How to Use a Rafter Square

Rafter Square at a Glance

A rafter square, better known as a speed square, marks 90° and 45° cuts instantly, finds any angle from 0 to 90 degrees using its pivot point, and guides a circular saw for clean crosscuts. Swanson invented it in 1925. Buy one for $12 and carry it in your back pocket. It's the most useful layout tool in your pouch, even if you've only been using it for square lines.

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In this guide:

• What all the markings mean
• Marking 90°, 45°, and any angle in between
• Using it as a circular saw guide
• Reading the rafter tables for plumb cuts and bird's-mouth

How to Use This Guide

Skill level: Beginner. No prerequisites. If you can hold a pencil and own a circular saw, you're ready.

You probably already use your speed square to draw square lines. That's maybe 20% of what it does.

The rafter tables on the hypotenuse are where most beginners get lost. Not because they're complicated, but because nobody explains what the three separate scales actually mean. Part 1 fixes that.

If you just need to mark angles: Jump to Part 2.

If you want to guide your circular saw: Jump to Part 3.

If you're framing a roof: Go straight to Part 4.

Part 1: Anatomy and the Three Scales Explained

Every rafter square is a right triangle. Two equal sides meet at a 90° corner; the hypotenuse is the longest side. That's the geometry. What matters is what's on each part.

The Six Parts

Lip / Fence. The raised thickened edge along one equal side. It hooks against the edge of lumber and gives you a consistent reference point. Quality models have inch markings on the lip. Hooking the square on the board means hooking this edge.

Body. The flat triangular plate. Buy aluminum, not plastic. Plastic models warp with temperature changes and wear out fast. The Swanson T0110 aluminum 7" square costs about $12 and will last decades.

Pivot point. Marked on the tool at the corner where the fence meets the hypotenuse. This is the rotation center for custom angles. It looks like a label you'd skip, but using the fence corner instead of the pivot is the most common speed square mistake. Part 2 explains the difference.

Hypotenuse. The long side. It has three different scales printed on it: the degree scale, the common rafter table, and the hip/valley rafter table. These look similar but mean completely different things.

Pencil notches. Small notches cut into the hypotenuse edge on most models, spaced 1/4" apart. Bob Vila's speed square guide explains the technique: insert your pencil into a notch and drag the square along the board edge to scribe a perfectly parallel line at that distance.

Diamond cutout / stud notch. On some Swanson models, there's a notch at the 3.5" mark (the width of a 2×4). Useful when you're doing stud layout and want a fast reference without measuring.

The Three Scales on the Hypotenuse

Three separate scales are printed on the hypotenuse. Per Wikipedia's Speed Square article, they measure completely different things.

The degree scale measures angles the way a protractor does. Straightforward.

The rafter tables measure roof pitch: the ratio of rise to run, not an angle in degrees. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. That's approximately 26.6 degrees, not 6 degrees. The "6" on the Common rafter table is the rise number in the ratio, not a degree value.

Confusing the degree scale with the rafter tables is the most common mistake beginners make when they try to cut rafters for the first time. Keep them separate in your head: degrees for angles, Common/Hip-Val for pitch.

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The Hip/Val table uses 17 inches as its reference run instead of 12 because hip and valley rafters run diagonally across the roof at 45° to the walls. The diagonal of a 12×12 square is approximately 16.97 inches, which carpenters round to 17. Same technique, different scale number.

Part 2: Marking Angles

Three methods. Each one has a specific use case.

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Method 1: Marking a 90° Cut

The fastest way to draw a square line on a board:

1. Hook the fence against the board edge. The full fence length should sit flush, no gaps.
2. Hold the square with your off hand, pressing the fence into the board.
3. Draw along the right-angle side (the unfenced equal side, perpendicular to the board edge).

That line is 90° to the board edge.

If the fence isn't flush, the line drifts. A 1/16" gap creates a visible error on wide boards. Press the fence down and keep it there while you draw.

For repetitive crosscut marks, this beats a combination square. No setting, no locking, just hook and draw.

RELATED: How to Read a Tape Measure Pair your rafter square with accurate tape measure reading for layout work.

Method 2: Marking a 45° Cut

1. Hook the fence against the board edge exactly as you would for a 90° line.
2. Draw a line along the hypotenuse instead.

That's the whole technique. The hypotenuse sits at exactly 45° to the fence by geometry. No measuring required.

To cut the angle in the opposite direction, flip the square over and repeat.

45° cuts come up constantly: miter joints for picture frames and door casings, crown molding corners, deck railing angles. The speed square marks them faster than setting a miter saw protractor for small layout work.

Method 3: The Pivot Method (Any Angle, 0–90°)

The pivot method marks any angle from 0 to 90 degrees. Most beginners never use it.

1. Identify the angle you need (say, 35°).
2. Place the pivot point on the edge of the board. It stays fixed.
3. Rotate the entire square around the pivot until the "35" mark on the DEGREES scale lines up with the top edge of the board.
4. Hold the square firmly.
5. Draw along the right-angle side.

The resulting line is at 35° to the board edge.

Why the pivot point, not the fence corner?

The pivot is the geometric center of rotation. Plant the fence corner instead and the fence's thickness shifts the rotation axis away from the board edge, giving you a slightly wrong angle. On a 12-inch board, that small gap becomes a visible error. It matters most for compound cut angles and framing, where 1° off means the joint won't fit.

Hook the marked pivot point, not the corner of the fence.

Tip: Press the pivot into the wood edge and hold that pressure while you draw. The square shifts under pencil pressure if you don't.

Part 3: Circular Saw Guide

A speed square is a functional saw guide for 90° and 45° cuts on boards up to about 5–6" wide. If you own a circular saw, this is one of the most used features.

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For 90° Cuts

1. Mark your cut line on the board.
2. Hook the fence over the far edge of the board (away from you, toward the back).
3. Align the right-angle side of the square with your pencil mark.
4. Hold the square and the board together with your off hand. The fence prevents the square from sliding.
5. Place the circular saw's baseplate against the square's body.
6. Push the saw steadily, keeping the baseplate against the square through the cut.

For 45° Cuts

Same setup. Run the saw along the hypotenuse instead of the right-angle side.

Width Limit

A 7" speed square can guide cuts on lumber up to about 5–6" wide, as Bob Vila notes in his speed square guide. Beyond that, the saw's baseplate slides off the square before the cut finishes. For boards wider than that (a 2×8 is 7.25" wide), use a track saw or straight edge setup instead.

One Safety Rule

Never let the blade contact the aluminum body. It damages the square and can snag and cause kickback. Keep the baseplate on the square; keep the blade away from it.

Hooking the fence on the far edge matters for control too. Your grip pulls the square toward you and the board, a stable braced position. Push the square away and you lose contact before the cut finishes.

Part 4: Rafter Layout

The Common and Hip/Val scales exist entirely for rafter layout. If you've never framed a roof, this section teaches you the logic. If you have, it shows you how to use the speed square instead of a full framing square.

Roof Pitch: The Foundation

Carpenters express roof pitch as rise:run: how many inches the roof rises per 12 inches of horizontal distance. A 6/12 pitch rises 6". Common residential pitches: 4/12, 5/12, 6/12, 8/12. Steeper roofs have higher numbers.

The rise number on the Common rafter table (1 through 30) corresponds directly to this rise value. For a 6/12 pitch, you use "6" on the Common scale.

Marking a Plumb Cut

The plumb cut is the angled cut at the ridge end of a rafter. It sits perfectly vertical when the rafter is installed.

1. Find your roof pitch (e.g., 6/12). Use the rise number, which is 6.
2. Place the pivot on the top edge of the rafter lumber.
3. Rotate until "6" on the COMMON scale aligns with the top edge of the board.
4. Draw along the right-angle side.

That line is the plumb cut. Cut along it and the rafter's ridge end will sit plumb when installed.

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The Bird's-Mouth

A bird's-mouth is the notch cut into the rafter where it sits over the top of the wall. It has two cuts: a vertical plumb cut and a horizontal seat cut. Together they let the rafter rest on the wall plate without rocking.

Step 1: Mark the plumb cut at the wall plate location. Use the same pitch setting as above. Place the pivot at the point on the rafter where the wall plate sits, rotate to the pitch number, and mark the vertical line.

Step 2: Mark the seat cut. Without changing your hand position or the square's angle relative to the rafter, rotate the square 90° and mark the horizontal line perpendicular to the plumb cut line.

The intersection of these two lines is the corner of your bird's-mouth notch.

Don't cut too deep. The seat cut depth should stay at 1/3 of the rafter's total depth or less. For a 2×6 rafter (5.5" actual depth), that's about 1.8" maximum. Cut deeper and you weaken the rafter at the point where it carries the most load.

Hip and Valley Rafters

Hip and valley rafters run diagonally across the roof at 45° to the walls. Their unit run is approximately 16.97" (the diagonal of a 12"×12" square), which Wikipedia confirms carpenters round to 17.

Use the HIP VAL scale instead of COMMON. The pivot technique is identical: same placement, same rotation, same marking. The only difference is which numbers you read on the hypotenuse. Use the Common scale for hip rafters and the angle will be wrong.

Part 5: Common Mistakes

Five mistakes beginners make consistently. Each has a simple fix.

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Using the fence corner instead of the pivot point. For custom angles, the pivot point is the rotation center, not the fence corner. Hook the marked pivot, not the corner. The corner sits several millimeters from the pivot; on a 12-inch board that gap produces a visible error.

Confusing the degree scale with the rafter tables. The DEGREES scale and the COMMON scale both have numbers on them. They measure different things. If you're marking a rafter cut and you read from the DEGREES scale instead of COMMON, your angle will be wrong. Double-check which scale you're on before marking.

Loose fence. If the fence isn't sitting flush against the board edge, the line drifts. Full contact, full length, before you draw.

Guiding a circular saw on boards wider than about 6". A 7" square can only cover so much width. On a 2×8 (7.25" wide), the saw's baseplate will run off the square near the end of the cut. Either use a 12" square or clamp a straight edge for wide cuts.

Mixing up Common and Hip/Val for rafter work. Both scales look similar on the hypotenuse. Label them in your head before you start: Common = standard rafters, Hip/Val = diagonal rafters. Using the wrong one gives a differently angled cut that won't fit.

Quick Reference

What This Unlocks

Once you can mark any angle confidently and read the rafter tables, you're ready for more demanding layout work. Cutting angles on a miter saw becomes easier because you can verify and mark your cut lines before the board ever reaches the saw. Roof framing and stair layout rely on the same rise-over-run logic you just learned. And a rafter square works alongside a tape measure and pencil to handle 90% of everyday layout without any other tools.

Sources

These guides and references informed this article.

• Wikipedia — Speed Square — history, anatomy, scale types, sizes
• Bob Vila — How to Use a Speed Square — fence, pivot, scribing notches, saw guide technique
• Swanson Tool Company — original manufacturer; T0110 is the standard 7" aluminum model