(1) Common rafters. These are framing members that extend at right angles from the plate line to the roof ridge. They are called common rafters because they are common to all types of roofs and are used as the basis for laying out other types of rafters.

(2) Hip rafters. These are roof members that extend diagonally from the corner of the plate to the ridge.

(3) Valley rafters. These rafters extend from the plate to the ridge along the lines where two roofs intersect.

(4) Jack rafters. These are a common rafter. The three kinds of jack rafter are the--

• Hip jack, which extends from the plate to the hip rafter.
• Valley jack, which extends from the ridge of the valley rafter.
• Cripple jack, which is placed between a hip rafter and a valley rafter. The cripple jack rafter is also part of a common rafter, but it touches neither the ridge of the roof nor the rafter plate.

(1) Scale or Measurement Method. The carpenter should first determine the length of the rafter and the length of the lumber from which the rafter may be cut. If he is working from a roof plan, he learns the rafter lengths and the width of the building from the plan. If no plans are available, the width of the building must be measured.

To determine the rafter length, first find one-half of the distance between the outside plates. (The amount of rise per foot has yet to be considered.) If the building is 20 feet wide, half the span will be 10 feet.

As an example, use a rise per foot of 8 inches. To determine the overall length of a rafter, measure on the steel carpenter's square the distance between 8 on the tongue and 12 on the blade (8 is the rise, and 12 is the unit run). This distance is 14 5/12 inches. This represents the line length of a rafter with a total run of 1 foot and a rise of 8 inches (see Figure 3-9 ).

Since the run of the rafter is 10 feet, multiply 10 by the line length for 1 foot (10 x 14 5/12 = 144 2/12). The answer is 144 2/12 inches or 12 feet 1/6 inch. The amount of overhang, normally 1 foot, must be added if an overhang is to be used. This makes the total length of the rafter 13 feet 1/6 inch. Use a 14-foot timber.

(2) Pattern Rafter Method. After the length has been determined, the timber is laid on sawhorses (saw benches) with the crown or bow (if it has any) as the top side of the rafter. If possible, select a straight piece for the pattern rafter. If a straight piece is not available, have the crown toward the person laying out the rafter. Figure 3-10 illustrates the five steps of the pattern rafter method.

• Hold the square with the tongue in the right hand, the blade in the left, and the heel away from the body. Place the square as near the upper end of the rafter a possible.
• In the example, the figure 8 on the tongue and 12 on the blade are placed along the timber edge, that is to be the top edge of the rafter as shown in step 1. Mark along the tongue edge of the square, which will be the plumb cut at the right.
• Since the length of the rafter is known to be 12 feet 1/6 inch, measure the distance from the top of the plumb cut and mark it on the timber. Hold the square in the same manner with the 8 mark on the tongue directly over the 12-foot 1/6-inch mark. Mark along the tongue of the square to give the plumb cut for the seat (see step 2).
• Next, measure off perpendicular to this mark, the length of overhang along the timber. Make a plumb-cut mark in the same way, keeping the square on the same edge of the lumber (see step 3). This will be the tail cut of the rafter. Often, the tail cut is made square across the timber.
• The level cut or width of the seat is the width of the plate, measured perpendicular to the plumb cut, as shown in step 4. Using the try square, square the lines down on the sides from all level and plumb-cut lines. Now the rafter is to be cut (see step 5).

(3) Step-Off Method. The rafter length of any building may be determined by "stepping it off" by successive steps with the square, as follows:

• Step off the same number of steps as there are feet in the run. For example, if a building is 20 feet 8 inches wide, the run of the rafter would be 4 inches over 10 feet. Figure 3-11 illustrates the four steps of the step-off method.

• This 4 inches is taken care of in the same manner as the full-foot run; that is, with the square at the last step position, make a mark on the rafters at the 4-inch mark (see Figure 3-11 , step 1).
• With the square held for the same cut as before, make a mark along the tongue. This is the line length of the rafter. The seat cut and hangover are made as described above and shown in Figure 3-11 , steps 2, 3, and 4.

(4) Table Method 1. To use the framing square to lay out rafters, the width of the building must first be known. Suppose the building is 20 feet 8 inches wide, and the rise of the rafters is to be 13 inches per foot of run. The total run of the rafters will be 10 feet 4 inches.

• Look at the first line of figures under the 13-inch mark (see Figure 3-12 ). You will see the number 17.69. This is the length in inches of a rafter with a run of 1 foot and a rise of 13 inches.

• To find the line length of a rafter with a total run of 10 feet 4 inches, multiply 17.69 inches by 10 1/3 and divide by 12 to get the answer in feet (17.69 x 10.333 = 182.79). The total of 182.79 inches is divided by 12 to equal 15 3/12 feet. Therefore, 15 feet 3 inches is the line length of the rafter.

(5) Table Method 2. The rafter table is on the back of the blade of some squares. Figure 3-13 shows the run, rise, and pitch of the rafters of the seven most common roof pitches. The figures are based on the length of the horizontal measurement of the building from the center to the outside (run). The rafter table on the outside edge, on the back of the square, gives both the body and the tongue in twelfths. The inch marks on the square may represent inches or feet, and the twelfth marks may represent twelfths of an inch or twelfths of a foot. The rafter table is used in connection with the marks and figures on the outside edge of the square. You will notice that at the left end of the table there are figures representing the run, rise, and the pitch.

• Run. In the first column, the figures are all 12. These may be used as 12 inches or 12 feet, because they represent the run of 12 inches.
• Rise. The second column of figure represents various rises per foot: 4, 6, 8, 10, 12, 15, and 18.
• Pitch. The third column of figures, in fractions, represents various pitches: 1/6, 1/4, 1/3, 5/12, 1/2, 5/8, and 3/4 (see Figure 3-14 ).

(1) Chord. A 1 by 6 or 2 by 4 chord is nailed across the rafters at the seat cut to tie them together. This chord forms a truss with the two rafters. A hanger or vertical member of 1 by 6 is nailed to the rafter joint and extends to the chord at midpoint, tying the rafter to the chord.

(2) Collar Beam. A tie or collar beam is a piece of stock (usually 1 by 4, 1 by 6, 1 by 8, or 2 by 4) fastened in a horizontal position to a pair of rafters between the plate and the ridge of the roof This type of beam tends to keep the building from spreading. Most codes and specifications require them to be 5 feet apart or every third rafter, which ever is less. Collar ties are nailed to common rafters with four 8d nails to each end of a 1-inch tie. If 2-inch material is used for the tie, they are nailed with three 16d nails at each end. This type of bracing is used on small roofs where no ceiling joists are used and the building is not wide enough to require a truss. The lower the collar beam or chord, the better it works.

(3) Support. In small roofs that cover only narrow buildings and in which the rafters are short, there is no need for interior support or bracing. In long spans, the roof would sag in the middle if it were not strengthened in some way. To support long rafters, braces or other types of supports must be installed.

(4) Rafter Support. In wide buildings, where the joists or chords must be spliced and there is no support underneath, the rafter and joists support one another (see Figure 3-16 ).