Example:
Determine the length of a common rafter of a house with a 25' span
and a quarter pitch, without tail.
Fig. 49. Framing Table for Common Rafter
Solution:
Run = 12#'
Length per foot of run for quarter pitch = 13.42"
12.5 × 13.42" = 167.75" = 13.98'
(Looking for the nearest fractional value of .98 in the Table of Decimal
Equivalents in Appendix III, 63/64 or practically 1')
The rafter would be framed 14' in length.
When a tail is a part of the rafter, proceed in the manner described adding the run of the tail, or length of lookout, to the run of the rafter.
Fig. 50. Framing Square Detail
[Fig. 50] shows a framing square, containing among other data, the rafter lengths per foot of run. To use the data pertaining to common or jack rafter lengths, (1) consider the run as 12" taken on the tongue; (2) select upon the blade along its outer edge the inch mark which represents the rise of the roof per foot of run required to give the pitch specified; (3) the number directly below this mark, reading across the blade in the space marked "Length of Common Rafter Per Foot of Run" gives the length per foot for that particular rise or pitch.
As a check for rafter length computations, the following procedure is suggested: Selecting the run as 12" on the tongue and the rise in inches per foot of run on the blade, place one square upon another as shown in [Fig. 51], using that side of the square divided into inches and twelfths. Do not use the end of the blade, the rounded corner makes it impossible to secure the accuracy demanded. Extreme accuracy is required if the constant is to be used for rafters of considerable length of run. Read the diagonal length between the numbers representing the run and rise. Read the whole number of inches as feet, and the fractions as inches, and take off any fractional remainder upon a very sharp pointed pair of dividers. Read this divider spacing by means of the hundredths scale on the framing square. The result should, if the work is very accurately done, be the same as that obtained by computation from the tables, even to the hundredths place decimal. Upon ordinary work where great accuracy is not required carpenters sometimes determine this constant for a given pitch by placing the framing square as in [Fig. 46] or 47, taking upon the tongue the run and on the blade the rise, marking along both tongue and blade. The distance between these marks is then read on a square placed along the edge.
