How to Replace a Bath Waste and Trap: A Plumber's Guide to Types, Sizes and Fitting

How to Replace a Bath Waste and Trap: A Plumber's Guide to Types, Sizes and Fitting

Bath waste and trap replacement is one of those jobs that looks straightforward until you are lying on your back under a bath panel with the wrong fitting in your hand. There are more variations than most people expect: click-clack, pop-up, flip plug, combined waste and overflow, slotted and unslotted — and that is before you get to trap depth and pipe connection size. This guide covers everything you need to know to specify and fit the right bath waste and trap, first time.

Bath Waste Types: What You Are Actually Choosing Between

The waste is the visible part — the fitting in the bottom of the bath that the water drains through. There are several mechanisms:

Slotted waste (with overflow): The standard UK fitting. The waste has a slot cut into it that connects to the overflow pipe so that water cannot rise above the overflow hole in the bath. Almost every domestic bath installation uses a slotted waste. If you are replacing like-for-like in a standard bath, this is what you want.

Unslotted waste (without overflow): Used where the bath has no overflow hole, or the overflow is handled separately. Less common in standard UK domestic work but sometimes encountered on freestanding baths or older cast iron baths.

Pop-up (click-clack) waste: A push-to-open, push-to-close mechanism. No plug required. These are increasingly common on modern baths and are often specified in bathroom refurbishments for a cleaner look. They come in chrome, brushed nickel, black, and gold finishes to match the rest of the bathroom.

Flip plug waste: A lever or tab on the waste body that flips to open or close. Similar in principle to pop-up but with a different actuator mechanism. Common on mid-range bath installations.

Combined bath waste and overflow kit: This is the full assembly — waste, overflow, the pipe connecting them, and sometimes the trap. These kits are the easiest way to replace a complete installation rather than sourcing individual components.

Trap Depth: 19mm vs 50mm Seal

The trap sits below the bath waste and holds a water seal that prevents drain gases entering the bathroom. For baths, there are two standard seal depths in the UK:

19mm seal trap: The most common for baths because space is tight between the bottom of the bath and the floor. A 19mm seal is enough to maintain an effective water seal under normal use conditions and is the correct specification for most standard UK bath installations. Building Regulations (Approved Document H) requires a minimum 19mm seal for baths.

50mm seal trap: Used where the installation is at higher risk of trap evaporation — a bath that is used infrequently, or where the trap is subject to negative pressure from nearby extract fans. The deeper seal takes longer to evaporate.

For the vast majority of residential bath replacements, a 19mm seal trap is correct.

The Top Access Bath Trap: A Time-Saver

Standard bath traps require access from below — either removing the bath panel or crawling underneath. On a retrofit or in a tight bathroom, this can be awkward. The McAlpine Top Access Bath Trap Waste combines the waste and trap in a single unit that can be installed and maintained from above through the plug hole — no bath panel removal required. The trap clean-out cover unscrews from the top, so clearing blockages does not require getting under the bath.

This is worth specifying on any bath replacement where future access will be difficult — an ensuite with a fully tiled surround, for example, or a bath built into a platform.

Pop-Up Wastes: Specifying the Right One

Pop-up and click-clack wastes have become the default on new bathroom installations. The key things to confirm when specifying one are:

• Slotted or unslotted — confirm whether the bath has an overflow hole. Most baths do, so a slotted waste is needed.
• Finish — chrome, black, brushed nickel, or gold. Match the basin and shower fittings.
• Cable length (for cable-operated pop-ups) — cable-operated pop-up wastes use a flexible cable run from the waste to the overflow cover plate where the actuator button sits. Cable length matters: a standard 1020mm cable covers most installations, but on deeper baths or unusual layouts, a 1050mm or longer cable may be needed.

The Embrass Peerless Black Bath Pop-Up Waste with Overflow is a clean, contemporary fitting that works well in modern bathroom schemes. The matte black finish is consistent across the waste body and overflow plate.

Complete Waste and Overflow Kits

If the existing waste and overflow assembly is old, corroded, or the customer wants a consistent finish, replacing the whole kit is cleaner than trying to match individual components. The McAlpine 1 1/2" Bath Combined Waste and Overflow includes waste, overflow fitting, and the pipe connecting them in a single kit. McAlpine is a well-regarded UK brand for bathroom waste fittings and their combined kits are straightforward to install.

Step-by-Step: Replacing a Bath Waste and Trap

1. Isolate water supply — not strictly necessary for waste work, but good practice. Ensure the bath is empty.
2. Remove the old waste — use a bath waste removal tool (a slotted bar that engages the cross-piece inside the waste) to unscrew the waste from below. On older installations the waste may be corroded in place — penetrating oil and patience.
3. Disconnect the trap — the trap connects to the waste tail above and the drain pipe below. Both connections should unscrew by hand or with a pair of grips. Have a towel ready for the water in the trap body.
4. Clean the seating surface — remove all old putty or sealant from around the waste hole in the bath. The new waste seals against this surface.
5. Fit the new waste — apply plumber's putty or silicone to the underside of the waste flange (check the manufacturer's instructions — some modern wastes use a rubber sealing washer and recommend against putty). Insert from above, tighten the back nut from below.
6. Connect the trap and overflow — connect the trap to the waste tail, then run the overflow pipe from the overflow fitting to the trap inlet. Ensure all connections are hand-tight plus a half-turn with grips. Do not overtighten plastic fittings.
7. Test — fill the bath and check all joints while the bath is full, then drain and check again.

Stock at Our Acton Trade Counter

We stock McAlpine and Embrass Peerless bath waste and trap fittings at our trade counter in Acton. Browse the full wastes and traps range and bath waste fittings on the website, or collect same day from 24 Western Avenue, Acton, London W3 7TZ.

APM Electricals — 24 Western Avenue, Acton, London W3 7TZ. Tel: 020 8702 8080. Open to the trade.