In previous summations, we illustrated how to calculate hip and valley rafters for a basic hip roof. We also identified the leading cause of humpy hips: High Heels. Or heel height on hip rafters that do not match the heel height of the common rafters.
Determine the heel height (height above the plate)
To find the heel height, square up from the outside of the wall to the top of the rafter and measure the height.
Write that down somewhere because you’re going to need it in a minute or so.
But first, let’s introduce the King Rafter. In the middle of the hip wall, a single common rafter will butt into the ridge, defining the slope of the roof. This common is called the King rafter, even though it is just a commoner like the rest of us.
On either side of this King rafter will be Hip rafters extending to the corners of the building, and supporting jack rafters.
Laying out a hip rafter
OK, let’s lay out that hip rafter. Just like a ridge, hips and valleys are usually one size larger than common rafters because they support a load. For this little practice shed, we will step up from 2x10 to 2x12 for the hips.
- Mark two plumb-cut lines 1-1/2 inches apart at one end of the hip.
- Cut both lines with a 45-degree bevel to form a point or cheek cut. Measure from that long point along the top of the rafter, mark the overall length calculated in the previous video, excluding the tail, and draw that plumb cut.
This is the outside of the wall.
The seat cut is not determined by the wall plate width like on a common rafter; the seat cut location is dictated by the heel height, which you were supposed to have written down somewhere.
As stated earlier, the heels must be the same, or the roof will be humpy. Our heel height is 7-⅜ inches, so we measure down from the top of the rafter to find the seat cut, and we mark that.
However, that seat cut is not REALLY the seat cut. It is kind of a decoy for rookies because we need to take into account a thing called hip-drop.
Before cutting the seat, adjust for hip drop
Hip drop is the difference between thick lumber that supports a load and skinny lines on a piece of graph paper.
The center of the hip aligns perfectly with the center of the pocket it needs to sit in, but because the hip is wider than a line, the edges of the hip are higher than the sloping pocket.
Hip drop is the distance the rafter needs to be lowered for square edges to plane out with the common rafters where the cheek cut sits. Rather than beveling both the hip's top edges, most carpenters drop the whole rafter down to plane them out. This equates to changing where the seat cut lands.
For reasons that someone understands, you can figure out hip drop without doing any math.
If you draw a parallel line to the plumb cut uphill on the rafter by half the width of the stock or ¾ inch in this case. Measure down the heel height on this new line and mark the dropped seat cut. The line is actually higher, but the effect is dropping the hip. The hip drop is 5/16 inch.
Lay out the rafter tail
We are most of the way there. Treat the overhang like a mini hip rafter. If using a calculator, you’ll need to know the run and the pitch. The pitch is still 6/12, so that’s easy. The run will equal the overhang of the common rafters or 6.5 inches.
Enter:
- 6.5 + RUN
- 6 + PITCH
- HIP/VAL
The result is the amount you must add to the rafter length to plane out with the common rafter tails. To determine the soffit cut at the bottom of the rafter, measure down from the top plate to the common rafter soffit cut and match that.
Slide the square down the rafter slope from the PLUMB cut and mark it. The plumb cut line is the overall length, but both edges must be beveled so that the line represents the center of two lines 1-1/2 inches apart.
That’s a lot of math for one piece of wood, but we’re going to leave it there and pick up next time how to cut and install all of those jack rafters, including the King common rafter.
