The first step is to prepare the triangles from which the lengths of the common and jack bars are obtained, also the lengths of the hip bars. After the drawings and patterns have been laid out full size according to the principles explained in Fig. 178, take a tracing of the triangle in the half section D C 4' and place it as shown by A 12 0, in Fig. 184. Divide 0 12, which will be 12 inches in full size, into quarter, half-inches, and inches, the same as on a 2-foot rule, as shown by the figures 0 to 12. From these divisions erect lines until they intersect the pitch A 0 which completes the triangle for obtaining the true lengths of jack and common bars for any size skylight. In similar manner take tracing of N R 4" in the diagonal elevation in Fig. 178 and place it as shown by B 12 0 in Fig. 183. The length 12 0 then becomes the base of the triangle for the hip bar in a skylight whose base of the triangle for the common and jack bars measures 12 inches as shown in Fig. 184, the heights A 12 in Fig. 184 and B 12 in Fig. 185 being equal. Now divide 12O in 12 equal spaces which will represent inches when obtaining the measurements for the hip bar. Divide each of the parts into quarter-inches as shown. From these devisions erect lines intersecting the hypothenuse or pitch line B 0 as shown.
To explain how these triangles are used in practice, Figs. 186 and 187 have been prepared, showing respectively a skylight without and with a ventilator whose curb measures 4 ft. x 8 ft. Three rules are used in connection with the triangles in Figs. 184 and 185, the comprehension of which will make clear all that follows.
Rule 1. To obtain the length of the ridge bar in a skylight without a ventilator, as in Fig. 186, deduct the short side of the frame or curb from the long side.
Rule 2. To find the length of the ventilator in a skylight deduct the short side of the frame from the long side and add the width of the desired ventilator (in this case 4 inches, as shown in Fig. 187).
Example: In Figure 187 take 8 feet (long side of frame) — 4 feet (short side of frame) = 4 feet. 4 feet + 4 inches (width of inside ventilator) = 4 feet 4 inches, (length of inside ventilator a' b'). To find the size of the outside ventilator h 1 and hood m p in Fig. 178 simply add twice the distance a b and a c respectively to the above size, 4 inches, and 4 feet 4 inches, which will give the widths and lengths of the outside vent and hood.
Rule 3. To find the lengths of either common or hip bar (in any size skylight) deduct the width of the ventilator, if any, from the length of the shortest side of frame and divide the remainder by two. Apply the length thus obtained on the base line of its respective triangle for common or hip bars and determine the true lengths of the desired bars, from the hypothenuse.
Example: As no ventilator is shown in Fig. 186, there will be nothing to deduct for it, and the operation is as follows: 4 feet (short est side of frame) _ 2 = 2 feet. We have now the length with which to proceed to the triangle for common and hip bars. Thus the length of the common bar c d will be equal to twice the amount of A 0 in Fig. 184, while the length of the hip bar b e in Fig. 186, will be equal to twice the amount of B 0 in Fig. 185. Referring to Figs. 186 and 187 the jack bars i j are spaced 16 inches, therefore, the length of the jack bar for 12 inches will equal A 0 in Fig. 184, and 4 inches equal to 4° 0; both of which are added together for the full length.
The lengths of the common and hip bars will be shorter in Fig. 187 because a ventilator has been used, while in Fig. 186 a ridge bar was employed. To obtain the lengths of the common and hip bars in Fig. 187 use Rule 3: 48 inches (length of short side)-4 inches (width of inside ventilator) = 44 inches; and 44 inches _ 2 — 22 inches or 1 foot 10 inches. Then the length of the common bar c' d' measured with a rule will be equal to A 0 in Fig. 184 and 10° 0 added together, and the length of the hip bar e' f in Fig.187 will be equal to B 0 in Fig. 185 and 10= 0 added together. Use the same method where fraction al parts of an inch occur. In laying out the patterns according to these measurements use the cuts shown in Figs. 178, 179, 180, and 181, being careful to measure from the arrowpoints shown on each pattern.
It will be noticed in Fig. 178 we always meas ure on line 4 in the patterns for the hip, common, and jack bars. This is done because the line 4 in the profiles E and E' come directly on the slant line of the triangles which were traced to Figs. 184 and 185 and from which the true lengths were obtained.
Where a curb might be used, as shown in Fig. 188, which would bring the bottom line of the bar 1i inches toward the inside of the frame b, all around, then instead of using the size of 4 x 8 feet as the basis of measurements deduct 3 inches on each side, making the basis of measurements 3 ft. 9 inches x 7 ft. 9 inches, and proceed as explained above.