Every sink in your home has a trap under it — the U-shaped bend of pipe that holds a small plug of water so sewer gas can’t drift back up through the drain. It’s simple, it’s required by Canadian plumbing codes, and it comes in exactly two sizes that cover nearly every sink you’ll ever own: 1-1/4 inch and 1-1/2 inch. Grab the wrong one at the store and you’re back in the car hunting for a reducing washer. Here’s how p-trap sizes actually break down, which material to buy for which job, and when the answer isn’t a P-trap at all.
The two sizes, and which fixtures use them
Trap size refers to the nominal diameter of the tubing, and it needs to match the drain outlet of the fixture above it. The convention is consistent across Canada:
| Fixture | Typical trap size |
|---|---|
| Bathroom sink (lavatory) | 1-1/4" |
| Kitchen sink | 1-1/2" |
| Bar or prep sink | 1-1/2" |
| Laundry tub | 1-1/2" |
| Shower or tub | Larger glued trap below the floor — sized at rough-in |
The logic is volume. A bathroom lav sees toothpaste and hand-washing, so a 1-1/4" pop-up drain and matching trap handle it easily. Kitchen and laundry fixtures move greasy water, food scraps from a basket strainer and washing-machine surges, so they step up to 1-1/2". Tubs and showers use a larger cemented trap below the floor, which is why they’re handled during rough-in rather than bought off the shelf — our shower base and drain sizing guide covers that side of things.
One practical wrinkle: many plumbers run 1-1/2" trap kits everywhere, including under bathroom sinks, and use a reducing slip washer to accept the 1-1/4" tailpiece. Going bigger than the fixture requires is fine. Going smaller — squeezing a kitchen sink onto a 1-1/4" trap — is not, and it’s the fastest route to a chronically slow drain.
Tubular vs. glued: buy the serviceable one
Under a sink cabinet you want tubular trap components: thin-walled ABS, PVC or brass tube joined with slip nuts and washers. Nothing is cemented, so the whole assembly comes apart by hand or with a gentle turn of the channel-locks. Dropped a ring down the drain? Loosen two nuts and the trap is off.
Glued (solvent-welded) Schedule 40 traps belong inside walls and under floors — tub and shower traps, mainly — where a permanent joint is exactly what you want. If a previous owner glued one under your kitchen sink, it works, but every future service call now involves a saw.
ABS plastic vs. chrome-plated brass
Inside a closed vanity or kitchen cabinet, plastic tubular is the sensible default: it’s inexpensive, it never corrodes, and nobody sees it. The trade-off is rigidity — plastic slip joints work loose more easily when the trap gets bumped by the recycling bin.
Where the trap is visible — a pedestal sink, a wall-hung basin, an open console — chrome-plated brass earns its keep. Heavier-gauge brass tube (17-gauge is the traditional benchmark; 20-gauge is the lighter economy grade) holds its shape, takes a polish, and looks intentional rather than left over. Canadian manufacturer OS&B (Oakville Stamping & Bending) is the reference point here: their range runs from basic ABS tubular kits up through 17-gauge chrome brass traps and decorative bottle traps, so you can match the trap to how exposed the installation is. Mountain Plumbing also makes handsome decorative waste fittings if you’re coordinating with a premium faucet finish.
Whichever material you choose, pair it with the right drain up top — pop-ups, grid drains and basket strainers all live in our sink drains collection.
Slip-joint basics: the five-minute skill
Slip joints seal with a tapered washer compressed by a nut. Three rules prevent nearly every leak:
- Washer direction matters. The bevelled edge faces away from the nut, into the fitting it seals against.
- Hand-tight, then a quarter turn. Plastic nuts especially. Over-tightening distorts the washer and creates the very leak you were trying to prevent.
- New trap, new washers. Old poly washers take a compression set; reusing them is false economy.
Skip the pipe dope and PTFE tape on slip joints — the washer is the seal, and thread compounds just make the nut slippery and harder to snug properly.
Matching the tailpiece and wall arm
A trap connects two other tubes: the tailpiece dropping from the sink drain, and the wall arm (trap arm) running to the drain stub-out in the wall. All three should be the same nominal size, with two common exceptions:
- A 1-1/2" trap can accept a 1-1/4" tailpiece using a reducing slip washer — the standard bathroom-sink setup mentioned above.
- At the wall, the arm usually enters a threaded trap adapter on the stub-out. Check whether yours is 1-1/4" or 1-1/2" before buying, and pick up a matching escutcheon to cover the wall opening if the trap will be visible.
Measure the horizontal distance from the tailpiece centreline to the wall, too. Trap arms can be trimmed with a tube cutter, but they can’t be stretched, and a trap yanked sideways to reach the stub-out will weep at the joints.
When a bottle trap is the answer
Open vanities, wall-mount basins and floating consoles put the trap on display, and a standard P-trap — even in chrome — reads as utilitarian. A bottle trap holds its water seal in a compact vertical cylinder instead of a U-bend, so the plumbing under the sink becomes a single tidy column. That’s why bottle traps are the default pairing for the exposed plumbing under wall-mount vanities and vessel-sink consoles.
Two honest caveats. First, the internal passages are tighter than a P-trap’s, so a bottle trap under a hard-working kitchen sink is a bad idea — keep them on bathroom lavs. Second, code acceptance varies: some Canadian jurisdictions treat certain decorative bottle traps as non-compliant, so buy one carrying a recognized certification mark and, on permitted work, confirm with your inspector before it disappears behind finished millwork.
Buying checklist
- Bathroom lav: 1-1/4" trap, or 1-1/2" with a reducing washer.
- Kitchen, bar, laundry: 1-1/2", no exceptions. Choosing the sink at the same time? Our stainless steel sink gauge guide covers the bowl end of the decision.
- Hidden in a cabinet: ABS or PVC tubular. Exposed: chrome-plated brass, heavier gauge if the budget allows.
- Tubular slip-joint under sinks; glued Schedule 40 only where it’s concealed and permanent.
- Fresh washers every time, and an escutcheon for any visible wall penetration.
Frequently asked questions
Can I use a 1-1/2" trap on a bathroom sink?
Yes. A 1-1/2" trap with a reducing slip washer at the 1-1/4" tailpiece is a completely standard bathroom installation, and many plumbers prefer it because they only stock one trap size. What you can’t do is the reverse — a 1-1/4" trap undersizes a kitchen or laundry drain.
Do P-traps need to be glued?
Not under a sink. Tubular slip-joint traps are designed to be serviceable and seal with compression washers — no cement involved. Glued traps are for concealed locations like tub and shower drains, where the joint must be permanent because nobody can reach it afterward.
Why does my new trap leak?
Nine times out of ten it’s a slip washer: installed backwards (the bevel should face away from the nut), pinched by over-tightening, or an old washer that was reused. Take it apart, seat a new washer bevel-first, and go hand-tight plus a quarter turn.
Are bottle traps legal in Canada?
Many are, but not all. Plumbing codes require a trap with an adequate water seal, and some decorative imports don’t carry a recognized certification. If the work is being inspected, choose a certified model from an established maker — OS&B’s bottle traps are the usual Canadian answer — and confirm acceptance with your local authority before installation.