Waste Traps: Which Type, What Size, and Why They Stop Your House Smelling Like a Drain

A bathroom that smells of drains two months after your extension is finished. That's the cost of getting waste traps wrong, and it happens more often than you'd think. The trap itself costs under a tenner. Ripping out a vanity unit to replace one that's siphoning, leaking, or undersized costs a plumber half a day. Getting the right trap on the right fixture in the right size is one of those small decisions that prevents an annoying, expensive problem later.

What it is and what it's for

Every fixture in your house that takes water away (sinks, basins, baths, showers, washing machines, dishwashers) has a trap fitted between the waste outlet and the pipe that carries water to the soil stack or drain. The trap is a curved or chambered fitting that permanently holds a small plug of standing water. That water plug is the seal. It blocks sewer gases from travelling back up the pipe and into your rooms.

The principle is simple. Water flows through the trap under gravity, but a portion always stays behind in the lowest point of the curve. As long as that water sits there, gases can't pass through. Break that seal (through evaporation, siphoning, or a crack in the trap) and you'll smell it immediately. Raw sewer gas contains hydrogen sulphide and methane. It's unpleasant and, in extreme cases, harmful.

Approved Document H of the Building Regulations governs trap requirements in England and Wales. It specifies minimum pipe sizes and water seal depths for every type of fixture. These aren't suggestions. Building control will check.

Types, sizes, and specifications

Traps come in several shapes, each designed for a different installation scenario. The shape determines which direction the waste pipe exits and how much space the trap needs.

P-trap

The most common type. Shaped like a letter P on its side. Water enters from the top, fills the U-bend, and exits horizontally through the wall. The outlet can usually be rotated through 270 degrees, which gives flexibility when aligning with the waste pipe behind the wall. P-traps are the default choice for basins, sinks, and any fixture where the waste pipe runs through the wall behind.

Plumbers prefer P-traps over bottle traps for kitchen sinks because they have a wider internal bore, restrict flow less, and are far less likely to clog with grease and food debris.

Bottle trap

A compact, cylindrical trap where water enters from the top and exits from the side near the top of the bottle. The water seal sits in the lower chamber. The base unscrews for cleaning, which makes clearing blockages straightforward, but the narrow internal passages mean bottle traps clog faster than P-traps, particularly on kitchen sinks.

Bottle traps exist primarily to save space. Under a pedestal basin or a wall-hung basin where there's no room for a P-trap's U-bend to swing, a bottle trap fits neatly. They're also the trap of choice where the pipework is visible, because chrome-plated bottle traps look considerably better than a white plastic P-trap.

S-trap

Same U-bend principle as a P-trap, but the outlet points downward through the floor instead of horizontally through the wall. Used where the waste pipe runs under the floor rather than through a wall. S-traps carry a slightly higher risk of self-siphoning (the downward pull of water in the vertical outlet can suck the seal water out) so they need careful installation with correct pipe gradients and adequate ventilation.

S-traps are still widely sold and installed in the UK. They're not prohibited under Approved Document H, despite some online sources (mainly US-focused) suggesting they're outdated. Where your waste exits through the floor, an S-trap is the correct fitting.

Bath trap

A shallow, flat trap designed to fit in the tight space between the underside of a bath and the floor. Standard bath traps have a 40mm outlet and a 50mm water seal. They connect to a flexible or rigid overflow tube from the bath's overflow hole. Space is the constraint here: a standard P-trap won't fit under most baths, so bath-specific traps are built with minimal vertical height.

Shower trap

Built into or sitting beneath the shower tray, with a flat chrome-plated or white cover at floor level. Standard shower traps are 90mm diameter with a 50mm water seal. Flow rate matters for showers: a basic trap handles around 11.5 litres per minute, which is fine for a standard mixer shower. If you're installing a high-flow power shower or rain head, look for a "turbo flow" or "fast flow" trap rated at 40-54 litres per minute. Without one, the tray will pool and overflow.

Washing machine / appliance trap

A 40mm trap with a vertical standpipe inlet that the washing machine or dishwasher hose clips onto. The standpipe prevents backflow, and the trap provides the water seal. Some models include a second inlet nozzle for a dishwasher alongside a kitchen sink trap.

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Size and seal depth requirements

Approved Document H, Table 1 sets the minimums. Get these wrong and building control will flag it.

The 32mm versus 40mm distinction is straightforward: washbasins and bidets use 32mm; everything else uses 40mm. If you buy a 32mm trap for a kitchen sink, it will physically restrict flow and back up under load.

Anti-siphon traps

Siphoning is the most common reason traps fail. When a large volume of water (a toilet flush, a bath emptying) travels down a shared soil stack, it can create a vacuum that sucks the water seal out of nearby traps. The result: a gurgling noise followed by drain smell in the room.

Anti-siphon traps (also called anti-vac traps) include a small air admittance valve built into the trap body. When negative pressure develops, the valve opens to admit air and break the siphon, preserving the seal. They cost a few pounds more than standard traps.

You need an anti-siphon trap (or a separate air admittance valve on the waste pipe) when:

• The waste run from fixture to stack is longer than about 1.7 metres for 32mm basin waste pipes (longer runs are permitted for larger pipe sizes; check AD H Table 3 for the limits)
• The pipe gradient is steeper than roughly 1 in 40 (25mm drop per metre)
• Multiple fixtures connect to a single branch before reaching the soil stack
• You've had gurgling or smell problems with a standard trap

If any of those apply, spend the extra on an anti-siphon model. It's cheaper than diagnosing the problem after installation.

How to work with it

Installation basics

Traps connect to the fixture waste outlet at the top (usually a threaded compression nut with a rubber washer) and to the waste pipe at the outlet (push-fit O-ring or solvent-weld, depending on your pipe system).

The trap must sit as close to the fixture as possible. Leaving even a short length of untrapped pipe between the fixture outlet and the trap creates a dead section where debris accumulates and smells develop. If a plumber positions the trap 300mm downstream of the sink because it's easier to route the pipe, that's wrong. The trap goes directly below the waste fitting.

Hand-tighten compression nuts. Overtightening cracks plastic threads. If a compression joint weeps, the washer is misaligned or damaged, not undertightened.

The 38mm problem

If you're fitting a trap to an older bath, Belfast sink, or period fixture, you may find the waste outlet is 38mm, not 40mm. This is a legacy UK sizing that predates metrication. Modern traps are 40mm. The 2mm gap is enough to leak.

McAlpine's "Multifit" range and Polypipe's adaptors handle this mismatch. When ordering, check whether the trap has a Multifit inlet (which grips both 38mm and 40mm waste fittings) or whether you need a separate reducing washer.

Connecting to solvent-weld waste pipe

Most extension plumbing uses solvent-weld waste pipe rather than push-fit. The trap outlet connects to the pipe system via a compression coupling or a solvent-weld adaptor. Don't solvent-weld a trap directly into the pipe run. You need to be able to remove the trap for cleaning without cutting pipe.

Clearing a blocked trap

P-traps: place a bucket underneath, unscrew the two compression nuts at each end of the U-bend, pull the U-section down. Clean out the debris, reassemble.

Bottle traps: unscrew the bottom cap. The contents of the bottle (water, hair, soap scum, and whatever else has accumulated) drop into a bucket. Clean, reassemble.

Both jobs take five minutes with no tools beyond a bucket and possibly a pair of grips if the nuts are stiff. If you can't clear a blockage by removing the trap, the blockage is further downstream in the waste pipe and needs rodding.

Cost and where to buy

Standard plastic traps are cheap. Expect to pay between £5 and £15 for any standard white plastic P-trap, S-trap, bottle trap, or bath trap from any major retailer.

Chrome-plated traps are a different price bracket entirely. A chrome-plated plastic bottle trap for a visible basin installation runs £15 – £25. Chrome-plated brass (heavier, more durable, better finish for exposed pipework) runs £20 – £56 depending on the design. The Wickes Contemporary Chrome bottle trap at £56 is the top end of retail pricing and only justified if the trap is fully on show under a wall-hung basin.

All major retailers carry waste traps. Screwfix, Toolstation, and Wickes all stock comprehensive ranges. McAlpine dominates retailer shelves. Toolstation alone lists over 40 McAlpine trap products.

Brand verdict

McAlpine is the brand UK plumbers trust. Founded in 1902, manufacturing in Scotland, and responsible for the first plastic traps sold in the UK (1957). Their compression seals are more reliable than budget alternatives, the threads are more precisely cut, and the range covers every conceivable configuration. Over 280 SKUs. If a plumber has a preference, it's almost always McAlpine.

FloPlast and Polypipe are solid mid-range options manufactured to BS EN 274-1. Widely available at builders' merchants and Wickes. Perfectly adequate for concealed installations. FloPlast's shallow bath trap (TSB42) at around £4.5 is a good budget option.

Budget/own-brand traps from major retailers attract consistently poor reviews from tradespeople. Multiple forum threads specifically warn against Screwfix own-brand traps, citing poor thread quality, leaking compression joints, and O-rings that fail within months. The price difference between a budget trap and a McAlpine is £2 – £3. Spend the extra.

Alternatives

HepVO waterless valves replace the water seal with a self-closing membrane. No water means no evaporation, no freezing, and no siphoning. They're designed for fixtures that are used infrequently (guest bathrooms, holiday homes, emergency overflow outlets) where a conventional water seal would dry out between uses. HepVO valves cost around £25 – £30 versus £5 – £9 for a standard trap, but they eliminate the most common failure mode of conventional traps in low-use situations. They also act as a local air admittance device, removing the need for secondary venting. In production since 1997 with over 5 million installations worldwide. Accepted under building regulations where a standard trap would be impractical.

For connecting your traps to the drainage system, solvent-weld waste pipe is the standard choice for extension plumbing.

Where you'll need this

• Second fix plumbing - installing traps under all sinks, basins, baths, showers, and appliance connections

Every water-using fixture in any extension or renovation project needs a trap. They're installed at second fix stage, after the waste pipes have been run during first fix and the sanitaryware has been positioned. A typical kitchen extension needs at minimum a kitchen sink trap and a washing machine/dishwasher appliance trap. Add a utility room, downstairs WC, or shower room and you're looking at four to six traps.

Common mistakes

Buying the wrong size. A 32mm trap on a kitchen sink restricts flow and causes pooling in the sink bowl. Check the table above: 32mm is for basins and bidets only. Everything else is 40mm.

Fitting a bottle trap on a kitchen sink. Bottle traps clog with grease and food particles far faster than P-traps. The narrow internal passages act as a filter for exactly the kind of debris a kitchen produces. Use a P-trap. Save the bottle trap for the bathroom basin where the worst it has to handle is soap scum and hair.

Positioning the trap away from the fixture. The trap goes directly below the waste outlet. Not 200mm along the pipe. Not wherever is convenient for the pipe routing. Untrapped pipe between fixture and trap collects debris and develops odour.

Confusing 38mm and 40mm fittings. Older UK baths and Belfast sinks have 38mm waste outlets. Modern traps are 40mm. A 40mm compression nut will not seal properly on a 38mm fitting. Use a Multifit trap or a reducing washer.

Ignoring shower trap flow rate. A power shower or large rain head pushes 12-16 litres per minute. At the flow rates described above, a shallow trap can lose its seal within minutes if water pools and overflows the tray rather than draining cleanly. Fast-flow traps rated at 40-54 litres per minute exist for exactly this situation, and they need to be specified at the time the shower tray is installed, not after the tray is tiled in.

Choosing chrome for a concealed trap. If the trap sits behind a pedestal, inside a vanity unit, or under a bath panel, nobody will see it. White plastic does the same job for a quarter of the price. Save the chrome-plated brass for wall-hung basins where the pipework is exposed.