The roof's main purpose is to keep out the rain, cold, or heat. It
must be strong enough to with stand
high winds; sloped to shed water; and, in areas of heavy snow, it must
be constructed more rigidly to bear the extra weight. This chapter will
familiarize carpenters with the most common types of roof onstruction
and materials. This chapter also covers reroofing.
Roofs for TOs are chosen to suit the building; the climate; the estimated
length of time the building will be used; and the material, time, and
skill required for construction. TO constraints dictate simple design
as shown in Figure 7-1.
When framing a roof (Figure 7-
2), carpenters must be familiar
with commonly used roofing
terms (Figures 7-3 and 7-4).
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.
Types of Rafters
Examples of most types of rafters are shown in Figure 7-3. The four
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.
Hip Rafters. These are roof members that extend diagonally
from the corner of the plate to the ridge.
Valley Rafters. These rafters extend from the plate
to the ridge along the lines where two roofs intersect.
Jack Rafters. These are a common rafter. The three
kinds of jack rafters 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.
Collar Tie and Beam
A collar tie or beam (Figure 7-7) is a piece of stock (usually 1 x 4,
1 x 6, l x 8, or 2 x 4) fastened in a horizontal position to a pair of
rafters between the plate and the ridge of the roof. This type of beam
keeps the building from spreading. Most codes and specifications require
them to be 5 feet apart or every third rafter, whichever 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.
In small roofs that cover only narrow buildings 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.
Rafters must be laid out and
cut with slope, length, and
overhang exactly right so
that they will fit when placed
in the roof.
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
Step 1. To determine the rafter
length, first find one-half of the
horizontal distance (total run)
of the rafter. The amount of
rise per foot will not be
considered yet. (For example, if
the building is 20 feet wide,
half of the span will be 10 feet.
See the example below.)
Step 2. After the length has
been determined, lay the
timber 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 away from
the person laying out the rafter.
Step 3. Hold the square with the tongue in your left hand, the blade
in your right, and the heel toward your body. Place the square as near
the upper end of the rafter as possible. (For example, in Figure 7-8,
page 7-6 (step 1) the figures 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.)
Step 4. Mark along the outside tongue edge of the square, which will
be the plumb cut at the ridge.
Step 5. Since the length of the rafter is known to be 12 feet and 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 (Figure 7-8, step 2).
Step 6. 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
timber (Figure 7-8, step 3). This
will be the tail cut of the rafter.
Often, the tail cut is made square
across the timber.
Step 7. The level cut or width of
the seat is the width of the plate
measured perpendicular to the
plumb cut, as shown in Figure 7-
8, step 4. Using a try square,
square the lines down on the
sides from all level and plumbcut
lines. Now the rafter is ready
to be cut (Figure 7-8, step 5).
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 1. 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.
Step 2. 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
rafters at the 4-inch mark
(Figure 7-9, step 1).
Step 3. 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 7-9, steps 2, 3, and 4.
NOTE: When laying off
rafters by any method, be
sure to recheck the work
careful1y. 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.
Table Method. 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 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 table gives the actual length of a rafter
for a given pitch and span.
Rafter Table, Type 1. Type 1 (Figure 7-10) appears on the face of the
blade. This type 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 first know what each figure represents.
· The row of figures in the first line represents the length
of common rafters per foot of run, as the title at the left-hand end of
the blade indicates.
· Each set of figures under each inch division mark represents
the length of 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.) See the Type 1 layout example below.
NOTE: The other five lines of figures
in the table will not be discussed, as they are seldom used in the TO.
The remaining procedure for laying out the rafters after the length
has been determined is as described previously.
Rafter Table, Type 2. Type 2 (Figure 7-11, page 7-8) appears on the
back of the blade of some squares. This shows the run, rise, and pitch
of rafters of the seven most common pitches of roof.
The figures are based on the horizontal measurement of the building
from the center to the outside.
The rafter table and the outside edge of the back of the square, both
the body and tongue, are in twelfths. (The inch marks may represent twelfths
of an inch or twelfths of a foot.) This table is used in connection with
the marks and figures on the outside edge of the square. At the left end
of the table are figures representing the run, the rise, and the pitch:
· In the first column, the figures are all 12 (12 inches or 12
feet). They represent the run of 12.
· The second column of figures represents various rises.
· The third column of figures (in fractions) represents the various
These three columns of figures show that a rafter with a run of 12 and
a rise of 4 has a 1/6 pitch, 12 and 6 has a 1/4 pitch, and 12 and 12 has
a 1/2 pitch. For example, use this scale for—
· A roof with a 1/6 pitch (or the rise of 1/6 the width of the
building) and a run of 12 feet. Find 1/6 in the table, then follow the
same line of figures to the right until directly beneath the figure 12.
Here appear the numbers 12, 7, 1O, which is the rafter length required
and which represents 12 feet 7 inches, and 10/12 of an inch. They are
written as follows: 12 feet 7 10/12 inches.
· A roof with a 1/2 pitch (or a rise of 1/2 the width of the building)
and a run of 12 feet. The rafter length is 16, 11, 6, or 16 feet 11 6/12
· A roof with a run of more than 23 feet. For example, if the run
is 27 feet, 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 1/12 inches.
NOTE: When the run is in inches, the rafter table
reads inches and twelfths instead of feet and inches.
See the Type 2 rafter table layout example at the top of the next page.
After the length of the rafter has been found, the rafter is laid out
as explained previously.
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 its length. Straight, sound timber should be used in trusses. The types
of trusses used in building construction are shown in Figure 7-12. (The
Howe and Fink trusses are most commonly used.) Truss terms are listed
in Figure 7-13, page 7-10.
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.
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 upon the length of the span
and the type of construction.
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
7-14. These braces tend to make a truss of the entire building by tying
the wall to the roof.
Purlins are used in roof construction to support corrugated sheet metal
if it is used, or to support the sheathing of roofs framed with trusses.
· In small roofs, purlins are inserted between the rafters and
nailed through the rafters.
· In large buildings where heavy trusses are used, the purlins
are continuous members that rest on the trusses and support the sheathing.
· In small buildings, such as barracks, mess halls, and small warehouses,
2 x 4s are used for purling, with the narrow side up.
To lay out a truss, use Figure 7-15 and the following steps:
Step 1. Get the material to a level spot of ground where work benches
will be almost level.
Step 2. Obtain from the blueprints the measurement of all pieces to be
used in the truss.
Step 3. Lay out the length on the different sizes of timber and cut
Step 4. After all lengths are cut, lay them in their correct position
to form a truss.
Step 5. Nail them together temporarily.
Step 6. Lay out the location of all holes to be bored. Recheck the measurements
Step 7. Bore holes to the size called for on the print. Use a brace
and bit or the woodborer that accompanies the air compressor. Bore holes
perpendicular to the face of the timber.
Step 8. After the holes have been bored, dismantle the truss and withdraw
Assembling a truss after it has been cut and bored is simple
(Figure 7-16). In most cases, timber connectors are used where different
members of the truss join. Straight, sound timber should be used in trusses.
· Assemble the truss with
the timber connectors in place.
· Place the bolts in the holes
and tighten them.
· Place washers at the head
and nut ends of each bolt.
Rafters are usually made into
trusses, as shown in Figure 7-
17. 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
A 1 x 6 or 2 x 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 x 6 is
nailed to the rafter joint and
extends to the chord at
midpoint, tying the rafter to
In wide buildings where the
joists or chords must be
spliced and there is no
support underneath, the
rafter and joists support one
another as shown in Figure 7-
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 rafter truss is set in
Rafter framing without the
use of ridgeboards may be
done rapidly by using a truss
assembly jig or template. The
template is laid out to form a
pattern conforming to the
exact exterior dimensions of the truss.
Layout. Lay out a template as shown in Figure 7-19
and as follows:
Step 1. Measure and mark a straight line on any selected surface. Mark
the exact length of the joists that will form the truss chord. This is
Step 2. From the center of the baseline and at right angles to it, lay
out a centerline (C) to form the leg of a right triangle, the base of
which is half the length of the baseline (A), and the hypotenuse of which
(B) is the length of the rafter measured as indicated.
Step 3. Nail 2 x 4 x 8 blocks flush with the ends of baseline A and
centerline C as shown in Figure 7-19. Mark the centerline on the center
Assembly. Assemble with a template as shown in Figure
7-19 and as follows:
Step 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.
Step 2. Lay one 2 x 4 joist or chord in place across the base blocks.
Step 3. Lay two 2 x 4 rafters in place over the joist.
Step 4. Center one end of a 1 x 6 hanger under the rafter peak. Center
the rafters against the peak block.
Step 5. Nail through the rafters into the hanger using six 8d nails.
Step 6. Line up one end of the chord.
Step 7. Nail through the rafter with 16d nails.
Step 8. Line up the other end of the chord.
Step 9. Nail as above.
Step 10. Center the bottom of the hangers on top of the chord and nail
with 8d nails.
Installation of Trusses
After the rafters are
assembled into trusses, they
must be placed on the
building (Figure 7-20, page 7-
14). Assemble the first set of
rafters either 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.
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:
Step 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).
Step 2. Rest one end of a truss assembly, peak down, on an appropriate
mark on the top plate on one side of the structure.
Step 3. Rest the other end of the truss on the corresponding mark of
the top plate on the other side of the structure.
Step 4. Rotate the assembly into position using a pole or rope.
Step 5. Line up and secure the rafter faces flush against the marks.
Step 6. Raise, align, and nail the three assemblies into position. Nail
temporary 1 x 6 braces across these three assemblies. Repeat with the
other assemblies as they are brought into position. Check the rafter spacing
at the peaks as the braces are nailed on.
Step 7. Braces may be used as a platform when raising those trusses
for which there is not enough room for rotation (Figure 7-21).