In laying out rafters, it is very important to set off the length on the work line, as deviations from this rule will often lead to mistakes. The lines indicating the run and rise of a rafter are easily traced, but the work line for the length of a rafter is sometimes lost to sight, particularly in cutting jack rafters. The framer must never lose the work line in cutting a rafter; if he does, he is like a mariner at sea without a compass or a ship without a rudder. The work line is an important part in obtaining the lengths of rafters, as will be shown.
Fig. 105 - Diagram Showing Importance of Work Line in Laying out Rafters.
In roofs which have a projection of the rafter for the cornice, the back of the rafter rises above the level of the plate whatever thickness may be allowed on the rafter for the support of the cornice. Referring to Fig. 105, A B represents the run of a common rafter, B C the rise, and A C the length and work line. Projections for the cornice must be added from the corner of the plate at A. Now suppose we square up from the corner of the plate at A to D, the back of the rafter, and measure the length to E the same as on the line A C. Now if we make the plumb cut at E, as shown by the dotted line, we find our rafter too short, as is plainly shown in the diagram.
Thus it will be seen that the work line is an essential point in laying out rafters.
We will now trace the work line in a jack rafter from the plate to the top bevel, as this is the place many mechanics are at a loss as to the proper point to which to measure.
Fig. 106. - Diagram Showing Work Line in a Jack Rafter.
Referring to Fig. 106, we can easily trace the work line and the lines forming the cut of the jack rafter. The work line is represented by A C, the plumb line or down bevel by D B', and is always the same as the down bevel of the common rafter. To find the bevel across the back of the rafter draw another plumb line the thickness of the rafter from the cutting line and measured square from it, as C E. Square across the back of the rafter to F ; connect F with D, and the lines to which to cut are F D B'. The proper point to which to measure on the line A C is from A to the scratch mark half way between the two plumb lines, this being the center of the rafter in thickness. In actual practice this little point need not be considered, and for convenience in measuring the length may be taken from A to C. So slight a deviation in the true length of a jack rafter does not cut any figure in framing or ever appear noticeable, from the fact that jack rafters can be moved forward or backward a little on the plate and hip and if they are all framed by the same rule will be of uniform distance apart.
We are instructed by some to deduct half the thickness of the hip or valley rafter in setting off the length of jacks. This is a point which may be disregarded, especially when hip and valley rafters are only 2 inches thick. It is evident that if we lay out a jack rafter setting off the length on the side which has the long corner of the bevel, it will be a little more than half the thickness of the rafter short when the bevel is cut.
Therefore, if jacks are cut according to the work line in Fig. 106, they will be near enough for all practical purposes in the usual order of building and without making any deduction in length for the thickness of hip and valley rafters. When roofs have a ridge pole deduct half its thickness from the length of the common rafter. Aside from this, it is seldom necessary to make any reduction in the lengths of rafters, as shown on the work lines in the plans.