G'day and Welcome back to Buildsum and in this video?
Which is the sixth video in my series on the timber framing code or the Australian Standard 1684.2
this one a quick look at the Roof Load Widths or RLW
so the roof load width is used as a convenient indicator for the roof loads that are carried by
some roof members and load-bearing wall members in their supportive or supporting structure
the RLW value shall be used as an input to the relevant wall framing and
substructure Span Tables, so it
does depend on what sort of roof construction you have what your formula is so
We'll go through a few different scenarios
starting with a
Truss Roof, so
You have a truss roof
and you want to know the RLW at A
What you need to do is take whatever span 1 is
add it to span 3 and
Divide that by two now you're going to say well, that's exactly the same as what span 1 is
Which it is in this situation, but if for some reason this was an uneven
Not the same as span 1 one then we'd get a different answer, so span 1 plus span 3
divide that by 2
And then add the overhang on the appropriate side so in this case we can add span 2 in
Now if you do b, then we'd be doing span 3
plus Span 1 divided by 2
and then add on span 4 to get our RLW at B
Remember we have to work out. We have to find out what the greatest
RLW is for our roof and then use that the whole way through
So that's for truss roof
Now we're gonna have a look at the cathedral or a non coupled roof
So in this case
again and starting at A and going to so Span 1 divided by 2
Plus Span 2
will give us the RLW at A and
If we do B its span 3 divided by 2
plus Span 4
Remember this for non coupled roof
if we have a ridge support
And want to know what the RLW is at C
and its span 1
Plus Span 3 so this span plus this span
Divided by 2 will give us the RLW at C
Ok let's go the next one for Skillion Roof
We want to know the RLW at D. So it's going to be span 1 which is the main span of the rafter
divided by 2 plus the overhang at this end and
if we want to know the RLW at span A
and it's going to be half the span of the main rafter plus the overhang at this end, so
They'll give you the RLW
For a skillion roof again a non coupled roof
So now we're moving to our couple roofs this one has no ridge supports
so to work out the RLW a it is span 1
Plus Span 2 which is the overhang
Will give us the arrow w in this situation, and if you go over to do B
it's going to be Span 3
plus span 4 so that's a coupled roof no ridge supports
We throw in a ridge support then
again for a it's span 1 divided by 2
just now half the load has been carried by average support, so it's a divided by 2
plus Span 2
give us the RLW at A and
For B. It's going to be span 3 divided by 2 plus span 4
which is their overhang to give us the RLW at B
If we go to a coupled roof that has under purlins couple of different scenarios here
Again this one has no ridge support, so for A it is
span 1 divided by 2 so it's basically that's being there plus span 2
Over on this side where we have multiple purlins ok we're going to need to have
Span 3 divided by 3 because there are three
sections
plus
our Overhang Span 4
ok
Now if we were to throw a ridge support into that scenario
We now have span 1 divided by 4
Okay, because this is going to Carry Halfway down there
And then that's going to get carried by the purlins and then the other end would be carried by the rest of the way
span 1 divided by 4 plus Span 2
On this side
We have 2 purlins
So it's going to be Span 3 divided by 6 plus span 4
will give us the RLW of B
in this situation, so what you really need to do is you need to choose
the right formula for your situation and
Then you can calculate
the largest RLW that you have in your roof and
Use that for your calculations