A speed square, also called a rafter square, is a triangular layout tool used in carpentry and workshop building. It helps mark straight cuts, square lines, angles, and roof rafter cuts quickly on boards and sheet materials. Its shape combines a ruler, try square, protractor, and saw guide in one compact tool.
Learning to use it well improves accuracy, saves time, and reduces wasted material.
Key Facts
- A speed square has three main parts: the blade, the fence, and the hypotenuse with angle markings.
- A square line is 90 degrees to the edge of a board and is drawn by holding the fence tight against the board edge.
- A 45 degree line can be marked by using the diagonal side of many speed squares or by aligning the pivot and degree scale.
- For angle marking, place the pivot point at the board edge and rotate until the desired degree mark meets the edge.
- Rafter pitch is often written as rise per 12 inches of run, such as 6 in 12.
- Slope ratio = rise / run, so a 6 in 12 roof has slope = 6 / 12 = 0.5.
Vocabulary
- Speed square
- A triangular carpentry tool used to mark square lines, angles, and rafter layouts on wood.
- Fence
- The raised straight edge of the speed square that hooks against the edge of a board.
- Pivot point
- The corner of the speed square used as the fixed point when rotating the tool to mark angles.
- Rafter pitch
- A roof slope measurement that gives the vertical rise for every 12 inches of horizontal run.
- Hypotenuse
- The longest side of the triangular speed square, opposite the right angle, often used for angle scales and guides.
Common Mistakes to Avoid
- Letting the fence lift off the board edge, which makes square and angle marks shift away from the intended reference line.
- Reading the wrong angle scale, which can give a complementary angle instead of the cut angle you meant to mark.
- Using a damaged or rounded board edge as the reference, which transfers that inaccuracy into every layout line.
- Marking with a thick or dull pencil, which creates a wide line and makes it hard to cut accurately to the measurement.
Practice Questions
- 1 A board needs a square cut 18 inches from one end. Describe how to measure the distance and use the speed square to mark a 90 degree cut line.
- 2 A roof has a pitch of 7 in 12. What is the slope ratio as a decimal, and how many inches does it rise over a 36 inch run?
- 3 You need to mark repeated 30 degree cuts on several boards. Explain why keeping the same board edge as the reference and using the pivot point consistently improves accuracy.