G'day and welcome back to Buildsum, and this is the twelfth presentation on the series on the Timber Framing Code
and this one's going to look at
the spans
working at the spans for Common Rafters
So a couple of rules for common rafters Rule 7.2.11 and 7.3.13
7.3.13 refers to Table 29 as the table that we need to work out common rafters
So rule 7.2.11says that rafters shall be single length members
or joined over supports, rafters in
Cathedral Roofs shall be designed to carry both roof and ceiling loads
Purlins that supports in loads and roof road shall be designed as rafters / Purlins with ceiling attached
7.2.11.2 Birdsmouthing
Rafters may be birdsmouth to add a depth of two a depth not exceeding one third the rafter depth
That's just something when you're actually cutting the rafters
Rule 7.3.13.1 says that the size of the rafters or purlins
Shall be determined from Spain table 29 of the supplements
So if you go to spend table twenty nine making sure you're in the right supplement for the timber you want to use
You'll see a few things we need to know we need to know
the spacing of the rafter
We also need to know what the roofing mass is for a particular job you
Know these were given in I gave these in the our
specifications, so we'll have a look at those in a sec
Also need to know the maximum span of our rafter
Okay, and we worked that out when we did our preliminary calcs so let's have a look at those things so
In our general information. We've got a roof mass of
10 kg per square meter
So we're going to use the 10 line and we've got a rafter spacing
Of 600 so we're going to use the 600 in either of these cases
Now the tables split into single span and continuous span and because at the moment I'm planning to use a purlin
We're going to go with the continuous span cause our rafter will run continuously over the Purlin
Our preliminary calcs said that half of our rafter span from purlin to ridge
or purlin to plate
Was 1.719
now the table is not that accurate so could just say 1800
So let's have a look you've got 600 is our spacing and in the continuous span area
So that's the column we need to use and if you come down the span side of this so we find
1800 or more and in the 10 line
Ok there is the first size. We can use so as far as that span goes we could use 70 x 45 now
The trick is that over here in the overhang side of the column. It's restricting us
to an overhang of 13 sorry
350
now
Now our eaves width on this job is 450 and by the time we take into consideration
the
Brick veneer in the cavity and the pitch of the roof. We actually end up with an eva overhang of
693
So we need to find an overhang
That's more than
693 so we need to come all the way there in our column on this side until we find an overhang and
There we go there's one there so
So now we're using 120 x 45
for our rafter now
as I did in the hanging beam video
We have the end section rule that sort of says that there's actually more timber in this
member the 120 x 45 then there is in the 140 x 35 so
We could
Actually use our 140 x 35 and for those of you
That are a little bit astute, and they will take a little bit of notice
you will now be thinking well if I look over here in the 600 column and
in the single sparing column, I can actually get away with a single span of
4.5 m and
If we just quickly jump back and have a look at our preliminary calques
Original calculation says that our span or rafter length was 4.1 so technically speaking
We could actually get away with no purling at all in this job
So we're going to use 140 x 35 for our rafter and
That means that we really don't need a purlin
this member here, and
If you don't need a purlin then we don't need struts. You don't need strutting beams, and there's a lot of saving in
work that goes with that so the extra cost of the large of rafter will save us a lot of physical work and
Cost of buying strutting beams to take the load of our roof
I'm still going to continue with the the working out for the purlins and the struts just as an exercise, but at this stage
They're not actually required for this roof