How to replace a bath waste

Video by Plumberparts. This walk-through is based on the video "How to replace a bath waste" from Plumberparts, a well-regarded UK plumbing tutorial channel. The video is particularly useful for showing how to diagnose whether the leak is at the waste itself or the trap — worth watching before you start undoing anything.

1. Find the source of the problem first

Before ordering parts, work out exactly where the water is coming from. Fill the bath, then pull the plug and watch underneath with a torch. If water tracks down around the plughole fitting itself, the waste body or its seal has gone. If it drips from the trap joint below, you may just need a new washer on the trap nut rather than a full waste replacement.

It is also worth pressing down on the chrome waste cover — if it flexes or rocks, the large backnut under the bath has come loose, which can sometimes be nipped up without replacing anything at all. That said, if the fitting is old and the chrome has corroded through, a full replacement is the tidier fix.

2. Gather the right parts and tools

Most UK baths take a 40 mm (1½ inch) waste fitting. Buy a kit that includes the new waste body, overflow cover, overflow pipe, and trap — it is worth replacing the lot if the bath is more than ten years old, because mixing new and old plastic parts often leads to a joint that never quite seals properly.

You will need a flat-blade screwdriver, a pair of adjustable grips or slip-joint pliers, plumber’s silicone sealant or plumber’s putty, and a bucket and towel for the residual water in the trap. A multi-grips is handy for the large backnut, which can be in a very awkward position depending on the bath design.

3. Remove the bath panel and drain the trap

Take the side bath panel off — most clip on or are held by a couple of screws at the bottom. Once you can see the trap, put a bucket underneath and unscrew the trap nut by hand. Slip-joint pliers help if it is tight, but go carefully; old plastic nuts crack easily and you may need the trap body to stay intact to take to a plumber’s merchant to match the outlet size.

Drain any remaining water into the bucket, then take the trap away and set it aside. You now have clear access to the underside of the bath and the large backnut that holds the waste fitting in place.

4. Undo the old waste fitting

From above, insert a flat screwdriver blade into the cross of the waste grid and hold it firm. From underneath, use multi-grips or a box spanner to turn the large backnut anti-clockwise. On older baths it may be stiff — a short burst of penetrating oil left for ten minutes can help considerably. Do not force it without the screwdriver engaged above, or you will just spin the whole fitting and round off the backnut.

Once the nut is off, lift out the waste body from above. Clean off any old putty, silicone, or limescale from around the hole in the bath. A plastic scraper is fine on acrylic; on a cast iron or steel bath you can use a flat blade but take care not to scratch the enamel.

5. Fit the new waste body

Roll a thin sausage of plumber’s putty and press it around the underside flange of the new waste grid before dropping it into the hole from above. Alternatively, run a bead of clear silicone — either works, though putty is easier to clean up if you overfill. Drop the rubber sealing washer and the fibre washer over the thread from below, then thread on the backnut finger-tight.

Hold the waste grid steady from above with the screwdriver and tighten the backnut from below until the putty just starts to squeeze out around the flange. Wipe away the excess putty or silicone cleanly. Do not overtighten — cracks in the bath surround are a real possibility on older acrylic baths if you go too hard.

6. Refit the trap and test thoroughly

Attach the new trap to the waste outlet, making sure the rubber cone washer is seated squarely inside the nut before you tighten it. Finger-tight plus a quarter turn is usually enough — traps do not need to be wrenched up hard. Connect the overflow pipe if your new kit includes one, checking the gasket is correctly located at the bath overflow hole.

Fill the bath about a quarter full, then pull the plug and crouch down with a torch to watch every joint as the water drains. Check the waste body seal, the trap nuts, and the waste pipe connection. If anything weeps, dry it off and nip it up a touch. Once it is dry and clear, refit the bath panel and you are done. Mind you, it is always worth checking again the following day — small seeps can take a fill cycle or two to show up.

When to call a handyman

Call Richard if the leak turns out to be at the waste pipe connection further along the run and not at the fitting itself, if the bath is cast iron and the backnut has seized solid, or if removing the old waste reveals cracking around the bath hole that needs attention before a new fitting will seal properly. A corroded overflow pipe that has started to weep inside the wall is also one to get sorted sooner rather than later.