This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
152. One of the most useful tools in the carpenter's kit is the steel square, as with it may be solved a number of problems which would otherwise require extensive drawings, or tedious calculations. Fig. 65 shows the regular form of this instrument; the length of the blade ac is always 24 inches, and that of the tongue a b varies, in different squares, from 12 inches to 18 inches. The corner a where the blade and tongue meet, is called the heel.
153. In marking the plumb-cut or foot-cut on a rafter, where the pitch of the roof is known in the proportion of rise to run, it is only necessary to so set the square upon the rafter in such a position that the rise in inches, as shown on the tongue, and the run in inches, as read on the blade, are both in line on the same edge of the rafter; the tongue will then mark the angle of the plumb-cut, and the blade will mark the bevel of the foot-cut.
In Fig. 65 let nop q be a rafter, the cuts of which are to be made for a roof in proportion of 10-inch rise to 12-inch run. The square is set so that the rise 10 inches is set off on the tongue at a d, and the run 12 inches is set off on the blade at a e; then the tongue will mark the plumb-cut dt at the ridge, and the blade at e s will mark the bevel of the foot-cut at the plate. If the rafter is to project beyond the plate, mark the foot-cut hj and draw h k at right angles to it. In the center of hk draw Im parallel to hj; then klm is cut out to notch over the plate, and the projection of the rafter beyond the plate is measured from h towards o. The length of the rafter may be determined by setting off the distance e d along the top of the rafter, once for each foot of half span in the roof, and the cuts at ridge and plate must be made from the ends of the rafter so determined. If the span is 24 ft. and the pitch is the same on each side of the ridge, the distance e d is set off twelve times from the plumb-cut dt to the top of the foot-cut h, beyond which the projection over the eaves is to be added, as explained above.
154. A very valuable appendage to the square is the simple device called a fence, shown at a in Fig. 66 (a). It consists of a piece of hard wood, preferably cherry or black walnut, 2 inches wide, 1 1/2 inches thick, and 2 feet 10 inches long, with saw kerfs bc and de cut in from each end to within 5 inches of the middle, thus leaving 10 inches of solid wood at c d. Two holes f and g are bored through each end, and two 1 1/4-inch No. 10 screws are then inserted, as shown at f, to bind the fence on the square.
The purpose of the fence is to do away with the use of the bevel in laying roof joists, angle beams, stair strings, etc. In the above case the fence can be set for the rise of the rafter at h i and the run laid off at h k; then, if the fence and square are applied to the scantling, as shown in Fig. 66 (b), the tongue will show the plumb-cut Im and the blade will give the bevel for the foot-cut gd.
In Fig. 66 (b), let eabcdfe be a rafter and hjk a steel square with the fence p q applied for the proper rise and run of the roof. The fence and square are then placed at the lower end of the joist, and the line gd is marked for the foot-cut. At the same time the point n is marked on the scantling at the point h of the square. The fence is now slid along until the point k on the square coincides with the mark at n on the rafter, and the point h on the square is then marked at o on the rafter. This operation is repeated once for every foot of run in the roof, as shown by the dotted lines; then the last position of the tongue at Im will be the plumb-cut of the rafter, and the distance Id will be its top length.