Rafter Square Markings Explained: Every Scale on Your Square

Most carpenters use about 10% of what's stamped on their rafter square. They mark 90-degree lines, maybe a 45, and that's it. The rest of those numbers — the common rafter scale, the hip-valley scale, the degree markings — might as well be hieroglyphics.

That's a shame, because those markings turn a simple triangle into a roof-framing computer. Everything you need to calculate rafter angles, find pitch, and lay out cuts is right there on the tool. You just need to know how to read it.

The Degree Scale

The degree scale runs along the hypotenuse of the square (or along a curved arc on some designs). It shows angles from 0° to 90°, with marks at every degree and typically with bolder marks at 5° or 10° increments.

How to Use It

1. Place the fence against the board edge
2. Pivot the square until the desired degree mark aligns with the board edge at the pivot point
3. Draw along the hypotenuse

The key detail most people miss: The degree reading is the angle between your marked line and the board edge. A 0° setting gives you a line parallel to the edge. A 90° setting gives you a line perpendicular to the edge (a square crosscut). A 45° setting gives you a perfect miter.

Common Angles and What They're For

Degrees	Use
90° (square)	Crosscuts, layout lines
45°	Miter joints, picture frames, 45° bracing
22.5°	Octagonal layouts, crown molding
30° / 60°	Decorative work, some structural connections
26.57°	6/12 roof pitch plumb cut
33.69°	8/12 roof pitch plumb cut
18.43°	4/12 roof pitch plumb cut

The Common Rafter Scale

This is the scale that earns the tool its name, and it's the one most people don't understand. The common rafter scale is a column of numbers — usually labeled "COMMON" or "COM" — running from 1 through 12 (or higher on some squares). Each number represents a roof pitch.

What the Numbers Mean

Each number on the common scale corresponds to the rise in a rise-over-12 pitch notation. The number "6" means 6/12 pitch — the roof rises 6 inches for every 12 inches of horizontal run.

When you align that number with the board edge (with the fence tight against the edge), the angle formed between the square's body and the board edge is the plumb cut angle for that pitch.

Scale Number	Pitch	Plumb Cut Angle
1	1/12	4.76°
2	2/12	9.46°
3	3/12	14.04°
4	4/12	18.43°
5	5/12	22.62°
6	6/12	26.57°
7	7/12	30.26°
8	8/12	33.69°
9	9/12	36.87°
10	10/12	39.81°
11	11/12	42.51°
12	12/12	45.00°

How to Use It: Marking a Plumb Cut

Let's say you're framing a 6/12 roof and need to mark a plumb cut line on a rafter.

1. Place the fence of the rafter square against the top edge of the rafter board
2. Find "6" on the common rafter scale
3. Pivot the square until the "6" aligns precisely with the board edge
4. Hold the square firmly and draw a line along the body of the square
5. This line is your plumb cut — it will be vertical when the rafter is installed at 6/12 pitch

How to mark the seat cut: Once you have the plumb line marked, the seat cut is perpendicular to it. Without moving the board, reposition the square so you can draw a line at 90° to your plumb line. Together, the plumb line and seat line form the birdsmouth.

The Hip-Valley Scale

Below (or beside) the common rafter scale, you'll find the hip-valley scale, usually labeled "HIP-VAL" or "H.V." This scale works exactly like the common scale but is calibrated for hip and valley rafters.

Why Hip-Valley Rafters Need a Different Scale

Hip and valley rafters don't run perpendicular to the ridge — they run at a 45° angle to it. This means their effective pitch is shallower than the common pitch, and their plumb cut angle is different.

For a roof with a 6/12 common pitch:

• Common rafter plumb angle: 26.57°
• Hip rafter plumb angle: 19.47° (it's as if the hip sees a shallower pitch)

The hip-valley scale automatically compensates for this. To mark a hip rafter plumb cut on a 6/12 roof, you align "6" on the hip-valley scale (not the common scale) with the board edge.

When to Use It

• Hip rafters: The diagonal rafters that run from the wall corners to the ridge on a hip roof
• Valley rafters: The diagonal rafters in the valley where two roof planes meet
• Jack rafters: Use the common scale for jack rafter plumb cuts (they're parallel to commons), but use the hip-valley scale if you need to mark the cheek cut angle

The Pivot Point and How to Align It

The pivot point is the corner of the square where the fence meets the body — it's your zero reference for all measurements.

Proper Alignment Technique

1. The pivot point sits at the exact point on the board edge where you want your mark to begin
2. The fence presses flat against the board edge, providing the reference surface
3. The scale marking you're reading aligns with the board edge at a point along the hypotenuse

Common mistake: Letting the pivot point float off the board edge. Even 1/32 inch of gap between the pivot and the edge changes your angle. Press firmly and hold steady.

For Degree Readings vs. Rafter Scale Readings

• Degree scale: Read where the board edge intersects the degree markings along the hypotenuse
• Rafter scales: Read where the board edge intersects the numbered scale printed on the body

Both use the same pivot point as the reference. The difference is which set of markings you're reading.

Reading Pitch Markings

Some rafter squares print pitch markings in a different format — showing the pitch as a ratio or the corresponding rafter length factor.

Rafter Length Per Foot of Run

Some premium squares include a scale showing the rafter length per foot of run. For a 6/12 pitch, this value is 13.416 inches. This means for every 12 inches of horizontal run, the rafter travels 13.416 inches along the slope.

To find total rafter length: multiply the horizontal run (in feet) by this factor, then convert to feet and inches.

Example: 14-foot run at 6/12 pitch = 14 × 13.416 = 187.82 inches = 15' 7-13/16"

For a complete reference table of rafter multipliers at every pitch, see our Roof Pitch Explained guide.

Worked Examples

Example 1: Mark a 4/12 Plumb Cut

You're cutting common rafters for a 4/12 pitched roof.

1. Place the fence against the top edge of the rafter board
2. Find "4" on the COMMON scale
3. Align the "4" with the board edge while keeping the fence tight
4. Draw along the body of the square — this is your 18.43° plumb cut line
5. For the birdsmouth seat cut, mark a perpendicular line from the plumb line

Example 2: Mark a Hip Rafter for an 8/12 Roof

Your common rafters are at 8/12. You need to cut hip rafters.

1. Place the fence against the top edge of the hip rafter board
2. Find "8" on the HIP-VAL scale (not the common scale)
3. Align "8" with the board edge
4. Draw along the body — this hip rafter plumb cut is at approximately 24.09°

Example 3: Set a 22.5° Miter for an Octagon

You're building an octagonal gazebo and need 22.5° miters.

1. Place the fence against the board edge
2. Find 22.5° on the degree scale along the hypotenuse
3. Pivot until 22.5° aligns with the board edge
4. Draw along the hypotenuse — this is your miter line

Example 4: Find Unknown Roof Pitch

You're tying into an existing roof and need to determine its pitch using a rafter square and a level.

1. Hold the rafter square against the underside of an existing rafter with the fence on the top edge
2. Place a torpedo level on the body of the square
3. Pivot until the level reads plumb (bubble centered)
4. Read the number on the common scale where the rafter edge crosses — that's your pitch

For more detail on this method, see How to Find Roof Pitch with a Rafter Square.

Frequently Asked Questions

What do the numbers on a rafter square mean?

The numbers on the COMMON scale represent roof pitches — each number is the rise per 12 inches of run. The number 6 means 6/12 pitch. When aligned with the board edge, it gives you the correct plumb cut angle for that pitch.

How do I read a speed square for roof pitch?

Find the COMMON scale on your square. Align the number matching your pitch (e.g., "8" for 8/12) with the board edge while pressing the fence tight. The line along the square's body is the plumb cut angle. For the level cut, mark perpendicular to the plumb line.

What is the difference between common and hip-valley scales?

The common scale gives angles for rafters running perpendicular to the ridge. The hip-valley scale gives angles for rafters running at 45° to the ridge (hip and valley rafters), which have a shallower effective pitch.

Why are my rafter cuts off even though I'm using the right pitch number?

The most common cause is the fence not being tight against the board edge. Even a small gap changes the angle. Other causes: reading the wrong scale (hip-valley instead of common), a square that's been knocked out of true, or measuring from the wrong reference point.

Can I use degree markings instead of the rafter scale?

Yes — they give the same angles. A 6/12 pitch is 26.57°. Some carpenters prefer degrees because that's what their miter saw uses. The rafter scale is convenient because you don't need to memorize the degree equivalent of each pitch. Either method works.

What does the diamond or triangle symbol mean on some rafter squares?

On some squares, diamond or triangle markers appear at specific intervals (commonly at 16" and 24") along the edges. These indicate standard framing spacing for rafters, studs, and joists — letting you quickly tick off layout marks without a tape measure.