The most important bit of obvious advice ever: soil and waste pipes need to be on a downhill gradient! The “fall” or “drop” should be between 1/40 (1cm down for every 40cm across) and 1/110.
If a 32mm waste pipe is longer than 1.5 mtrs, a 40mm pipe 3 mtrs, and a 50mm pipe 4 mtrs, then the options are to increase the pipe size to the next size, and/or install an air admittance valve.
Gradients from 1 in 40 to 1 in 110 will normally give adequate flow velocities. If the gradient is steeper than 1 in 40, the liquid may run faster than the solids in the sloping foul water pipe.
The maximum fall for a waste pipe is 1 in 40. The minimum fall is 1 in 110 (apart from vertical pipes, but that's a different story). So pipes with a gradient between 1 in 40 and 1 in 110 should have adequate flow to prevent blockages from occurring.
If your waste pipes connect to the soil stack, a 40mm discharge pipe should be sufficient to facilitate this for a maximum of two groups of appliances per floor.
Soil and waste pipe system design and installation
The most important bit of obvious advice ever: soil and waste pipes need to be on a downhill gradient! The “fall” or “drop” should be between 1/40 (1cm down for every 40cm across) and 1/110.
Cast Iron
Despite the heat resistance and noise-dampening properties of cast iron, it's not as suitable for underground water supply as newer materials. Cast iron is not resistant to corrosion, which can compromise the quality of the water supply and damage the integrity of the piping, leading to leaks and breaks.
The term slope is also frequently used and has the same meaning as pitch. If you've ever wondered what the minimum slope for water drainage is, it is generally accepted that 1/4″ per foot of pipe run is the minimum for proper pitch on a sewer line.
These calculations just mean that for every 80 or 60 units of distance, your patio should slope down by 1 equivalent unit. For a 1 in 80 fall, this is a 12.5mm drop in level per metre and for a 1 in 60 fall, it's a 16mm drop per metre.
Vertical Fall (meters) = (Horizontal Distance / Gradient)
This formula divides the horizontal distance by the gradient to find the vertical fall in meters.
Branch and waste pipe gradients The gradient of a waste pipe should be even along its length and adequate to drain the appliance without loss of trap seal due to induced syphonage. The minimum gradient for 32, 40 and 50mm nominal size pipes should be 18mm/metre.
With a diameter of 40/40mm and an angle of 45 degrees, this waste pipe tee is the ideal choice for any sewage installation project. Its size and shape make it versatile and adaptable to various plumbing systems and layouts.
a 1 in 40 drop is 25mm drop in a 1 metre run. To measure that, I have a standard 1 metre long level, and I tape a small bit of 2*1 timber to one end of the level, then set the pipe so the level reads level and you have a 1 in 40 fall.
Rise = 1 unit. Run = 80 units. Fall per meter = 1/80 = 12.5mm.
Pipes must slope slightly downhill to drain properly. The standard slope is anywhere from ¼ inch to 3 inches per foot depending on the use. An accurate calculation is essential for properly functioning plumbing.
The 1% slope means that the vertical changed 1 unit for every 100 changes in the horizontal. A 1% slope is very slight. It means the ramp's elevation goes up 1 inch for every 100 inches it goes out in the horizontal direction.
1 in 80 Fall: Used for larger pipes or higher flow rates, with a 12.5 mm drop per metre. 1 in 60 Fall: Suitable for intermediate situations, with a 16.67 mm drop per metre.
A five degree pitch is equivalent to 9mm every 100mm or 87mm per meter.
“Over sloped pipes (greater than ½ in. per ft.) are as likely to clog as under sloped ones because the liquids move too fast and leave the solids behind.” Why it's costly: Without the proper slope, drain lines may clog more frequently.
Recommended Slope for Toilet Drain Pipes
An adequate slope allows gravity to effectively carry waste and water through the drain pipes and into the sewer or septic system. The universally accepted standard for a toilet drain pipe slope is a quarter inch per foot of pipe (1/4 inch per foot).
Australian standards dictate 100mm sewer drains (as in the video) must be laid to a minimum grade of 1.65% (or 165mm per 10m of drain) The angles of bends waste pipe lengths should be marked out prior to excavation to minimise the usage of multiple bends for a single direction change.
In short, pvc drainage pipes are the best option for your underground drainage system.
Underground Drainage Hoppers – Similar to a gully, these allow the downpipe or waste pipes into the underground drainage system. These however do not come with a trap built in. Rodding Eyes – These are installed to allow rodding of the system in case of blockages.
Dig-up test results in the U.S. and around the world indicate that PVC pipe can be expected to provide reliable service in excess of 100 years.