Fig. 91. - Conical Tower Roof with Rafters Concave in Form.
Fig. 92. - A Convex Mansard Roof.
Fig. 93. - An Ogee Veranda Roof.
Having presented to the reader a practical system for almost every conceivable form of straight work in roof framing, the next step will be to show an easy system of framing curved, or molded, roofs, as they are sometimes called. Curved roofs usually take the form of concave, convex or ogee. An ogee is a form having a double curve, and is both concave and convex. Fig. 91 shows a conical tower roof, the rafters being of the concave form. Fig. 92 shows a convex mansard roof. Fig. 93 shows an ogee veranda roof. These are the principal forms, of curved or molded rafters, though they are variously combined and applied. The lengths, bevels and shapes are, however, developed in much the same manner, and when once it is understood how to develop the shape in one form any shape desired can be readily worked by the same method. The plan Fig. 94, represents the corner portion of a roof with ogee rafters. The lines A B and B C represent the wall plates and D E and D F the deck plates. A D is the run of common rafter, D E the rise, and A E the length of common rafter on the working line. This line governs the pitch of roof and the bevels. E is the down bevel at the top and A the bottom bevel. Connect B D for the run of the hip, square up the rise, D G, and connect B G for the length and working line of hip rafter. G is the down bevel at the top and B the bottom bevel. To lay out the curved rafter, referring now to Fig 95, set off the run A D, the rise D E, the length and work line A E. Draw the desired curves, as shown. H I indicates the bottom edge of the rafter, and J H shows the width of lumber necessary for making the curved rafter. To economize in the width of lumber, the convex portion above the work line may be worked out separately and nailed on. As a guide in laying out the corresponding curves in the hip rafter divide the length of the common rafter on the work line into any number of equal spaces, as 1, 2, 3, etc. From these points on the work line plumb up or down, as the case may be, to the curve line of the rafter.
Now we are ready to develop the shape of the hip.
Fig. 94.- Plan of Corner of a Roof with Ogee Rafters.
Referring to Fig. 96, set off the run B D, the rise D G, and connect B G for the length and work line of the hip. Divide the work line of the hip into the same number of equal spaces as numbered on the work line of the common rafter 1, 2, 3, etc, and plumb up or down, as the case may be, the same distances as shown on the common rafter. Then a line traced from B through these points to G will be the profile of the hip rafter. Fig. 97 represents the corner portion of a roof having two pitches. In this the angle and run of the hip are changed, without changing the method of finding the profiles of the rafters. Take the run, rise and length of common rafter on one side of the hip, and draw the desired shape. Then find the profile of the common rafter on the opposite side of the hip by dividing the work line into the same number of spaces and proceeding as before. The run of the hip being changed, we obtain a different length for the work line. When this is divided into the same number of equal spaces as were the common rafters, and the curved lines traced through the points, we obtain the shape of hip which will correspond to the profiles of the common rafters from either side. In roofs of two pitches it is evident that there must be two sets and two bevels of common and jack rafters. Now in curved roofs the lengths and bevels may be found by following the work lines of the common rafters, which maybe drawn straight, as has been shown in Fig. 95.
Fig. 95. - Laying out a Curved Rafter.
The lengths and bevels of the jacks for the different pitches may be found as shown in Figs. 62, 63 or 64. Again, it is evident that a jack rafter must be the same shape as the common rafter on the same side of roof from the bottom, or plate, up to the point where it joins the hip. Hence its length may be found in the following manner by measuring on the work line of the common rafter.
Fig-. 96. - Developing the Shape of the Hips.
Referring now to Fig. 98, A D is the run of the common rafter, D E the rise and A E the length and work line. To find the length of jack, set off the run of jack A B and square up the rise B C to the work line of the common rafter; then A C is the length of jack on the work line. This method is very simple, yet as it is a new and novel way of finding the length of jack rafters it will be well to point out a common mistake which the inexperienced might chance to make. Bear in mind that A E is the length of common rafter. B C is not the length of jack, as some might suppose, but the rise of jack ; A C is the length of jack. The down bevel is the same as that of the common rafter. To find the bevel across the back, set off from D the length of common rafter to F, and connect F with A, which shows the work line of the hip. Now continue the line B C to the work line of the hip, and the bevel at G will be the bevel across the top of jack. B G is also the length of jack, and will be found to be the same as A C.
Fig. 97. - Plan of Corner Portion of a Roof having Two Pitches.
When the bevel of the jacks is known all that is necessary is to square up the rise of each jack from the base line of common rafter A D to the work line A E and take the length from A to the point where the rise of each jack joins the work line of common rafter, as shown. Many lines and much time may be saved in finding the bevels of jack rafters on roofs of different pitches by using the plan shown in Fig. 60, which is the simplest and easiest of all to remember and is applicable to roofs of any pitch.