Rafter Calculations for a Hip Roof: Hip Rafters and Jack rafters

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A few videos ago, we covered calculating basic rafters for a simple gable roof. That’s simple because simple gable roofs have rafters that are all the same. That’s what makes them simple. Hip roofs are more complicated.

Not only do we need the common rafter length, but we also need lengths for two other types of rafter: hip rafters, which connect the ridge to the corners, and jack rafters, which are modified common rafters connecting the hips to the wall plates.

The bottom of a jack rafter is like a common rafter, but the length and bevel directions vary.

Hips have a lower pitch than common rafters because they cover a longer distance the same height. For this reason, it is easier to walk up and down hips and valleys than straight up or down the roof.

You can determine the Hip pitch by looking at your speed square or calculating it on a construction calculator. There are two scales on a speed square: commons and hip/val, which means Hips and valleys. We’ll talk about valleys later, but you can see a 6/12 on the common scale is a little under 27 degrees.

A 6/12 on the hip/val scale is a little under 20 degrees. With different geometry, you need to make sure the roof does not come out looking humpy.

Misaligned hips are usually caused by high heels. Wait. The heel height of the hips must match that of the common rafters; otherwise, the hips will be humpy.

Let’s run through the calculations and see if we can rise to the challenge of perfect pitch.

Process for calculating common rafters
Into your construction calculator, punch these numbers and keys:

14 [feet]
11 [inches] 1/4 [run]
6 [inch] and then [pitch]
[diagonal] = 16’ 8-7/16” — common rafter length, excluding the tail, from the face of the ridge to the outside of the wall sheathing.
More steps to calculate hips and jacks
[Hip/Val] = hip and valley length again, excluding the tail: 22’ 4-⅞”
If you punch [hip/val] again, you get the plumb cut angle of 19.5 degrees. Brrr.
Punching it again gives you the level cut or the seat cut—a much more civilized temperature.
To find the Jack rafter lengths, we’re gonna start punching the Jack button.
Punch it once to set the rafter spacing. 16 inches on center is the default.

If you like that, keep punching Jack. If you don’t like it, pick another number.

Punch Jack again to get the difference in length of each jack rafter.

Punch it a third time to get the length of the first jack rafter, 15 feet 2 and a half inches.

You can keep punching it to get every jack rafter length until you get to the corner, and the length is zero. Remember to write down those numbers, as we’ll start laying out and cutting rafters in the next one.

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