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A rafter square, also called a speed square or layout square, earns its place in a tool belt because it can mark common rafter cuts, check 90-degree corners and guide a circular saw. Some models also have a hip-valley scale for laying out those rafters. The right choice depends less on extra features than on whether the square is accurate, easy to read and suited to the stock you work with.
What a rafter square does
The square’s fence sits against the edge of a board. For a common rafter, you use the degree scale to set the angle and draw the plumb cut across the face. The number on the scale represents the roof’s rise in inches per 12 inches of run: a 6-in-12 pitch is 26.6 degrees, while a 12-in-12 pitch is 45 degrees.
Many squares also have a diagonal scale labeled for hip and valley rafters. It gives a starting angle for those cuts, which are steeper than common-rafter cuts at the same roof pitch. Treat the scale as a layout aid, not a substitute for understanding the cut: roof geometry, thickness, overhang and the connection at the ridge can all affect the finished fit.
Choosing a square
For routine framing, a standard 7-inch aluminum square is light, inexpensive and easy to carry. It is a practical choice if you mark boards up to about 2 inches thick and do not need a long guide edge. A 12-inch model is easier to register on wider stock and can guide a saw through a longer cut, but it takes up more belt or pouch space. Check the size you actually use before buying; a large square is not automatically more accurate.
Look for crisp, durable graduations, a fence that sits firmly against the board edge and a body that is flat rather than bowed. A high-contrast or laser-etched scale is easier to read in shade and when the tool is dusty. A thicker body may feel robust, but can be less convenient for some saw-guide setups. Plastic models can work for occasional marking, though the fence and edges are more vulnerable to wear and impact than metal.
It is worth paying more for clear markings and reliable machining if the square gets daily use. For a few home projects, a basic metal square is usually enough. Compare the common options in a 7-inch rafter square search and a 12-inch rafter square search; check the listed dimensions and scale layout rather than relying on a feature count.
| Type | Best for | Trade-off |
|---|---|---|
| Basic 7-inch metal | General framing, belt carry and common cuts | Shorter guide edge; cramped on wide stock |
| 12-inch metal | Wider boards and longer saw-guide cuts | Bulkier and heavier to carry |
| Square with hip-valley scale | Marking hip and valley rafters without calculating the angle separately | Scale is only useful if it is legible and you know which reference to use |
| Plastic square | Light-duty work and occasional DIY use | More susceptible to edge damage and wear |
Marking common rafters
Start by confirming the roof pitch from the plan or measuring the existing roof. Set the square’s pivot point on the board edge, align the selected pitch number with that edge, then hold the fence tight and draw along the blade. For a 6-in-12 roof, use the 6 mark on the degree scale. Mark the plumb cut at the ridge end, then lay out the birdsmouth and tail cut from the rafter length and overhang dimensions.
Keep your pencil against the same edge of the square throughout the mark. A gap between fence and board, or a square that shifts while you draw, changes the angle. Before cutting a full set, mark and cut one rafter, test it in place, and adjust the pattern if needed. A small error repeated across ten rafters becomes a visible mismatch at the ridge or eaves.
Marking hip rafters
A hip rafter runs diagonally from an outside corner to the ridge, so its plumb cut is steeper than the common-rafter cut on the same roof. Use the hip-valley scale where provided, following the square’s printed instructions and the pitch reference the scale assumes. For a simple roof with matching pitches on both sides, the scale can save calculation and speed repetitive layout.
Do not assume every hip cut is identical. A roof with unequal pitches, a different ridge arrangement or compound bevel requirements may need calculated angles or a full-size layout. The square’s scale does not account for every framing detail, and a correct plumb angle alone does not guarantee the cheek cut or birdsmouth will fit. If the scale is hard to read, verify the angle with a framing calculator or a bevel gauge before cutting expensive stock.
Accuracy and care
Check a new square before trusting it. Butt its fence against a straight board and draw a line along the blade. Flip the square over, align the fence to the same edge and draw a second line. If the lines diverge, the square may be out of true or the edge may be irregular. Repeat the check on a known-straight board. Also inspect the fence for dents: one small burr can hold the body away from the work and spoil every mark.
Use a sharp pencil or a marking knife for fine layout, and keep sawdust out of the graduations. A square used as a saw guide should be held firmly, with hands clear of the blade path; check that the saw base does not catch on the square’s edge. For repeated cuts, clamp a straight guide or use a suitable saw guide rather than relying on a short square to control a long cut.
If you frame regularly, a well-made aluminum square with readable common and hip-valley scales is the most useful all-round buy. If you only mark a few rafters, a basic 7-inch metal square will do the work—provided it is square, the scale matches your layout, and you test the first cut before repeating it.