Lesson 5.2 Framing Members
Joists, rafters, truss, purlins, and braces are considered the main framing members of a roof system.
Ceiling joists form the framework of the ceiling of the room. They are usually lighter than floor joists but large enough to remain rigid. Ceiling joists are usually installed 16 or 24 inches on center, with the first ceiling joist placed on the outside edge of the top plate. The second joist is placed 16 inches on center lines from the outside edge of the first joist, and the remaining joists are placed 16 inches on the center lines continuing across the building. Extra joists, if needed, may be paced without affecting the spacing of the prime joists. Joists that lie beside rafters on a plate are cut at the same pitch as the rafter, flush with the top of the rafter (see Figure 3-5 ). The ceiling joists are nailed to both the top plates and the rafters (see Figure 3-6 ).
Figure 3-5. Ceiling joists
Figure 3-6. Nailing ceiling joists
Rafters make up the main framework of all roofs. They are inclined members spaced from 16 to 48 inches apart. They vary in size, depending on length and spacing. The tops of inclined rafters are fastened to the ridge or another rafter, depending on the type of roof. Rafters rest on the top wall plate. Rafters are nailed to the plate, not framed into it. Some are cut to fit the plate, while in hasty construction they are merely laid on top of the plate and nailed in place. They may extend a short distance beyond the wall to form the eaves and protect the sides of the building. Sometimes, metal anchor are used to connect joints and rafters to the top plate (see Figure 3-7 ). Metal anchors permit rapid installation of joist and rafters, eliminating the need for nailing them. Metal anchors are fastened with 1 1/4 inch nails.
Figure 3-7. Metal anchors
Types. Examples of most types of rafters are shown in Figure 3-8 . The four types of rafters used are common, hip, valley, and jack.
Figure 3-8. Roof framing terms
(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.
Rafter Layout. Rafters must be laid out and cut with the slope, length, and overhang exactly right so that they will fit when placed in the roof.
(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 ).
Figure 3-9. Steel carpenter's square
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.
Figure 3-10. 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.
Figure 3-11. 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.
When laying off rafters by any method, be sure to recheck the work carefully. When two rafters have been cut, it is best to put them in place to see if they fit. Minor adjustments may be made at this time without serious damage or waste of material.
(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.
Figure 3-12. Table Method 1
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.
Figure 3-13. Table Method 2
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 )
Figure 3-14. Pitch on the rafter table
Assembly. Rafters are usually made into trusses. Two rafters are connected at the top, using a collar tie well nailed into both rafters. Before any ties or chords are nailed, the rafters should be spread at the lower end to equal the width of the building. This is done by using a template or by measuring the distance between the seat cuts with a tape (see Figure 3-15 ).
Figure 3-15. Assembling a truss
(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 ).
Figure 3-16. Rafter support detail
Knee Brace. If no additional bracing is needed, the truss is set in place on the plates. If additional bracing is needed, a knee brace is nailed to the chord. The knee brace forms a 45° angle with the wall stud. For easier erection, the knee brace may be omitted until the rater truss is set in place (see Figure 3-16 ).
A truss is a framed or jointed structure composed of straight members connected only at their intersections in such a way that if loads are applied at these intersections, the stress in each member is in the direction of it length. Straight, sound timber should be used in trusses. Figure 3-17 shows various types of trusses used in construction. (The Howe and Fink trusses are most commonly used.) Trusses are used for large spans to give wide, unobstructed floor space for such large buildings as shops and hangars. Sometimes small buildings are trussed to save material. These small trusses act as rafters and give the roof rigidity.
Figure 3-17. Types of trusses
Placement. After the rafters have been assembled into trusses, they must be placed on the building (see Figure 3-18 ). Assemble the first set of rafters in the end section of the building or at the center. Raise rafter trusses into position by hand and nail them into place with 16d nails. (Temporary workbenches may be built for the workers to stand on while erecting trusses.) These trusses must be temporarily braced at the end section of the building until the sheathing is applied. Knee braces are not used on every rafter truss unless needed. Install trusses as follows:
Figure 3-18. Installing trusses
(1) Mark the proper positions of all truss assemblies on the top plate. The marks must show the exact position on the face of all rafters (such as south or north) (see Figure 3-18 , A).
(2) Rest one end of a truss assembly, peak down, on an appropriate mark on the top plate on one end of the structure (see Figure 3-18 , A).
(3) Rest the other end of the truss on the corresponding mark of the top plate on the other side of the structure (see Figure 3-18 , B).
(4) Rotate the assembly into position using a pole or rope (see Figure 3-18 , C).
(5) Line up and secure the rafter faces flush against the marks.
(6) Raise, align, and nail the three assemblies into position. Nail temporary 1 x 6 braces across these three assemblies. Repeat this procedure with the other assemblies as they are brought into position (see Figure 3-18 , D). Check the rafter spacing at the peaks as the braces are nailed on.
(7) Braces may be used as a platform when raising those trusses for which there is not enough room to permit rotation.
Web Members. The web members of a truss divide it into triangles. The members indicated by heavy lines normally carry tensile stresses for vertical loads. Sometimes the top chords of these trusses slope slightly in one or two directions for roof drainage, but this does not change the type of truss. The necessary number of subdivisions, or panels, depends on the length of the span and the type of construction.
Terms. These terms should be understood before proceeding further with this lesson.
(1) Bottom chord. A member that forms the lower boundary of the truss (see Figure 3-19 ).
Figure 3-19. Truss
(2) Top chord. A member which forms the upper boundary of the truss.
(3) Chord member. A member that forms part of either the top or the bottom chord.
(4) Member. The component that lies between any adjacent joints of a truss. It can be of one or more pieces of structural material.
(5) Web member. A member that lies between the top and bottom chords.
(6) Joint. Any point in a truss where two or more members meet; sometimes called a panel point
(7) Panel length. The distance between any two consecutive joint centers in either the top or bottom chords.
(8) Pitch. The ratio of the height of the truss to the span's length.
(9) Height of Truss. The vertical distance at midspan from the joint center at the ridge of a pitched truss or from the centerline of the top chord of a flat truss to the centerline of the bottom chord.
(10) Span length. The horizontal distance between the centers of the two joints located at the extreme ends of the truss.
Uses. Trusses are used for large spans to give wide, unobstructed floor space for such large building as shops and hangers. The Howe and Fink trusses are most commonly used (see Figure 3-20 ).
Figure 3-20. Howe and Fink trusses
Support. Trusses are supported by bearing walls, posts, or other trusses. To brace a truss to a wall or post, knee braces are used as shown in Figure 3-21 . These braces tend to make a truss of the entire building by tying the wall to the roof (see Figure 3-21 ).
Figure 3-21. Knee braces
Layout. Use the following steps to lay out a truss:
(1) Build the truss on workbenches that are paced on a level spot on the ground.
(2) Obtain the measurement of al material from the blueprints.
(3) Lay the pieces in their correct position t form a truss.
(4) Nail them together temporarily (see Figure 3-22 ).
Figure 3-22. Truss layout
(5) Lay out the location of all holes to be bored.
(6) Bore the holes to the size called for on the blueprint.
(7) Dismantle the truss and withdraw the nails after the holes have been bored.
Assembly. Assembling a truss after it has been cut and bored is simple. In most cases, timber connectors are used where different members of the truss join. Assemble the truss with the split rings in place. The bolts are then placed in the holes and tightened. Place washers at the head and nut ends of each bolt. Use straight, sound timber trusses (see Figure 3-23 ).
Figure 3-23. Split rings on a truss
Purlins are used in roof construction to support corrugated sheet metal if it is used or to support the sheathing of roofs famed with trusses. In small roofs, short purlins are inserted between the rafters and nailed through the rafters. In large buildings where heavy trusses are used, the purlins are continuos members that rest on the trusses and support the sheathing. In small buildings, such as barracks, mess halls, and small warehouses, 2 by 4s are used for purlins, with the narrow side up (see Figure 3-24 ).
Figure 3-24. Purlins
Bracing is used to stiffen framed construction and make it rigid. Bracing may be used to resist winds, storms, twists, or strains. Good bracing keeps corners square and plum Bracing prevents warping, sagging, and shifting that could otherwise distort the frame and cause cracked plaster and badly fitting doors and windows. In small roofs that cover 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 rafter, braces or other types of supports must be installed. The three methods commonly used to brace frame structures are let-in, cut-in, and diagonal-sheathing bracings.
Let-In Bracing. Let-in bracing is set into the edges of studs, flush with the surface. The studs are always cut to let in the braces; the braces are never cut. Use 1 by 4s or 1 by 6s set diagonally from top plates to sole plates, or between top or sole plates and framing studs.
Cut-In Bracing. Cut-in bracing is toenailed between studs. It usually consists of 2 x 4s cut at an angle to permit toenailing. They are inserted in diagonal progression between studs running up and down from corner posts to the sill or plates.
Diagonal-Sheathing Bracing. The strongest type of bracing is diagonal sheathing. Each board braces the wall. If plywood sheathing 5/8 inch thick or more is used, other methods of bracing may be omitted.
3-10. Roofing Terms
When framing a root carpenters must be familiar with commonly used roofing terms. The following are the most common of those terms:
Basic Triangle. The basic triangle is the most elementary tool used in roof framing (see Figure 3-25 ). When framing a roof, the basic right triangle is formed by the horizontal lines (or run), the rise (or altitude), and the length of the rafter (the hypotenuse). Any part of the triangle can be computed if the other two parts are known. Use the following equation:
Figure 3-25. Basic triangle
The square of the hypotenuse of a right triangle is equal to the sum of the squares of the two sides. In roofing terms:
Rafter length2 = run2 + rise2
Bird's Mouth. A bird's mouth is a cutout near the bottom of a rafter, that fits over the top plate. The cut that fits the top of the plate is called the seat; the cut for the side of the plate is called the heel (see Figure 3-26 ).
Figure 3-26. Bird's mouth
Cut of a Roof. The cut of a roof is the rise over the run (such as 4/12 roof) or the pitch of the roof (see Figure 3-27 ).
Figure 3-27. Roofing terms
Span of a Roof. The span of any roof is the shortest distance between the two opposite rafters' seats (see Figure 3-27 ).
Line Length. In roof framing, line length is the hypotenuse of a triangle whose base is the run and whose altitude is the total rise (see Figure 3-27 ).
Horizontal Line. A horizontal line is one level with the building foundation.
Overhang. The overhang is that part of a rafter that extends past the outside edge of the walls of a building. When laying out a rafter, this portion is in addition to the length of a rafter and is figured separately. The overhang is often referred to as the tailpiece.
Total Rise. The total rise is the vertical distance from the wall plate to the top of the ridge.
Run. Run always refers to the level distance any rafter covers--normally, one-half the span.
Unit of Run (or unit of measurement). The unit of measurement, 1 foot (or 12 inches), is the same for the roof as for any other part of the building. Using this common unit of measurement, the framing square is used in laying out large roofs.
Pitch. Pitch signifies the amount that a roof slants and the ratio of rise (in inches) to run (in inches). Using this method, 4, 6, or 8 inches of rise per foot of run would give a pitch of 4:12, 6:12, or 8:12. There are two methods of indicating pitch.
(1) Method 1. The pitch is indicated as a ratio of the rise to the span of a roof, stated in fractions (3/4, 5/8, 1/2, and 5/12). The units of span and rise must be the same (inches or feet), and the faction is reduced to its lowest common denominator (see Figure 3-11 ).
To obtain the unit rise, multiply the pitch by 24. For example, if the pitch is given as 1/3, multiply 1/3 by 24 (1/3 x 24 = 8). Therefore, the unit rise is 8 inches per foot (8-12 pitch).
If the pitch is given as 5/12, multiply 5/12 by 24 (5/12 x 24 = 10). Therefore, the unit rise is 10 inches per foot (10-12 pitch).
(2) Method 2. The pith is stated as the ratio of rise (in inches) per 1 foot of run (12 inches). Using this method, 4, 6, or 8 inches of rise per foot of run would give a pitch of 4-12, 6-12, or 8-12. A roof with 1/2 pitch can be said to have a 12-12 pitch. Remember 1/2 x 24 = 12.
Rise. The rise of a rafter is the vertical (or plumb) distance that a rafter extends upward from the plate.
Plumb Line. The line is the line formed by the cord on which the plumb bob is hung (see Figure 3-28 ).
Figure 3-28. Roof pitch
Plate. The plate is the wall-framing member that rests on the top of the wall studs (see Figure 3-8 ).
Ridge. The ridge is the highest horizontal roof member. It ties the rafters together at the upper end (see Figure 3-8 ).
3-11. Rafter Tables on a Framing Square
The framing square may have one or two types of rafter tables on the blade. One type gives both the line length of any pitch of rafter per foot of run and the line length of any hip or valley rafter per foot of run. The difference in the length of the jack rafter, spaced 16 or 24 inches (on center), is also shown in the table. Where the jack, hip, or valley rafter needs side cuts, the cut is given in the table. The other type of rafter table gives the actual length of a rafter for a given pitch and span.
Line length. The rafter table (see Figure 3-29 ) is used to determine the length of the common, valley, hip, and jack rafters, and the angles at which they must be cut to fit at the ridge and plate. To use the table, the carpenter must know what each figure represents.
Figure 3-29. Line lengths on the rafter table
(1) The row of figures in the first line represents the length of common rafters per foot of run (look at the left end of Figure 3-29 ), as the title at the left-hand end of the blade indicates.
(2) Each set of figures under each inch division mark represents the length of a rafter per foot of run, with a rise corresponding to the number of inches over the number. For example, under the 16-inch mark appears the number 20.00 inches. This number equals the length of a rafter with a run of 12 inches and a rise of 16 inches. Under the 13-inch mark appears the number 17.69 inches, which is the rafter length for a 12-inch run and a 13-inch rise.
The other five lines of figures in the table are seldom used in the theater of operations.
Actual Length. At the left end of the table (see Figure 3-30 ) are figures representing the run, rise, and the pitch of a roof.
Figure 3-30. Actual lengths on the rafter table
(1) The figures show that a rafter with a run of 12 and a rise of 4 has 1/6 pitch. A 12 run, 6-inch rise has 1/4 pitch. A 12 run, 8-inch rise has 1/3 pitch.
(2) To use the rafter table to determine the length of a rafter with a 1/6 pitch (or a rise of 1/6 the width of the building) and a run of 12 feet, find the 1/6 in the table, then follow the same line of figures to the right until directly beneath the figure 12. The numbers that appear beneath this figure are 12, 7, and 10, which show the rafter length required and which represent 12, 7, and 10 mean 12 feet, 7 inches, and 10/12 of an inch. Therefore, the length of the rafter required is 12 feet 7 10/12 inches long.
(3) Using rafter table method 2, assume you have a roof with a 1/2 pitch (or a rise of 1/2 the width of the building) and a run of 12 feet (see Figure 3-30 ). Find 1/2 pitch on the table. Follow the same line of figures to the right until directly beneath the figure 12. The numbers that appear beneath this figure are 16, 11, and 6, which represents 16 feet 11 6/12 inches. The length of the rafter required is 16 feet 11 6/12 inches long.
(4) When the run is in inches, the rafter table reads inches and twelfths instead of feet and inches. If the pitch is 1/2 and the run is 12 feet 4 inches, add the rafter length of a 12-foot run to that of a rafter length of 4-inch run (see Figure 3-30 ). For a run of 12 feet and 1/2 pitch, the length is 16 feet 11 6/12 inches. For a run of 12 feet and 1/2 pitch, the length is 5, 7, and 11. In this case, the 5 is inches, the 7 is twelfths, and the 11 is 11/12 of 1/12 (which is nearly 1/12 of an inch). Add the 1/12 to the 7 to make it 8, making a total of 5 8/12 inches. Add the two lengths together (16 feet 11 6/12 inches + 5 8/12 inches = 17 feet 5 1/12 inches)
(5) If the run of a building is over 23 feet, the table is used as follows: Using a run of 27 feet, with a 1/4 pitch (the framing square blade is 24 inches long), find the length for 23 feet, then find the length for 4 feet, and add the two. The run for 23 feet with a pitch of 1/4 is 25 feet 8 5/12 inches. For 4 feet, the run is 4 feet 5 8/12 inches. The total run for 27 feet is 30 feet 2 1/2 inches.
(6) The lengths that are given in the rafter table are line lengths. The overhang must be added.
(7) When the roof has an overhang, the rafter is usually cut square to save time. If the roof does not have an overhang, the rafter is cut plumb, but no notch is cut in the rafter seat.
(8) A level cut is made on the rafter long enough to extend across the plate and the wall sheathing. This type of rafter allows very little protection to the sidewalls.
3-12. Using Templates
Rafter framing without the use of ridge boards may be done rapidly by using a truss assembly jig or template. The template is laid out t form a pattern conforming to the exact exterior dimensions of the truss. Lay out a template as follows (see Figure 3-31 ).
Figure 3-31. Laying out a template
Lay Out. Lay out a template as shown in Figure 3-31 and as follows:
(1) Measure and mark a straight line on a selected surface. Have the exact length of the joists that will form the truss chord. This is baseline A (see Figure 3-31 ).
(2) From the center of the baseline and at right angles to it lay out the centerline (C) to form the leg of a right triangle, the base of which is at half the length of baseline A, and the hypotenuse of which (B) is the length of the rafter measured as indicated (see Figure 3-31 ).
(3) Nail 2- by 4- by 8-inch blocks flush with the ends of baseline A and centerline C as shown in Figure 3-31 . Mark the centerline on the center jig blocks.
Assembly. Assemble with a template as shown in Figure 3-31 and as follows:
(1) Start the assembly by setting a rafter in the jig with the plate cut fitted over the jig block at one end of the baseline. The peak is flush with the centerline on the peak jig block. Nail a holding block outside the rafter at point D as shown in Figure 3-31 .
(2) Lay one 2- by 4-inch joist or chord in place across the base blocks.
(3) Lay two 2- by 4-inch rafters in place over the joist.
(4) Center one end of 1- by 6-inch hanger under the rafter peak. Center the rafters against the peak block.
(5) Nail through the rafters into the hanger using six 8d nails.
(6) Line up one end of the chord.
(7) Nail through the rafter with 16d nails.
(8) Line up the other end of the chord.
(9) Nail as above.
(10) Center the bottom of the hangers on top of the chord and nail with 8d nails.
3-13. Roof Openings
Major roof openings are those that interrupt the normal run of rafters or other roof framing. Such openings may be for ventilation, chimneys, trap-door passage, skylight, or dormer windows. Roof openings are framed by headers and trimmers. Double headers are used at right angles to the rafters, which are set into the headers in the same way as joists in floor-opening construction. Trimmers are actually double rafter construction in roof openings. Nailing steps may be added if needed. Figure 3-32 shows roof-opening construction.
Figure 3-32. Roof opening