Lay out the birds-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 birds-mouth.
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.
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
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.
Last, lay out and cut the plumb cut line and the seat
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
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.