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PLUMBING CONNECTION
SUMMER 2015 65
3.7.7.1 HYDRAULIC CAPACITY
The hydraulic capacity of an overflow device shall be not
less than the design flow for the associated gutter outlet.
Overflow devices shall discharge to the atmosphere.
This means whatever the flow capacity of the gutter is that
the overflow must be able to discharge the amount of water in
the event of a 1 in 100 year storm if the downpipe is blocked.
5.3.2 GUTTER INSTALLATION
(e)
Freeboard
Box gutters calculated in accordance with AS/
NZS 3500.3 include a 30 mm allowance for freeboard to
prevent wind driven spillages.
(G)
Sizing
Box gutters to be sized in accordance with AS/
NZS 3500.3 to effectively collect and discharge all roof
water with an overflow risk of 1 in 100 y. As an example
for commercial or industrial installations, gutters to
have a minimum size of not less than 300 mm wide and
75 mm deep at the high end; for domestic installations
a minimum size of not less than 200 mm wide and 75
mm deep at the high end commensurate to the roof
catchment area serviced by that gutter.
NOTE: Box gutters 200 mm wide are more prone to
blockages and should be subject to more frequent
inspections and maintenance.
(i)
Fall or slope
Box gutters to be installed with a minimum
uniform fall of 1 in 200 towards the outlet.
This effectively states that all box gutters must allow for
30mm of “freeboard” which is from the overflow point eg.
rainhead overflow or specific box gutter overflow must be at
least 30mm below the height of the back of the box gutter.
All domestic box gutters must be a minimum of 200mm
wide and 75mm deep.
AS/NZS 3500.3: 2015 – 3.7.4 LAYOUT
(e) For the sump/high capacity overflow device, the depth
of the sump (hs) shall be not less than 150 mm
regardless of the position of the normal outlet. Changes
are not required, provided the sump/side overflow device
is used
(G) Box gutters shall—
(i) be straight (without change in direction);
(ii) have a horizontal constant width base (sole) with
vertical sides in a cross-section;
(iii) have a constant longitudinal slope between 1:200 and
1:40;
(iv) discharge at the downstream end without change of
direction (i.e., not to the side); and
(v) be sealed to the rainheads and sumps.
This section tells us that the (h
s
) or depth of sump
cannot be less than 150mm and it does not matter where
the sump is as long as the gutter falls to it and you provide
an overflow device.
This next point is a big one and affects a lot of current
designs:
Box gutters shall be straight (Without change
of direction).
This means that every box gutter must
discharge independently and cannot follow the outline of
the building until you find a rainhead. Each box gutter run
must discharge independently to ether a sump or directly
out of the building without changing the size of the gutter or
changing direction.
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