A framing square, also known as a steel square, is one of the staples of the carpenter's toolbox. Not only can it be used to determine right angles, it is also used to calculate the length and angle of rafter ends, as well as the angle and placement of stair risers and treads on the stringer. In the hands of a skilled professional, it is both an alignment tool and a framing calculator.
Framing Square Anatomy
A framing square is made up of two main parts: the blade, a 24-inch long steel rule 2 inches wide, and the tongue, a 16-inch long steel rule, 1 1/2 inches wide. The two meet at a right angle at a point called the heel. Each of the rules is marked with standard inch and fraction measurements stamped into their faces. The center of the blade typically contains rafter charts and other useful measurements.
The most basic use of the framing square, which even beginning carpenters are typically familiar with, is aligning square, right or 90 degree angles. This is the most common corner angle found in carpentry. The square is fitted into the corner of the angle so that the heel fits the inside corner with the blade aligned to one angle and the tongue aligned to the other. Adjust the pieces until they are perfectly in line with the arms of the square. It can also be used to plumb vertical elements extending up from a level horizontal plane by fitting it into the corner.
Rafters are angled according to run and rise. The run is the horizontal distance the roof runs, rise is the vertical climb. Pitch is stated as a number of inches of rise per foot of run, as in 4 in 12, which indicates a 4-inch rise for every 12 inches of run. The framing square is marked at 12 inches on the tongue and the number of inches of rise on the blade. A diagonal line between the two indicates the angle of the pitch. Position the square with the two marks aligned with the top edge of the board. Mark along the outside of the blade to indicate the angle of the rafter end.
A similar technique is used to figure the height of stair risers and the depth of the treads. The riser measurement is marked on the blade, while the tread measurement is marked on the tongue. Align the square with both marks on the top edge of the stringer board. Mark along the outside of the square along the blade around the heel and up the tongue to mark the position of the tread and riser. This is then repeated for each step on the stair.
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