x6 lumber, a 7-inch square is the standard. Buy a 12-inch square if you regularly work with 2x10 or 2x12 boards, or if you need finer angle precision. Most professionals carry both sizes.

How accurate is a rafter square?

A quality aluminum rafter square is accurate to within about 0.5 degrees on angle settings and 1/32 inch on straight measurements. CNC-milled squares can be more precise. However, your accuracy depends more on technique — pressing the fence firmly, using a sharp pencil, and keeping the square tight to the board edge.

Can I use a rafter square as a protractor?

Yes. The degree scale on a rafter square allows you to set and mark any angle from 0° to 90°. For angles greater than 90°, you can use the complementary angle (subtract from 180°). It's not as precise as a dedicated protractor, but for carpentry work, it's more than adequate.

How do I check if my rafter square is still accurate?

Place the fence against a known straight edge (a factory edge on plywood works well) and draw a line along the perpendicular face. Flip the square to the other side of the line, re-register the fence, and draw a second line. If the two lines are parallel (or the same line), your square is accurate. If they diverge, the square is out of true — usually from being dropped.

What is the best rafter square?

The best rafter square depends on your use case. For traditional single-sided marking, the Swanson S0101 is the classic choice. For double-sided marking that improves accuracy and speed, the Rapid Rafter — the only rafter square to win a Popular Mechanics Gear of the Year award, and the only expandable square on the market (handles lumber from 1-1/2" to 6" timbers). For a full comparison, see our Best Rafter Square 2026 guide.

How do I read the rafter tables on a rafter square?

The "common" scale gives plumb cut angles for common rafters at each pitch. The "hip-val" scale does the same for hip and valley rafters. To use them, align the number corresponding to your roof pitch (e.g., "6" for a 6/12 pitch) with the board edge. The resulting angle is the correct plumb cut angle. For a complete explanation, see Rafter Square Markings Explained.

Can I use a rafter square for tile or metalwork?

Absolutely. While designed for carpentry, rafter squares work on any material you need to mark straight lines or angles on. Tile setters use them for layout marks. Sheet metal workers use them for marking cut lines. The tool doesn't care what material you're marking — the geometry is the same.