Fig. 70-b. Example. - Given to frame an octagonal silo with a span of 21' and a roof pitch of 11" to the foot.
To Lay Out Miter Cut Of Plate. (Cf. small table Fig. 70-b.) Eight sided polygon, plate miter = 67½ degrees. Set a T-bevel by means of a protractor* to this angle.
To Find Length Of A Side For Plate. (Cf. small table, Fig. 70-b.) 21' (2 x run) x .414 (cotangent of octagon) = 8.69' = 8¾' (.69 = ¾, columns 3 and 4.)
To Lay Out Common Rafter. a. Plumb cut. - (Cf. column 5) =47 deg. 30 min. Set T-bevel by means of protractor to same.
b. Length - (Cf. column 11). Take 10½ (run) x 16.28" (length of common rafter per foot of run) = 14' 2.94" = 14' 3". (.94= 1, columns 3 and 4.) c. Seat cut. - (Cf. column 8) 42 deg. 30 min. Set TV bevel by means of protractor.
d. Tail. (1) Length - Determined as is length of common rafter (Cf. column 11), horizontal projection of tail or length of lookout becoming the multiplier.
Use same number of degrees as for plumb cut.
To Lay Out Hip Or Valley Rafter. a. Plumb cut. - (Cf. column 7) = 49 deg. 50 min.
b. Side cut - (Cf. column 17) = 17 deg. 40 min. This also gives miter cut at end of hip tail for fascia, and also the miter cut of the bird's mouth joint of valley rafter, where it must be cut out to fit the angle of the plate.
*Starrett's "Framing Tool" is strongly recommended for framing in degrees, Cf. Figs. 81 and 82.
C. Length of hip or valley rafter. - (Cf. column 13) 10½ (run of common rafter) x 17.03" (length of octagon hip or valley per foot of run of common rafter) = 14' 10.82"= 14' 10⅞" (.82=⅞, columns 3 and 4).
d. Seat cut. - (Cf. column 10) = 40 deg. 10 min. Use protractor and T-bevel.
e. Tail for hip or valley rafter. -
Determined as for hip or valley, horizontal projection of tail or length of lookout of common rafter becoming multiplier.
Use same number of degrees as for plumb cut of hip or valley.
f. Backing of hip. - (Cf. column 22) = 74 deg. 50 min.
g. Drop of hip when no backing is used. (Cf. column 24)
a. Plumb cut. - Same as for common rafter.
b. Side cut. - (Cf. column 15) = 17 deg.
c. Length. - (Cf. column 20) = 12/5" that for square roof of same pitch. When set 16" centers =12/5x21¾" = 52¼", common difference. Proceed accordingly.
d. Tail for jack. - Same as for common rafter.