The Easiest Way to Calculate Irregular Hip Rafters
There are several ways to calculate hip rafters and jack rafter lengths. I’ll cover the most efficient method i’ve found, which uses the RedX Roof App to automatically calculate every measurement.
Measurements needed
The measurements we will need to to enter into the app are as follows, We will go into details on all of these later on.
- Roof pitch 1
- Roof Pitch 2
- Facia overhang
- The Bottom Run of the common rafter
- Jack rafter spacing
These 4 values are all we need to generate a 2D plan view of our corner hip That will look like the image below.
Customization:
There are many things we can customize like
- Hip rafter thickness
- Hip rafter depth
- Jack rafter depth
-Facia depth
- How deep the birds mouth cut will be
I’ll go over what all of those later on in the post, I just want to show you all of the other measurements the app will generate before we get into many details.
Measurements The App Will Get
The app will generate just about every measurement you can think of and it will also help you visualize all of the measurements by generating an image of every rafter. Let’s start with the hip rafter.
If you are cutting bevel angles on your hip rafter we will get these angles later from the jack rafter bevel angles as they will be the same. The app also show you the backing angles if you wanted to make it really fancy and bevel the top of your hip rafter.
Jack Rafters 6/12 Side
Similarly to the hip rafter the app will also generate a zoomable image and all of the rafter details
- A list of every Jack Rafter Length
- The length of the common rafter (Dark Blue on the plan view image at the beginning of the post)
- The bevel angles if you wanted to cut the bevels on your jack rafters, this will also be one of the bevel angles on your hip rafter
Jack Rafters 9/12 Side
Just like the other rafters the app will generate an image with all of the rafter details.
The only difference with this rafter that the birds mouth has been replaced with a Rafter Support Block. This is because the app assumes both of your walls the 6/12 side and the 9/12 side have the same height, and for the wall to have the same height and same overhang with the selected roof pitch your rafter will need a support block underneath the rafter. You can think at this support block as a truss heel of some sort.
Want a Birds Mouth Instead
If you wanted to raise your wall on your 9/12 side instead to keep a birds mouth you could do that as well. You can find the height difference of your two walls by comparing the height above plate from the 6/12 side to the height above plate from the 9/12 side. For example on the 9/12 side the height above plate is 7 & 1/8" and on the 6/12 side the height above plate is 3 & 3/4". So the difference in wall height would be 7 & 1/8" minus 3 & 3/4" which is 3 & 3/8". You would raise your 9/12 side wall 3 & 3/8"
Lets Go Over The Details
Now that you’ve seen what the app can do let me show you all of the details. First thing that you should know is that you can zoom into any of the rafters to get a closer look at all of the measurements. When you zoom in all the way, to the overhang section of any rafter you will see that there are more measurements to be found.
The Measurements You Enter Into The App
- Roof Pitch 1 or Rise for this first input you can enter either the roof pitch of the first side if you know it, or the rise from the top of the wall to the top of the ridge or in other words to the top of where you’r common rafter will finish.
- Run 1 This is the the distance from the outside of the wall to where your common rafter will meet the top ridge, this is the run for the first side.
- Roof Pitch 2 Or Run 2 Here you can enter either the roof pitch of your second side if you know it, if you don’t then you can enter the run of this second side (the distance from the outside of the wall to where your common rafter for side number to will meet the ridge).
- Overhang Here you enter the distance from the outside of the wall to the the inside of your facia.
- Facia Drop This is the distance from the top of your facia to the top of your wall. This number can be a positive or negative number. This number will determine the depth of your birds mouth or Height Above Plate for every rafter.
- Rafter Spacing This is the distance from the centre of one rafter to the centre of another rafter.
- Jack Rafter Depth This is the depth of the common rafter and all of the jack rafters. For example if you where using a 2x6 the rafter depth would be 5 & 1/2 inches.
- Hip Rafter Thickness This is the thickness of the hip rafter. For example if you where using a 2x6 the rafter depth would be 1 & 1/2 inches.
- Hip Rafter Depth This is the depth of the hip rafter.
- Facia Depth This is the depth of the the facia.
As of right now these are all of the input measurements and customization that you can do. There might be some changes in the future.
Measurements
Here is a bit of explanation on what some of the measurements mean.
- Total Rafter Length This is the total length of the hip rafter. If you have a look at the topdown plan view you will see that the hip rafter total length is to a square end with out any bevels. So if you are cutting the bevels on your hip rafter keep in mind that the measurements are to the shot side of the bevel.
- Hip Drop This is the vertical distance you need to drop you’r hip if you are not cutting backing angles on the top of your hip rafter.
- Rafter Support Block Depending on your rafter you will either have a rafter support block measurement or a Birds Mouth Depth Measurement. Keep in mind that if you are cutting the backing angles on the top of your hip rafter that you will need to add the hip drop measurement to this measurement since that will bring up you hip rafter.
- Rafter Height Above Plate Keep in mind that if you are cutting the backing angles on the top of your hip rafter that this measurement will be to the short side of the bevel.
- Rafter Pitch This is the vertical rise for every foot of horizontal run or horizontal distance travelled.
- Rafter Angle This is the degrees of the plumb cut of the rafter.
Conclusion
As you can see the RedX Roof App makes it super easy to get every single measurement and rafter detail to build an irregular hip roof or a same pitch hip roof. There are lots of other great features like Valley Builder, Rafter Calculator, Turret Roof Builder, Plywood Angles, Dormer Builder and others…
Do you think this is something that you would use? Let me know in the comments.
Thanks to all of you who made it this far!
