Fig. 87. Plan of Uneven Pitches
Fig. 87. Plan of Uneven Pitches
The backing of hips on roofs of uneven pitches, while the amount to be removed on each side of the hip will vary, is determined by the principles developed inSec. 39. The amount to be removed from each side must be separately determined according to the angle the hip makes with the plate.
Lengths of hip and valley jacks are determined as inSec. 42. Lengths of cripple jacks will not be of uniform length, as in even pitched roofs. The runs for such jacks may be obtained with sufficient accuracy by measurements taken from an accurately made scale drawing.
The following example will make clear the method of attack where it is desired to develop a constant for hip or valley in terms of the common rafter of one of the pitches.
Example:Given: Main roof, Rise = 8', Run = 12' = ½pitch.Minor roof, Rise = 5', Run = 6' = 5/12 pitch.
(1) Find the run of valley rafter overc,Fig. 88-a, in terms of the run of the common rafter overb.
Solution:c²=a²+b²= (7½)² + 6² (a:12::5:8,whencea= 7½)c= 9.60'Expressing this run of valley in terms of 12" of run of common rafteroverb.9.60':6'::x:12".x= 19.20". (Check this value by scale drawing.)
Length of valley rafter, then, is found by taking 19.20" on the tongue with 10" (5/6 of 12") on the blade, advancing the setting as many times as there are feet in the run of the common rafter overb.
Fig. 88-a. Uneven Pitches
Fig. 88-a. Uneven Pitches
The numbers just given will give the plumb cut and the seat cut of this valley rafter.
(2) Find the side cut of the valley rafter when it rests against the ridge of the minor roof.
Solution:The side cut of the valley when allowed to rest in the plane of theplate = angleB,Fig. 88-a, whose tangent =b/a= 6/7½ (= 12/15 = .800 = 38° 40')
Therefore, take 7½" on the blade (always the run of intercepted common rafter of major roof), and 6" on the tongue (always the run of the common rafter of the minor roof); scribe on the blade.
For side cut of valley rafter when it is to be fitted to a ridge of the major roof, the lay-out when in the plane of the plate is obtained by means of these same numbers, but the scribing is done on the tongue. This lay-out in either case is for the rafter whenlying in the plane of the plate. Having secured this, proceed as inSec. 35, (2).
Side cuts of jacks for the minor roof are determined by the length of the ridge overa,Fig. 88-a, and the length of the rafter overb. But the length of the ridge overa= the run of the intercepted common rafter of the major roof, 7%' here. A general rule may be derived, therefore: For the side cut of a jack on a minor roof take on the blade of the square the run of the intercepted common rafter of the major roof (inches for feet), and on the tongue take the length of rafter of the minor roof; scribe on the tongue,Fig. 88-b. For the side cut of jacks on a major roof, take on the tongue of the square the run of the common rafter of the minor roof, and on the blade the length of the intercepted common rafter of the major roof; scribe on the blade,Fig. 88-c.
Fig. 88-b. Fig. 88-c.Side Cuts of Jacks for Uneven Pitches
Fig. 88-b. Fig. 88-c.Side Cuts of Jacks for Uneven Pitches
A new problem arises in connection with uneven or irregular pitches, the problem of making the projecting cornice member one with another. Manifestly, if one part of a roof is steeper than another and the plate the same height all around the building, the cornice cannot be made to meet in the same plane. This difficulty is overcome by raising the plate of the steeper roof an amount equal to the difference in the rises of the two pitches for a run equal to that of the projecting cornice. For example, in a half-pitch the rise would be 24" for a run of 24" in the cornice. In a quarter-pitch rise for a 24" run of cornice would be 12", a difference in risesof 12". The plate of the steeper roof must be raised that much higher than that for the lower pitch.
Fig. 89. Deck Frame
Fig. 89. Deck Frame
48. Decks; Chimney Openings.—Decks are sometimes used in roof framing to prevent some part of the roof from rising too high above the remainder. Such decks are framed of joist stock spiked together with butt joints. Upon the top of such deck joists are placed feathering strips so sawed as to give the deck a slight "fall" in each direction, two to three inches in eight to ten feet. Upon these feathering strips flooring is laid.
The deck is framed, raised and braced, being held up by studding placed under each corner. Next, the hip rafters are placed, then the flooring on the deck, and finally the common and jack rafters. Should common rafters be placed before the deck flooring, the joists must be braced to withstand the pressure against their sides. On large decks the outer deck frame is doubled.
Chimney openings are framed as shown inFig. 89.