d. Tail for jack. Same as for common rafter.
6. Note.—
All rafter lengths are measured down the middle of the top edges theoretically. On the square cornered roof, measurements of hip or valley rafters may be made from the long point without further reduction in length providing a 2" thick ridge is used. Jacks need no further reduction in length on a square cornered building provided the hips or valleys are of the same thickness as the jacks and the measurements are made from the long points of the jacks. Otherwise suitable reductions must be made for rafter and ridge thicknesses.
Directions for Using Table for Protractor.—Fig. 70-b.
Example.—Given to frame an octagonal silo with a span of 21' and a roof pitch of 11" to the foot.
1. 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[B] to this angle.
[B] Starrett's "Framing Tool" is strongly recommended for framing in degrees, Cf. Figs. [81] and [82].
2. 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' = 8%' (.69 = %, columns 3 and 4.)
3. 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) × 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 T-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. (2) End cut. Use same number of degrees as for plumb cut. |