Bathroom Vanity Plumbing Problems: What the Plumber Finds When the Old Vanity Comes Out

1. The drain is at the wrong height for the new vanity

The old vanity had a door and a shelf, and the drain came out of the wall at 18 or 20 in. and nobody cared. The new one has three drawers to the floor, and the P-trap sits exactly where the second drawer wants to be. This is the most common vanity problem in Ontario right now because of how vanities are built now. Fixes, cheapest first: a vanity whose top drawer is notched around the trap (most prefab doubles are); a shallower second drawer; an offset or “space-saver” trap kit ($40 to $90) that moves the trap tight to the wall; a bottle trap, which is compact and allowed in Ontario when it is a listed one; or moving the drain stub-out up or down inside the wall, which is the plumber opening the drywall, $400 to $900, plus the patch. The fit checker on the sizes page asks for the stub-out height for this reason.

2. The shut-off valves do not shut off

The two little valves under the sink, one hot, one cold. If they are the multi-turn type (a round handle, many turns), and over 15 years old, there is a fair chance they will not close fully, or will close and then weep from the stem when turned back on. The plumber then has to shut the main, drain down, and replace them with quarter-turn ball valves: $40 to $80 each in parts and about an hour for the pair. It is the best money in the whole job, because the next time there is a problem under that sink, the water goes off in two seconds. If the main shut-off is also an old gate valve that does not close, that is a bigger conversation and a different day.

3. The supply lines are the old ones

Braided stainless supply lines have a rubber hose inside and a life of about 10 years; the chrome-plated copper or plastic ones from the 1990s are worse. Replace both, every time, with new braided lines rated for the job: $15 to $30 each. The flooded bathrooms that insurers pay for are mostly a supply line that let go at 3 a.m. Any install quote that reuses the old ones is a quote to query.

4. The trap is an S-trap, or there is no vent

A P-trap goes down from the sink, round the U, and then horizontally into the wall to a drain that has a vent. An S-trap goes down, round the U, and straight down through the floor, with no vent, and it can siphon itself dry so the sewer smell comes up through the sink. S-traps are not permitted by the Ontario Building Code in new work and they are still found under a lot of older vanities, especially where the drain is in the floor. A proper fix is a P-trap and a vent connection, which may mean an air admittance valve (an “AAV” or “cheater vent”) where the code allows it, or a real vent tied into the stack, which can be an afternoon with the wall open: $300 for the AAV route to $1,500 for a vent run. Not every plumber will insist on fixing an S-trap during a vanity swap; the honest ones will tell you it is there.

5. The drain is in the floor and the new vanity is floating

A floor drain under a freestanding vanity is hidden by the cabinet. Under a floating vanity it is in plain view with 12 in. of air around it. The drain has to be moved into the wall at vanity height, which on a wood-framed floor with access from below is $600 to $1,000, and on a slab is a concrete saw and considerably more. Covered in detail on the floating vs. freestanding page, and the reason to check the drain before ordering a floating vanity rather than after.

6. The trap and tailpiece are corroded or the wrong size

Chrome-plated brass traps from the 1970s and 1980s corrode from the inside and crumble when touched. Some older sinks drain through 1-1/4 in. and newer vanity faucets with pop-ups are 1-1/4 in. too, but the wall stub-out might be 1-1/2 in., and the adapter is a $5 part the installer should have in the truck. A new PVC or ABS trap kit is $15 to $30; a chrome one to match an exposed trap under a floating vanity or a wall-hung sink is $60 to $150. Budget a new trap on every vanity swap and be pleasantly surprised if the old one is reusable, which it is not.

7. The slow drain was never the trap

If the old sink drained slowly, the new one will too unless the problem was in the trap (hair, mostly, and it will be cleaned out by the swap). If the problem is in the wall or past it, the new vanity does nothing. A slow drain that does not fix with a clean trap is a line that needs a snake or a camera, which is $150 to $400 and a different trip. Mention a slow drain to the plumber before the day so the snake comes with them.

8. Water damage under the old vanity

The old vanity comes out and the subfloor under it is soft, stained, or has a hole where the drain was. Or the drywall behind it is swollen at the bottom. A supply line or a trap dripped for years into a cabinet that hid it. Minor: a sheet of 3/4 in. plywood patch and a day to dry, $150 to $400. Major: the subfloor and joist ends, and if there is a finished ceiling below, that too. It is not the plumber’s job to fix; it is the carpenter’s, and the vanity install stops until it is done. Nobody can quote this before the vanity comes out, which is why a vanity swap quote should have an allowance line or a note saying “subject to condition of subfloor”.

What the plumber needs you to have decided

• Which vanity, with its drawings, so the trap height is known before the drywall is open.
• Which faucet, because the drain assembly comes with it and the hole spacing (single hole, 4 in. centreset, 8 in. widespread) is the top’s problem and the plumber’s.
• Whether the shut-offs are being replaced (say yes).
• Whether a second sink is going in, and whether it can share a trap; see single vs. double.
• Who patches the drywall after, if the wall is opened.

With those settled, a vanity swap is the two-hour job on the installation cost page. Without them, it is the plumber standing in the bathroom calling the supplier.