A speed square is a triangular measuring and marking tool with a raised fence that hooks over the edge of a board, letting builders and DIYers draw quick square and angled lines, set roof pitches, scribe parallel marks, check corners, and guide a circular saw; it combines functions of a try square, miter square, protractor, and layout guide in one compact piece.
Speed Square
What is a Speed Square?
A speed square is a compact, triangular layout tool used to measure, mark, and verify angles on wood and other building materials. It has a short lip (called a fence) that hooks over the edge of a board so you can quickly mark a perfect 90-degree line or a 45-degree line. It also includes degree markings and roofing pitch scales for laying out miters, rafters, and stair parts. Because it doubles as a saw guide, it’s a handy way to make fast, repeatable cuts with a circular saw.
A speed square is sometimes called a rafter square or carpenter’s triangle. While “Speed Square” is a brand name popularized by Swanson, the term has become a common way to describe this style of triangular layout tool.
Anatomy and Scales
Understanding the parts makes the tool much easier to use:
- Fence: The thick, raised edge that hooks over the board. This keeps the square aligned so your marks are accurate.
- 90° and 45° edges: The two main straight edges let you strike square and 45-degree lines instantly.
- Pivot point: A small mark or hole near the corner. Place your pencil at the desired degree on the scale and rotate around this pivot to mark any angle.
- Degree scale: Graduations (often 0–90°) for setting common miter angles.
- Common and hip/valley scales: Roofing pitch scales used to lay out rafters based on rise over run (for example, 6/12). This helps translate roof pitch into cut angles.
- Scribe notches: Small notches at set intervals (often 1/4 inch) for drawing parallel lines along a board’s edge.
- Diamond cutout: On many models, a diamond or other cutout marks a frequently used reference (such as 3-1/2 inches for laying out studs with 2x lumber). Uses vary by manufacturer.
Common Uses in DIY and Home Projects
A speed square earns its keep in many tasks:
- Marking crosscuts: Hook the fence and draw a 90-degree line across a board for straight cuts.
- Marking 45-degree miters: Flip to the 45-degree edge to lay out trim or picture frames.
- Setting any angle: Use the pivot and degree scale to mark angles like 22.5°, 30°, or 60° for miters or braces.
- Guiding a circular saw: Hold the square’s edge as a fence for straight, accurate cuts and repeatable lengths.
- Roofing and rafters: Use the common and hip/valley scales to translate roof pitch to plumb and seat cuts on rafters.
- Stair layout: Mark consistent rises and runs on stair stringers using the degree scale and scribe notches.
- Checking square: Ensure corners on boxes, cabinets, or frames are true by comparing to the 90-degree edge.
- Scribing parallel lines: Use the notches to draw a straight line a fixed distance from an edge for rips or reveals.
Types and Variations
Speed squares come in a few variations to fit different tasks and preferences:
Size:
- 7-inch: The most common size. Fits comfortably in a tool pouch and works well with 2x lumber.
- 12-inch: Better for wider boards, sheet goods, and as a longer saw guide. Heavier but more stable.
- Mini (under 7 inches): Pocket-friendly for trim and tight spaces, though less useful as a saw guide.
Material:
- Aluminum: Lightweight, durable, and usually etched markings that won’t wear off easily.
- Stainless steel: Tough and stable; resists corrosion but can be heavier and pricier.
- Plastic/composite: Light and inexpensive. Adequate for light-duty use, though markings may wear and edges can nick.
Markings:
- Etched/engraved: Long-lasting and easier to read under jobsite wear.
- Printed/painted: High contrast initially, but may fade over time.
- Metric/imperial: Some squares include both for mixed-measure projects.
How to Choose One
Consider these points before buying:
- Accuracy and legibility: Look for clear, high-contrast markings that are etched or deeply engraved.
- Size match: A 7-inch square covers most framing and trim tasks; choose 12-inch if you cut sheet goods often.
- Edge quality: Straight, crisp edges improve marking precision and saw-guiding.
- Scribe notches: Useful if you frequently mark parallel lines or lay out stair stringers.
- Finish: Non-glare surfaces are easier to read outdoors. Anodized aluminum resists wear.
- Feel: The fence should sit flush on the board with no wobble. Try it on a flat edge if possible.
How to Use It: Step-by-Step Examples
1) Square crosscut with a circular saw
- Hook the fence against the board edge with the square’s 90-degree side across the face.
- Align your cut line with the square’s edge.
- Hold the square firmly with your non-cutting hand, keeping fingers clear of the saw path.
- Slide the saw’s shoe against the square and make the cut. The square acts as a guide for a straight, clean crosscut.
2) Mark a 30-degree miter on a board
- Place the fence on the board’s edge.
- Put your pencil at the 30-degree mark on the degree scale.
- Pivot the square around the pivot point until the pencil meets your desired start point.
- Draw the line along the square’s edge. You now have a precise 30-degree layout line.
3) Lay out a 6/12 roof pitch cut (common rafter)
- Hook the fence on the board.
- Find “6” on the common scale (representing 6 inches of rise per 12 inches of run).
- Rotate the square until the “6” aligns with the board’s edge.
- Draw the plumb cut across the board. Flip as needed to mark the seat cut using the same alignment.
4) Scribe a parallel line 1 inch from the edge
- Place your pencil in the appropriate scribe notch (four 1/4-inch notches equal 1 inch).
- Slide the fence along the board’s edge while keeping the pencil in the notch.
- You’ll draw a straight, parallel line for rips, reveals, or offset fastener lines.
5) Check a box corner for square
- Place the square’s 90-degree edge inside the corner.
- If both edges touch evenly with no gaps, the corner is square. Adjust clamps or fasteners if you see daylight.
Mistakes to Avoid
- Misreading pitch vs degrees: Roof pitch numbers are not degrees. Use the correct scale for the task.
- Not seating the fence: If the fence isn’t fully hooked, your marks and cuts will drift out of square.
- Pushing the saw into the square: Let the saw’s shoe glide; excessive sideways force can shift the square.
- Using a damaged edge: Nicks or bent corners reduce accuracy. Replace or file lightly if needed.
- Flipping the square inconsistently: Marking from different faces can introduce small errors. Stay consistent.
- Forgetting kerf: The saw blade removes material. Mark on the waste side and guide the saw accordingly.
- Overheating plastic squares: Hot saw shoes can melt or warp plastic edges. Use metal for frequent saw-guiding.
Care and Maintenance
- Keep it clean: Wipe off dust and resin so the fence seats fully.
- Protect the edges: Store it in a pouch or toolbox to prevent dings.
- Check accuracy: Occasionally compare against a known square edge or a framing square; replace if it’s out.
- Light lube: A small wipe of dry lubricant helps it slide smoothly as a saw guide without attracting dust.
Related Terms
- Rafter square: A general term for triangular layout squares; often used interchangeably with speed square.
- Combination square: A multi-part tool with a sliding head and ruler for 90°/45° and depth measurements.
- Try square: A fixed 90-degree layout tool, usually for woodworking joints.
- Framing square: Large L-shaped square used for framing, stair, and rafter layout.
- Protractor: Angle-measuring tool; a speed square includes a quick-read protractor scale.
By learning the scales and practicing a few quick layouts, you’ll use a speed square to save time, reduce measuring mistakes, and produce cleaner cuts across a wide range of DIY projects—from shelves and trim to decks, stairs, and simple roof framing.