What Is The Required Slope For Toilet Pipes

Each house has a vertical stack of waste that runs down to the main sewer. Toilets are connected by branch lines, which in many cases are almost horizontal. Plumbing rules dictate that the waste must flow downhill. Therefore, the slope of branch lines should not be too steep. It could cause water to flow too fast and cause the waste material to block the pipes.


System for disposing of toilet waste

Through the toilet flange (a plumbing fitting which holds the toilet on the floor), water enters the waste system. The water flows from there into the branchline, which is usually bent to make it horizontal depending on how it’s configured. It then feeds into the T-fitting that is attached to waste stack. This branch line can not exceed 3m (10 feet) in length. Its length is limited by the diameter of the pipe. To allow air to flow and prevent water from rushing, the branch line should be ventilated.

  • The toilet flange is a fitting in the plumbing that holds the toilet flush to the ground.
  • It then flows to the branch line, which can, depending on its configuration, have a bend that brings it nearly horizontal.


Size of the pipe and water flow

Two sizes of plumbing waste pipes are available: 7.5cm (3in) or 10cm (4in). It may sound logical to expect that bigger pipes will allow more water flow and better waste disposal. A smaller pipe can hold more water, so it is easier to fill. Larger diameter pipes will have a steeper slope in order to allow water to flow slowly enough to lift the solids to the stack. They then fall into the sewer.

  • Two sizes of plumbing waste pipes are available: 7.5cm (3in) or 10cm (4in).
  • Therefore, pipes with larger diameters have to be sloped slightly to allow water to flow slowly enough to lift the solids.


Ski slope required

Toilet waste can be trapped if its slope is too steep. If it’s too gentle the water won’t flow as fast and will settle. It is required that there be a minimum of three millimeters of slope for each 30 cm (or one-quarter inch) of pipe. However, it may go higher than this to allow water to flow more quickly. The outlet of a three meter (10 foot) pipe must be between 3.1 and 6.2cm (1 1/4 to 2 1/2 in) lower than the inlet. Pipes with larger diameters require a more gentle slope. The slope could be required to increase if they are made from cast iron.

  • If the toilet waste lines slope is too steep, it may leave behind solid matter. However, if they are too gentle water will not flow as quickly and settles out.
  • A minimum slope of three millimeters per thirty cm (or 1/8 inch per ft) is required by plumbing codes. However, it may be higher than that at 6 millimeters per 30 cm (1/4 in per ft).



Not only is incorrect sloping the brach waste lines a cause for frequent toilet blockages but so are other causes. When the branch line vents become blocked, it can cause a vacuum inside, which holds back the water, and allows solid matter to settle. You can clear your vents by spraying water on the roof. This will help you to get rid of any clogs before you decide to re-plumb those waste lines. Insufficient water can cause blockages in pipes due to low-flow toilets. These cases can lead to pipes that are too steep or not sloping correctly.

  • Toilet blockages can be caused by incorrect sloping at the brach waste drain line. However, there are many other reasons.