How to Connect a Basin Trap: Steps and Sealing Tips

The basin trap serves a double function: it drains the water while forming a water seal that blocks smells coming back up. If badly fitted, it leaks at every connection, or lets smells through a dried-out gasket. Connecting a basin trap is a plumbing operation within everyone's reach, provided you respect the assembly order and never tighten a fitting dry.

The parts of the trap and the equipment

• The trap itself: tailpiece, P-trap bowl and drain tube
• The basin waste, with its grid and rubber gasket
• The sealing rings and plastic cone washers
• An adjustable wrench and a flat screwdriver
• PTFE tape and a bucket to catch the water

Choosing the right trap

Two main families exist: the classic tube trap and the bottle trap, more compact and easier to clean, very common under basins. The standard drain diameter is 32 mm, versus 40 mm for the kitchen sink. Check the distance between the centre of the waste and the wall drain connection: bottle traps adapt by offsetting, but a large gap requires an adjustable tube trap.

Traps are fixed in two ways: push-fit models assemble without tools and come apart in a single gesture for cleaning, while screw models (cap nuts) offer better long-term sealing but need a wrench. Also check the material: polypropylene resists limescale and cleaning products better than plain PVC. Finally, measure the height under the basin: a bottle trap needs at least 15 cm of clearance under the waste to fit the cleaning bowl.

Fitting the basin waste

1. Clean around the basin drain hole and remove the old gasket.
2. Thread the rubber gasket under the grid, ceramic side.
3. Place the grid in the hole and hold it with one hand.
4. Screw the waste nut under the basin, without forcing so as not to crack the ceramic.
5. Wipe off the excess silicone sealant if the model requires it.

Assembling and connecting the trap

1. Push the tailpiece into the waste nut and lock it by hand.
2. Clip the bowl into place checking that the cone washers are properly seated.
3. Connect the drain tube to the wall pipe or to the 32 mm pipe in the wall.
4. Hold the tube with your hand and tighten the fitting in successive half-turns.
5. Align the assembly checking that the bowl stays accessible for cleaning.

Testing the seal and checking the flow rate

Reopen the cold then hot tap, and let it run for a minute. Place a sheet of kitchen paper under each fitting and check for any weeping after a few minutes. Then fill the basin completely and empty it all at once: this test reveals leaks that a simple trickle of water does not show. Finally, check that the water drains without gurgling, a sign of a good water seal.

Replacing an existing trap

If a trap leaks continuously or lets smells through despite new gaskets, complete replacement is often faster than repair. Unscrew the waste and drain nuts, unclip the bowl and remove the assembly. Clean the bottom of the unit under the basin and the wall drain opening first, then fit the new trap in the order presented above. Keep the original collars and spacers: certain configurations require an intermediate fitting to make up a difference in height between the waste and the wall drain.

The mistakes to avoid

• Tightening the plastic fittings with a wrench, which crushes the gaskets
• Forgetting a cone washer, a cause of invisible leaks
• Connecting the trap to the wrong drain pipe (32 mm instead of 40 mm)
• Neglecting the slope of the drain tube, which must slope gently downwards towards the wall

Maintaining the trap to avoid bad smells

Once connected, the trap needs minimal maintenance. Once a month, pour a litre of hot water into the basin to top up the water seal, that reserve of water that blocks smells rising back up and evaporates if the room stays unoccupied for several days. If smells persist despite the water seal, dismantle the bowl, clean it with white vinegar, and check that the drain pipe is not on a reversed slope, which would stop the water staying in it.

Connecting a basin trap takes about twenty minutes and comes down to a few simple moves: fitting the waste with its gasket, assembling the trap in the right order, tightening each fitting with a feel for it. The full-load sealing test is the last step, and the most important. In the event of a persistent leak, dismantle the fitting concerned, replace the cone washer and reassemble without rushing: the cause is almost always a badly positioned gasket.