What is a trap in plumbing?

A plumbing trap is a curved section of waste pipe fitted immediately below every sanitary appliance - basin, bath, shower, sink, WC, and floor drain - that retains a small amount of water after each use. This water seal physically blocks the passage of foul gases and odours from the drainage system into the building. Without a trap, sewer gases (which include hydrogen sulfide, methane, and bacteria) would enter the room directly through the waste outlet.

What are the different types of trap?

The main trap types are: P-trap (the most common - the outlet runs horizontally to the wall); S-trap (outlet runs vertically downward - used where the drain is in the floor); bottle trap (a cylindrical trap under basins - easy access for cleaning but prone to siphonage); tubular trap (deep-seal, used where siphonage risk is high); and WC trap (integral to the WC pan, not a separate fitting). Running traps are used for floor gullies. Deep-seal traps (75mm water seal instead of 38mm) are used where siphonage is a risk.

Why does my trap smell even when there is water in it?

A trap can smell for several reasons: biofilm and soap scum coating the inside of the trap (clean with a bottle brush and bicarbonate of soda); a partial blockage that causes water to sit in the trap for extended periods; or a trap that is rarely used (e.g. a guest bathroom) where the water seal evaporates and should be replenished by running water regularly. If the smell is sulfurous and appears when water runs away quickly, the trap seal may be siphoning out - this suggests the branch waste pipe is too long or lacks adequate ventilation.

What is trap siphonage?

Trap siphonage occurs when the flow of water down a waste pipe creates a negative pressure (partial vacuum) that sucks the water seal out of the trap - often audible as a gurgling sound as the trap empties. It is most common on long or steep branch waste pipes without adequate ventilation. Approved Document H limits branch pipe lengths and gradients to prevent siphonage. Where longer runs are unavoidable, a deep-seal trap (75mm seal depth), an anti-siphon trap, or a branch ventilation pipe is required.

Plumbing & Heating noun

Trap

/ trap /

Also known as: water trap, P-trap, U-bend, bottle trap, deep-seal trap

Definition

A trap is a curved section of waste pipe fitted immediately below every sanitary appliance - basin, bath, shower, sink, WC, and floor drain - that retains a small amount of water after each use. This water seal blocks the passage of foul gases, odours, and vermin from the drainage system into the habitable space above. Building Regulations require all waste appliances to have a trap with a minimum water seal depth of 38mm (or 75mm in deep-seal applications).

In practice

The most common trap type for basins and baths is a P-trap (horizontal outlet) or bottle trap (cylindrical body, horizontal outlet). Under kitchen sinks, a tubular trap with cleaning eye is standard. WC pans have an integral S-trap built into the porcelain, with the outlet running either horizontally (back-to-wall) or vertically downward (close-coupled or traditional). Floor gullies use a running trap or a trapped gully pot.

The water seal depth is the key performance parameter - standard traps retain 38mm of water, deep-seal traps retain 75mm. Trap siphonage (where the rush of water down the waste pipe sucks the seal out) is prevented by limiting branch pipe lengths and gradients, using deep-seal traps, or installing an anti-siphon trap or vent pipe. A gurgling sound from a basin or bath after the water empties is the classic sign of partial or complete trap siphonage.

Building Regulations

Relevant PartPart H - Drainage and waste disposal

Minimum seal depth (standard)38 mm water seal

Deep seal trap75 mm - required where siphonage risk is high

Floor traps / gullies75 mm minimum seal

Approved Document H Table 1 sets out the requirements for traps, including minimum seal depths and the need for all appliances to be trapped. Branch pipe lengths and gradients are limited by AD H Table 2 to prevent siphonage of the 38mm standard trap seal. Where branch lengths exceed these limits, deep-seal traps (75mm) or branch ventilation pipes are required. All traps must be accessible for clearing blockages.

Full Building Regulations guidance

Trap

Just so you know...