A trap is a plumbing fitting that holds water to block sewer gas, bacteria, and insects from moving up a drain into a building. Every water-using fixture, from a kitchen sink to a toilet, has a drain line, and each drain line needs a trap that seals it. The seal keeps the indoor environment fresh by stopping foul gas from the sewer main, and it does the same job in reverse by keeping vermin from climbing up the pipe. Traps appear under sinks, bathtubs, toilets, and washbasins, usually built into the fixture connection or installed with a few joints of pipe.
The trap principle extends beyond plumbing. Any device that intercepts a nuisance at the point of entry uses the same logic: catch it before it spreads. Above ground, that means soapy pans and sticky boards, and the same intercept-at-the-source thinking powers DIY stink bug traps that clear a window sill in a single evening.
What a Plumbing Trap Does and How the Water Seal Works
A trap is designed to retain a small amount of water after every flush, and that retained water creates the seal. Each time the fixture is used, the water forming the seal is pushed out and replaced with fresh water, so the barrier renews itself constantly. The minimum water seal depth for a floor trap is typically 50 mm, 75 mm, or 110 mm depending on the fixture and local code.
Self-Scouring by Design
Traps are engineered to clean themselves. The velocity of waste moving through the trap matters: if the flow is too fast, the water that should stay in the trap gets siphoned down the drain; if it is too slow, solids settle in the bottom of the trap and eventually clog it. Getting that balance right is why people should not fabricate traps from loose piping and fittings; a homemade trap either siphons dry or collects debris.
Traps as Pest Barriers
The water seal stops sewer gas, but it also stops the insects and rodents that travel through drain lines. The same barrier logic applies outside the plumbing system, where closets and pantries need their own defenses: natural cedar repellents and pheromone traps intercept moths before they reach wool and stored food, the way a water seal blocks anything from climbing up a waste pipe.
The 14 Types of Plumbing Traps
Plumbing codes and fixture designs have produced a family of traps, each shaped for a specific location. The 14 most common types are:
| Trap Type | Typical Location | Key Notes |
|---|---|---|
| Floor trap (Nahni trap) | Bathroom and shower floors | UPVC or cast iron, removable grating, 50-110 mm seal |
| Gully trap | Outside, before the sewer connection | Heavy cast iron, 7.25 kg minimum, 30 cm depth |
| P trap | Toilets and sinks with wall outlets | UPVC or cast iron, classic U shape |
| S trap | Toilets and sinks with floor outlets | Same shape as P, outlet through the floor |
| Q trap | Water closets on upper floors | Similar to S trap, used above ground level |
| Intercepting trap | Between building drain and public sewer | Blocks sewer gas from the municipal line |
| Bottle trap | Pedestal and wall-hung sinks | Compact vertical design for tight spaces |
| Grease trap | Kitchen drains and food service | Separates fats, oils, and grease from wastewater |
| Drum trap | Older bathtubs | Cylindrical body with removable cover for cleaning |
| Running trap | Long horizontal drain runs | In-line trap built into the pipe run |
| Straight-through trap | Drains with limited vertical space | Minimal depth and low flow resistance |
| Low-level bath trap | Bathtubs with shallow clearance | Flat profile under the tub outlet |
| Bell trap | Floor drains | Bell-shaped cover, older design |
| Building trap | Building drain outlet | Final trap before the sewer, cleanout access |
Fixture Traps
The P trap is the workhorse: it connects to water closets and sinks with a wall outlet and is made from UPVC or cast-iron sheet. The S trap looks almost identical but discharges through the floor, which suits toilets installed where a wall outlet is not available. The Q trap sits under a water closet on an upper story, and the bottle trap packs the same seal into a compact vertical body used under pedestal and wall-hung sinks.
Drain and Building Traps
Floor traps, also called Nahni traps, collect wastewater from bathroom floors, showers, and washbasins, with a removable grating on top for cleaning. Gully traps sit outside the building before the connection to the sewer line; heavy cast-iron gully traps weigh at least 7.25 kg, with the cover not less than 4.53 kg and the frame not less than 2.72 kg, and the total depth runs at least 30 cm. Intercepting traps, running traps, and the building trap all work on the same principle at different points in the drainage network.
Choosing the Right Trap for the Job
Selection follows the fixture and the outlet location, not preference. A wall outlet calls for a P trap, a floor outlet for an S trap, and a tight cabinet for a bottle trap. The same selection logic appears in pest control, where the best mouse traps range from snap and glue boards to live-catch boxes rated by capture success and cleanup ease.
Pest Traps Around the Building
Pest traps around a building use attractants instead of water seals. Light, heat, carbon dioxide, pheromones, and food baits pull insects toward a capture chamber or a sticky surface, and placement decides success more than the trap brand.
Mosquito Trap Placement
Understanding how mosquito traps work helps homeowners put them where CO2 and heat plumes overlap with activity zones. Female mosquitoes seeking a blood meal follow the carbon dioxide trail a person exhales, so traps that emit CO2 and mimic body heat draw them away from patios and doorways. Traps placed at the edge of the yard outperform units set next to the seating area, because the plume pulls insects away from people rather than toward them.
Matching the Trap to the Pest
- Snap traps for mice and rats: fast, reusable, and inexpensive.
- Glue boards for crawling insects: no poison, but messy to service.
- Light traps for flies and moths: effective at night, less so in daylight.
- Live-catch boxes for larger pests: humane, but require daily checking.
- Bait stations for ants and roaches: slow acting, so the colony shares the poison.
Placement and Rotation
Rotate bait types every few weeks. Pests learn to avoid a trap that stays in the same spot with the same lure, and a clean trap catches nothing. Wash sticky surfaces, refresh water-based lures, and move the unit when capture rates drop.
Traps for Structural Pests
Some pests attack the building itself, and traps for structural pests protect the wood, not just the occupants. Carpenter bees drill into fascia, soffits, and decking to lay eggs, and the damage accumulates over several seasons.
Carpenter Bee Trap Boxes
A carpenter bee trap is a wooden box with a hole that mimics a nest entrance; the bee enters, cannot find the exit, and drops into a collection jar. Protecting wooden structures from carpenter bee damage relies on these trap boxes plus preventive painting, because the bees avoid finished, painted surfaces when selecting nest sites.
Termite Monitoring and Baiting
Termite protection uses in-ground monitoring stations around the foundation. The stations contain untreated wood that attracts foraging termites; once activity is confirmed, the wood is swapped for a bait that the workers carry back to the colony. Baiting is a long game, and the stations need quarterly inspection to stay effective.
A Layered Defense Routine
- Seal entry points: caulk gaps, repair screens, and close crawl space vents.
- Remove harborage: clear brush, stacked lumber, and debris from the foundation line.
- Install the right trap for the pest confirmed on site.
- Monitor weekly and record capture locations.
- Escalate to a licensed pest control operator when captures stay high for a month.
Grease Traps and Kitchen Drainage
Grease traps solve a different problem: keeping fats, oils, and grease out of the drainage system. Wastewater from kitchen sinks and dishwashers carries FOG that congeals in pipes, so the trap cools the water and lets the grease float to the surface while baffles hold it back.
How a Grease Trap Works
A grease trap is a flow-through tank with inlet and outlet baffles. Wastewater enters, slows down, and the grease rises while solids settle; the clarified water exits to the drain line. Capacity is rated in gallons, and the trap must be sized to the peak flow of the fixtures it serves, not the total volume of the kitchen.
Maintenance and Pumping
A neglected grease trap becomes a source of odor and backups. The maintenance schedule depends on volume: light residential use needs inspection monthly, while commercial kitchens pump every 30 to 90 days depending on local FOG program rules.
Grease Traps and Sustainability
Food service operators watch the grease stream closely because recovered FOG can be diverted to rendering and biofuel instead of the sewer. The debate over grease traps in green kitchens covers pumping frequency, waste diversion, and the fate of the recovered material, and it shows how a plumbing fitting connects to the wider waste stream.
Sizing a Grease Trap
Size by flow-through rate, not by the number of sinks. A single-compartment sink with a pre-rinse sprayer typically needs a 20 to 25 GPM trap, and a three-compartment sink with a disposer needs more. Sizing upward reduces pumping frequency, but an oversized trap allows grease to congeal and harden before it can be pumped.
Building a Trap Maintenance Routine
All traps, plumbing and pest alike, need a maintenance routine. A dry trap seal is no seal at all, and an empty bait station catches nothing.
Seasonal Checks
Twice a year, pour water into floor drains and unused fixture traps to refill the seals. Check gully traps for debris, confirm the grating seats properly, and inspect the vent stack for nests. In pest season, walk the perimeter with a flashlight and note any new entry points.
Buying Traps and Supplies
Traps and trap parts are commodity purchases once the spec is known, and contractors buy them by the case at the start of the season. Tool deals for contractors show how shopping smart avoids out-of-stock traps when a job is on the clock, and the same principle applies to homeowners keeping spares for the bathroom and kitchen.
Matching the Trap to the Problem
The final layer of defense is matching the control product to the infestation. When roaches have established harborage, matching roach killers and traps to the species and infestation level is the last step in a layered defense, and it works best after the water seals and exclusion work are already done. A trap only earns its place when the rest of the building is sealed, drained, and maintained.