Plumbing Materials

Plumbing runs hidden behind walls and under floors for decades. Using the right pipe for the right job, and jointing it correctly, is the difference between a trouble-free installation and a slow leak that causes thousands of pounds of damage years later.

These guides cover the plumbing materials used in a domestic extension: the pipes that carry hot and cold water, the fittings that connect them, the waste systems that drain away water, and the isolation valves that let you turn off sections without shutting down the whole house. Each page explains what the component does, where it is used, and what your building control inspector will check.

Where to start

The decision that shapes most plumbing quotes is copper or plastic. Read 15mm copper pipe and 15mm push-fit pipe side by side, then push-fit fittings and compression fittings. Push-fit is quicker and forgiving to install, and copper is harder to damage later. Most extensions end up with a mix, and knowing which goes where lets you follow the quote.

The guides, grouped by job

Water supply pipe. 15mm, 22mm and 28mm copper, 15mm and 22mm push-fit pipe, and UFH pipe if you are putting wet underfloor heating in the new floor.

Joints and connections. Push-fit fittings, compression fittings, brass tap connectors, plumber's jointing compound and, for older threaded work, hemp and compound.

Control and safety. Stop valves and isolation valves so a leak does not mean turning off the whole house, double check valves to stop backflow from outside taps and appliances, and thermostatic mixing valves to prevent scalding where building regulations require them.

Waste and soil. 110mm soil pipe, solvent-weld waste pipe and its fittings, waste traps, a strap-on boss for joining into an existing stack, a washing machine standpipe, and an air admittance valve where a vent through the roof is not practical.

Gas. 22mm gas supply pipe. Any gas work must be done by a Gas Safe registered engineer. This guide is there so you know what they are doing and what to check on the certificate.

Where these fit in the build

Pipe routes are decided in plumbing layout planning, the pipes go in at first fix plumbing (with underfloor heating laid before the screed), and taps, traps and appliances are connected at second fix plumbing. Below-ground drainage is a separate job, covered in the drainage stage of the build.