Many plumbers guidelines recommend a ratio of around 1 500 which is roughly 20mm for every 5m of guttering.
Box gutter fall ratio.
I fall or slope box gutters to be installed with a minimum uniform fall of 1 in 200 towards the outlet.
For example calculate the fall in a 50 metre section of foul water pipework if the gradient is to be 1 in 80.
Roof drainage systems should be designed and detailed by a suitably qualified trade or professional in accordance with the bca and the australian standards.
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.
Fall gradient x distance.
If gutters slope too severely they don t hold water and water could splash over the sides in heavy rain.
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.
Calculate the gutter slope by dividing the length of the gutter needed by 10.
The above recommendations assume that you have properly sized downspouts every 40 feet.
As with gutters a downspout s capacity must match or exceed the expected runoff.
The standard is about inch per 10 feet.
Fall gradient x distance.
The amount of collected water increases until it spills over the sides of the gutter.
Simplified suitable for flattish roofs with little or no vertical faces or steep roofs draining to one side of box gutter only or any roof if you are in a hurry and make the necessary allowances for all catchments.
This effectively states that all box gutters must allow for 30mm of freeboard which is from the overflow point eg.
If gutters don t slope water gathers in them.
The gutter slope also called the pitch is the amount the gutter tilts down to let rainwater flow out of it.
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.
The fall is the measurement by which the gutter drops to allow water to drain towards the outlet downpipes.
1 80 0 0125.
Fall 0 0125 x 50.
Quantity and placement of downpipes.
Fall 0 625 metres or 625mm.
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.
Particular reference should be made to the correct sizing of gutter.
A gradient of 1 in 80 is converted to a number instead of a ratio.
