Steel square
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The steel square is a tool that carpenters and other tradesmen use. Today the steel square is more commonly referred to as the framing square. It consists of a large arm and a smaller one, which meet at an angle of 90 degrees (a right angle). It can also be made of metals like aluminum, which is light and resistant to rust.
The wider arm, two inches wide, is called the blade; the narrower arm, one and a half inches wide, 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 pitches and fractional equivalents.
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[edit] Use in 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. Residential framing in Connecticut, for example, requires the maximum rise to be 8¼ inches (210 mm) and the minimum tread to be 9 inches (229 mm). These regulations can vary from state to state. 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 image to the right illustrates how a stringer is laid out.
- The rise (vertical measurement) is 24 inches (610 mm), and the run (horizontal measurement) is 21 inches (533 mm). Note that 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 8-inch (203-mm) and 10½-inch (267-mm) marks meet the edge of the board. The outline of the square is traced. The square is slid up the board until the 10½-inch (267-mm) mark is placed on the old position of the 8-inch (203-mm) mark. Now the process is repeated.
- The board is cut along the dotted lines, and the top plump 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 5/4-by-6 (25-by-140-mm) tread stock. This gives an overhang of about ¾ inch (19 mm). There is also a space in between the boards. The bottom of the stringer must be cut to the thickness of the tread, which is one inch (25 mm) in this case. If this is not cut, there will be three different riser heights, as shown in the sketch. (This step is called dropping the stringer.)
After one stringer is cut this piece becomes the pattern that is traced onto the remaining stringers.
[edit] Use in 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 (pitch) and contains the following information
(see figure 5) .
- Rafter Table: Image 1 shows a cut away view of the Steel Square. The top number is the pitch of the rafter. Ie.The intersection of a 10"/12" Pitch displays 15.62". This indicates that the length of a common rafter measures exactly 15.62" per foot of run. Run is the horizontal or level distance the rafter travels. A 20Ft. wide house has a run of 10 FT per rafter.
- Common rafter length per foot run: The common rafter connects the peak of a roof (the ridge) to the base of a roof (the plate). This number gives the length (hypotenuse) of the common rafter per twelve units of horizontal distance (run). See Image 2
- Hip or valley rafter per foot 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 seventeen units of run. See Image 3
- Image 3 illustrates the relationship between hip,jack and common rafters and how they tie into the ridge and bottom plate.
- Difference in length 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. See Image 3
- Side cut length of jack rafters: In carpentry, the side cut is the beveled angle at the end of a board (in this case, the jack rafter). This angle can be cut on the fly by aligning this given number on the blade of the steel square and the twelve-inch mark on the tongue, and drawing a line along the tongue.
See Image 6
- Cutting Rafters: Common,hip and valley criple rafters are all cut in a similar way. The rafters rise
is marked on the tontue and 12 is placed on the edge of the refter. The Hip and valley use 17 as referance line and the corresponding rise is marked where the tongue intersects the edge of the rafter. See Image 5
 3.Hip rafter elevation |
 4.Side cut of hip rafter |
 5.Common Rafter layout |
 6.Jack rafter side cut layout |
[edit] Use of the octagon scale
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Image 7 shows the scale from 0 to 25 on the face of the framing square.The scale is graduated from 0 inches to 67(this view is cut away) inches for the diameter of the octagon. There are four indented dots and a slash. The slashes denote multiples of five, and the dots represent the increments of 1. Place one point of the compass on the 0 indented slash and the other on the desired number for the size of the octagon. This scale is located on the tongue of the framing square |
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Image 8 This is an illustration is for a flat octagon. This can alternately be accomplished with a protractor without using the Steel square. |
[edit] Use of the diagonal scale
The diagonal scale gives the diagonal, or the hypotenuse, for the different legs of the triangle for which a brace is to be cut. The long points of the brace are always cut at a 45-degree angle, with one exception.The brace is the longer measurement between these following numbers. The brace will fit between the legs that measure 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57 or 60 inches. There is also a triangle whose legs measure 18 and 24 inches and whose diagonal is 30 inches. However the square does not reveal the angles. The angles are 37°and 53° Respectively. Laying out 18 and 24 units and drawing a diagonal on a piece of plywood is another of many ways to determine the angles.This scale is located on the tongue of the framing square
