- Today, we're looking at how to frame a common rafter roof
by only using the numbers found on a framing square.
There are many different ways
to calculate the length of rafters,
and I've shown them all here on this channel.
However, I've never shown in detail
how to use the rafter tables on a framing square.
If you look on the blade of a framing square,
you will see the rafter tables,
which are basically pre-calculated numbers
used to find the lengths of common rafters,
hip and valley rafters, and even jack rafters.
As with every roof,
you first need to determine the total run of the building,
which is half the distance of the span
minus half the width of the ridge board material.
For our mock-up,
the total span or total width of the building is four feet.
And the ridge thickness is an inch and a half.
So half the thickness of the ridge is three quarters.
And half the span is two feet.
Therefore, two feet minus three quarters
gives us a total run of 23.25 inches.
So now that we have the total run, which is here,
the next number to figure out
is the length of the rafter itself,
which is this diagonal measurement here.
In order to solve for that,
we need to determine the ratio of rise to run,
which is also sometimes referred to as the pitch.
For example, if you want a roof
with a steeper slope or pitch,
then going with a higher number on the rise,
like 10, for an example,
would produce a steeper roof.
On the other hand, if you want a shallower roof,
you may want to go with something more like
a four-inch rise, like this.
The rise of the roof is normally determined
by an architectural design or codes
or personal preference from the builder.
And in this case you might be the builder.
So for our example, let's just go with a six-inch rise.
Back at the framing square, the first line of numbers
are labeled as "length, common rafters, per foot of run."
And if we slide over to find our six-inch rise,
which is here,
directly below that is our unit length
for a six-inch-rise roof.
And that number is 13.42.
What that means is that
for every 12 inches of unit run on a six-inch-rise roof,
the diagonal length will be 13.42.
And you can see that if I pull a tape
between those two numbers.
Back at the drawing,
we need to translate the unit length of 13.42
into our theoretical rafter length here.
To do that, we first need to convert
our run, that is in inches, to feet.
And that's easily done
by dividing our total run of 23.25 by 12,
which of course is how many inches are in one foot.
And that number is 1.94, if I round up.
Again, this number is now in units of feet.
What that means is that there's 1.94,
or almost two full units of the 13.42 in our rafter length.
And I know that sounds confusing, but the math is simple.
Multiply the 1.94 times the 13.42
for a theoretical rafter length of 26 inches.
With that number figured out,
we now can see the length of our rafter,
which is from the side of the ridge board
down to the outside wall.
This measurement does not include the overhang amount,
and that's why it's called the theoretical rafter length.
Like the slope of a roof,
the overhang amount, including the fascia and soffit cuts,
are determined by architectural design codes
or personal preference.
All right, let's lay out and cut this rafter to size.
With the framing square held
to the six-inch mark on the tongue
and the 12-inch mark on the blade,
mark out the ridge plumb cut line
and cut it out with a circular saw.
Hook a tape measure over the end
and measure down the diagonal length of 26 inches,
and make a mark.
With the square still holding a six-twelve,
slide it up or down until it lines up with your mark
and draw the exterior building line, or heel cut.
For this roof, the overhang amount wants to be five inches.
So measure over five and draw the fascia cut line.
From there, measure down four inches on the fascia line
and draw the soffit cut.
The last thing to do is mark out the seat cut length,
which makes up the birdsmouth cut here.
The length of the seat cut is normally the same thickness
as the exterior wall and plywood.
So that makes ours four inches.
To lay out the seat cut of the birdsmouth,
flip the square over
and line up the square with the heel line here.
Then slide the square up or down
until the length of the seat cut measures four inches,
and draw your seat cut, completing the birdsmouth.
The birdsmouth then gets cut out,
and of course the overhang amount as well.
And when the ridge is installed,
these common rafters are looking great.
Before we end for the day, I do want to show you
one more set of numbers found on the framing square,
and that is the length of hip and valley rafters
per foot of run.
In a previous video, we built a hip roof,
which actually had the same run as our example today.
And in that video, we used a calculator to work out
the length of the hip rafter,
which goes from the bottom corner here
all the way up to the ridge.
The length of that hip rafter was 34 and seven eighths.
If we look back at our framing square,
we can see that just under the number
for our common rafter length
is the length for the valley or hip rafter,
which for our six-inch rise is 18 inches.
Therefore, if we use the same total run of 1.94 feet
and multiply that by 18,
which again is the unit length for a hip rafter,
we get 34.92.
To convert the 0.92 into a fraction,
so it makes more sense on your tape measure,
all you have to do is multiply the 0.92 times 16.
The 16 represents sixteenths,
which is the smallest unit of measurement
we normally use here in the US for building.
So 0.92 times 16 is 14.72.
To get the top number of our fraction,
we simply use the first number here,
and then put that over 16.
So that would be 14 sixteenths, or seven eighths of an inch,
which means that 34.92 is the same as 34 and seven eighths.
So as you can see, the framing square works perfectly
for figuring out all of your rafter lengths.
I hope this video has been helpful.
If you still have questions,
be sure to watch my rafters playlist,
where I go over all of the other methods available.
I hope everybody is doing well.
And I pray God's blessing upon you and your family
during this holiday season.
(upbeat music)