Plumbing Connection Winter 2022

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5 0 PLUMBING CONNECTION Winter 2022
Ken Sutherland CPEng,
MIEaust, RPEQ has been a
Hydraulic Consultant for
over 30 years, and now offers
many Free Design Programs
on and
DIAGRAM 4
Providing the starting point is on
rainfall line A-B, and the ending point
is on rainfall line C-D, it will intersect
the same amount of rain, and therefore
have the same catchment area.
CATCHMENT AREA
To determine what the real catchment
area is, we must determine the area of
the vertical rise, and add half that to the
roof plan area (as shown in Diagrams 1,
2, and 3).
Now the hard part – we must do
some mathematics, trigonometry, and
geometry. (All that fun stuff.) Or we
can leave it to Excel, or we can look up
a multiplying factor in AS/NZS 3500.3
Table 3.4.3.2.
So, for those of us interested in the
maths, lets plough on and give it a go.
We know the rain falls at an angle of
2:1, therefore in the Diagram 1 above
the length of the ‘slope effect added’ is
half the ‘vertical rise’ (2:1 remember).
This is also true for the roof areas,
that is the area of the slope effect
added is half the area of the vertical
rise.
So, all we need do now is find the
area of the vertical rise.
Anyway, to avoid these calculations
you can look up the multiplying factor
‘F’ in the Plumbing Code AS/NZS 3500.3
Table 3.4.3.2 and multiply the roof plan
area by this factor, or simply measure
the area from the architect’s drawings.
For a straight roof the architect
has normally shown this slope on the
drawings.
But if there are vertical drops or
different slopes, then we can take the
total vertical rise as shown in Diagram 1.
This will intersect the same amount
of rain as if we calculated each roof
individually.
However, it works both ways. We can
have a rain shadow where we subtract
half the area of any vertical rise/face.
(See Diagram 4.)
For a complicated roof, think of it this
way: ‘What is the area on the ground
where the rain would have fallen if the
roof wasn’t there?’
This will be our design catchment
area.
If you can remember your trigonometry,
The vertical rise area (VA)
= (roof plan area) * tan (roof slope) ... (1)
Catchment area (CA)
= roof plan area + 1/2(VA) ... (2)
Substitute eqn 1 in eqn 2
Therefore CA
= roof plan area + (1 +1/2* tan (roof slope)
= roof plan area * F,
where F = (1 + tan (roof slope) / 2)