- In our first hip roof video,
we learned how to lay out a hip roof
using a construction calculator and a framing square.
And if you missed that video, I'll leave a link below
so you can watch it now.
Today we'll be answering two important questions
that came out of the last video and they are,
why do we use a 17 inch run for a hip rafter layout?
And why do we add 7/16 of an inch to a jack rafter
when using a Construction Pro calculator.
First up, why do you use a 17 inch run
for a hip or valley rafter?
It starts off by understanding that there is a difference
between a common rafter layout and a hip rafter layout.
And that difference is a five inch run.
If you remember, common rafters
are installed 90 degrees to the ridge,
which means that the run is always 12 inches.
However, hip and valley rafters
are not installed 90 degrees to the ridge,
but are installed at a 45 degree angle.
Which means you can't use the same 12 inches
because the hip rafter is actually climbing
at a slower, more gradual rate than a common rafter.
To better explain this,
if we look at an overhead shot of our roof,
imagine with me for a minute that this is a 12 inch
by 12 inch square.
Again, this is for illustration purposes only.
But if we look at the square, each side would be 12 inches.
And the measurement for the common rafters run
would be 12 inches
and the measurement for the king rafters run
would be 12 inches as well.
For our hip roof here,
which has a six inch pitch or rise
for every 12 inches of horizontal run
these common rafters rise six inches.
However, if we now look at the diagonal measurement
of the 12 inch by 12 inch square,
that number is actually 16.97 or simply 17 inches.
Therefore for this roof for every 17 inches of run
on the hip rafter, it rises six inches.
So you can see that the 17 inches actually comes
from the diagonal measurement
of a 12 inch by 12 inch square.
With that knowledge, now, you know that
if a rafter is coming off the ridge at a 45 degree angle,
like a hip or a valley,
you must use 17 inches and not 12 inches.
Next step. Why do we need to add 7/16 to the jack rafters
on our roof?
If you use the numbers coming right off of
the Construction Pro calculator,
your jacks will actually be about 7/16 of an inch short.
If you're trying to make them land 16 inches on center.
For example, if I pull a tape from this king common rafter,
over 16 inches to locate the jack rafter,
you can see that it's off and needs to move to the right
and actually be longer.
Now, if you look at the right side jack,
where I added the 7/16,
you can see that the 16 inches on center layout
is working perfectly.
So where does this mysterious 7/16 come from?
Well, it has to do with the dual bevel angle
we made at the top of the hip rafter.
This can be a complicated topic to teach on.
And honestly, there are other carpenters
that know roof framing better than I do
but let me give you my cliff notes version.
The simplest way to come up with the 7/16
is to subtract the rafters thickness
with 1/2 it's diagonal length.
To get the diagonal length,
you simply draw a 45 degree line
and measure 1/2 that distance.
And for an inch and 1/2 rafter like this,
the diagonal measurement is two and 1/8.
So half of that would be an inch and a 1/16,
therefore, an inch and a 1/2
minus an inch, and 1/16 equals 7/16.
Which then would be added to the jack rafter lengths
coming directly from the calculator.
I find this method very simple to understand,
but for the fun of it,
let me give you one more answer to why
the 7/16 is there.
If you look at the top of our roof,
I've extended the hip rafter line, which is this line here.
I've also added this line,
which extends the king common rafter.
And if you look now really close,
you can see that the lines actually create
a little triangle,
which is really a mini roof.
That has a run and it has a pitch.
And guess what the run of this little roof is?
That's right, 7/16.
And the measurement then from this point to this point
is an inch and a 1/16,
which is exactly 1/2 the diagonal distance
we came up with earlier.
So hopefully that sheds a little bit more light
on where all of these numbers are coming from.
Now I'm not quite sure why this calculator
can't make those jack rafter adjustments,
but I'm sure it's for a good reason.
Personally, I find it really easy
to just to add the 7/16
to all my jack rafter lengths.
But I do understand that some of you
like to do it a different way.
So in that spirit,
let me give you one more work around
so that your jack rafters are coming off
of the Construction Pro calculator correctly.
Because the 7/16 is a run measurement
you can add that run to your existing run.
But this can only be done after you've already calculated
your common rafter and your hip rafters.
For example, the run of this roof
is 23 and 1/4 inches.
So if we add the 7/16,
we come up with a alternative run if you will,
of 23 and 11/16.
This is solely for the purpose of just figuring out
the correct jack lengths.
So once we work through all of the calculations
using the new run,
the jack lengths,
as they're coming off of the calculator
now are more accurate
and no longer need further adjustments.
As you can imagine,
there are many different ways to approach
coming up with the right numbers.
And honestly, in rough construction,
sometimes getting close enough works too,
but now at least you have a little bit more knowledge
so you can move forward more easily.
Well, that's it.
I hope those two questions have been clarified for you.
If you need further assistance,
you can drop me a comment below,
or you can find me on Instagram.
That platform works really well too for sharing information.
Thank you so much for watching today.
God bless.
(upbeat music)