Quantity and placement of downpipes.
Box gutter fall ratio.
The amount of collected water increases until it spills over the sides of the gutter.
The fall is the measurement by which the gutter drops to allow water to drain towards the outlet downpipes.
The standard is about inch per 10 feet.
And the provision of appropriate overflow devices.
Allowable box gutter width slope and length required by the australian plumbing code as nzs 3500 3 2 box gutters allowable width slope and length to as nzs 3500 3 2018 stormwater drainage acceptable solutions.
This effectively states that all box gutters must allow for 30mm of freeboard which is from the overflow point eg.
Fall gradient x distance.
A gradient of 1 in 80 is converted to a number instead of a ratio.
Particular reference should be made to the correct sizing of gutter.
The gutter slope also called the pitch is the amount the gutter tilts down to let rainwater flow out of it.
For example calculate the fall in a 50 metre section of foul water pipework if the gradient is to be 1 in 80.
1 80 0 0125.
Fall 0 0125 x 50.
With the steeper fall i now have the water departs the gutters much more rapidly effectively increasing the capacity.
If gutters slope too severely they don t hold water and water could splash over the sides in heavy rain.
I also had a problem that my new flat roofed loft conversion dumps so much water into the valley on the front roof slope that it formed a fast flowing stream that jumped over the gutter.
If gutters don t slope water gathers in them.
If the length of the gutter is 10 5 metres 35 feet the amount of slope required is 1 25 cm 1 4 inch multiplied by 1 metres 3 5 feet which when rounded up is 2 5 cm 1 inch.
Rainhead overflow or specific box gutter overflow must be at least 30mm below the height of the back of the box gutter.
I fall or slope box gutters to be installed with a minimum uniform fall of 1 in 200 towards the outlet.
Fall gradient x distance.
Increasing the pitch increases a gutter s handling capacity but the gutter may look askew over a long run.
Box gutter with rain water head at end of box gutter.
Calculate the gutter slope by dividing the length of the gutter needed by 10.
As with gutters a downspout s capacity must match or exceed the expected runoff.
Roof drainage systems should be designed and detailed by a suitably qualified trade or professional in accordance with the bca and the australian standards.
The above recommendations assume that you have properly sized downspouts every 40 feet.
