(a) These three columns of figures
show that a rafter with a run of 12 and a rise of 4 has
A collar beam or tie is a piece of stock (usually 1
one-sixth pitch, 12 and 6 has one-fourth pitch, and 12
by 4, 1 by 6, or 1 by 8) fastened to a pair of raters in a
and 12 has one-half pitch. To use this scale for a roof
horizontal position part way between the top plate
with one-sixth pitch (or a rise of one-sixth of the
and the roof ridge as shown in figure 2-18. These
width of the budding) and a run of 12 feet, find 1/6 in
beams keep the side walls of the building from
the table, follow the same line of figures to the right
spreading. The lower the collar beam or chord, the
until directly beneath the figure 12, which is the run
better it fulfills its purpose. This type of bracing is
of the rafter. Under the figure 12 appear the numbers
used in the roofs of small buildings where no ceiling
12, 7, 10, which is the rafter length required and
joists are used.
which represents 12 feet, 7 inch and 10/12 of an inch.
They are written as follows: 12 feet 7 10/12 inches.
For a pitch of one-half (or a rise of one-half the width
of the building) and a run of 12 feet, the rafter length
is 16, 11, 6 or 16 feet, 11 6/12 inches.
(b) If the run is greater than 23 feet, the
table in figure 2-17 is used as follows: Using a run of
27 feet, find the length for a run of 23 feet, then find
the length of 4 feet and add the two. The run for 23
feet with a pitch of one-fourth 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.
When part of the run is in inches, the rafter table
reads inches and twelfths instead of feet and inches.
For example, if the pitch is one-half and the run is 12
feet, 4 inches, add the rafter length of a 12-foot run to
that of a rafter length of a 4-inch run, as follows: For
a run of 12 feet and one-half pitch, the length is 16
Figure 2-18. Collar beam.
feet, 11 6/12 inches. For a run of 4 inches and one-
TRUSS TYPE RAFTERS
half pitch, the length is 5, 7, 11. In this case the 5 is
inches, the 7 is twelfths, and the 11 is 11/12 of 1/12,
a. General. Rafters will seldom be put up
which is nearly 1/12. Add it to the 7 to make it 8,
singly; they are usually assembled into trusses, as
making a total of 5 8/12 inches, then add the two
shown in figure 2-19. Two rafters are connected at
lengths together. This sum is 17 feet, 5 2/12 inches.
the top by using a rafter or collar tie well nailed into
The lengths that are given in the table are the line
both rafters. Before any ties or chords are nailed,
lengths; the overhang must be added. After the
accuracy of the rafter cuts should be checked by
length of the rafter has been found, the rafter is laid
spreading the lower ends of the rafters to correspond
out as explained in a above.
to the width of the building.
This may be
accomplished by a template, or by measuring the
(c) When the roof has an overhang, the
distance between the seat cuts with a tape.
rafter is usually cut square to save time. When the
roof has no overhang, the rafter cut is plumb, but no
b. TO type trusses. Trussed rafters for a
notch is cut in the rafter for a seat. The level cut is
20-foot-wide, TO type building are made from the
made long enough to extend across the plate and the
following members (fig. 2-19).
wall sheathing. This type of rafter saves material,
although little protection is given to the side wall.