Short definition
A lavatory is the code and trade word for a bathroom sink — the small basin used for hand and face washing. Code distinguishes it from a kitchen sink by trap size: a lavatory uses a 1¼-inch trap, a kitchen sink uses 1½-inch. You’ll see the term on permits, plumbing quotes, and fitting boxes; in everyday speech, most people call it a bathroom sink.
What it is
In US plumbing code, a lavatory is any small wash basin used for personal hand and face washing. It can mount in many styles — set into a vanity countertop, hung from the wall, supported by a pedestal column, supported by two front legs (a console), or sitting on top of the counter as a vessel. The mount style doesn’t change the code definition.
What does change is what’s underneath. Every lavatory connects to a 1¼-inch tailpiece, drains through a 1¼-inch P-trap, and has a drainage fixture unit (DFU) load of 1 under the Uniform Plumbing Code that Washington adopts. Hot and cold supplies stub out behind the sink with angle-stop shutoffs.
For accessibility, the rim height is typically 31 inches in standard residential and 34 inches maximum for ADA forward-approach. ADA bathrooms also require 27-inch knee clearance below the apron and insulated hot supply and drain lines so a wheelchair user doesn’t burn against bare metal.
The 1¼-inch trap matters because the fittings and tailpieces are not interchangeable with kitchen-sink hardware. Walking into a parts counter and asking for “lav supply” or “lav drain” gets you the small parts; “kitchen drain” gets you the larger ones.
Why it matters to a homeowner
You’ll see the term in three places: on a building permit (“1 lav, 1 wc, 1 tub”), on a plumbing quote, and on the box of a replacement faucet (“lavatory faucet”). Recognizing it stops you from buying the wrong-sized fittings — putting kitchen-sized 1½-inch parts on a lavatory is the most common DIY mistake at the parts counter.
It also matters for accessibility planning. If you’re renovating a bathroom for aging-in-place or for a family member who uses a wheelchair, the rim-height, knee-clearance, and supply-insulation rules all hang on the lavatory dimensions, not the room dimensions.
Common variants and what a lavatory is not
- Lavatory vs. kitchen sink. Lavatory = bathroom, 1¼-inch trap. Kitchen sink = food prep, 1½-inch trap. Not interchangeable for fittings.
- Lavatory vs. lavatory faucet. The basin is the lavatory; the faucet that serves it is a “lavatory faucet” or “lav faucet” on the spec line.
- Lavatory vs. bidet. Different fixtures with different code rules — a bidet has backflow-protection requirements that a lavatory does not.
- Lavatory vs. “the lavatory” (older usage). In some older US texts and in British English, “lavatory” refers to the room containing a toilet, not the basin. Modern US plumbing code uses lavatory only for the basin itself.
Common failure modes
- Pop-up stopper not seating — bent lift rod, worn pivot ball, or mineral deposits.
- Slow drain — hair and soap scum at the pop-up cross-bar and J-bend.
- Cracked porcelain rim — impact damage; cosmetic only unless the seal fails.
- Faucet base or handle leak — different repair path; see the faucet entries.
- Loose mounting on a wall-hung lavatory — usually missing wall blocking from the original install.