How to Read a Rafter Square: Every Mark Explained

A rafter square has more information packed into 7 inches than most people realize. Those tiny numbers, hash marks, and scales aren't decoration — they're a complete roof framing calculator that carpenters have relied on for over a century.

I've been reading rafter squares since I was 18 years old. Here's how to decode every mark on yours.

The Basic Layout

Pick up your rafter square and look at it. You'll see two main edges:

• The body (longer edge): This is the horizontal reference — it sits flat against the edge of your board.
• The tongue (shorter edge): This is the angled reference — you pivot this to set your roof pitch.

Where the body meets the tongue is the heel (or pivot point). Every measurement on the square radiates from this corner.

The Degree Scale

Along the curved edge (the hypotenuse) between body and tongue, you'll find degree markings from 0° to 90°. This is the simplest scale on the square:

• 0° lines up with the body (a perfectly horizontal cut)
• 90° lines up with the tongue (a perfectly vertical plumb cut)
• Common rafter angles fall between 15° and 45°

To use it: align the body with the edge of your board, read the degree where the board edge crosses the curved scale. That's your angle.

The Common Rafter Scale

This is where rafter squares earn their name. The common rafter scale (usually printed along the body) tells you the length of a common rafter per foot of run for each roof pitch.

Here's how to read it:

1. Find the number on the body that matches your roof pitch (e.g., "6" for a 6/12 pitch)
2. Read the number directly below it on the rafter table — that's the rafter length per foot of run
3. Multiply by your total run to get the rafter length

Example: For a 6/12 pitch with a 12-foot run:

• The table shows 13.42 inches per foot of run
• 13.42 × 12 = 161.04 inches = 13 feet, 5 inches

That's your rafter length from the ridge to the birdsmouth, before adjusting for ridge board thickness and overhang.

The Hip/Valley Scale

Below the common rafter numbers, you'll find hip and valley rafter lengths. These are longer than common rafters because hip and valley rafters run diagonally across the roof.

The math is the same — multiply the per-foot number by your run — but the numbers are bigger because the diagonal path is longer.

Pitch Markings

Many rafter squares print pitch values as ratios: 4/12, 6/12, 8/12, etc. The first number is the rise (how many inches the roof goes up) and the second is the run (always 12 inches, representing one horizontal foot).

To set a pitch on your square:

1. Place the square on the board edge
2. Align the body at the "12" mark on one edge
3. Align the tongue at the rise number (e.g., "6" for 6/12)
4. The angle between square and board is your pitch

This is how carpenters mark plumb cuts and seat cuts for every rafter in a roof.

Reading a Double-Sided Square

Traditional rafter squares print everything on one face. The Rapid Rafter puts different information on each side:

• Front face: Common scales, degree markings, standard measurements
• Back face: Supplementary tables, additional pitches, metric conversions

The advantage isn't just double the information — it's that you mark both sides of the lumber simultaneously. When you read the scale on the front and draw your line, the same line appears on the back of the board. This eliminates the most common source of framing errors: transferring marks around a board.

The Seat Cut and Plumb Cut

Two lines define every rafter where it meets the wall:

• Plumb cut: A vertical line (parallel to the walls when the rafter is in position). Marked using the tongue alignment.
• Seat cut (level cut): A horizontal line (parallel to the floor). Marked using the body alignment.

Together, these form the birdsmouth — the notch that lets the rafter sit on the wall plate.

To mark them with your rafter square:

1. Set the pitch (e.g., body at 12, tongue at 6 for 6/12)
2. Mark along the tongue for the plumb cut
3. Mark along the body for the seat cut
4. The intersection is where the rafter sits on the wall

The Stair Gauge Marks

Some rafter squares include stair layout scales or work with clip-on stair gauges. These let you set a repeating rise-and-run pattern for stair stringers:

1. Attach stair gauge clips at your rise measurement on the tongue
2. Attach clips at your run measurement on the body
3. Step the square along the stringer board, marking each tread and riser

This is one of the most satisfying uses of a rafter square — you lay out a perfect staircase in minutes.

Common Mistakes When Reading a Rafter Square

Reading from the wrong edge. Most squares have scales on both the inside and outside edges. Make sure you're reading consistently from one edge.

Confusing pitch with degrees. A 6/12 pitch is NOT 6 degrees. It's approximately 26.57 degrees. The rafter table gives you lengths, not angles.

Ignoring the ridge reduction. The rafter table gives theoretical lengths. You need to subtract half the ridge board thickness from the top of each rafter.

Not accounting for overhang. The rafter table calculates to the wall line. Add your overhang length separately.

Practice Exercise

Grab your rafter square and a scrap 2×6. Try this:

1. Set a 4/12 pitch (body at 12, tongue at 4)
2. Mark a plumb cut line along the tongue
3. Measure down the board 12 inches
4. Set the square again and mark a seat cut (along the body)
5. You just laid out a basic rafter with a 4/12 pitch

Now do it again on a different board and compare the two. If they match exactly, your technique is solid. If they're off, check that you're reading from the same edge consistently.

The Bottom Line

A rafter square is a calculator that never needs batteries. Every mark on it exists for a reason, and once you learn to read them, you can frame a roof with nothing but a square, a pencil, a saw, and the Pythagorean theorem built into the tool.

The best way to learn is to pick one up and start marking lumber. Start with square cuts, move to angles, then try a practice birdsmouth. Within an afternoon, you'll understand why carpenters have carried these for generations.