Tub-to-Shower Conversion: What Plumbing Actually Changes
You've measured the old tub a dozen times in your head. Out it comes, in goes a walk-in shower, and the cramped bathroom finally feels like one you'd want to use. Then a plumber crouches by the tub, points at the floor, and says the drain has to move. That one sentence is where a tub-to-shower conversion stops being a swap and turns into actual plumbing work, and it's the part the glossy ads and big-box install chains tend to skip right past.
The fixtures you can see are the easy half. What changes behind the wall and under the floor is the half that decides whether the job goes smoothly or turns into a surprise the morning after demo.
The Drain Has to Move, and That's the Whole Job
A bathtub drains from one end. You stand at the faucet, and the drain and overflow sit at your feet against the wall. A shower drains from the middle of the floor. Those two spots aren't in the same place, and they're usually a foot or two apart.
That gap is the heart of the conversion. The old tub drain gets capped or removed, and a new drain has to be opened, positioned, and plumbed where the shower floor will actually sit. On a tub, the trap is a tub-style trap and shoe assembly tucked against the end wall. A shower wants a two-inch drain with its own trap, centered or set to one side, depending on the pan you're using. Moving that connection means getting into the floor.
How hard that is depends on what's under you. If there's a basement or crawlspace below, a plumber can often reach up from underneath, cut the old line, and run new pipe to the right spot with minimal damage to your finished floor. If the bathroom sits on a concrete slab, the new drain location has to be chased into the slab: cut, dug, plumbed, and patched. Same goal, very different workday. This is the single biggest reason two conversion quotes for "the same job" can look so different: one home has open access below, the other has six inches of concrete in the way.
Slope, Trap, and Vent: the Stuff You Never See
Once the drain is in the right place, it has to drain properly. A shower pan needs the floor pitched toward the drain, usually about a quarter-inch fall per foot, so water doesn't pool in the corners. Too flat and you get a puddle that never quite dries; too steep and the pan feels off underfoot. That slope gets built into the mortar bed or the pre-formed pan, and the drain height has to match it exactly.
Then there's the trap. Every drain needs one: the curved bit that holds a little water and blocks sewer gas from rising into the room. When the drain moves, the trap moves with it and has to sit at the correct depth below the new pan. Get it wrong and you either can't connect the pan or you end up with a trap that siphons dry and lets odor in.
The piece people forget entirely is the vent. Drains don't just need a downhill path for water; they need air coming in behind it, or the draining water pulls a vacuum and sucks the traps dry. The old tub had a vent. When you relocate the drain far enough, that vent connection sometimes has to be re-tied or extended so the new shower trap still breathes. In an older home where the venting was marginal to begin with, this is where a quick "swap" quietly becomes a real plumbing job, and where cutting corners shows up later as a gurgle and a smell.
The Valve Comes Out Too
While the wall is open, the faucet changes. A tub had a tub spout and often a separate valve set low for filling. A shower runs off a single mixing valve set at chest height, and most conversions are the right time to install a modern pressure-balancing or thermostatic valve so the water doesn't scald you when someone flushes a toilet elsewhere in the house.
That valve swap means new supply connections inside the wall: hot and cold lines re-routed up to valve height, the spout dropped, and the showerhead arm stubbed out. None of it is exotic, but all of it lives behind the tile, which is why it has to be done before the wall closes up, not after. A good crew sets the valve, pressure-tests the connections, and only then lets the tile go on.
Where the Surprises Hide in Older Homes
In a home built decades ago, opening the wall and floor for a conversion can reveal what's actually back there. The supply lines might be old galvanized pipe that's corroded down to a pencil-width opening, choking your flow long before it reaches the new valve. The drain you're tying into might be cast iron that's scaled or cracked at the joint. The framing around a long-leaking tub can be soft with rot, especially at the base of the wall where splash collected for years.
None of this is a reason to dread the project. It's the reason to want it found now, with the wall open, rather than after the fresh tile is on. A plumber who works on old homes expects these and prices the conversion based on what's likely lurking, rather than quoting a clean swap and hitting you with change orders once the demo dust settles.
| Step in the conversion | What's actually involved |
|---|---|
| Remove the tub | Pull the tub, cap the old supply and drain, expose the wall and floor |
| Relocate the drain | Open floor, run a new 2-inch line, and trap to the shower drain position |
| Set slope and trap depth | Build ~1/4-inch-per-foot pitch to the drain; set trap at correct height |
| Confirm the vent | Re-tie or extend venting so the new trap doesn't siphon dry |
| Swap the valve | Install a pressure-balancing or thermostatic valve, re-route supply, and stub showerhead |
| Pressure-test and close | Test all connections under pressure before the pan and tile go in |
If a conversion quote doesn't mention moving the drain, ask where the new drain will sit and whether your bathroom is over a basement or on a slab. The answer tells you most of what separates a half-day swap from a multi-day re-pipe.
Frequently Asked Questions
Almost never. A tub drains from the end where the faucet is; a shower drains from the center of its floor. Those locations are usually a foot or more apart, so the drain has to be opened in a new spot and re-plumbed. The only time the old location works is the rare case where the shower pan is designed to drain at exactly that end, which most aren't.
For a clean job with good access below the bathroom, the plumbing and pan work often run a few days, plus tile and finish time after. If the bathroom sits on a concrete slab and the drain has to be chased into the slab, add time for cutting and patching. Old supply or drain lines that need replacing while the wall is open can extend it further. A site visit is the only honest way to put a number on it.
You'll need a shower valve. A tub faucet sits low for filling and usually isn't a mixing valve built for a shower. Most conversions are a natural time to install a pressure-balancing or thermostatic valve, which keeps your temperature steady when water is used elsewhere in the house. That's a real comfort and safety upgrade, not just a cosmetic one.
A drain needs air behind the water; otherwise, it creates a vacuum that sucks the trap dry, letting sewer gas into the room. When the drain moves, the vent connection sometimes has to move or extend with it so the new trap keeps breathing. In older homes where the venting was already marginal, skipping this is what causes a gurgling drain and an odor weeks after the job looks finished.
It depends on how many bathrooms you have. A home with only one bathtub can lose appeal to buyers with kids, so keeping at least one tub is the usual advice. If you have a second tub elsewhere, converting a cramped or unused tub to a walk-in shower is often a net gain in everyday usability. It's a household-by-household call, not a blanket rule.
Most often, it's access and what's behind the wall. A bathroom over an open basement is far easier to re-drain than one on a slab. A quote that includes replacing old galvanized supply or cracked cast iron found during demo will run higher than one that quietly assumes everything back there is fine, and only one of those quotes won't surprise you later.
Look Past the Fixtures
A tub-to-shower conversion sells itself on what you can see: the open glass, the bench, the easy step-in. The work that makes it last is all the stuff you can't: the relocated drain, the right slope, a trap at the correct depth, a vent that breathes, and a proper mixing valve set in the wall. Get a plumber who treats the drain move as the real job and checks what's behind an older wall before quoting, and the finished shower is exactly what you pictured, with no second act in the ceiling below.
Planning to turn a tub into a walk-in shower — Get a conversion plan that accounts for moving the drain, the vent, and whatever's behind an older wall. AT Plumbing Services serves Scarborough, South Portland, Portland, and the surrounding areas. Call (207) 707-3170.
