This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
The standard steel square, shown in Fig 15, is the one known to the trade as No. 100, but catalogued by some dealers as No. 1,000. The square consists of two parts, the blade, generally 24 in. long and 2 in. wide, and the tongue, usually 18 in. long and 1 1/2 in. wide. The outside edges on one face are divided into inches and sixteenths, and on the other face the inches are divided into twelfths. On the inside edge the graduations are to eighth inches on one side and to thirty-seconds on the other.
On the tongue, near its junction with the blade, Fig. 15 (b), will be seen a diagonal scale (shown enlarged in Fig. 16), used for taking off hundredths of an inch. The line ab is here 1 in. long, and is divided into 10 parts; the line cd being also divided into 10 parts, diagonal lines are drawn connecting the points of division as shown. For example, to take off .76 in., count off seven spaces from c, c g equaling .70 in.; now count up the diagonal line until the sixth horizontal line ef is reached; then ef is equal to .76 in.
On the same side of the tongue is the brace scale, which may be seen at C, Fig. 15 (b). This scale gives the length of a brace of given rise and run, or, in other words, the length of the hypotenuse of a right-angled triangle with equal I For instance, the hypotenuse of a triangle each of whose sides is 57 in., is 80.61 in. The length and end cuts for a brace of any rise and run may be found by using the square in a similar manner.
On the blade, Fig. 15 (b), is shown the board-measure scale, the use of which will be explained by aid of an example. Let it be required to find the number of board feet in a 1" X 7" board, 13 ft. long. Under the 12" mark, find the number 13, and follow the horizontal space in which 13 is found to the 7" mark; the answer is there found to be 7 7/12 ft. B. M. If the board is over 1 in. thick, the problem is solved in the same way, the result being multiplied by the thickness in inches. If the length of the board is greater than any number given under the figure 12, it should be divided into parts, as in the following example: Required the contents in board measure of a 2" X 9" plank, 23 ft. long. Divide the length into two parts, 10 ft. and 13 ft.; the contents of the 10' part is found, as before shown, to be 7 6/12 ft. B. M., that of the 13' board to be 9 9/12:, ft. B. M. Therefore the total contents
(if 1 in. thick) is 7 6/12+9 9/12 = 17 3/12, ft. B. M.; but as the board is 2 in. thick, the contents are 2 X 17 3/12 or 34 1/2 ft. B. M.
The octagonal scale, found on the tongue at A B, Fig. 15
(a), is used in inscribing an octagon in a square. The scale is marked 10, 20, 30, etc. To inscribe an octagon in a 12" square (see Fig. 17), draw the lines ab and cd, bisecting the sides; from d mark d e and df, each equal to 12 divisions on the octagonal scale; mark bg, etc., in the same way, and draw eg, a side of the required octagon. The other sides may be similarly found. For a 10" square, make d e equal to 10 divisions; for a 7" square, equal to 7 divisions, etc.
In Fig. 18 (a) is shown an adjustable fence, a strip of hard
Fig18 wood about 2 in. wide, 1 1/2 in. thick, and 2 1/2 ft. long. A saw kerf, into which the square will slide, is cut from both ends, leaving about 8 in. of solid wood near the middle. The tool is clamped to the square by means of screws at convenient points, as shown. Let it be required to lay out a rafter of 8' rise and 12' run. Set the fence at the 8" mark on the blade, and at the 12" mark on the tongue, clamping it to the square with 1 1/4" screws. Applying the square and fence at the upper end of the rafter, we get the plumb-cut d e at once. By applying the square, as shown, twelve times successively, the required length of the rafter and the foot-cut c 6 are obtained. In this case the twelve applications of the square are made between the points c and d. Run and rise must also be measured between these points. If run is measured from the point b, which will be the outer edge of the wall plate, it will be necessary to run a gauge line through & parallel to the edge of the rafter, and subtract a distance eg from the height of the ridge, to give us the correct rise. The square must then be applied to the line bg. A rafter of any desired rise and run may be laid off in this manner by selecting proportional parts of the rise and run for the blade and tongue of the square. For a half-pitch roof, use 12 in. on both tongue and blade; for a quarter-pitch, use 6 in. and 12 in.; for a third-pitch use 8 in. and 12 in., etc. The terms half-pitch, quarter-pitch, etc., refer to the height of the ridge expressed as a fraction of the span.