Waste Traps and Bottle Traps — P-Traps, S-Traps, and Bath Waste Kits for UK Plumbers

Every sink, basin, bath, and shower tray needs a trap — a water-sealed fitting that blocks sewer gases from entering the building while allowing waste water to drain freely. Choosing the wrong trap type or the wrong depth seal can mean call-backs, foul smells, or a failed Building Control sign-off. This guide covers the main trap types, sizing, seal depth requirements, and the most common installation scenarios you'll encounter on a domestic or light commercial job.

Why Traps Matter: The Water Seal

A trap works by retaining a small volume of water (the "water seal") in a U-shaped section of pipework. This water blocks the passage of sewer gases — hydrogen sulphide, methane, and other odorous compounds — back into the property. If the water seal evaporates (in infrequently used appliances), is siphoned out by pressure surges, or is physically displaced, foul odours will result.

Building Regulations Approved Document H and British Standard BS EN 12380 govern trap seal depths and anti-siphon requirements in the UK. For domestic installations:

• Minimum seal depth: 25mm for most appliances (75mm recommended for ground-floor and close-coupled connections)
• Anti-siphon traps: Required where pipe runs exceed 1.5m horizontal or 2m vertical without adequate ventilation
• Resealing traps: Used where negative pressure events are likely (open-plan wet rooms, high-rise installations)

P-Trap vs S-Trap: When to Use Each

The most common distinction is between P-traps and S-traps, determined by the outlet direction:

P-Trap (Horizontal Outlet)

The outlet exits horizontally into the wall. P-traps are used for:

• Under-sink basin waste where the soil stack or branch pipe is at wall level
• Kitchen sinks connecting to a 40mm horizontal branch
• Shower trays connecting through a wall to a vertical stack

P-traps are the most common type in modern UK construction because waste pipes typically run horizontally through partition walls to a soil stack or hopper.

S-Trap (Vertical Outlet)

The outlet exits vertically downward through the floor. S-traps are used for:

• Ground-floor installations where the drain is directly below the appliance
• Replacing old lead or iron traps connected to floor drains
• Bath wastes in some older properties where the drain exits through the floorboards

S-traps are inherently more prone to self-siphonage than P-traps because the full column of water in the S-curve can pull out the seal when draining. For this reason, modern regs prefer P-traps with correct pipe gradients, or deep-seal P-traps (75mm) for S-trap replacements.

Trap Types by Appliance

Basin Bottle Trap

The bottle trap is the standard choice for washbasins and cloakroom basins. It comprises a vertical chrome or white bottle-shaped body that drops down from the waste outlet, with a horizontal outlet to the wall. Advantages:

• Compact — fits neatly under pedestal basins and vanity units
• Easy to disassemble for clearing hair or soap blockages
• Chrome versions look neat under wall-hung basins where pipework is visible

Bottle traps are typically 32mm waste bore. Most have a 75mm seal depth (deep seal) — adequate for most domestic installations without anti-siphon protection, provided the horizontal branch run is under 1.5m and the pipe gradient is between 1:36 and 1:12.

Note: Standard bottle traps are NOT suitable for use in anti-siphon applications or where the horizontal run exceeds 1.5m. Use a resealing bottle trap or fit an air admittance valve (AAV) on the branch pipe if the run is longer.

Bath Trap and Overflow Kit

Bath traps are low-profile P-traps designed to fit in the limited space beneath an acrylic or steel bath. They connect to the bath overflow (via a flexible overflow pipe) as well as the waste outlet, creating a combined overflow and waste assembly.

Key specifications for bath traps:

• Outlet diameter: 40mm (standard UK bath waste)
• Seal depth: 38mm or 75mm — specify deep seal (75mm) where the branch run is long
• Overflow connection: Usually 22mm or 28mm flexible corrugated hose
• Compression or push-fit outlet: Compression is more reliable; push-fit requires the correct tail alignment

Many bath waste and overflow kits are sold as complete sets including a chrome overflow cover plate, flexible overflow pipe, slotted waste, and the trap. This simplifies ordering and ensures all parts are compatible.

Shower Trap (Shower Waste)

Shower traps sit flush within or just below the shower tray, with a grating at floor level. The key variable is the floor height available beneath the tray:

• Standard shower trap (50mm deep): For trays with a raised plinth or timber deck — at least 60mm underfloor clearance needed
• Slimline/low-profile trap (25mm deep): For stone-resin or acrylic trays with minimal underfloor depth — requires careful attention to the 25mm minimum seal
• Wet room linear trap: Channel-format drain for level-access wet rooms — requires a 40mm or 50mm horizontal or vertical outlet

Shower waste diameters are typically 40mm or 50mm (UK), matching the branch pipework. Do not reduce a 50mm shower to 40mm branch without checking that the run length and gradient still provide adequate flow — under-sized traps can cause slow drain complaints.

Kitchen Sink Trap

Kitchen sinks require a 40mm P-trap. Where a washing machine or dishwasher standpipe is nearby, a 40mm two-in-one swivel trap with an integral washing machine inlet spigot (the "Belfast" or "kitchen combinator" trap) simplifies the waste connections.

For kitchen sinks with twin bowls, a connector pipe runs from the second bowl to the single trap via a running trap or swept connection — never use a straight coupling between the two bowl outlets without a proper trap arrangement, or gases can enter from one bowl to the other.

Urinal Trap

Urinal traps (35mm or 50mm, P-trap or S-trap) have a deeper water seal than domestic traps and are designed for commercial installations. They must include a gully trap or anti-siphon device in healthcare and food premises under separate Water Regulations guidance.

Seal Depths at a Glance

Anti-Siphon and Resealing Traps

In multi-appliance bathrooms or where branch pipe runs exceed 1.5m, the risk of trap siphonage increases. Options to maintain the water seal:

Deep Seal Trap (75mm)

The simplest upgrade. A 75mm seal depth provides significantly more resistance to siphoning than the minimum 25mm. Use deep-seal traps as standard on basin and kitchen waste runs in older properties with long horizontal branch runs.

Resealing (Anti-Siphon) Trap

A resealing trap has an internal float mechanism that admits air into the trap body if negative pressure threatens to pull out the seal, then reseals once the pressure equalises. Brands such as McAlpine manufacture resealing bottle traps and resealing P-traps for both 32mm and 40mm applications. Slightly more expensive than standard traps but eliminate the need for an AAV in awkward locations.

Air Admittance Valve (AAV)

An AAV fitted to the branch pipe allows air in when negative pressure develops, preventing siphonage in the downstream trap. AAVs must be:

• Accessible for maintenance (not buried in a wall)
• Above the water seal level of the highest fitting served
• Installed in a ventilated void (not a completely sealed cavity)
• BS EN 12380-compliant (the valve must be marked)

Trap Materials

ABS Plastic (White or Grey)

The most common material for hidden-waste applications. ABS is lightweight, chemical-resistant, and compatible with both compression and push-fit fittings. Most Aquaflow and McAlpine traps are ABS. Use ABS traps under baths, in vanity units, and in any application where the trap is not visible.

Chrome-Plated ABS or Brass

For visible applications under wall-hung basins or freestanding baths, chrome bottle traps and P-traps provide an acceptable finish. Chrome ABS is lighter and cheaper than brass; chrome brass is more durable and provides a higher quality appearance. Both require finger-tight adjustment only — do not overtighten chrome plastic components or the threads will crack.

Solvent-Weld ABS

For permanent installations where the trap will never need cleaning access, solvent-weld traps can be used. These are less common in domestic work but appear in commercial or high-velocity drain installations. Note that a solvent-welded trap requires cutting out to access — not recommended unless you have a specific technical reason.

Waste Outlets and Wastes

The trap connects to the appliance via a waste outlet (waste fitting). Common types:

• Slotted waste: Has overflow slots cut into the top — used with basins and sinks that have an overflow hole. The slot allows water from the overflow to drain through the waste without running over the rim.
• Unslotted waste: A plain waste fitting with no overflow slot — for use with baths that connect the overflow directly to the trap via a separate pipe, or with appliances with no overflow.
• Pop-up waste: A click-to-open/close waste controlled by a push-rod from a knob or lever on the tap body. Common in basin mixer taps with integrated waste systems. Seal is achieved by a rubber plug on a vertical rod — clean regularly as hair accumulates.
• Plug and chain: A simple flat waste fitting with a chrome ball or plug on a chain. Standard for Belfast sinks, laundry tubs, and some butler sinks.
• Clicker/click-clack waste: A push-to-open, push-to-close design without a rod. Cleaner look than a pop-up system and compatible with most basin mixer taps. Very popular in contemporary bathrooms.

Waste thread sizes are standardised — 1¼" BSP (basin and bidet, 32mm bore) and 1½" BSP (bath, kitchen sink, and shower waste, 40mm bore). Most traps are sold with the correct female BSP thread to accept these fittings.

Bath Waste and Overflow Kits: What's Included

A bath waste and overflow kit typically contains:

1. A 40mm bath trap (P-trap or S-trap, 38mm or 75mm seal)
2. An overflow cover plate (chrome or white, to match the tap finish)
3. A flexible overflow pipe (to connect the overflow plate to the trap)
4. A slotted or unslotted bath waste with backnut
5. A compression connector or push-fit adapter for the 40mm outlet

Before ordering a bath waste kit, confirm:

• The bath overflow hole position (end or side mounted)
• The floor clearance beneath the bath (determines trap height and type)
• The outlet direction required (horizontal P-trap or vertical S-trap)
• Finish required (chrome, white, or brushed nickel)

Step-by-Step: Fitting a Basin Bottle Trap

1. Fit the waste: Apply plumber's putty or silicone to the underside of the waste flange. Push through the basin waste hole from above. Below the basin, fit the sealing washer and backnut, then tighten finger-tight plus a half turn with grips — do not overtighten on acrylic or ceramic.
2. Attach the trap: Thread the bottle trap tail onto the waste fitting thread (1¼" BSP). Hand-tighten — traps with a plastic nut should never be spanner-tightened or the thread will strip.
3. Connect to waste pipe: Align the horizontal outlet with the wall waste pipe. If using a compression fitting, insert the rubber olive and tighten. If push-fit, ensure the pipe is fully inserted to the stop mark.
4. Fill and test: Run the basin to full and then release the plug. Watch under the sink for drips at the waste-to-trap joint and the trap-to-pipe joint. A single drip indicates the olive is not seated or the waste backnut needs tightening.
5. Check seal depth: After draining, pour a small amount of water and wait 10 minutes. Foul smell indicates the seal has drained — check for siphonage or evaporation (rare on first fit — suspect incorrect pipe gradient or missing anti-siphon provision).

Common Problems and Fixes

Foul Smell from Unused Basin

The water seal has evaporated. Run water for 10 seconds to recharge. If this happens regularly, the appliance is infrequently used — install a resealing trap or consider a gully trap with a cover.

Gurgling When Other Appliances Drain

Siphonage from the branch pipe. Fit an AAV on the branch above the basin waste connection, or upgrade to a resealing trap.

Persistent Slow Drain

Hair and soap in the bottle trap base. Unscrew the bottle section (it unscrews at the waist), clean with a bottle brush, and refit. If still slow, rod through to the stack connection — accumulation in the horizontal branch is common in hard-water areas.

Leak at Waste-to-Trap Connection

The BSP thread is cross-threaded or the rubber washer is absent or perished. Remove, inspect, and replace the washer — waste fittings typically come with two washers (one above, one below the appliance). Never use PTFE on BSP waste threads — the thread is designed for a face seal with a rubber washer.

Trap Won't Seal Correctly on Bath

The overflow pipe may be kinked, allowing air into the trap body. Re-route the overflow pipe with a smooth curve and confirm the overflow plate is seated correctly on the bath rim with its sealing washer.

Anti-Bacterial Bottle Traps for Healthcare

In healthcare, care home, and food-preparation environments, standard plastic traps can harbour biofilm (Legionella, Pseudomonas) in the warm, wet environment at the trap seal. For these applications:

• Specify antimicrobial-coated traps (not widely stocked — enquire from specialist medical plumbing suppliers)
• Ensure weekly flushing of infrequently used outlets per HTM 04-01
• Consider copper traps — natural antimicrobial properties, more expensive but approved for healthcare
• Install TMVs to keep hot water at 55°C at the outlet to prevent Legionella growth in the trap water

Relevant Products

The following products from our catalogue are relevant to waste trap installations:

Summary

Choosing the right trap comes down to three decisions: bore size (32mm or 40mm matching the appliance), seal depth (75mm for most domestic applications, 25mm minimum for low-profile shower traps), and outlet direction (P-trap for horizontal-to-wall, S-trap for vertical-to-floor). Keep a range of standard bottle traps, bath waste kits, and 40mm P-traps on the van — they're replacement items on every bathroom renovation and are regularly damaged or seized on older properties.

Shop Waste Traps at APM Electricals

Browse our full range: Waste Fittings | Plumbing Fittings

APM Electricals
24 Western Avenue, Acton, London W3 7TZ
Phone: 020 8702 8080
Web: www.apmi.uk
Same-day collection available for West London trades.