How to Use a Framing Square for Rafters and Stairs

A framing square works by letting you mark a rise on one arm and a run on the other, then draw along the edges to get a repeatable angle. For rafters, you set the roof pitch on the two arms and step the square along the board. For stairs, you set the riser height and tread depth, clamp stair gauges at those marks, and trace each step. Check your math on scrap first, because every later cut copies the first mark.

Know the parts of the square

A framing square is an L. The long arm is the blade, usually 24 in. The short arm is the tongue, usually 16 in. The corner where they meet is the heel. Our best framing square page lists the Johnson CS7HD as a 16 in by 24 in square, so that is the common size to learn on.

Both arms have an outer edge and an inner edge, and the faces carry scales in inches and fractions. On many squares, one face also has rafter tables and other conversion charts. The Johnson CS7HD lists integrated tables for pitch to degrees, number of joists, studs or rafters in a given run, and a brace table, along with laser-etched markings in 1/16 in steps.

A small speed square can do some of the same angle work. The Swanson S0101 in our data has a 7 in blade, a pivot point, and rafter tables. It is handy for a quick common rafter mark, but it cannot span a full stringer layout the way a 24 in blade can.

Rise, run, and pitch in plain terms

Run is the horizontal distance. Rise is the vertical distance. Pitch is the rise for every 12 in of run, so a 6/12 roof climbs 6 in for every 12 in it travels sideways.

Stairs use the same idea. The riser is the rise of one step and the tread is the run of one step. Once you pick those two numbers, you can lay out every step with the same two marks on your square.

How to use a framing square for rafters

The step-off method for a common rafter uses the 12 mark on the blade as the run and the pitch number on the tongue as the rise. Here is the process for a 6/12 roof.

Safety first: work on the ground or a stable bench when you lay out and cut rafters. Wear eye and hearing protection when you use a saw, and keep your free hand behind the blade path.

Pick a straight board and set it crown up on sawhorses. Hold the square with the heel toward you and the tongue pointing along the board.
Align the 12 mark on the blade and the 6 mark on the tongue with the same edge of the board. Both marks must touch the edge, or your angle will drift.
Draw along the tongue. This is the plumb cut line, which sits against the ridge board.
Slide the square along the edge until the tongue lines up with your pencil mark, then mark again. Each step is one foot of run.
Repeat for the number of feet in the run. A building 12 ft wide has a 6 ft run, so you make six steps.
Subtract half the thickness of the ridge board at the top, then mark the seat cut where the rafter rests on the wall plate.

Read the rafter table

The rafter table saves you from stepping off every board. On a 6/12 roof, the diagonal length for each 12 in of run is about 13.42 in, because it is the hypotenuse of a 12 by 6 triangle. Multiply that by the run in feet and you get the rafter length along its edge, before you deduct for the ridge.

For a 6 ft run, that is about 80.5 in. Most tables print the per-foot length for each pitch on the blade, so you read across from your pitch and multiply. Read the label on your own square before you trust any row, because makers format these tables differently.

If your square lacks tables, the step-off method above gets you the same answer. The table is a shortcut and a good way to double check your marks.

How to use a framing square for stairs

Stair layout starts with the total rise, which is the vertical distance from the lower finished floor to the upper finished floor. Divide it by a target riser height of about 7 to 7-3/4 in to get a rough number of risers, then round to a whole number and divide again for the exact riser height. Local building codes set limits on riser height and tread depth, so check yours before you cut.

Here is a worked example. Say the total rise is 108 in. Fifteen risers gives 7.2 in each, which is about 7-3/16 in. Choose a 10 in tread. A stringer has one fewer tread than risers, so 14 treads at 10 in gives a total run of 140 in.

Clamp stair gauges (small brass fittings) to the square at 7-3/16 on the tongue and 10 on the blade. Use the same edge of the board for both.
Lay the square on a 2x12 stringer board with the gauges hugging the board edge. Trace the outside of the square to mark the first riser and tread.
Slide the square so the tongue lines up with the end of the tread line you just drew. Trace again. Repeat for all 14 treads.
Drop the bottom of the stringer by the thickness of one tread so the first step matches the rest. Do the same adjustment at the top as needed.
Cut the first stringer and test it in place. If it fits, use it as a pattern for the others.

Quick comparison: rafters and stairs

The tool is the same, but the numbers you put on each arm change.

Task	Blade mark	Tongue mark	What you trace
Common rafter	12 (run per foot)	Pitch rise, such as 6	Plumb cut and each step along the board
Stair stringer	Tread depth, such as 10	Riser height, such as 7-3/16	Each riser and tread
Checking a corner	Edge of the blade	Edge of the tongue	Whether the corner is square

Safety and accuracy tips

Rafter and stair work involves heights and power tools, so build in a few habits.

Wear safety glasses and hearing protection when you run a circular saw. Support the stringer or rafter so the offcut cannot pinch the blade. Do not finish a stair inside corner cut with a circular saw, because the blade overruns the line. Finish those corners with a handsaw.

On a roof, use proper fall protection and a stable ladder. A matte or anodized finish helps you read marks in bright sun, and our framing square comparison notes which models use those finishes. Keep your pencil sharp and mark on the same side of the line every time, because a thick line can shift your cut by a sixteenth of an inch.

Frequently asked questions

Can I use a speed square instead of a framing square for stairs?

You can mark a single step with a speed square, but it is awkward for a full stringer. A 24 in blade lets you hold the run and rise marks against the board in one pass and keeps your layout consistent.

Why do I subtract one tread from the number of risers?

A staircase starts at the floor and ends at the upper floor, so the top step is the floor itself. That means there is always one fewer tread than risers.

What are stair gauges for?

Stair gauges clamp to the square at your riser and tread marks. They hold the same position against the board edge for every step, so you do not have to realign the square by eye each time.

How do I check that my square is accurate?

Draw a line along the blade on a board with a straight edge, flip the square over, and line up the blade with that same line. If the edges still match, the square is true. If they do not, replace it.