A vanity swap is the most common DIY bathroom project in Ottawa. The new vanity is an affordable upgrade, from basic big-box units up to designer pieces. The old one comes out in an hour. The new one looks great in the showroom. What could go wrong?
The answer is: the part nobody photographs. The plumbing rough-in inside the wall was set for the old vanity. The new vanity has different drain and supply locations. The countertop has a different overhang. The floor is no longer level after 25 years of settling. None of that is visible until the new vanity is on the floor and you're trying to connect the p-trap.
Here's what actually goes wrong on a DIY vanity swap, and how to plan around it before you buy.
The plumbing rough-in centre is the first thing to measure
Every vanity has a rough-in opening in the back panel where the supply lines and drain pipe pass through. That opening is typically 6 to 8 inches wide and centered somewhere on the back of the cabinet. The rough-in plumbing in your wall has to fit inside that opening.
The drain stub-out coming out of your wall is usually centered on the wall behind the existing vanity. If you replace the vanity with a new one of the same width and you're using a single-bowl sink in the same position, the rough-in usually clears. If you change anything (vanity width, sink position, double-bowl, vessel sink, wall-mounted floating vanity), the rough-in math changes and you might find the drain stub falls outside the cabinet's back panel cutout.
Before you buy a new vanity:
- Measure horizontally from a corner or wall to the centre of the drain stub-out.
- Measure vertically from the finished floor to the centre of the drain stub-out.
- Measure the supply line locations the same way.
- Compare those measurements to the back-panel cutout on the vanity you're considering.
If the drain doesn't fall inside the cutout, you have three options: cut the back panel of the new vanity to relocate the cutout, hire a plumber to relocate the rough-in plumbing inside the wall, or pick a different vanity. Most DIYers discover this problem after buying the vanity and end up with a partially open back panel hidden behind the cabinet.
The wall-mounted vanity surprise
Floating wall-mounted vanities have become common. They look great and they expose the floor underneath. What they require is wall blocking inside the studs at the right height to support 200 to 400 pounds of vanity, sink, and water. If your wall is standard drywall over 16-inch studs with no blocking, a wall-mounted vanity will tear out of the wall within a year. If you're swapping a floor-standing vanity for a wall-mounted one, plan to open the wall, install solid blocking, patch and re-tile or re-paint. This is not a one-day job.
Water shutoffs: the part that always fights you
Every vanity has two angle stops on the wall (one for hot, one for cold) that turn the water on and off to that fixture. Those shutoffs were installed when the house was built and might never have been turned off in 20 to 40 years.
What happens when you try to close them: the handle turns but the valve doesn't close. The packing inside has hardened. You shut the angle stops, disconnect the supply lines, and water keeps trickling out because the valve isn't actually closed.
The fix: replace the angle stops as part of the vanity job. Compression-fit angle stops are inexpensive at any plumbing supply shop. To replace them you have to shut the main water supply to the house (or to that bathroom if you have a sub-shutoff), pull the old angle stop, and install a new quarter-turn ball-valve angle stop with new compression rings.
Quarter-turn angle stops are the right product. They cost a little more than the multi-turn style and they actually close. The multi-turn style is what failed in the first place.
Faucet height matters more than DIYers think
The faucet sits on top of the vanity countertop. The spout reaches out over the sink bowl. Two things have to align: the spout has to clear the front edge of the bowl so water doesn't spray on the countertop, and the spout height has to clear how you actually wash your hands.
Modern faucets are taller than the original 1970s and 1980s units they're often replacing. A "tall faucet" with a 12-inch spout height will splash water all over the countertop if the sink bowl is shallow (under 5 inches deep). A counter-mount vessel sink raises the bowl another 4 to 6 inches, which means the faucet has to be a vessel-style (16+ inch height) to clear, and now you're looking at pricier vessel faucets, not entry-level ones.
Match the faucet to the bowl. Standard drop-in or undermount sinks (5 to 7 inches deep) work with mid-height faucets (8 to 11 inch spout height). Vessel sinks need vessel-height faucets. Wall-mounted faucets need to be installed before the wall is closed up, so plan for that on a renovation, not a swap.
P-trap alignment: the puzzle nobody mentions
The drain pipe coming out of the wall is at one height. The new sink's drain tailpiece is at a different height. The p-trap has to connect them with the right slope (1/4 inch per foot toward the wall) and without putting any sideways stress on the trap.
What happens on a DIY swap: the new sink's drain is 3 inches lower than the old one. The wall stub-out is at the height of the old sink. The DIYer cobbles together extra extensions, pipe nipples, and slip joints to bridge the gap. The trap ends up with weird angles, every joint is a potential leak point, and the configuration violates plumbing code (a p-trap should not be more than 24 inches below the fixture outlet).
If the new vanity's drain height differs from the old by more than an inch or two, the wall stub-out should be relocated. That's a plumbing job. Having a plumber move the stub is a modest, contained cost. Fixing the water damage when the trap leaks behind the new vanity for six months before you notice is a far larger one.
Level the cabinet, then caulk the perimeter
The two finishing details that DIYers skip on the last day:
Level. Old bathroom floors are rarely level. Tile installations from the 1980s tilt 1/4 to 1/2 inch across a 30-inch vanity base. If you set the vanity directly on the floor without shimming, the countertop tilts, the drawers don't sit square, and water pools toward whichever side is low.
Use cedar shims under the vanity base to level it both directions (front-to-back and side-to-side). Check with a 24-inch level on the top of the cabinet before you set the countertop. The shims get hidden by the toe-kick or scribed trim.
The whole assembly should be level, not just plumb against the wall. The wall is also probably not perfectly plumb. The cabinet has to be level even if it pulls away from the wall slightly at the top. Scribe and trim the side panel to fit the wall after the cabinet is level.
Caulk. The seam where the countertop meets the wall and where the vanity base meets the floor needs a continuous bead of mildew-resistant silicone caulk. Skip this and water from the sink area runs down the back of the vanity, sits against the wall, and rots the drywall over a few years. The caulk is a 30-minute job. The damage from skipping it is a wall reframe that costs far more.
The vanity isn't done when it's bolted to the wall. It's done when the level reads zero, the silicone is tooled, and the trap holds water for an hour with the faucet running.
What this job actually takes
For a like-for-like vanity swap with no plumbing relocation, a careful DIYer can do it in 4 to 6 hours including mess cleanup. Budget another hour the next day to check for leaks and re-tighten any compression fittings that have weeped slightly.
For a vanity change that involves a sink position move, a different bowl style, or wall-mounting where there was none, this is where most homeowners are better off bringing in a plumber for the rough-in part and doing the cabinet install themselves. Vanity replacement done as part of a full bathroom renovation is also where the plumbing changes get folded into the larger scope and the whole assembly is built right the first time.
If you're in any doubt about whether the rough-in plumbing matches your new vanity, take photos of the wall behind the existing vanity (with it pulled out) and bring them to the showroom before you buy.