G'day and welcome back to Buildsum and in this video, which is the next video in the estimating series
I'm gonna look at estimating the rafter length
So as I said we only going to do
Estimating the length I'm not going to work out the actual
Cutting length there is another video or a pattern rafter video that goes into those details, and I'll put that links up for that
So here you can see our
Rsfter runs from our ridge up the top here
down through our wall plate down here, and you can see that essentially that forms a triangle from the
center of our ridge up here
down to a
line, that's
Comes in line with the back of the plate this line across here is called our XY line
okay, so we can actually work out the length of our rafter for there and
As you can see with that triangle, there's a couple of ways you could do it
We could use Pythagoras if you knew the rise or the length of this side and a half span
But I'm going to use COS
So a half span of our roof
COS and the roof pitch will give us the length of our rafter and estimated length of their after
So here's the half span dimension from the center of our roof to the outside of our
supporting plate and
Our roof pitch in this case is 30 degrees
so
put the formula in
Sorry, our a half span in this case it's
3.464mm
And we use the formula
So 3.464mm divided by COS 30 will give you rounded up 4.000m
So that's 4.000m from the center of the reach at the top of the roof down to point that is
In line with and plumb up from the back of the plate now obviously, that's not the length of the rafter we still have our
overhang here
So the easiest way to do that is to then add
what we call our eaves width
Okay, so if you had a brick veneer building you would have to add the eaves width plus the brick and the cavity
In this case I don't have that so I'm just going to go to the back of my plate
The back of the supporting plate the outside of my rafter
horizontal measurement in this case 600 so
3.464 + 600
Gonna, give us 4.064m by that again by COS and the roof pitch
will give us a
overall rafter length of
4.693 mm
okay, now because
that length comes to the short point on this end and
The long point up here
Common practice is then to get our actual order length. We add 100mm on each in
Just to allow for the plumb cuts on on this on this end and this end they're actually allowing the extra material there
So that would give us 4.893, and we rounded up to the nearest orderable length
So in this case we'll be ordering links of 5.1
Okay, so there you go, that's how we work out an estimate lenght not the cut length
Just that overall length of the timber that we would need the order for our rafters
G'day again, I'm back
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