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.
Body and tongue 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.
Use
Calibration 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.
Stair framing
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.
The rise (vertical measurement), and the run (horizontal measurement). The stringer will rest partially on the horizontal surface. This is a two-by-twelve piece of lumber. A framing square is placed on the lumber so that the desired rise and tread marks meet the edge of the board. The outline of the square is traced. The square is slid up the board until the tread is placed on the mark and the process is repeated. The board is cut along the dotted lines, and the top plumb cut and the bottom level cut are traced by holding the square on the opposite side. The stringer in this example has two pieces of tread stock. This allows for a slight overhang. There is also a space in between the boards. The bottom of the stringer must be cut to the thickness of the tread. This step is called dropping the stringer. After one stringer is cut this piece becomes the pattern that is traced onto the remaining stringers.
Roof framing
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: Common rafter per foot of run. The common rafter connects the peak of a roof (the ridge) to the base of a roof (the plate). This number gives the unit line length (hypotenuse) of the common rafter per twelve inches of horizontal distance (run). Hip or valley rafter per foot of run. The hip or valley rafter also connects the ridge to the plate, but lies at a 45-degree angle to the common rafter. This number gives the length of the hip or valley rafter per twelve inches of run. Difference in lengths jacks. The jack rafters lie in the same plane as the common rafter but connect the top plate (the wall) or ridge board to the hip or valley rafter respectively. Since the hip or valley rafter meets the ridge board and the common rafter at angles of 45 degrees, the jack rafters will have varying lengths when they intersect the hip or valley. Depending on the spacing of the rafters, their lengths will vary by a constant factor—this number is the common difference. This angle can be cut on the fly by aligning this given number on the body of the steel square and the twelve-inch mark on the tongue, and drawing a line along the tongue. Cutting hip and valley cripple rafters are all cut in a similar way.
Octagon scale The octagon scale allows the user to inscribe an octagon inside a square, given the length of the side of the square. The markings indicate half the length of the octagon's sides, which can be set to a compass or divider. Arcs drawn from the midpoints of the square's sides will intersect the square at the vertices of the planned octagon. All that remains is to cut four triangular sections from the square.
Diagonal scale Knee bracing is a common feature in timber framing to prevent racking under lateral loads. The diagonal scale is useful for determining the length of the a knee brace desired for a given distance from the joint between the post and beam.
Calculators in roof framing
In addition to use the square tool, construction calculators are also used to verify and determine roofing calculations. Some are programmed to calculate all side cuts for hip, valley and jack regular rafters to be exactly 45° for all rafter pitches. The rafter table is expressed in inches, and the higher the numerical value of the slope, the greater the difference between side cut angles within a given slope. Only a level roof, or a 0 slope will require a 45° angle side cut (cheek cut) for hip and jack rafters.
… excerpt ends here. Continue reading the full article.
