Waste pipes are an important part of any construction project involving household plumbing. This is because they are necessary to transport “greywater” from your home to the proper disposal area.
If you’re redoing a bath or need to repair some pipes, knowing the proper dimensions is key. Use this guide to learn more about proper waste pipe dimensions and their necessary fittings.
Common Waste Pipe Sizes
Waste pipes in the UK are measured in millimetres. Take a look at some of the standard waste pipe sizes and their uses below:
|Waste Pipe Dimensions||Uses|
|21.5mm||Overflows for water tanks in lofts and cisterns|
|32mm||Low water volume appliances (e.g., small hand basins)|
|40mm||Kitchen sinks, showers, baths|
|50mm||Commercial appliances such as heavy-duty washers; connecting multiple waste pipes to one flowing system|
If you’re uncertain of your pipe’s size, always remember to measure from the outside of the pipe. The dimensions don’t refer to the size of the hole, but the external edges of the pipe end.
Two Types Of Waste Pipes
There are two types of waste pipes you can use for both new waste systems and pre-existing pipes. These are:
- Solvent weld
Some systems may use only one type of pipe or the other. But it is possible to have a mix of both types of pipe in one system. Read on to find out more about each type.
Push-fit pipes don’t require additional fittings or even tools to install. This is because they have a captive ring seal that creates watertight joints on its own.
But these seals are still flexible enough to allow for thermal movement. It’s also easy to remove and replace these pipes without additional tools.
Solvent weld pipes use a permanent solvent cement to fit two pipes (or a pipe and fitting) together. Like push-fit, they are easy to install, but may require more effort to remove.
While they don’t require extra tools or hardware, they do require a solvent, as the name suggests.
You coat the outside of the internal pipe with the solvent, then the inside of the external pipe or fitting. Then you push the two parts together to form a seal.
To create a permanent weld, you need to twist the two components together. Then you’ll get a tight and long-lasting fit. If there’s any chance of impact damage to the plumbing system, solvent weld pipes are a reliable option.
Fittings For Waste Pipes
Once you know the dimensions of your waste pipe, you can find the correct fittings for your needs.
Because the fittings typically wrap around the pipe, the size you need will usually match the diameter of your pipe. For example, if you have a 35mm waste pipe, then you can use a 35mm pipe clip.
Below are some of the common fittings used for waste pipes.
- Coupling – Also called slip couplings, these fittings connect two pipe lengths together in a ring. They can be double or single-socketed.
- Pipe clips – These are flexible clips that secure waste pipes to walls or other piping and add support around joints.
- Tees – These connectors can join two (single tee) or three (cross tee) waste pipes at a 92.5-degree angle.
- Conversion bends – Like regular bend pipes, conversion bends change the direction of a waste pipe run. But they also allow you to convert between push-fit and solvent weld pipes and fittings.
- Caps – Sometimes the perfect fit for a pipe system can include extra outlets for wastewater. To avoid spillage, you can use caps to close off these unused outlets.
- Waste traps – These are tight vertical bends that create a shallow pool of water at all times. This pocket of water blocks bad odours from coming back up your pipes and out of the drain.
- Air admittance valve – Air admittance valves are used in very long waste systems. The longer the system, the more bends there are and the potential for changes in air pressure. These valves allow air to enter the pipes as needed to balance the air pressure.
Because push-fit and solvent weld connections are fast and easy to install and remove, compression fittings are now less common.
But while some fittings are specific to either push-fit or solvent weld pipes, compression fittings are compatible with both. This is particularly good for systems that use both forms of pipe.
So if you want to update your plumbing, compression fittings can be very useful to customise it exactly how you want.
Compression fittings consist of a sleeve, nut and the fitting body. They require a wrench to install and remove, but create a watertight steel that can withstand high pressure.
When you screw the nut into the fitting body, the sleeve underneath compresses and forms a watertight seal.
Uncommon Waste Pipe Dimensions And Their Uses
There are other sizes of pipes available, but they’re often for more specific situations than the common household waste system.
For instance, 38mm pipes are more common throughout Europe, instead of the 32mm and 40mm common in the UK. If you buy appliances from abroad, keep this in mind as they may not be easily compatible with your system.
68mm pipes are only meant for rainwater, not household waste. 100, 120, and 125mm pipes are not meant for water at all, but for ventilation and ducting.
Finally, 160mm is very rarely seen in private domestic homes. Instead, it is meant to carry sewer water, so you can find it near flats and other larger buildings.
Difference Between Waste Pipes And Soil Pipes
While soil pipes also remove waste from a residence, it’s a different kind of waste. Waste pipes remove “greywater,” which is the leftover water from appliances like dishwashers, washing machines and showers.
These pipes are small because they are only meant to carry water, not solid material. You shouldn’t use a waste pipe for a toilet, for example.
Soil pipes, on the other hand, can transport solid waste material as well as water into the sewage system. These pipe systems, also called soil vent pipes (SVP) are used for things like toilets, urinals and bidets.
Because they carry sewage, these pipes need larger dimensions than greywater waste pipes.
Common Soil Pipe Dimensions
|Soil Pipe Dimensions||Uses|
|110mm||Above-ground sewage systems; single homes|
|150mm||Below-ground for toilets and kitchen; often made of cast iron|
|160mm||Below-ground; multiple-home systems|
There are smaller sizes available, but you should never use a pipe smaller than 100mm for a toilet soil pipe. These pipes are too small to handle the solid waste load of a normal toilet.
Understanding the different dimensions of waste pipes and what they mean doesn’t have to be complicated. For everyday household wastewater, there are only a handful of standard sizes from which to choose.
In addition, most of the fittings you need for your pipes will have the same dimensions as the pipes themselves.