- In today's video,
we're laying out and cutting rafters for a lean to roof,
plus learn how to avoid a very common mistake
when building this style of roof.
A lean to roof, sometimes called a shed roof
or a mono pitch roof,
pitches or slopes in one direction.
Now, this style or type of roof can be freestanding
or in this case,
it can attach to an existing structure using a ledger.
A ledger, for those that don't know,
is a framing board or beam
that is attached to the existing structure first,
so that the new structure or roof can be attached.
For today's example,
this roof needs to have a pitch
of 4 inches or 102 millimeters,
which means that for every 12 inches
or 30.5 centimeters of horizontal run,
this roof will rise 4 inches vertically.
Not all lean two roofs have a predetermined pitch
but for today's example, we do.
With that said, however,
we do need to determine a few things
and that is, we need to figure out the total run,
we need to figure out the diagonal length of the rafter,
and we need to figure out the specific location
or the height of the ledger.
In order to get the total run of our new structure,
we simply measure from the existing exterior wall
or old structure, which is this 2 x 4 wall
over to the exterior wall of the new structure,
which is this top plate here.
That measurement is 44 inches or 112 centimeters.
Because our roof design utilizes a ledger,
we need to deduct for its thickness,
which is an inch and a half or 3.8 centimeters from the run.
The math then is 44 minus 1.5 inches
for a total of 42.5 inches or 108 centimeters.
With that number established, we now have two numbers:
the total run and the pitch,
which is all we need to start using the calculator.
But before we start crunching a bunch of numbers,
let's take a quick look at a drawing
just to make sure that we're all on the same page.
At this point,
we figured out the total run of our new structure,
which is from the ledger here
to the outside edge of the wall here.
Again, we deducted our 1.5 or an inch and a half ledger,
which left us with a total run of 42 1/2 inches.
What we still don't know is the rise here
and the diagonal here.
At the calculator, enter 42 1/2 inch run,
4 inch pitch, and hit the diagonal button
for an answer of 44 13/16" or 114 centimeters.
From there, all we need is the rise,
which is a simple click of the rise button
for an answer of 14 3/16" or 36 centimeters.
Back at the drawing, we now have three numbers:
the rise, the run, and the diagonal.
However, the rise that's on this drawing
represents a rise up to the theoretical measuring line.
And if you've seen my other rafter videos,
this line runs from the edge of the wall
or at a corner of the birdsmouth
all the way up the rafter,
which means that there's still more rafter above this line.
And that measurement has a name
and it's called the height above plate,
or sometimes called the rafter stand.
Height above plate changes depending on the birdsmouth cut
and the width of the framing material used.
For example, if I show you one of the rafters used
to make this roof,
you can see that if I extend a line
up from the corner of the birdsmouth,
it's measurement is 6 1/8" or 16 centimeters.
This number is really important
and can be found on height above plate charts.
One of the best places I've found for this information
is from a website from SBE Builders.
Here it shows you the height above plate, the pitch,
and it gives you the ability
to enter in your own information.
So, for our roof, we need to enter a 4 inch pitch,
and then we need to enter a seat cut length,
which is this measurement here.
In general, the seat cut length
is whatever your exterior wall thickness is,
plus the thickness of the sheathing.
So, for our roof,
let's go with a 4 1/2 inch or 11.5 centimeter seat cut.
This, of course, is a great place for me to mention
that building codes are different all around the world,
so be sure to check your local building codes
before you build anything in your area.
Next is the rafter material depth or width,
depending on how you look at it.
Our material is 2 x 8, which at my lumberyard
measures 7 1/4 inches or 18 centimeters.
From there press submit and if you scroll down,
you can see the height above plate,
which is 6.14" or 156 millimeters.
So, why do I show you where to get all of these numbers?
Well, in order to figure out the exact height of your ledger
or the height of any roof you're building,
you need to know the height above plate
for your specific rafter size.
And if we look back at our picture,
you can now see that our rafter
will actually be sitting 6.14 inches higher
than the theoretical rafter line.
And this is where a lot of people make a mistake.
They forget about the height above plate.
So, what's different between a lean to roof
and say, a more traditional gable roof
is that we're building to an exact height.
That ledger has to be installed exactly where it needs to be
in order for the roof to work out,
so if you forget the height above plate,
your height of your ledger will be off.
Therefore, in order to get the exact location of our ledger,
we need to add to our rise of 14 3/16" to 6.14 inch
height above plate to get a total height of the ledger,
which is 20 5/16" or 52 centimeters.
With that, measure up the 20 5/16" from the bottom plate,
make a mark, and install the ledger at that location.
Because I've done a lot of videos
on how to cut and lay out rafters,
we're gonna move through this section pretty quickly.
Using the numbers we've already figured out,
lay out a framing square with the four on the tongue,
which is our pitch, and 12 on the blade,
and draw a plumb cut line.
For those of you that don't know
where the 12 inch number came from,
the 12 is actually the unit run for a common rafter
and these are common rafters.
So for all common rafters, the unit run is always 12.
Next, our diagonal measurement,
which was 44 and 13/16" or 114 centimeters,
so we're gonna measure down from the ridge plumb cut line
and draw another line.
Flip the square around,
so that you can measure over for the 4 1/2 inch seat cut,
draw the line, and this is the birdsmouth,
which gets cut out.
Next, a quick 8 inch or 20 centimeter measurement
for the overhang amount
and then down 6 1/2 inches or 16.5 centimeters
for the facia cut.
Again, these numbers are based on personal preference
or by design.
With everything cut out and with the rafters installed,
you can see the plumb cut lines fit great with the ledger
and the seat cut, it's sitting perfectly as well.
Well, that's it for today's video.
If you still have some questions
and you're still a bit confused,
start by watching my other rafter videos first.
And then if you still have questions,
feel free to reach out to me in the comments
or on Instagram.
That's a great place to connect with me.
Thank you so much for watching
and supporting Training Hands Academy.
God bless.
(country music)