G'day welcome back to Buildsum in presentation 22 in the series on the timber-frame code
AS 1684 and in this one we're going to look at our top plates
so the rules associated with top plates are Rule 6.2.2.1 and
6.3.4 and the table, we're going to use for my example that we're using is
Table 15
So rule 6.2.2.1 says the top plates shall be provided
Along the full length of all walls including over openings, and there's a bit more about bottom plates, but we'll look at that next
The size of top plates for single story or upper story of a two-story construction shall be determined
from span Table 15 and 16
of the supplements respectively for sheet and tile roofs so she proves table 15
tile roofs Table 16
if you need top plates for the lower story of two story construction, and you need span table 46 and
There's a note there that wall plates sizes in the span tables are appropriate for wall plates
Supporting defined Roof loads located in any position along the length of the plate
rule 6.3.4
says the top plates may be a minimum of 35mm deep multiplied by
the width of the stud for any stress great so
In other words if you stud is 70 then your bottom plate obviously has to be 70
If your stud 90 then your bottom plate obviously has to be 90
they are not required to resist wind up lift forces as
were rafters of trusses are normally fixed
Or we're tied and spacing is 0 see note 7
in Span Table 15 and 16 and
Loads from roof trusses rafters floor joists and similar members are located directly above studs or at
Or within 1.5 times the depth of a plate from the stud so if you can guarantee
That any roof trussed rafter floor joist is going to land within
one and a half times the depth of the top plate from the stud then you can just use a minimum of
35mm
All right 70 x 35 in our case. That's a that's a given alright if you can guarantee
Everything's going to land
in that sort of region
top plates fully supported on masonry Walls Shall be determined from the span tables assuming a stud spacing of
300mm and a tie-down spacing
equivalent to the tight end spacing of the plate and
Double or multiple top plates known as Ribbon plates may be used
Provided allowable roof load width is determined in accordance with the span tables for members
indicated as being made up of multiples, so
2 / 35 x 70 or 2 / 38 x 75
That's obviously there's multiples plates in there
so here's our span Table, Table 15 top plates sheet Roof and
Obviously you have to make sure it's for the right timber species
We have to look at the rafter spacing. So ours is 600
Bring this up our truss joist rafter spacing 600. So we're obviously going to use this column and
our tie-down spacing
So if we're going to tie our top plate down to our wall
and the standard spacings that's 1200 so we'll use 1200 and
we can see here our stud spacing is 600 okay, so
You have to use a 70 x 35mm top plate that's how it works even if our
Tie down spacing was 600 or nine hundred or even zero it's not going to change
using a 70
35 x 70mm
Member the reason it's written backwards like that is of course what they always put the height first so because the timber top plates
essentially on its flat so 35mm high 70mm wide
Okay, so 35 x 70 is our top plate
and there we go. We've worked out the top plate size of the top plate that we need for this particular job