Fitting waste pipes to sinks, basins, baths and showers is often one of the trickiest parts of any kitchen or bathroom plumbing project. Getting the waste pipe route right from the start saves you from leaks, blockages and unpleasant smells later. Before you begin any plumbing work inside your home, it helps to understand how the various plumbing systems inside a house connect together and where your waste pipe ultimately discharges.
How Waste Traps Work and Why They Matter
Every sink, basin, bath or shower that drains water must have a waste trap installed. The purpose of a waste trap is simple yet essential. It creates a water seal that prevents foul-smelling gases and odours from the main drainage system from entering your home. Without a trap, every drain in your property would allow sewer gases to flow back into your kitchen and bathroom.
The core principle of a waste trap is a U-shaped section of pipe that holds a small amount of standing water. This water forms a barrier between the room and the drain. When you empty a sink or run a bath, the incoming water pushes the old water out of the trap and replaces it with fresh water, maintaining the seal. You can find a full explanation of the science behind these fittings in this detailed guide on how waste traps operate.
All waste pipes from your home eventually connect to the foul water drainage system, which handles everything except rainwater. The gases that build up inside these drains are under pressure and will try to escape through any available opening. The water trapped inside your waste pipe is the only thing standing between your living space and those drain odours.
Without water in the trap, the seal is broken and smells will enter the room. This can happen if a fixture is unused for a long period and the trap water evaporates, or if the waste pipe is not installed with the correct fall to allow proper drainage.
Types of Waste Traps and How to Choose the Right One
There are several types of waste traps available, each suited to different installations and space constraints. The most common types you will encounter include:
- P traps – The most widely used type under kitchen sinks. The pipe forms a P shape with the outlet running horizontally through or along the wall. These traps usually come in two parts that screw together, making them easy to install and dismantle for cleaning.
- S traps – Similar to a P trap but the outlet drops vertically into the floor instead of running through the wall. Common in older homes where the waste pipe goes through the floor rather than the wall.
- Bottle traps – A compact cylindrical design often used under bathroom basins where cupboard space is limited. These take up less room than a P trap but can be more prone to blockages if not cleaned regularly.
- Shower traps – Low-profile traps designed to fit within the shallow space under a shower tray. They are wider and flatter than standard traps to maximise water seal depth while fitting in a confined area.
- Bath traps – Larger traps designed to handle the high volume of water discharged when a bath is emptied. Many bath traps incorporate an overflow connection as well.
When selecting a trap, consider the space available, the type of fixture and whether you need to connect additional appliances. P traps remain the most versatile option for most kitchen sink installations, especially when working with plastic pipe joints and fittings that allow flexible routing under the cupboard.
| Trap Type | Best For | Outlet Direction | Space Required |
|---|---|---|---|
| P trap | Kitchen sinks | Horizontal through wall | Moderate |
| S trap | Older homes with floor waste | Vertical through floor | Moderate |
| Bottle trap | Bathroom basins | Horizontal or vertical | Compact |
| Shower trap | Shower trays | Horizontal | Low profile |
| Bath trap | Baths | Horizontal | Large |
Most kitchen sinks come supplied with a waste kit containing the trap and all necessary connectors. The kit usually includes enough components to position the down pipe close to the back wall, keeping the space under the sink free for storage.
Setting Up an Under-Sink Waste System with Appliance Connections
Modern kitchens typically have a washing machine and dishwasher that also need to drain into the waste system. Most sink waste kits include appliance fittings that allow you to connect the drain hoses from these machines directly into the under-sink pipework. This is a neat solution when your appliances are located close enough to the sink. For more detailed guidance on how drainage piping is structured throughout a property, see our article on building drainage systems and their design.
The waste pipe configuration under a 1½ bowl sink can become more complex. These sinks have two bowls but share a single waste outlet, requiring additional connectors and adaptors. A typical setup uses a P trap positioned at the lowest point so the waste can run straight out through the wall. The space under the sink fills up quickly with pipework, so careful planning is essential.
Here are the key considerations when assembling an under-sink waste system:
- Position the P trap as close to the back wall as possible to maximise storage space inside the cupboard.
- Use the supplied adaptors to connect washing machine and dishwasher hoses if your appliances are nearby.
- Ensure all screw-fit connections are hand-tight only – over-tightening can crack the plastic fittings.
- Test for leaks by running water through the sink and checking every joint before closing the cupboard.
- If your appliances cannot reach the under-sink connections, you will need a separate drainage setup. Read the guide on dishwasher plumbing setup and washing machine connections for more details.
There are no strict rules about how the pieces must fit together, as long as the connections are watertight and the pipework has a continuous downward slope toward the outlet. The flexibility of plastic push-fit and compression fittings means you can adapt the layout to suit your specific cupboard space.
Drilling Through Walls and Running the Waste Pipe Outdoors
Once the internal waste pipework is assembled, the next step is to create a hole through the external wall so the waste can discharge into the outside drainage system. This is where many DIY enthusiasts encounter difficulty. The most effective method is to hire a heavy-duty electric drill with a diamond core 50 millimetre drill bit. A core drill of this type will cut through brick, block and stone in under five minutes without the need for hammering or chiselling.
If the sink unit is fitted against an external wall, you will first need to cut a hole in the back panel of the cupboard to access the wall itself. A 100 millimetre hole saw attachment on a standard drill will create enough clearance for the core drill bit to pass through.
Follow these steps for drilling through the wall:
- Mark the position of the hole on the internal wall, ensuring it aligns with your waste pipe outlet.
- Drill through the cupboard back panel with a hole saw if needed.
- Fit the diamond core bit to the drill using an arbor, as core bits do not have a standard shaft for the chuck.
- Attach an extension piece to reach through the full thickness of the wall.
- Drill slowly and do not push hard – diamond-tipped bits cut through masonry easily when allowed to work at their own pace.
- As you approach the outer face of the brickwork, reduce pressure significantly to avoid blowing the face off the brick.
For a complete walkthrough of joining pipe sections after the hole is drilled, refer to this practical guide on connecting PVC pipes using various methods. Once the hole is through, push the pipe through from inside, mark the external wall face on the pipe with a pencil, then withdraw the pipe to make the external connection.
Maintaining Your Waste Pipes and Preventing Blockages
A well-fitted waste pipe system requires minimal maintenance, but blockages can still occur if debris builds up over time. Kitchen sink wastes are the most vulnerable because food scraps, grease and cooking oil can accumulate inside the pipes. Using a sink strainer in the plug hole is the single most effective way to prevent solid food waste from entering the drain.
One advantage of screw-fit waste systems is that they are easy to dismantle for cleaning. If a blockage occurs in the trap or pipework under the sink, you can simply unscrew the connections, remove the debris and reassemble the system. This is much more straightforward than dealing with a blockage in a solvent-welded or push-fit system where the joints are permanent.
When using push-fit or solvent-welded waste pipes, always install access points or inspection eyes at key locations so that blockages can be cleared without cutting the pipework. For a comprehensive overview of the various connector types you might need, check out the range of pipe fittings available for home plumbing projects.
Bathroom wastes tend to block less often than kitchen wastes but can still accumulate hair and soap scum over time. Regular flushing with hot water and a mild cleaning solution helps keep the pipes clear. If you notice slow drainage, address it promptly before the blockage becomes complete.
Finally, ensure that your waste pipe has the correct fall or gradient toward the main drain. A fall of approximately 20 to 40 millimetres per metre of pipe run is generally recommended for 40 millimetre waste pipes. Pipes that are too flat will not drain properly, while pipes with too steep a fall can cause the water to run off faster than the solids, leaving debris behind. To continue with the external connection work after your hole is drilled, see the second part of this guide on fitting wastes outdoors and connecting to drainage.
By taking the time to plan your waste pipe route carefully, choosing the right trap for each fixture and using the correct drilling technique for the wall, you can achieve a reliable waste system that will serve your home for years without problems.