- Today, we're learning how to lay out and cut
common rafters using a speed square and a basic calculator.
Gabled roofs like our mock-up here
are made up of common rafters,
and in order to lay out those rafters,
you'll need to know the slope of the roof first.
In order to get the slope
you'll need to measure and understand four things:
span, total run, unit run, and unit rise.
First is the span.
The span is the total measurement
of the building from its outside edges.
So the span of our mock-up here is four feet.
Next is the total run.
This is an easy number to get
because it's half the distance of the span,
or in our case, two feet.
To understand unit rise and unit run,
we're gonna use this visual aid.
Unit run, which is represented by this side of the triangle,
is always 12 inches
for rafters that are 90 degrees to the ridge,
like our common rafters.
And then last is the unit rise.
So for every 12 inches of unit run,
a rafter will rise a certain amount
which is normally predetermined on plans or drawings.
If not, you can determine this for yourself
simply by choosing how steep or shallow
you'd like your roof.
But for our roof here
we're gonna stay with a nine-inch rise.
Now that we have some of the numbers,
and now that you know some of the terms and concepts,
it's time to determine the last missing piece
which is the unit length or slope.
The unit length or slope is the ratio between the unit run
and the unit rise.
There are many methods to calculate unit rise.
Let's look at two of them.
The first is the Pythagorean theorem.
In order to solve for unit length,
we'll need to use the equation
C equals the square root of A squared plus B squared.
On the calculator type 12 squared plus nine squared equals,
then the square root button
for total unit length of 15 inches.
The second method utilizes rafter tables
which can be found online or on a framing square
if you have one.
If we look at my framing square,
length of common rafter per foot of run is located here
below the inch marks.
The inch marks represent units of rise.
So if we look under the nine, which is what our roof is,
we can see that the unit length for a 9:12 slope roof
is 15 inches.
As you can see, both methods come up with the same number,
but if you're still a bit confused
about what all these numbers are about
let me add this thought.
What I'm holding here is one complete unit.
Look at this not only as a visual aid,
but a tool for figuring out any rafter length
for a 9:12 slope
simply by adding more of these units together.
Let me show you what this looks like in our mock-up.
If you remember, our total run was two feet,
and our unit run here is one foot.
Therefore, it will take two of these units
to complete our building's total run
which also means that it'll take two of these units
to complete our total needed rafter length,
which is 15 plus 15 for a total of 30 inches.
Now that we have our rafter length
it's time to do some layout,
and this is where our speed square comes in.
Starting at the top, place the speed square
on the top side of the rafter.
Now pivot this square to the nine on the common scale.
Draw a line, and make a cut with a circular saw.
Next hook your tape measure over the long point of the miter
and measure down 30 inches and make a mark.
Using the same speed square set up at nine;
draw a line.
As a side note,
it's okay to cut this rafter a little bit long,
like 30 1/8, or something like that.
You can always recut a rafter,
but you can never stretch one back out.
The other thing I want you to know
is that we're actually making a pattern,
a test, if you will,
and we may need to make some tweaks and adjustments
along the way.
Jumping back in, the line that we just marked
represents the outside building line
and the heel cut of the bird's mouth.
But if you were to make a cut there
we'd be left with a rafter with no overhang.
Therefore, we need to do a little bit more work
and it starts with marking out the seat cut.
Generally the seat cut of the bird's mouth
is the same width as the top plate,
or the top plate and exterior sheathing added together.
To mark the seat cut slide, the square up the heel line
until you've reached your desired measurement.
Take extra caution to make sure that the two lines
are exactly 90 degrees to each other.
With that done, slide the square down
continuing to hold that same nine slope,
and with the aid of a tape measure,
mark out your desired overhang.
The last line to be marked out before cutting begins
is the soffit.
Because we're going with a four-inch fascia,
this line will be our soffit cut.
While cutting it's very important to take your time,
keeping your cuts straight and clean.
Then once you've completed one rafter
use it as a pattern for the second rafter.
And once the second rafter's done
it's time to test the fit.
Holding them in place,
look to see how well they're fitting.
And is there any minor adjustments that are needed?
For example, look to see
how well the ridge miters are lining up.
Look to see if the seat cuts are sitting flat
to the top plate,
and look to see if the rafters are too long,
leaving a gap then between the rafter and the top plate.
Small adjustments are possible at this point
because the rafter is actually cut
3/4 of an inch too long.
You see the rafters were cut too long
because we didn't account for the Ridge board yet.
And I did that on purpose.
I have found that when you're learning
it's nice to have the ability to correct minor mistakes
without having to scrap a whole piece of lumber
just because you cut it too short.
Now, if your rafter was a good fit from the start,
go ahead and lay out to remove half the thickness
of the ridge from both rafters.
So in our case, that's about 3/4 of an inch.
But if you had to make some drastic changes
you will most likely need to take off less,
and you may even need to use a piece of ridge board
to help you gauge the proper changes.
It can be a bit of trial and error,
and if you have to start all over,
that's okay, you're learning.
Just remember whatever adjustments you make to one rafter
be sure to do the exact same to the other.
Then once you're satisfied with those two rafters,
and before you begin cutting more,
double check to see which rafter will be the best pattern
and make sure it's marked clearly.
After you've cut a few more rafters with a pattern,
start installing to see how they look.
Keep the tops flush with the ridge
and the heel cuts tight to the top plate.
And lastly, remember this: math is perfect,
and oftentimes what we build is not.
So some minor adjustments are gonna have to be made.
With a little bit of practice and a handful of mistakes,
you're gonna be able to build
and create some amazing things.
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Thanks for joining me.
See you next week.