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

The first row gives the length of common rafters per foot of run. Find your roof pitch (rise per foot) along the blade edge, then read the number on the first line of the table — that is the rafter length per foot of run. Multiply by the total run for full rafter length.

How do I check if my framing square is accurate?

Draw a line along the tongue on a straight board edge. Flip the square and draw another line from the same point. If the square is true, both lines will be identical. If they diverge, the square is out.

What are stair gauges and how do I use them?

Stair gauges (stair buttons) clamp to the framing square at your rise and run measurements, creating a repeatable template for marking stair stringers. Set the rise on the tongue and the run on the blade, clamp the gauges, and walk them along the stringer board.

Measuring & Layout

$8-$15 for basic steel, $15-$25 for quality aluminum (Swanson, Stanley), $20-$35 for premium stainless or precision-ground, $30-$50 for kits with stair gauges.

The Complete Guide to Framing Squares (2026)

Prepared by Lumberama's Tools & Equipment Team

|Last updated May 26, 2026|Sources & methodology

A framing square (also called a carpenter's square or steel square) is an L-shaped layout tool with a 24-inch body (blade) and a 16-inch tongue. Stamped with rafter tables, it is the traditional tool for laying out rafters, stairs, and checking large assemblies for square. Despite its simplicity, a framing square is one of the most mathematically powerful tools in a carpenter's kit.

Key Takeaways

A framing square (also called a carpenter's square or steel square) is an L-shaped layout tool with a 24-inch body (blade) and a 16-inch tongue.
Stamped with rafter tables, it is the traditional tool for laying out rafters, stairs, and checking large assemblies for square.
Despite its simplicity, a framing square is one of the most mathematically powerful tools in a carpenter's kit.
Framing squares are used for laying out common, hip, and valley rafters, marking stair stringers, checking large assemblies for square (using the 3-4-5 rule), marking 90-degree crosscut lines on wide boards, calculating roof angles using the rafter tables, marking bird's mouth cuts, and as a straightedge for layout.
Material (steel, aluminum, or stainless steel), accuracy of the 90-degree corner, legible rafter tables, measurement markings on both sides, tongue and blade widths, protective finish, and compatible stair gauges (stair buttons).

What It's Used For

Framing squares are used for laying out common, hip, and valley rafters, marking stair stringers, checking large assemblies for square (using the 3-4-5 rule), marking 90-degree crosscut lines on wide boards, calculating roof angles using the rafter tables, marking bird's mouth cuts, and as a straightedge for layout.

Types & Variations

Standard Steel (most common, stamped rafter tables), Aluminum (lighter, corrosion-resistant), Stainless Steel (premium, will not rust), Johnny Square (compact version), and Rafter Square sets with stair buttons. Available in standard inch markings and metric.

Key Features to Look For

Material (steel
aluminum
or stainless steel)
accuracy of the 90-degree corner
legible rafter tables
measurement markings on both sides
tongue and blade widths
protective finish
and compatible stair gauges (stair buttons).

How to Use

1) For checking square: Place the corner of the square in the corner of your assembly. Both legs should be flush against the edges. 2) For stair layout: Attach stair gauges at the rise and run dimensions. Walk the square along the stringer, marking each step. 3) For rafter layout: Use the rafter tables to determine the length per foot of run. Set the square to the rise and run values on the tongue and blade. 4) For marking wide boards: Use the blade as a straightedge for long, accurate lines.

⚠️ Safety Tips

Be careful of sharp corners — they can scratch or cut. Wear gloves when handling new steel squares. Do not drop — impacts can knock the square out of true. Keep the square away from edges where it could fall on someone. Use caution on roofs — a falling square is a hazard.

Maintenance

Check squareness periodically by drawing a line and flipping the square — both lines should align. Coat steel squares with light oil or paste wax to prevent rust. Store flat to prevent bending. Clean stamped markings with a wire brush if they become obscured. Handle with care — even small bends destroy accuracy.

Price Range & Recommendations

Price Range: $8-$15 for basic steel, $15-$25 for quality aluminum (Swanson, Stanley), $20-$35 for premium stainless or precision-ground, $30-$50 for kits with stair gauges.

Beginners should get the Swanson NA202 16x24-inch steel square ($15) — it includes clear rafter tables and works for all basic framing layout. Add a set of stair gauges for stringer layout. Pros will appreciate the Starrett No. 2 for precision or the Swanson Speed Square + Framing Square combo for complete job-site layout capability.