So welcome up to the building site today my job is to cut a pattern rafter
So that I can construct a gable roof on this little large shed that we're going to construct
so
the dimensions of the shed as we can see 5.920m long
and 3.920m wide that's the overall dimensions from the outside of the Brickwork
the outside of Brickwork
So the first thing we need to work out for our pattern rafter is what our span of our roof is
so if you have a look at a plan view
because this is a brick veneer construction we actually have to deduct a
110m for our brickwork
and
then a further 40mm
for our
cavity so effectively we have to deduct 150m of this side and
150mm of this side to give us our span of our roof which in this case will be 3.620m
Once we've got that we can halve it
which will give us our half span so 1/2 span for this roof will be 1810 or 1.810m
Okay, so that gives us our and half span
[hat's the first thing we need we then need obviously we need to know what the pitch of our roof is in
this case our pitch is going to be 30 degrees
So we can then use the formula
TAN Roof pitch which in this case is 30
times by our half span
Which in this case is 1810 will give us a rise of our roof of 1.045m
so 1/2 span
do the Pythagoras or sorry do the formula and you get a rise of 1.045
That known as the half Span 1810 rise 1.045m
so using this later on we can actually work out what the length of our common rafter is and
similarly we also need to work out what our
eave overhang is. In this case we had a 450 eave width
On our plan, but because it's brick veneer
We've effectively well one thing we're going to add that 150mm on
so 450 plus 150 going to give us 600 and
we can then work out what this distance here is which is what we call the drop off
we're not going to need that for these calculations today, but
which is commonly used so we can work out where a brickworks can I finish and also so we can
put our window heads and that sort of thing into our walls
same formula okay, so at this time we using the eave width this dimension here as
Instead of our half span and we can get the rise
Okay, so that's the information we need to know now the other thing if you want to put your bevels on to a steel square
We can then use the formula TAN times the Roof pitch
so that's TAN 30 gives us 577 and
of course
577 won't fit on the tongue of the square we divide that by 2
So that gives us 289
So we put 289 on the square there. We have 500 on here
That's 0.5m because when we do TAN 30 that's based on a metre of run
So 50 half of that
okay 289 is half of 577 so effectively we've
we've created the same triangle of the same proportions onto the steel square
Okay, so now what we have to do. We have to get our timber decide what the actual timber we need to
to cut the rafters out of
So the best way to do that once again now we have all this information
We can use
Pythagoras
So we know that our half span
was 1.810
We know that our rise was 1.045 so we can simply use Pythagoras
so to work out what the actual length of
that rafter was
We then also have to do the same process with our overhang, so pythagoras
our eave width remember adding the for the 150mm and
We can then work out what our overhang distance was and we can add those two together to
give us
an actual length of 2.782
now we're going to add we're going to cut angles on both ends of the
both ends of the rafter so we can allow 100mm for cutting the angles that gets us up to 2.982 and
We go to order orderable lengths so we'll just round that up to 3m. So we're going to order 3m lengths
the alternative to doing it that way if you like is to use COS and the way we use COS we take our half spin
and we divide it by COS and then the roof pitch in this case 30 so half span was 1810
COS 30 gave us the same answer 2.090
did the same for the overhang so instead of half span should be eaves width
COS and the roof pitch will give us the answer so 600 divided by COS 30 gives us
692mm add them together and again we end up with a 3m length of timber
So we've got our 3m length of timber
So the next step where we want to start we want to start up at the Ridge
So we're going to take our steel square now you can quite easily use a
Sliding bevel set it whatever your Roof pitch had to be
Whatever method you want to use to get your angle
I'm going to use steel square because so that we can show you a couple of things with it, but
you could use a sliding bevel or whatever you want to get your these bevels with
So I'm going to start with a sliding bevel and I'm actually going to draw in
Just a line across here make it a dotted line
This is not going to be a line that you cut it's just somewhere to start your setout
now the unique thing with the
Steel Square is I don't actually need to know
What this distance long here is all i need to know is my half span so I can actually read across the square from here
across 500
Put a mark and then I can move the square down
Okay, so there's my centerline I've come down 500 go down another 500
All right, so we've got a metre another 500. We're down to 1500 and on the last step
because our half span was 1810 we go instead of going 500 we go
310 and that gives us the back of our birdsmouth. We don't even know need to know what dimension that was
You can if you do know, and you we do know because we've worked it out. We actually know that
That dimension there is
2.090 so there's no reason why I can't just mark your centreline of your ridge on your roof
measure that all the way down
and then just draw that angle in with again your sliding bevel or what ever it is you are using to get your angle
Okay, so we've got the we've got the back of the birdsmouth marked so the next thing we need to do is we need
to draw in our birdsmouth now the maximum depth of our birdsmouth is
1/3 the width of the timber so we break the timber up into these widths
okay, three equal widths and with this being a 90mm piece of timber, they're going to be 30mm each
Okay, and we can then
transfer that along to where it intersects with our
back of our Birds mouth line and
effectively there are that dimension there is about 34m
Okay, so you can go less, but the maximum is one-third the width of the timber
Okay, so once we've got that there we can then draw in a line long here on the level bevel and that will give us
the top of our birdsmouth
like such
okay, once we've done that we can then
Wworry about our overhang
so in this case, our overhang was
600mm on the
plan length so with the
Steel square we can mark out 600mm and once again there's no reason why we can't measure it
We did work that dimension out if we go back to our calcs it should be
692mm so we could measure
from back of the Birdsmouth
down the rake
692mm and come up at the same point
Okay, so we've got that all marked in we then need to go back up to
the top of our roof
So here we've got the centerline of our roof
but we have to deduct
half the thickness of our ridge
otherwise the the rafter won't fit. So we've got to come square off our original line
whatever that dimension happens to be could be 10mm could be more
Once again, you can use the steel square or use your sliding bevel
whatever to mark that shortening and
end up well along like that
Okay, so now we've done that the next step is just to go and cut all our all our marks
so we have to cut it off here where our
Ridge shortening is
We're going to have to cut out
Cut out our birdsmouth
[get rid of that line] cut out our bird's mouth and
Cut our overhang off now sometimes we leave the overhang depending on what what facsia
we're going to use if you're going to use a timber fascia
generally, you're better off to leave that overhang and cut it off to a string line later if you're going to use a metal
fascia which has got the adjustable clips then there's no reason you can cut that off at this point
so you cut off a rafter and
there we have
A pattern rafter that we can now go and use to mark out the remaining rafters
So that's all you need to do for a Gable roof
There are a couple more steps for a hip roof you actually have to mark out where our
Crown end and our creepers are cut off, but we'll cover those in another video