Fig. 46. Position in Laying off Plumb Cut when Laid off before Seat Cut
Figs. [45], [46] and [47] illustrate the proper position of the worker relative to his work. Such a position will seem awkward to the beginner but he should learn to visualize his work while in this position that the efficiency of framing may not be reduced thru the awkward position first likely to be assumed.
Fig. 47. Laying off Plumb Cut when Seat Cut is First Laid off
17. To Find the Length of a Common Rafter.—First Method: The theoretic length of a rafter is indicated by the center lines in Figs. [45-a] and [48]. In estimating the total length of stock for a rafter having a tail, the run of tail or length of lookout must be considered.
The pitches most commonly used are the half, third, and quarter. From an examination of [Fig. 43] it will be seen that the length of a common rafter is the hypotenuse of a right triangle whose legs are the rise and the run of the roof. The problem, then, of finding the length of a common rafter when the rise and run are known is merely that of solving the equation c² = a² + b².
Fig. 48. Rafter Length
Practical carpenters would not consider it economy to take time to solve for rafter lengths in this manner, for every variation in rise or run would necessitate a rather long solution. Instead, they have discovered that for every foot of run of a rafter the length of the rafter increases proportionately, the ratio of rise to run remaining the same, [Fig. 44.] With a table, therefore, in which the length of rafter for each foot of run, for each of the common pitches is given, the length of rafter for any given pitch can be found by merely multiplying the constant given by the amount of run for that particular rafter.
[Fig. 49] shows such a table worked out for a rather extended number of pitches. From this table it will be seen that the number to take as a constant for the run is 12", and that the rise in inches per foot of run is taken upon the other member of the framing square. A jack rafter as will be illustrated later is but a shortened common rafter, therefore, what is said of the common rafter is also true of the jack rafter. The jack, however, has an additional cut which will be discussed in another section.