APPENDIX IV
(Short Cuts to Roof Framing)
Griffith's Framing Tables For the Square and Octagonal Roof
Directions for Using Table for the Steel Square.—Fig. 70-a.
Example.—Given a square hipped roof, that is, a roof with square corners, having a span of 25 ft. and a pitch of ½.
1.To Lay Out Miter Cut of Plate.—Take 12" on tongue of framing square and 12" on blade; scribe along blade. (Cf. small table,Fig. 70-a.)
2.Length of Ridge.—Length of long plate diminished by length of short plate, increased by thickness of ridge piece plus diagonal thickness of hip.
3.To Lay Out Common Rafter.—
a. Plumb cut. (Cf. column 5, beginning at left,Fig. 70-a.) Take 12" on tongue of square and 12" on blade (column 2), scribe along blade.b. Length of common rafter. (Cf. column 11) 12½ (run of common rafter) x 16.97" (length of common rafter per foot of run) = 17' 8.12" = 17' 8⅛ (.12" = ⅛", columns 3 and 4).c. Seat cut. (Cf. column 8) Take 12" on tongue and 12" on blade; scribe along tongue.d. Tail for common rafter.(1) Length determined as was that of common rafter, the horizontal projection or length of look- out becoming multiplier. 173(2) End cut use same numbers as for plumb cut but invert position of square.e. Reduction for ridge. Measure straight back from plumb cut a distance equal to 3/ the thickness of ridge piece.
a. Plumb cut. (Cf. column 5, beginning at left,Fig. 70-a.) Take 12" on tongue of square and 12" on blade (column 2), scribe along blade.
b. Length of common rafter. (Cf. column 11) 12½ (run of common rafter) x 16.97" (length of common rafter per foot of run) = 17' 8.12" = 17' 8⅛ (.12" = ⅛", columns 3 and 4).
c. Seat cut. (Cf. column 8) Take 12" on tongue and 12" on blade; scribe along tongue.
d. Tail for common rafter.
(1) Length determined as was that of common rafter, the horizontal projection or length of look- out becoming multiplier. 173
(2) End cut use same numbers as for plumb cut but invert position of square.
e. Reduction for ridge. Measure straight back from plumb cut a distance equal to 3/ the thickness of ridge piece.
4.To Lay Out Hip or Valley Rafter.--
a. Plumb cut. (Cf. column 6) Take 17" on the tongue and 12" on the blade; scribe along the blade.b. Side cut. (Cf. column 16) Take 17" on the tongue and 20.78" or 20-13/16" (columns 3 and 4) on the blade; scribe along the blade.This also gives the miter cut at the end of the tail of the hip for fascia, and also the miter cut at the bird's mouth joint of valley rafter, where it must be cut out to fit the angle of the plate.c. Length of hip or valley rafter. (Cf. column 12) 12½ (run of common rafter) x 20.78" (length of hip or valley rafter per foot of run of common rafter) = 21' 7.75" = 21' 7¾" (.75 = ¾, columns 3 and 4.)d. Seat cut. (Cf. column 9) Take 17" on tongue and 12" on the blade, scribe along the tongue.e. Tail for hip or valley rafter.(1) Length determined as was that of hip or valley rafter, the horizontal projection of tail or length of the lookout of common rafter becoming the multiplier.(2) End cut use same numbers as for plumb cut of hip or valley but invert square.f. Reduction for ridge. Measure straight back from plumb cut ½ diagonal thickness of ridge.g. Backing of hip. (Cf. column 21) Take 12" (rise of common rafter) on the tongue and 20-13/16" on the blade scribe along the tongue for angle of backing.h. Drop of hip when no backing is given. (Cf. column 23) Reduce at seat cut 9/16", measured at right angles to seat cut.
a. Plumb cut. (Cf. column 6) Take 17" on the tongue and 12" on the blade; scribe along the blade.
b. Side cut. (Cf. column 16) Take 17" on the tongue and 20.78" or 20-13/16" (columns 3 and 4) on the blade; scribe along the blade.
This also gives the miter cut at the end of the tail of the hip for fascia, and also the miter cut at the bird's mouth joint of valley rafter, where it must be cut out to fit the angle of the plate.
c. Length of hip or valley rafter. (Cf. column 12) 12½ (run of common rafter) x 20.78" (length of hip or valley rafter per foot of run of common rafter) = 21' 7.75" = 21' 7¾" (.75 = ¾, columns 3 and 4.)
d. Seat cut. (Cf. column 9) Take 17" on tongue and 12" on the blade, scribe along the tongue.
e. Tail for hip or valley rafter.
(1) Length determined as was that of hip or valley rafter, the horizontal projection of tail or length of the lookout of common rafter becoming the multiplier.
(2) End cut use same numbers as for plumb cut of hip or valley but invert square.
f. Reduction for ridge. Measure straight back from plumb cut ½ diagonal thickness of ridge.
g. Backing of hip. (Cf. column 21) Take 12" (rise of common rafter) on the tongue and 20-13/16" on the blade scribe along the tongue for angle of backing.
h. Drop of hip when no backing is given. (Cf. column 23) Reduce at seat cut 9/16", measured at right angles to seat cut.
5.To Lay Out Jacks.—
a. Plumb cut. Same as that for the common rafter!b. Side cut. (Cf. column 14) Take 12" on the tongue and 16.97" or 17" on blade (.97" = 1", columns 3 and 4), scribe on the blade.Scribing along the tongue will give the face cut of roof boards, also of plancher.c. Length. Jacks set 24" centers, beginning with shortest jack =2' 10" (column 19). Second shortest jack = length of shortest increased 2' 10", etc.Beginning with longest jack = length of common rafter diminished by 2' 10", etc.d. Tail for jack. Same as for common rafter.
a. Plumb cut. Same as that for the common rafter!
b. Side cut. (Cf. column 14) Take 12" on the tongue and 16.97" or 17" on blade (.97" = 1", columns 3 and 4), scribe on the blade.
Scribing along the tongue will give the face cut of roof boards, also of plancher.
c. Length. Jacks set 24" centers, beginning with shortest jack =2' 10" (column 19). Second shortest jack = length of shortest increased 2' 10", etc.
Beginning with longest jack = length of common rafter diminished by 2' 10", etc.
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.
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 tableFig. 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 indegrees, Cf. Figs.81and82.
[B]Starrett's "Framing Tool" is strongly recommended for framing indegrees, Cf. Figs.81and82.
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.—
4.To Lay Out Hip or Valley Rafter.—
5. To Lay Out Jacks.—
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