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As we noted earlier, the four most common roof designs you will encounter as a Builder are gable, hip, intersecting, and shed. In this section, we will examine various calculations, layouts, cutting procedures, and assembly requirements required for efficient construction.

GABLE

Next to the shed roof, which has only one slope, the gable roof is the simplest type of sloping roof to build because it slopes in only two directions. The basic structural members of the gable roof are the ridgeboard, the common rafters, and the gable-end studs. The framework is shown in figure 2-13.

Figure 2-13.—Framework of a gable roof.

The ridgeboard is placed at the peak of the roof. It provides a nailing surface for the top ends of the common rafters. The common rafters extend from the top wall plates to the ridge. The gable-end studs are upright framing members that provide a nailing surface for siding and sheathing at the gable ends of the roof.

Common Rafters

All common rafters for a gable roof are the same length. They can be precut before the roof is assembled. Today, most common rafters include an overhang. The overhang (an example is shown in fig. 2-14) is the part of the rafter that extends past the building line. The run of the overhang, called the projection, is the horizontal distance from the building line to the tail cut on the rafter. In figure 2-14, note the plumb cuts at the ridge, heel, and tail of the rafter. A level seat cut is placed where the rafter rests on the top plate. The notch formed by the seat and heel cut line (fig. 2-15) is often called the bird’s-mouth.

Figure 2-14.—Typical common rafter with an overhang.

Figure 2-15.—A "bird’s-mouth" is formed by the heel plumb line and seat line.

The width of the seat cut is determined by the slope of the roof: the lower the slope, the wider the cut. At least 2 inches of stock should remain above the seat cut. The procedure for marking these cuts is explained later in this chapter. Layout is usually done after the length of the rafter is calculated.

CALCULATING LENGTHS OF COMMON RAFTERS.— The length of a common rafter is based on the unit of rise and total run of the roof. The unit of rise and total run are obtained from the blueprints. Three different procedures can be used to calculate common rafter length: use a framing square printed with a rafter table; use a book of rafter tables; or, use the step-off method where rafter layout is combined with calculating length.

Framing squares are available with a rafter table printed on the face side (fig. 2-16). The rafter table makes it possible to find the lengths of all types of rafters for pitched roofs, with unit of rises ranging from 2 inches to 18 inches. Let’s look at two examples:

Figure 2-16.—Rafter table on face of a steel square.

Example 1. The roof has a 7-inch unit of rise and a 16-foot span.

Look at the first line of the rafter table on a framing square to find LENGTH COMMON RAFTERS PER FOOT RUN (also known as the bridge measure). Since the roof in this example has a 7-inch unit of rise, locate the number 7 at the top of the square. Directly beneath the number 7 is the number 13.89. This means that a common rafter with a 7-inch unit of rise will be 13.89 inches long for every unit of run. To find the length of the rafter, multiply 13.89 inches by the number of feet in the total run. (The total run is always one-half the span.) The total run for a roof with a 16-foot span is 8 feet; therefore, multiply 13.89 inches by 8 to find the rafter length. Figure 2-17 is a schematic of this procedure.

If a framing square is not available, the bridge measure can be found by using the Pythgorean theorem using the same cut of 7/12: 7² + 12² = 193²; the square root of 193 is 13.89.

Two steps remain to complete the procedure.

• Step 1. Multiply the number of feet in the total run (8) by the length of the common rafter per foot of run (13.89 inches): 13.89 x 8 = 111.12 inches.
• Step 2. To change .12 of an inch to a fraction of an inch, multiply by 16: .12 x 16 = 1.92.

The number 1 to the left of the decimal point represents 1/16 inch. The number .92 to the right of the decimal represents ninety-two hundredths of 1/16 inch. For practical purposes, 1.92 is calculated as being equal to 2 x 1/16 inch, or 1/8 inch. As a general rule in this kind of calculation, if the number to the right of the decimal is 5 or more, add 1/16 inch to the figure on the left side of the decimal. The result of steps 1 and 2 is a total common rafter length of 111 1/8 inches, or 9 feet 3 1/8 inches.

Example 2. A roof has a 6-inch unit of rise and a 25-foot span. The total run of the roof is 12 feet 6 inches. You can find the rafter length in four steps.

• Step 1. Change 6 inches to a fraction of a foot by placing the number 6 over the number 12:

(1/2 foot = 6 inches).

• Step 2. Change the fraction to a decimal by dividing the bottom number (denominator) into the top number (numerator):

(.5 foot = 6 inches).

• Step 3. Multiply the total run (12.5) by the length of the common rafter per foot of run (13.42 inches) (fig. 2-16):

• Step 4. To change .75 inch to a fraction of an inch, multiply by 16 (for an answer expressed in sixteenths of an inch).

.75 x 16 = 12

The result of these steps is a total common rafter length of 167 3/4 inches, or 13 feet 11 3/4 inches.

SHORTENING.— Rafter length found by any of the methods discussed here is the measurement from the heel plumb line to the center of the ridge. This is known as the theoretical length of the rafter. Since a ridgeboard, usually 1 1/2 inches thick, is placed between the rafters, one-half of the ridgeboard (3/4 inch) must be deducted from each rafter. This calculation is known as shortening the rafter. It is done at the time the rafters are laid out. The actual length (as opposed to the theoretical length) of a ratler is the distance from the heel plumb line to the shortened ridge plumb line (fig. 2-18).

Figure 2-18.—The actual (versus theoretical) length of a common rafter.

LAYING OUT.— Before the rafters can be cut, the angles of the cuts must be marked. Layout consists of marking the plumb cuts at the ridge, heel, and tail of the rafter, and the seat cut where the rafter will rest on the wall. The angles are laid out with a framing square, as shown in figure 2-19. A pair of square gauges is useful in the procedure. One square gauge is secured to the tongue of the square next to the number that is the same as the unit of rise. The other gauge is secured to the blade of the square next to the number that is the same as the unit of run (always 12 inches). When the square is placed on the rafter stock, the plumb cut can be marked along the tongue (unit of rise) side of the square. The seat cut can be marked along the blade (unit of run) side of the square.

Figure 2-19.-Steel square used to lay out plumb and seat cuts.

Rafter layout also includes marking off the required overhang, or tail line length, and making the shortening calculation explained earlier. Overhang, or tail line length, is rarely given and must be calculated before laying out rafters. Projection, the horizontal distance from the building line to the rafter tail, must be located from drawings or specifications. To determine tail line length, use the following formula: bridge measure (in inches) times projection (in feet) equals tail line length (in inches). Determine the bridge measure by using the rafter table on the framing square or calculate it by using the Pythagorean theorem. Using figure 2-20 as a guide, you can see there are four basic steps remaining.

Figure 2-20.—Laying out a common rafter for a gable roof.

Step 1. Lay out the rafter line length. Hold the framing square with the tongue in your right hand, the blade in the left, and the heel away from your body. Place the square as near the right end of the rafter as possible with the unit of rise on the tongue and the unit of run on the blade along the edge of the rafter stock. Strike a plumb mark along the tongue on the wide part of the material. This mark represents the center line of the roof. From either end of this mark, measure the line length of the rafter and mark the edge of the rafter stock. Hold the framing square in the same manner with the 6 on the tongue on the mark just made and the 12 on the blade along the edge. Strike a line along the tongue, his mark represents the plumb cut of the heel.

Step 2. Lay out the bird’s-mouth. Measure 1 1/2 inches along the heel plumb line up from the bottom of the rafter. Set the blade of the square along the plumb line with the heel at the mark just made and strike a line along the tongue. This line represents the seat of the bird’s-mouth.

Step 3. Lay out the tail line length. Measure the tail line length from the bird’ s-mouth heel plumb line. Strike a plumb line at this point in the same manner as the heel plumb line of the common rafter.

Step 4. Lay out the plumb cut at the ridgeboard. Measure and mark the point along the line length half the thickness of the ridge-board. (This is the ridgeboard shortening allowance.) Strike a plumb line at this point. This line represents the plumb cut of the ridgeboard.

Step-Off Calculations and Layout

The step-off method for rafter layout is old but still practiced. It combines procedures for laying out the rafters with a procedure of stepping off the length of the rafter (see fig. 2-21). In this example, the roof has an 8-inch unit of rise, a total run of 5 feet 9 inches, and a 10-inch projection.

Figure 2-21.-Step-off method for calculating common rafter length.

First, set gauges at 8 inches on the tongue and 12 inches on the blade. With the tongue in the right hand, the blade in the left hand, and the heel away from the body, place the square on the right end of the rafter stock. Mark the ridge plumb line along the tongue. Put a pencil line at the 12-inch point of the blade.

Second, with the gauges pressed lightly against the rafter, slide the square to the left. Line the tongue up with the last 12-inch mark and make a second 12-inch mark along the bottom of the blade.

Third, to add the 9-inch remainder of the total run, place the tongue on the last 12-inch mark. Draw another mark at 9 inches on the blade. This will be the total length of the rafter.

Finally, lay out and cut the plumb cut line and the seat cut line.

Roof Assembly

The major part of gable-roof construction is setting the common rafters in place. The most efficient method is to precut all common rafters, then fasten them to the ridgeboard and the wall plates in one continuous operation.

The rafter locations should be marked on the top wall plates when the positions of the ceiling joists are laid out. Proper roof layout ensures the rafters and joists tie into each other wherever possible.

The ridgeboard like the common rafters, should be precut. The rafter locations are then copied on the ridgeboard from the markings on the wall plates (fig. 2-22). The ridgeboard should be the length of the building plus the overhang at the gable ends.

Figure 2-22.—Ridgeboard layout.

The material used for the ridgeboard is usually wider than the rafter stock. For example, a ridgeboard of 2- by 8-inch stock would be used with rafters of 2-by 6-inch stock. Some buildings are long enough to require more than one piece of ridge material. The breaks between these ridge pieces should occur at the center of a rafter.

One pair of rafters should be cut and checked for accuracy before the other rafters are cut. To check the first pair for accuracy, set them in position with a 1 1/2-inch piece of wood fitted between them. If the rafters are the correct length, they should fit the building. If, however, the building walls are out of line, adjustments will have to be made on the rafters.

After the first pair of rafters is checked for accuracy (and adjusted if necessary), one of the pair can be used as a pattern for marking all the other rafters. Cutting is usually done with a circular or radial-arm saw.

COLLAR TIE.— Gable or double-pitch roof rafters are often reinforced by horizontal members called collar ties (fig. 2-23). In a finished attic, the ties may also function as ceiling joists.

Figure 2-23.—Calculation for a collar tie.

To find the line length of a collar tie, divide the amount of drop of the tie in inches by the unit of rise of the common rafter. This will equal one-half the length of the tie in feet. Double the result for the actual length. The formula is as follows: Drop in inches times 2, divided by unit or rise, equals the length in feet.

The length of the collar tie depends on whether the drop is measured to the top or bottom edge of the collar tie (fig. 2-23). The tie must fit the slope of the roof. To obtain this angle, use the framing square. Hold the unit of run and the unit of rise of the common rafter. Mark and cut on the unit of run side (fig. 2-24).

Figure 2-24.—Laying out end cut on a collar tie.

METHODS OF RIDGE BOARD ASSEMBLY.— Several different methods exist for setting up the ridgeboard and attaching the rafters to it. When only a few Builders are present, the most convenient procedure is to set the ridgeboard to its required height (total rise) and hold it in place with temporary vertical props (fig. 2-25). The rafters can then be nailed to the ridgeboard and the top wall plates.

Figure 2-25.-Setting up and bracing a ridgeboard when only a few workers are available.

Plywood panels should be laid on top of the ceiling joists where the framing will take place. The panels provide safe and comfortable footing. They also provide a place to put tools and materials.

Common rafter overhang can be laid out and cut before the rafters are set in place. However, many Builders prefer to cut the overhang after the rafters are fastened to the ridgeboard and wall plates. A line is snapped from one end of the building to the other, and the tail plumb line is marked with a sliding T-bevel, also called a bevel square. These procedures are shown in figure 2-26. The rafters are then cut with a circular saw.

Figure 2-26.-Snapping a line and marking plumb cuts for a gable-end overhang.

This method guarantees that the line of the overhang will be perfectly straight, even if the building is not.

Over each gable end of the building, another overhang can be framed. The main framing members of the gable-end overhang are the fascia, also referred to as "fly" (or "barge") rafters. They are tied to the ridgeboard at the upper end and to the fascia board at the lower end. Fascia boards are often nailed to the tail ends of the common rafters to serve as a finish piece at the edge of the roof. By extending past the gable ends of the house, common rafters also help to support the basic rafters.

Figures 2-27 and 2-28 show different methods used to frame the gable-end overhang. In figure 2-27, a fascia rafter is nailed to the ridgeboard and to the fascia board. Blocking (not shown in the figures) rests on the end wall and is nailed between the fascia rafter and the rafter next to it. This section of the roof is further strengthened when the roof sheathing is nailed to it. In figure 2-28, two common rafters arc placed directly over the gable ends of the building. The fascia rafters (fly rafters) are placed between the ridgeboard and the fascia boards. The gable studs should be cut to fit against the rafter above.

Figure 2-27.-Gable-end overhang with the end wall framed under the overhang.

Figure 2-28.-Gable-end overhang with the end wall framed directly beneath the rafters.

End Framing

Gable-end studs rest on the top plate and extend to the rafter line in the ends of a gable roof. They may be placed with the edge of the stud even with the outside wall and the top notched to fit the rafter (as shown in fig. 2-28), or they maybe installed flatwise with a cut on the top of the stud to fit the slope of the rafter.

The position of the gable-end stud is located by squaring a line across the plate directly below the center of the gable. If a window or vent is to be installed in the gable, measure one-half of the opening size on each side of the center line and make a mark for the first stud. Starting at this mark layout the stud spacing (that is, 16 or 24 inches on center [OC]) to the outside of the building. Plumb the gable-end stud on the first mark and mark it where it contacts the bottom of the rafter, as shown in figure 2-29, view A. Measure and mark 3 inches above this mark and notch the stud to the depth equal to the thickness of the rafter, as shown in view B.

Figure 2-29.—Calculating common difference of gable-end studs.   

The lengths of the other gable studs depend on the spacing.

The common difference in the length of the gable studs may be figured by the following method:

and, 2 x 6 inches (unit of rise) or 12 inches (common difference).

The common difference in the length of the gable studs may also be laid out directly with the framing square (fig. 2-29, view C). Place the framing square on the stud to the cut of the roof (6 and 12 inches for this example). Draw a line along the blade at A. Slide the square along this line in the direction of the arrow at B until the desired spacing between the studs (16 inches for this example) is at the intersection of the line drawn at A and the edge of the stud. Read the dimension on the tongue aligned with the same edge of the stud (indicated by C). This is the common difference (8 inches for this example) between the gable studs.

Toenail the studs to the plate with two 8d nails in each side. As the studs are nailed in place, care must be taken not to force a crown into the top of the rafter.

HIP

Most hip roofs are equal pitch. This means the angle of slope on the roof end or ends is the same as the angle of slope on the sides. Unequal-pitch hip roofs do exist, but they are quite rare. They also require special layout methods. The unit length rafter table on the framing square applies only to equal-pitch hip roofs. The next paragraphs discuss an equal-pitch hip roof.

The length of a hip rafter, like the length of a common rafter, is calculated on the basis of bridge measure multiplied by the total run (half span). Any of the methods previously described for a common rafter may be used, although some of the dimensions for a hip rafter are different.

Figure 2-30 shows part of a roof framing diagram for an equal-pitch hip roof. A roof framing diagram may be included among the working drawings; if not, you should lay one out for yourself. Determine what scale will be used, and lay out all framing members to scale. Lay the building lines out first. You can find the span and the length of the building on the working drawings. Then, draw a horizontal line along the center of the span.

Figure 2-30.—Equal-pitch hip roof framing diagram.

In an equal-pitch hip roof framing diagram, the lines indicating the hip rafters (AF, AG, BI, and BK in figure 2-30) form 45° angles with the building lines. Draw these lines at 45°, as shown. The points where they meet the center line are the theoretical ends of the ridge piece.

The ridge-end common rafters AC, AD, AE, BH, BJ, and BL join the ridge at the same points.

A line indicating a rafter in the roof framing diagram is equal in length to the total run of the rafter it represents. You can see from the diagram that the total run of a hip rafter (represented by lines AF-AG-BI-BK) is the hypotenuse of a right triangle with the altitude and base equal to the total run of a common rafter. You know the total run of a common rafter: It is one-half the span, or one-half the width of the building. Knowing this, you can find the total run of a hip rafter by applying the Pythagorean theorem.

Let’s suppose, for example, that the span of the building is 30 feet. Then, one-half the span, which is the same as the total run of a common rafter, is 15 feet. Applying the Pythagorean theorem, the total run of a hip rafter is:

Ö (15² + 15²) = 21.21 feet.

What is the total rise? Since a hip rafter joins the ridge at the same height as a common rafter, the total rise for a hip rafter is the same as the total rise for a common rafter. You know how to figure the total rise of a common rafter. Assume that this roof has a unit of run of 12 and a unit of rise of 8. Since the total run of a common rafter in the roof is 15 feet, the total rise of common rafter is the value of x in the proportional equation 12:8::15:x, or 10 feet.

Knowing the total run of the hip rafter (21.21 feet) and the total rise (10 feet), you can figure the line length by applying the Pythagorean theorem. The line length is:

Ö (21.21² + 10²) = 23.45 feet, or about 23 feet

To find the length of a hip rafter on the basis of bridge measure, you must first determine the bridge measure. As with a common rafter, the bridge measure of a hip rafter is the length of the hypotenuse of a triangle with its altitude and base equal to the unit of run and unit of rise of the rafter. The unit of rise of a hip rafter is always the same as that of a common rafter, but the unit of run of a hip rafter is a fixed unit of measure, always 16.97.

The unit of run of a hip rafter in an equal-pitch roof is the hypotenuse of a right triangle with its altitude and base equal to the unit of run of a common rafter, 12. Therefore, the unit of run of a hip rafter is:

Ö (12² + 12²) = 16.97

If the unit of run of a hip rafter is 16.97 and the unit of rise (in this particular case) is 8, the bridge measure of the hip rafter must be:

Ö (16.97² + 8²) = 18.76

This means that for every unit of run (16.97) the rafter has a line length of 18.76 inches. Since the total run of the rafter is 21.21 feet, the length of the rafter must be the value of x in the proportional equation 16.97:18. 76::21.21:x, or 23.45 feet.

Like the unit length of a common rafter, the bridge measure of a hip rafter can be obtained from the unit length rafter table on the framing square. If you turn back to figure 2-16, you will see that the second line in the table is headed LENGTH HIP OR VALLEY PER FT RUN. This means "per foot run of a common rafter in the same roof." Actually, the unit length given in the tables is the unit length for every 16.97 units of run of the hip rafter itself. If you go across to the unit length given under 8, you will find the same figure, 18.76 units, that you calculated above.

An easy way to calculate the length of an equal-pitch hip roof is to multiply the bridge measure by the number of feet in the total run of a common rafter, which is the same as the number of feet in one-half of the building span. One-half of the building span, in this case, is 15 feet. The length of the hip rafter is therefore 18.76 x 15, or 281.40 inches—23.45 feet once converted.

Step off the length of an equal-pitch hip roof just as you do the length of a common rafter, except that you set the square to a unit of run of 16.97 inches instead of to a unit of run of 12 inches. Since 16.97 inches is the same as 16 and 15.52 sixteenths of an inch, setting the square to a unit of run of 17 inches is close enough for most practical purposes. Bear in mind that for any plumb cut line on an equal-pitch hip roof rafter, you set the square to the unit of rise of a common rafter and to a unit of run of 17.

Sstep off the same number of times as there are feet in the total run of a common rafter in the same roof; only the size of each step is different. For every 12-inch step in a common rafter, a hip rafter has a 17-inch step. For the roof on which you are working, the total run of common rafter is exactly 15 feet; this means that you would step off the hip-rafter cut (17 inches and 8 inches) exactly 15 times.

Suppose, however, that there was an odd unit in the common rafter total run. Assume, for example, that the total run of a common rafter is 15 feet 10 1/2 inches. How would you make the odd fraction of a step on the hip rafter?

Remember that the unit of run of a hip rafter is the hypotenuse of a right triangle with the other side each equal to the unit of run of a common rafter. In this case, the run of the odd unit on the hip rafter must be the hypotenuse of a right triangle with the altitude and base equal to the odd unit of run of the common rafter (in this case, 10 1/2 inches). You can figure this using the Pythagorean theorem

Ö (10.5² + 10.5²)

or you can set the square on a true edge to 10 1/2 inches on the blade and measure the distance between the marks. It comes to 14.84 inches. Rounded off to the nearest 1/16 inch, this equals 14 13/16 inches.

To layoff the odd unit, set the tongue of the framing square to the plumb line for the last full step made and measure off 14 13/16 inches along the blade. Place the tongue of the square at the mark, set the square to the hip rafter plumb cut of 8 inches on the tongue and 17 inches on the blade, and draw the line length cut.

Rafter Shortening Allowance

As in the case with a common rafter, the line length of a hip rafter does not take into account the thickness of the ridge piece. The size of the ridge-end shortening allowance for a hip rafter depends upon the way the ridge end of the hip rafter is joined to the other structural members. As shown in figure 2-31, the ridge end of the hip rafter can be framed against the ridgeboard (view A) or against the ridge-end common rafters (view B). To calculate the actual length, deduct one-half the 45° thickness of the ridge piece that fits between the rafters from the theoretical length.

Figure 2-31.-Shortening a hip rafter.

When no common rafters are placed at the ends of the ridgeboard the hip rafters are placed directly against the ridgeboard. They must be shortened one-half the length of the 45° line (that is, one-half the thickness of the ridgeboard When common rafters are placed at the ends of the ridgeboard (view B), the hip rafter will fit between the common rafters. The hip rafter must be shortened one-half the length of the 45° line (that is, one-half the thickness of the common rafter).

If the hip rafter is framed against the ridge piece, the shortening allowance is one-half of the 45° thickness of the ridge piece (fig. 2-31, view C). The 45° thickness of stock is the length of a line laid at 45° across the thickness dimension of the stock. If the hip rafter is framed against the common rafter, the shortening allowance is one-half of the 45° thickness of a common rafter.

To lay off the shortening allowance, first set the tongue of the framing square to the line length ridge cut line. Then, measure off the shortening allowance along the blade, set the square at the mark to the cut of the rafter (8 inches and 17 inches), draw the actual ridge plumb cut line. (To find the 45° thickness of a piece of lumber, draw a 450 line across the edge, and measure the length of the line and divide by 2.)

Rafter Projection

A hip or valley rafter overhang, like a common rafter overhang, is figured as a separate rafter. The projection, however, is not the same as the projection of a common rafter overhang in the same roof. The projection of the hip or valley rafter overhang is the hypotenuse of a right triangle whose shorter sides are each equal to the run of a common rafter overhang (fig. 2-32). If the run of the common rafter overhang is

Figure 2-32.—Run of hip rafter projection.

18 inches for a roof with an 8-inch unit of rise, the length of the hip or valley rafter tail is figured as follows:

1. Find the bridge measure of the hip or valley rafter on the framing square (refer to figure 2-16). For this roof, it is 18.76 inches.
2. Multiply the bridge measure (in inches) of the hip or valley rafter by the projection (in feet) of the common rafter overhang:
3. Add this product to the theoretical rafter length.

The overhang may also be stepped off as described earlier for a common rafter. When stepping off the length of the overhang, set the 17-inch mark on the blade of the square even with the edge of the rafter. Set the unit of rise, whatever it might be, on the tongue even with the same rafter edge.

Rafter Side Cuts

Since a common rafter runs at 90° to the ridge, the ridge end of a common rafter is cut square, or at 90° to the lengthwise line of the rafter. A hip rafter, however, joins the ridge, or the ridge ends of the common rafter, at other than a 90° angle, and the ridge end of a hip rafter must therefore be cut to a corresponding angle, called a side cut. The angle of the side cut is more acute for a high rise than it is for a low one.

The angle of the side cut is laid out as shown in figure 2-33. Place the tongue of the framing square along the ridge cut line, as shown, and measure off one-half the thickness of the hip rafter along the blade. Shift the tongue to the mark, set the square to the cut of the rafter (17 inches and 8 inches), and draw the plumb line marked "A" in the figure. Then, turn the rafter edge-up, draw an edge centerline, and draw in the angle of the side cut, as indicated in the lower view of figure 2-33. For a hip rafter to be framed against the ridge, there will be only a single side cut, as indicated by the dotted line in the figure. For one to be framed against the ridge ends of the common rafters, there will be a double side cut, as shown in the figure. The tail of the rafter must have a double side cut at the same angle, but in the reverse direction.

Figure 2-33.—Laying out hip rafter side cut.

The angle of the side cut on a hip rafter may also be laid out by referring to the unit length rafter table on the framing square. (Look ahead to figure 2-41.) You will see that the bottom line in the table is headed SIDE CUT HIP OR VALLEY USE. If you follow this line over to the column headed by the figure 8 (for a unit of rise of 8), you will find the figure 10 7/8. If you place the framing square faceup on the rafter edge with the tongue on the ridge-end cut line, and set the square to a cut of 10 7/8 inches on the blade and 12 inches on the tongue, you can draw the correct side-cut angle along the tongue.

Bird’s-Mouth

Laying out the bird’ s-mouth for a hip rafter is much the same as for a common rafter. However, there are a couple of things to remember. When the plumb (heel) cut and level (seat) cut lines are laid out for a bird’s-mouth on a hip rafter, set the body of the square at 17 inches and the tongue to the unit of rise (for example, 8 inches-depending on the roof pitch) (fig. 2-34, view A). When laying out the depth of the heel for the bird’s-mouth, measure along the heel plumb line down from the top edge of the rafter a distance equal to the same dimension on the common rafter. This must be done so that the hip rafter, which is usually wider than a common rafter, will be level with the common rafters.

Figure 2-34.-Backing or dropping a hip rafter:

A. Marking the top (plumb) cut and the seat (level) cut of a hip rafter;

B. Determining amount of backing or drop;

C. Bevel line for backing the rafter;

D. Deepening the bird’s-mouth for dropping the rafter.

If the bird’s-mouth on a hip rafter has the same depth as the bird’s-mouth on a common rafter, the edge of the hip rafter will extend above the upper ends of the jack rafters. You can correct this by either backing or dropping the hip rafter. Backing means to bevel the top edges of the hip rafter (see fig. 2-35). The amount of backing is taken at a right angle to the roof surface on the top edge of the hip rafters. Dropping means to deepen the bird’s-mouth so as to bring the top edge of the hip rafter down to the upper ends of the jacks. The amount of drop is taken on the heel plumb line (fig. 2-34, view D).