Pipes & Fittings: PEX

Pipes and fittings: choosing the right material for your network

An undersized pipe or a fitting that’s incompatible with the existing network material is a guaranteed leak within six months. This category brings together pipes in PEX, multilayer, copper and PVC, along with their matching fittings — crimp, push-fit, compression or solvent-welded — for water supply, drainage and heating. Each family serves a specific purpose: PEX for sanitary water under underfloor heating, copper for exposed installations that need to last 50 years, pressure PVC for outdoor irrigation and drainage.

PEX and multilayer pipe: the standards for new builds and renovation

PEX (cross-linked polyethylene) now dominates sanitary plumbing projects thanks to its flexibility: a 25 or 50 metre coil unrolls directly from the manifold to the water point, with no elbow to solder. Common diameters are 12 mm for individual supply lines, 16 mm for secondary distribution, and 20 to 26 mm for riser columns. Standard NF EN ISO 15875 governs its pressure rating (PN10 or PN16 class depending on wall thickness) and temperature resistance, with continuous performance at 60°C for domestic hot water.

Multilayer pipe combines an aluminium core sandwiched between two layers of PE-RT, giving it a real advantage over pure PEX: it holds its shape once bent, with no shape memory pulling it back to its original position. This makes it the preferred choice for underfloor heating, where the laying pattern needs to stay stable while the screed is poured.

Crimp or push-fit fittings: two techniques, two toolsets

A crimp fitting requires a specific press tool (TH or U jaw depending on the manufacturer) that compresses a stainless steel or brass ring around the pipe once it’s inserted onto the fitting’s spigot. The joint is irreversible once crimped, so it’s essential to check the pipe insertion depth before crimping. Push-fit fittings work the other way round: a PEX ring, expanded cold with an expander tool, retracts back into place afterwards, allowing more tolerance for pipe lengths cut a little short.

Copper: the go-to material for exposed pipework

Hard-drawn (rigid) or annealed (soft, coiled) copper tube remains the material of choice for visible installations, especially in renovations where looks matter as much as performance. Diameters 12/14, 14/16 and 16/18 mm (inner/outer diameter) cover almost all domestic needs. Fittings are soldered with a torch using soft solder (rosin flux, up to 200°C) or capillary brazing for gas lines, or fitted with compression rings for a flame-free installation — useful when flammable materials are nearby.

One thing many people don’t realise: never connect copper directly to galvanised steel without a dielectric union. The galvanic reaction between the two metals speeds up corrosion of the steel, with pinholes that can perforate the pipe in just a few years.

Pressure PVC and drainage PVC: two ranges not to confuse

Pressure PVC (usually grey or blue, rated PN10 or PN16) is used for pressurised water supply for irrigation, pool filling or outdoor networks. It differs from drainage PVC (white or orange, non-pressurised), used for wastewater and rainwater in diameters of 32, 40, 100 and 125 mm. Both are joined with a specific solvent cement after cleaning and priming the surfaces, allowing a drying time of at least 24 hours before pressurising — a joint glued too early will leak under load even if it looks watertight to the eye.

• PEX: domestic water, underfloor heating, conduit-run installation, PN10/PN16
• Multilayer: underfloor heating, networks requiring a stable bend
• Copper: exposed pipework, gas, 50-year lifespan with minimal upkeep
• Pressure PVC: irrigation, pools, buried outdoor networks
• Drainage PVC: wastewater, rainwater, non-pressurised

How to avoid diameter mistakes when ordering

The diameter shown on the packaging sometimes refers to the outer diameter, sometimes the inner, depending on the material — a 16 mm PEX pipe refers to the outer diameter, whereas a 16/18 copper fitting specifies both measurements. Before ordering, measuring the existing pipe with callipers rather than relying on memory or a visual estimate saves you a product return and a job stalled for half a day.

Can you mix materials on the same network?

Yes, as long as you use the right transition fittings: a brass mixed fitting to go from PEX to copper, a dielectric union between copper and steel, a threaded adapter to connect PVC to taps and valves. The same rule applies at every junction point: the fitting’s working pressure must match that of the weaker material on either side of the joint.