Steel square

The steel square is a tool used in carpentry. Carpenters use various tools to lay out structures that are square (that is, built at accurately measured right angles), many of which are made of steel, but the name steel square refers to a specific long-armed square that has additional uses for measurement, especially of various angles. It consists of a long, wider arm and a shorter, narrower arm, which meet at an angle of 90 degrees (a right angle). Today the steel square is more commonly referred to as the framing square or carpenter's square, and such squares are no longer invariably made of steel (as they were many decades ago); they can also be made of aluminum or polymers, which are light and resistant to rust.

The longer wider arm is 50 millimetres (2.0 in) wide, and is called the body; the shorter narrower arm, is 37 millimetres (1.5 in) wide, and is called the tongue. The square has many uses, including laying out common rafters, hip rafters and stairs. It has a diagonal scale, board foot scale and an octagonal scale. On the newer framing squares there are degree conversions for different slopes and fractional equivalents.

Framing squares may also be used as winding sticks.

In traditional timber frame joinery, mortises and tenons were typically 50 millimetres (2.0 in) wide and 50 millimetres (2.0 in) from the edge of the timber when working with softwoods, giving rise to the width of the body. Likewise, mortises and tenons were traditionally 37 millimetres (1.5 in) wide when working in hardwoods, explaining the width of the tongue. This allowed for quick layouts of mortise and tenon joints when working both hard and softwoods.

A steel square is self-proving and self-calibrating in that you can lay out a perpendicular line, flip the square over, and determine the size and direction of the error. The error can be corrected by opening or closing the angle with a center punch.

Stairs usually consist of three components. They are the stringer, the tread and the riser. The stringer is the structural member that carries the load of the staircase, the tread is the horizontal part that is stepped on, and the riser board is the vertical part which runs the width of the structure. There are many types of stairs: open, closed, fully housed, winding, and so on, to mention a few of them.

Laying out a staircase requires rudimentary math. There are numerous building codes to which staircases must conform. In an open area the designer can incorporate a more desirable staircase. In a confined area this becomes more challenging. In most staircases there is one more rise than there are treads.

There is a table of numbers on the face side of the steel square; this is called the rafter table. The rafter table allows the carpenter to make quick calculations based on the Pythagorean theorem. The table is organized by columns that correspond to various slopes of the roof. Each column describes a different roof inclination (slope) and contains the following information: