Waste Traps: P-Traps, Bottle Traps, Bath Traps, and Shower Traps — A UK Plumber's Guide

Waste Traps: P-Traps, Bottle Traps, Bath Traps, and Shower Traps — A UK Plumber's Guide

A waste trap is one of the most frequently replaced plumbing components in domestic and commercial buildings. Every sanitary appliance — sink, basin, bath, shower, urinal — requires a trap to prevent sewer gas (primarily hydrogen sulphide) entering the building through the waste outlet. Understanding the different trap types, seal depths, and correct sizing is essential for any plumbing installation or maintenance job.

Why Traps Are Required

A waste trap retains a small volume of water in a U-shaped section of pipe. This water seal physically blocks the passage of air from the drain and sewer system back into the room. Without a functioning trap, sewer gases — which include hydrogen sulphide, methane, and other compounds — can enter the property. These gases are unpleasant at low concentrations and potentially toxic at higher levels. Building regulations and BS EN 274 (waste fittings for sanitary appliances) require traps on all waste outlets.

The seal depth — the vertical height of water retained in the trap — determines how resistant the trap is to losing its seal through siphonage or evaporation. Standard installations require a minimum 50mm seal depth. In some situations (particularly single-stack drainage without separate ventilation) a 75mm deep seal trap is specified to reduce the risk of seal loss through induced siphonage when adjacent appliances drain simultaneously.

P-Traps

The P-trap is the standard trap for most domestic waste installations. It takes its name from its cross-sectional profile — a U-bend with the outlet running horizontally into the wall. P-traps connect the waste outlet of a sink, basin, or bath to a horizontal branch waste pipe that runs through or into the wall.

Key P-trap configurations:

• Compression P-trap: Standard domestic fitting. The trap body connects to the waste outlet with a 32mm or 40mm push-seal nut, and the outlet connects to the waste pipe with a compression fitting. Available with or without a cleaning eye (rodding point). Standard for kitchen sinks (40mm) and bathroom basins (32mm or 40mm).
• Solvent weld P-trap: Fixed joint version for permanent installations in concealed locations. Solvent welded to the waste pipe. No rubber seals to fail — used in commercial installations and where access for future maintenance is very limited.
• Swivel P-trap: The horizontal outlet section can be rotated to align with the incoming waste pipe without cutting or repositioning the pipe. Useful in retrofit situations where the pipe position does not line up precisely with the trap outlet.
• Tubular P-trap: Exposed chrome or white tubular trap for visible undersink installations where aesthetics matter — typically under a pedestal basin or in a cloakroom where the pipework is visible.

Bottle Traps

The bottle trap has a cylindrical body (the "bottle") rather than a U-bend shape. Water is retained in the bottom of the cylinder, and the outlet connection is at the side of the cylinder near the base. Bottle traps are popular in modern bathrooms because:

• The cylindrical shape is visually cleaner and easier to clean than a U-bend configuration
• Many designs are available in chrome, brushed nickel, black, and gold finishes to match tap and accessory finishes
• The bottom of the bottle is removable (usually a screw-on cap) for clearing blockages without disconnecting the pipework

Bottle traps are available in 32mm and 40mm sizes. The outlet can typically be adjusted to connect into a wall waste pipe or into a floor-standing waste pipe. Standard inlet connection is via a backnut to the basin waste fitting.

Limitation of bottle traps: Bottle traps are more susceptible to siphonage (loss of water seal when the appliance drains) than P-traps because the seal depth is shallower in some designs. In single-stack drainage installations without ventilation, check the manufacturer's seal depth specification before specifying a bottle trap. If in doubt, use a P-trap with a 75mm seal instead.

S-Traps

The S-trap has a U-bend with the outlet running downward into the floor rather than horizontally into the wall. Once common where waste pipes ran under the floor rather than through walls, S-traps have largely been superseded in modern UK plumbing because they are more prone to self-siphonage — the full bore of water draining through the trap can suck the seal out as it passes. Current best practice and BS EN 12056 favour P-traps connected to ventilated branch waste pipes over S-traps.

S-traps are still found in older properties and are available as replacements. If replacing an S-trap in an older property, consider whether it is possible to reformat the waste to a P-trap connection instead.

Bath Traps

Bath traps are a low-profile variant of the P-trap designed to fit in the restricted space beneath a bath panel. Standard bath waste outlets are 40mm. Two main types:

• Standard bath trap: A conventional 40mm P-trap, 75mm or 38mm seal depth. The bath overflow must connect into the trap or into the waste outlet above the trap using a combined bath waste and overflow kit.
• Bath overflow and waste trap kit: A combined assembly including the bath plug waste, a flexible overflow pipe connecting the overflow outlet on the bath to a T-connection on the trap, and the trap body itself. Available as push-pull (pop-up) click waste versions or traditional plug and chain. The overflow connection is critical — do not simply cap off the overflow connection on a bath trap, as the bath overflow is a safety device preventing flooding.

Minimum clearance for a bath trap is approximately 100mm between the bottom of the bath waste outlet and the floor — check this during planning as some low-profile baths have very limited clearance.

Shower Traps

Shower traps are low-profile by design, as shower trays are typically close to the floor. Two main configurations:

• Standard shower trap: 90mm or 60mm diameter body with a 40mm outlet. The low profile typically gives only a 40–50mm water seal. Must connect to an adequately vented waste system to prevent seal loss — on single-stack systems, check the drain loading and use 75mm deep-seal where possible.
• Wetroom / linear drain trap: Slot drain or tile insert drain with a removable liner covering a trap body in the screed. Used in level access wet rooms and walk-in shower areas. The trap body is set in the screed during construction — ensure the trap outlet connects to a 40mm or 50mm waste run with correct fall (minimum 1:40 gradient).

For walk-in showers and wetrooms, ensure the trap is rated for the expected water flow from the shower head — a power shower or multiple body jets with high flow rates requires a trap with adequate flow capacity, not just a domestic shower trap.

Trap Sizes: 32mm vs 40mm

UK waste pipe and trap sizing follows a practical convention:

• 32mm traps and waste: Wash hand basins (cloakroom and bathroom). 32mm is the standard for basin waste in domestic installations.
• 40mm traps and waste: Kitchen sinks, baths, shower trays, bidets, and in some cases bathroom basins where higher flow is expected. 40mm is also standard for bath and shower waste to handle the volume of water draining from a full bath or power shower.
• 50mm waste: Not a trap size — 50mm is the branch drain and soil stack connection size. Waste pipes from traps (32mm or 40mm) connect into 50mm branch drains (see article on 50mm solvent weld and push-fit drainage).

Do not use 32mm waste on a kitchen sink — the food debris and grease in kitchen waste requires the larger bore to avoid repeated blockages. Always use 40mm for kitchen applications.

Seal Loss: Causes and Prevention

A trap that has lost its water seal is immediately identifiable by the smell of drains in the room. Causes of seal loss:

• Self-siphonage: The trap siphons itself as the appliance drains, pulling water past the seal point. More common with S-traps and deep bottle traps on long horizontal waste runs.
• Induced siphonage: Another appliance draining on the same stack creates negative pressure in the branch pipe that draws water from the trap. Prevented by proper ventilation of the waste system (air admittance valve or ventilated stack) and by keeping branch waste pipe lengths within limits (typically: 32mm waste max 1.7m, 40mm waste max 3m without ventilation).
• Evaporation: Traps on rarely used appliances — a guest cloakroom, a floor drain in a cellar — can evaporate over weeks. The solution is to add water regularly, or to install a trap primer or anti-evaporation trap insert on permanently at-risk drains.
• Capillary action: Debris such as hair wrapped around the base of the trap body can wick water out of the seal over time. Clear the trap regularly.

Cleaning Access

All traps should be accessible for periodic clearing. Kitchen sink traps accumulate grease and food debris; bathroom basin traps accumulate hair and soap. The standard compression P-trap allows the U-section to be removed by unscrewing two compression nuts — always fit a bucket below before removing. Bottle traps with a screw-off base are even quicker to clear. Where traps are concealed (under fitted furniture, behind panels), ensure access hatches are provided or specify traps with a cleaning eye in the accessible position.

Waste Traps at APM Plumbing & Electrical

APM stocks a full range of waste traps for domestic plumbing — 32mm and 40mm P-traps (compression and solvent weld), bottle traps in white and chrome, bath waste and overflow kits, shower traps, and anti-siphon (HepvO) traps for single-stack installations. All standard seal depths (38mm, 50mm, 75mm) are available.

Browse our waste traps and waste fittings or contact the Acton trade counter for project quantities. Trade orders placed before 2pm ship same day.

Products Available from APM Plumbing & Electrical

Key waste traps and waste fittings available for trade collection or delivery:

Get It from APM Electricals

APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or visit www.apmi.uk for same-day trade counter collection and next-day delivery across London and the UK.