Roof Framing 101
Cut the Pattern
Make a duplicate rafter from the pattern. Then lay the rafters out on a smooth, flat surface, with a 2-by between them at the ridge line. Measure to make sure the rise, run and span are correct. You may wish to test these on the building before cutting the rest of the rafters. Once you’re sure these two pattern rafters are correctly cut, mark them as patterns and mark and cut the necessary number of rafters. If the building has hanging or “fly” rafters for the gable ends, cut them as well. They do not, however, have the bird’s mouth cut.
Make sure you carefully follow the pattern rafter. A number of years ago I was constructing a two-story building. One carpenter laid out and began to cut the rafters. He became ill from the extreme heat of the day and another carpenter took over for the last third of the rafters. When the roof framing was completed and decking installed, there was a built-in sag. I don’t know if the second carpenter didn’t use the pattern rafter, or simply wasn’t as precise, but it was a costly mistake.
Pivoting Square Method
The new C.H. Hanson Pivot Square makes the chore of laying out a roof quite simple. I wish I had this tool a number of years and buildings ago. This quality tool is basically a small “adjustable” square. It comes with its own heavy-duty belt holder that is also designed to hold a carpenter’s pencil and the instruction booklet.
The Pivot Square has three edges corresponding to the three sides of a triangle. Degrees and rise are marked on a blade attached to the pivoting arm. With the common rise figures facing you, and the raised fence on the right, the bottom represents the base of the triangle (the run) and the right side the altitude (the rise). The long adjustable edge represents the hypotenuse of the triangle, or the line length.
The Pivot Square can be set to any whole number pitch from 1/12 to 12/12. Simply adjust the square to the desired pitch and lock in place with the knurled knob. You can then use the square to transfer the angle for the cut to the lumber. Or you can hold the square in place and use it as a sturdy guide for running a portable circular saw. One great feature is that the pitch is also marked on the tool in degrees of angle. Determine the pitch, then you can set a miter saw or compound miter saw to make cuts in degrees that conform to the desired pitch. The Pivot Square can also be used to lay out pitches steeper than 12/12, as well as to lay out hip-valley rafters. These figures are determined on the back side of the square. This tool takes a lot of the figuring and guessing out of creating hips and valleys. And, the tool is even more versatile and can be used for other cuts as well. A series of numbers run along the center of the graduated arc of the adjustable blade on both sides of the tool. Each of these numbers represents the complement of the corresponding angle along the edge. The sum of these two figures always equal 90 degrees. For instance, when you set the square for 20 degrees, the complementary angle of 70 is directly aligned. You can make the 20-degree cut following the set angle, flip the tool over and make the complementary 70-degree cut to create a perfect 90 degrees. The square also comes with three leveling vials which make it easy to measure the pitch of existing structures. To measure pitch, first position the hypotenuse of the square on the roof, running straight up the pitch. Make sure it’s not off angle because of the shingle edges. You may use a 4-foot level to set it on for more accurate measurements. The outer end, or number 9 on the hypotenuse or adjustable blade, should be at the top or upper side of the pitch. Loosen the lock and adjust the square until the level vial on the base reads level, then lock the setting. You can now read the pitch and degrees on the “Deg. Common Rise” scale of the square.
To lay out a rafter with thePivot Square, first determine the pitch and set the square to the correct pitch on the Deg. Common Rise scale. Position the square with the base or raised edge on top of the rafter board. Mark the plumb line for the ridge line cut near the end of the rafter board. Measure the length of line on the rafter (less 1/2 the ridge board), and make another plumb line mark parallel to the first. Determine the rafter overhang, move the square to that position and make another plumb cut mark. To create the bird’s mouth cut, position the square at the bottom of the middle plumb line with the altitude side against the plumb cut and mark across the board using the square base, and beginning at the bottom of the plumb cut. Once this level line is established, measure and mark the wall thickness, plus about 1/4-inch for any slight imperfections along the level line. Then use the square to draw a short plumb line parallel to the others, down from this mark to the bottom edge of the board. This completes the bird’s mouth layout.