How to cut and join soil pipe

Video by PlumberParts. This walk-through is based on "HOW TO CUT AND JOIN SOIL PIPE THE EASY WAY" from PlumberParts, a long-running UK plumbing tutorial channel. The demonstration covers cutting 110mm soil pipe square and chamfering it correctly — two steps where most DIY leaks actually start. Well worth watching before you pick up the saw.

1. Understand the system before you start

Most UK homes built after the 1970s use 110mm push-fit plastic soil pipe (sometimes called PVC-u or uPVC) for the main soil stack and branch connections. It handles waste from toilets, and connects to 40mm or 32mm waste pipes from baths, showers, and basins via boss fittings or branch junctions.

The key thing to grasp is gradient. Soil pipe needs a consistent fall — typically 1 in 40 — to carry waste cleanly. Too steep and liquids run ahead of solids. Too shallow and things sit in the pipe. Mark your fall before you cut anything.

2. Gather the right tools and fittings

You will need a fine-tooth saw or a pipe cutter designed for plastic, a tape measure, a marker pen, a deburring tool or file, and pipe lubricant or washing-up liquid. For fittings, decide whether you need a straight coupler, a 45-degree swept bend, a 90-degree bend, or a branch junction before you buy.

Push-fit soil fittings use rubber ring seals that rely on a smooth, square cut end. Do not scrimp on prep time here. A ragged or angled cut will rock the pipe in the socket and eventually leak.

3. Mark and cut square

Wrap a sheet of paper around the pipe to mark a perfectly square cut line — the paper edge gives you a straight guide all the way around. Mark with a permanent marker, then cut along the line using a fine-tooth saw, turning the pipe as you go rather than sawing through in one pass.

To be fair, with practice you can cut a straight line freehand, but the paper trick is quicker for most people. A soil pipe cutter tool gives the cleanest result of all if you have access to one.

4. Chamfer the cut end

This step is frequently skipped and is the reason many push-fit joints leak under pressure. Use a deburring tool, a file, or a sharp craft knife to cut a 15-degree chamfer around the outside edge of the cut end. This allows the pipe to slide past the rubber ring without folding or displacing it.

Run your finger around the inside edge too and remove any burr. A sharp internal burr will catch waste and start a blockage in time.

5. Mark the insertion depth

Before you push the joint on, mark the correct insertion depth on the pipe. Most soil fittings show the socket depth on the fitting itself — replicate that mark on the pipe. When the pipe is fully home, the mark should sit at the face of the fitting socket.

Why does this matter? Pull-back. After insertion, pull the pipe back by 10mm to allow for thermal expansion. Without this gap, hot waste can cause the pipe to buckle or push fittings apart over time.

6. Lubricate and assemble

Apply pipe lubricant or a small amount of washing-up liquid to the chamfered end and the rubber seal inside the socket. Push the pipe in firmly with a slight twisting action until it reaches the insertion mark, then pull back 10mm as described.

Check the pipe has not knocked the rubber seal out of its groove. A displaced seal looks like it is seated but will leak the moment waste passes through. Open the socket and visually check the ring if you are unsure.

7. Support the run properly

Soil pipe needs a bracket at every 1.2 metres minimum, and always within 150mm of each fitting. Unsupported runs will bow, stress the joints, and eventually work loose. Use the correct pipe clips for the pipe diameter, and make sure they are secured into something solid — timber noggins, masonry, or solid blocking.

On external stacks, use brackets with rubber inserts to reduce noise transmission into the structure.

When to call a handyman

Call Richard if you are dealing with cast-iron soil pipe, if the stack passes through a structural element, or if you need to break into the existing stack where there is no existing access point. Adding a new branch connection to an active stack is also a job best done with a second pair of hands.