This discussion will explain how much to drop a hip rafter. Let me explain….. a hip rafter rises from the corner of a building at a 45 degree angle. Because of this, if the carpenters used the same HAP (height above plate) on that rafter as he did the commons then the corners of the hip rafter would rise up above the plane of the roof causing a hump. To stop this the carpenters can either drop the rafter by the proper amount (which is why they call it “dropping the hip”) or chamfer the edges to match the incline of the hip. Study the diagram below.
Dropping the Hip Using Math
Take the rise of the main roof and multiply it by 1/2 the thickness of the hip. Then divide the answer by 16.97 (framers use 17). Then multiply that answer by 16 to get sixteenths of an inch.
For example main roof is an 8 in 12,
1/2 rafter thickness is 3/4″. That’s .75″ inch in decimal equivalent.
8 x .75 = 6;
6 divided by 16.97 =.3535;
.3535 x 16 = 5.65 or 5/16″.
Now simply measure up from the seat line of the birds mouth 5/16″ for the correct drop.
You may wonder why we use 16.97. This is because 16.97 is the diagonal of a 12″ square and 12″ is the run of the common rafter. Thus 16.97 (use 17) is the run of the hip.
Dropping the Hip Using a Framing Square
There is a way to do this without all that math. (in case you left your calculator at home)
The amount of drop is determined by laying a level line anywhere on a hip rafter and measuring 1/2 the thickness of the rafter stock on the level line and then measuring the plumb distance at that point. The seat line is raised this amount so the rafter drops accordingly. Remember on a hip rafter we use 17 to mark the level cut. For example 8/12 on the common rafter but 8/17 on the hip and valleys. Marking the 8 will give you a plumb line; 17 a level line.
Chamfering the Hip Rafter
Occasionally a hip rafter cannot be dropped (because it is very thick) or part of it needs to be chamfered. In this case measure the distance of the calculated drop at the top edge of the rafter along a plumb line of course. Set your saw to the incline of the hip and rip the beveled chamfer. The incline of the hip is calculated as follows.
rise divided by 16.97 = TAN;
Inverse function of tan on the calculator gives us the incline of the roof.
For example 8/16.97 = .471420;
press inv tan .471420 and the answer is 25.2401 degrees.
Set your saw to 25 degrees and rip the chamfer.
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