The optimal slope for drain pipes typically ranges from 1/4 inch to 3 inches of fall per foot of length, depending on the pipe's diameter and the type of waste it carries. The larger the pipe, the less slope it requires to move wastewater effectively.
In general plumbing practice (based on common codes such as IPC or UPC in the United States), the minimum slope for a 4-inch horizontal drainage pipe is typically 1/8 inch per foot (1%) or steeper. This slope ensures proper drainage and prevents the buildup of debris or stagnant water in the pipe.
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.
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. Too steep (1/10) then the water runs quicker than the solids so doesn't wash them away (ugh!).
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.
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 it's too steep, water and solids can separate, causing solids to accumulate in the pipe.
In general, the maximum distance of the toilet from the main drain should not exceed 4 meters, with a 2% minimum slope. Within these limits, the plumbing system can guarantee an effective draining, avoiding bad smells and the problem of emptying the siphons.
A gradient of 1:60 means that there will be 1 unit of fall for every 60 units of patio width. The patio is to be 4.2m wide, so if that distance (the run) is divided by 60, the result is the 1 unit of fall. We'll work in millimetres rather than metres... So, there needs to be 70mm of fall across the patio.
4″ waste line can support 90 DFU's, Toilets can be 3–4 DFU's depending on how many gallons per flush. So, 22 toilets if they use a lot of water.
Horizontal drain piping, including building drains and building sewers, shall be installed in uniform alignment at the following uniform slopes: not less than 1/4 inch per foot for 1-1/4 inch. 1-1/2 inch and 2 inch pipe size. not less than 1/8 inch per foot for 3 inch, 4 inch, 5 inch, and 6 inch pipe size.
To calculate drainage fall, divide the total length by the fall ratio. For a 1 in 40 fall, divide the length of the pipe by 40.
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. A pipe that doesn't slope enough won't drain while a pipe that slopes too much drains water too quickly, leaving solids behind.
That's right, it is possible to have too much slope in your drain lines. According to Redwood Kardon, a former plumbing inspector, “Over sloped pipes (greater than ½ in. per ft.)
Guidelines for Toilet Drain Pipes
The universally accepted standard for a toilet drain pipe slope is a quarter inch per foot of pipe (1/4 inch per foot). This means for every foot the pipe travels horizontally, it should drop in elevation by one quarter of an inch to facilitate proper flow.
Since these pipes shouldn't be carrying contaminated water (stop peeing in the sink when the toilet is occupied!), the waste system doesn't need to be vented the way a soil system does, as no harmful gasses are produced.
The typical rule of thumb according to code requires you to run a horizontal pipe for no more than one-third of the total length.
Connecting gutters and gullies
Gutters and gullies should only collect rainwater and connect to the rainwater drain. Wastewater pipes should not be connected to the rainwater system. If the rainwater gutters and gullies are connected to the wastewater drain, rainwater could overwhelm the drain and cause flooding.
This means a 1:80 fall refers to an 80-inch rise in the horizontal plane and a 1-inch rise in the vertical plane. The resulting angle is approximately 0.7° from the horizontal. A good online tool for calculating the degree rise from a gradient ratio can be found here.
2% is 1 in 50, so a 2% slope is roughly 1/4″ rise per foot.
For instance, residential sewer line depth ranges from 18 to 30 inches. However, city sewer line depths are different. Your city's sewer lines must always be deeper than the deepest residential pipe to allow waste to flow downhill. As such, these lines can go as deep as needed.
Two-percent slope is required for 21/2-inch (64 mm) diameter and smaller pipe, and 1-percent slope is required for 3-inch (76 mm) diameter and larger pipe.
As a general rule, the recommended slope for most plumbing pipes is 1/4 inch vertical drop over every 1 horizontal foot, or about 2 percent. Pipes with a diameter of 4 inches or greater can have a slope of 1/8 inch per foot downhill from the source.