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PLUMBING CONNECTION Winter 2022
HOWDOES ROOF SLOPE AFFECT THE
CATCHMENT AREA?
T
he design catchment area is not
the actual area of the roof sheets.
So how do we figure out what
the design catchment area is? And
why is this so? I will attempt to offer an
explanation.
The crucial thing to understand is:
Rainfall measurements are taken as the
water depth in millimetres of rain falling
on a horizontal surface, over a given time.
That is the way the Bureau of
Meteorology measures it. The angle of
the rain is not important to them. All
that matters is the depth of rain that
has fallen over a given time.
So, when thinking about this, we need
to calculate the area on a horizontal
plane, where the rainfall would have
fallen, if the roof wasn’t there. The roof
intersects this amount of rainfall.
This way, our method of measurement
will be the same as the way the Bureau
of Meteorology measures rainfall.
Therefore, we will both be on the same
page, and we can use their data in our
calculations.
However, to do this, somewhere along
the line, someone must dream up at
what angle the rain is falling.
Fortunately for us, the powers that be
have come up with an angle of 2:1, as
shown in the Diagram 1.
However, just like anything to do with
rainfall, there is no standard rainfall
event.
The Plumbing Codes devote a lot of attention to the topic of roof slope and how it affects a catchment
area, but it can be confusing.
Ken Sutherland
provides a clear overview.
BOX GUTTERS -
KEN SUTHERLAND
DIAGRAM 1
DIAGRAM 3
DIAGRAM 2
All we can do is base the design on
averages and figures pulled out of the
air.
For instance, we design eaves gutters
based on a rainfall event that may, or
may not, occur once in every 20 years.
A rainfall angle of 2:1 is as good as
any; in fact, this makes the calculations
much easier. Looking at Diagram 1, if we
have a vertical rise of 2m, we will have a
slope effect (horizontal distance) of 1m.
(2V: 1H).
Also from Diagram 1, a roof from A
to D also intersects the same amount of
rain as the main roof.
In fact, any roof between rainfall lines
B and C will intersect the same amount
of rain, and therefore have the same
catchment area.
But what is interesting is that it
doesn’t matter what the roof does to
get from point A to point D.
It can go up and down, or round and
round.
DIAGRAMS: SUPPLIED BY KEN SUTHERLAND.