- In today's video we're looking at the simplest
and easiest way to lay out a common rafter.
For those of you that don't know,
there are four main methods for laying out common rafters.
And they are the Pythagorean-theorem, unit length,
a calculator and the step-off method,
which is what we're covering today.
Out of all of them, the step-off method
might be the easiest one to understand.
And that's why a lot of trade schools actually begin
with teaching this method.
To understand this method, and really all the other methods,
it's important to know the difference
between the theoretical rafter length
and the actual rafter length.
Theoretical simply means the measurement
or the rafter size before you add
or subtract anything else to it.
And the actual rafter length is
what the rafter will actually be cut to
in order to get it to fit.
To better explain this, let me make a simple drawing.
Here you can see a drawing of a common rafter.
A ridge, an exterior wall with a double top plate
and an overhang.
The distance from the outside wall here to the center line
of the ridge board is the theoretical rafter length.
The theoretical rafter length is technically found
along the measuring line, which is here,
which runs from the corner of the birdsmouth,
all the way up the rafter.
And this is the method that the step-off method
and other methods calculate.
The actual rafter length on the other hand
is the theoretical length plus the overhang amount,
whatever you choose that to be, minus half the thickness
of the ridge board.
Again, whatever that might be.
Bottom line, the theoretical length is a fixed measurement
because the overhang amount and the ridge thickness can vary
from roof to roof.
So now that you know that, let's look at the step-off method
in more detail.
The first number you need to know is the total run,
which is simply half the width of the building's total span.
And for our mock-up, the span is four feet
or 122 centimeters, which means that half
that distance would be the total run, two feet
or 61 centimeters.
The second number we need to know is the unit run,
not to be confused with the total run.
The unit run is really easy to calculate
because it's always the same for common rafters,
and that is 12 inches or 30.5 centimeters.
The third number to determine is the unit rise.
Unit rise is the amount that the rafter rises per 12 inches
of horizontal run.
Now, unit rise is normally determined by a set of plans
or drawings, or you can simply go
with what you think looks the best for your project.
So for our little project here, I'm gonna go
with a seven inch or 18 centimeter unit rise.
All right, so now that we have a unit rise of seven inches
and a unit run of 12, it's time to take those measurements
to the rafter using a framing square.
For this example, we'll be using the inside numbers
of the framing square and marking on the outside.
However, depending on the width of your framing material
or just personal preference,
you can totally use the outside numbers
of the framing square as well.
With the square position with the tongue side
on the right, line up the seven inch mark with the top
of the lumber here.
And on the blade side of the square,
line up the 12 inch mark.
I like to use these framing jigs by SquiJig
to hold my numbers so that I can easily slide
the framing square up or down, or to move it
to another board without losing my settings.
The first line to draw is the ridge cut,
which is this line here.
Then measure over 12 on the blade and make a mark
for your first 12 inch unit of run.
Now, slide the square down and line up your tongue
with the 12 inch mark and draw another line.
Remember that our total run of the building was two feet
or 61 centimeters.
So we're going to need to lay out two full units of run
to equal our total run.
So with our first unit here, measure over 12 inches again,
make a mark and slide down the square
to step off the second unit run.
Now we have a total of two units each at 12 inches
for a total of two feet, which is the total run
of the building.
Our theoretical rafter length then is the measurement
from the ridge line here down to the outside building line,
or heel cut of the birdsmouth.
More on that in a moment.
Because our mock-up has a total run of two feet,
it was really easy to lay out because the total run
worked out perfectly with our two 12 inch units of run.
But sometimes buildings don't end up like this
and can be a little bit more, say, two feet, one inch
or 63.5 centimeters.
How do you lay out for that?
It's actually really easy.
Once you draw your ridge line, instead of moving right
into marking out the whole unit of run, which is 12 inches,
you would first measure over one inch or 25 millimeters.
Then from that line, you measure over 12 inches
on the square, make a mark
and draw your first full unit of run.
If you do it this way, it takes care of any smaller
or odd numbers first, before stepping off
the full 12 inch units of run.
All right, with the theoretical lines marked out,
let's go ahead and mark out for the birdsmouth
and the overhang.
Now the birdsmouth is made up of two cuts, a heel cut
and a seat cut.
The seat cut is what sits on the top plate
of the exterior wall.
And the heel cut is what fits tight
to the exterior wall sheathing.
If you look back at our rafter, we can see
that the last line that we drew was
both the exterior building line as well as the heel cut.
Therefore, all we have to do is mark out the seat cut
to have our completed birdsmouth.
The seat cut is often the same length as the exterior stud
and the sheathing combined.
In our example, the stud is 3.5 inches or 89 millimeters,
and the wall sheathing is a half inch or 13 millimeters,
which means that the total length
of our seat cut is four inches or 102 millimeters.
To lay that out, simply use the scale on the bottom
of the square to measure over from the heel line four inches
and draw a line.
With that done, now you can see the birdsmouth,
which will get cut out later on.
Before we move on, I do wanna quickly bring up
the one-third rule.
The idea is that you never want the corner
of the birdsmouth to be more than one-third the width
of the rafter.
So if I break our rafter up into thirds,
you can see that the birdsmouth is within the first third
of the rafter.
Codes vary from place to place, so be sure to double check
all your local codes before building your roof.
Moving on, the next thing to work on is the overhang,
which gets added to the theoretical line here,
making the rafter longer.
Overhang amounts like unit rise can change from building
to building, from project to project, because of design
or personal preference.
For this project, we're gonna go with an eight inch
or 203 millimeter overhang.
With the square still set up as before,
slide it down and measure over
from the outside building line, or theoretical line,
eight inches and draw another line.
This line represents the fascia or sub-fascia line.
The fascia line is where the fascia trim gets installed,
but sometimes some people like
to install a sub-fascia first, meaning another piece
of framing lumber that gets installed first
before the finished fascia goes over that.
Now, some framers like to do this, myself included,
because it helps to stiffen that trim as well
as remove any waviness that might be in the rafters.
In order to mark out our fascia,
we need to measure down the fascia line six
and a half inches or 165 millimeters from the top
of the rafter and make a mark.
And just like laying out for the seat cut
of the birdsmouth, slide the square over to the six
and a half mark and draw the soffit cut line or level line.
So this will get cut out here and so will this,
and of course the birdsmouth as well.
With the first rafter cut out
and the second one patterned off of the first,
you can see that the rafters are
now fitting together nicely.
However, there is one more adjustment that we need to make,
and that is at the theoretical ridge line.
Every roof will have what is called a ridge board.
And for this roof, a ridge board that is larger
than a two by eight, like a two by 10 will work great.
Because a two by 10 is an inch and a half
or 38 millimeters thick, we need
to reduce the overall rafter length by half that distance,
which in this example is three quarters of an inch
or 19 millimeters.
To do that, you simply slide the square up
and measure back three quarters of an inch
from the theoretical ridge line
and draw the actual ridge cut line.
From there, use a saw to remove that material.
Well, now with the ridge board installed,
everything is fitting as it should.
It's tight to the ridge and tight to the seat cuts.
At this point, you could use one of these as a pattern
to make all the other rafters for the entire roof.
If you have any questions, which I assume you will,
please hit me up in the comment section below.
You can always find me on Instagram,
send me videos or photos if you find yourself getting stuck
on a project.
And of course, be sure to watch all
of my other rafter videos so that you can view
and learn those other three methods.
Thank you so much for watching.
I'll see everybody in the next video.
(gentle music)