Johnson Level & Tool Mfg. RAS-1 - Manual (Page 8)

6
RAFTER MEASUREMENTS
The use of our square for rafter layout is based on two simple and
common building measurements: (1) the rafter run, and (2) the
rafter rise. These are available from either the building blueprints,
drawings, or actual measurements. The tables included in the back
of this book are also based on these two simple measurements
(see Figures 2 and 3, and also the tables starting on page 40).
RAFTER RUN: Run is the horizontal or level distance the rafter
will span. It is measured in feet.
RAFTER RISE: Rise is the vertical distance of a rafter between
its highest and lowest points. It is measured in feet.
INCH RISE: The rise measured in inches per foot run. It is also
called “Inch Rise Per Foot Run.” It can be calculated with the
following formula:
INCH RISE RAFTER RISE (ft) x 12
OR =
(SCALE NUMBER) RAFTER RUN (ft)
The Inch Rise gives you the corresponding scale number to use
on the square (for the common & hip-val scales). It is also the “one
number by which you can frame the roof.”
The inch rise, the rafter run and the tables in the back of the book
are all that are needed to lay out rafter lengths and the cuts for
common, hip, valley and jack rafters. The instructions outlined
below show the step-by-step procedure to follow for basic roof
construction.
STEP 1. OBTAIN RAFTER RUN: Measure or calculate the
horizontal distance the rafter will span, starting at the outside of
the wall on which it will rest. Include any boarding on the wall if it
extends to the wall top plate (see Figure 3).
When measuring building width to obtain rafter run, use a steel tape
if possible so that you can measure the full width of the building.
Measure from outside to outside of the wall or the top plate on
which the rafter will rest. If boarding extends up to the top plate,
measurement is to be taken from outside of the boarding.