How to Convert a Tub to a Low-Threshold Shower

Why this matters

Stepping over a tub wall is the single most common reason an older customer stops bathing safely, and taking the tub out is the obvious fix. The trouble is that a tub is a sealed box: water stays inside it no matter how badly anyone aims the showerhead. Take the box away and the floor itself has to keep the water in, which means the slope, the drain and the waterproofing now do the tub's job. A conversion that gets the threshold right and those three wrong ends with a wet subfloor, a stained ceiling below, or water in the hallway.

Three trades touch this job and the failures happen in the handoffs between them. The steps below are in build order, and each says who owns it. Where the water goes once it leaves the showerhead is the subject of a sibling: see related, Curbless Showers and Where the Water Goes.

Before anything: shut it down and plan the demolition

• Water and power. Shut the supply to the tub valve and drain the lines. Turn off and lock any bathroom circuit whose box is in the tub wall or the floor you will open, and prove it dead on a known live source before and after (NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices at 29 CFR 1910.333(b)(2)).
• The tub. A cast iron tub weighs several hundred pounds. Plan the path out, use enough people or a dolly, and never lift it on a stair alone. Breaking it in place with a sledge throws sharp shards and enamel chips; full face protection, cover it with a moving blanket, and keep everyone else out of the room. Old enamel and glazes can contain lead, so treat the chips as lead debris: contain them, clean up with a HEPA vacuum and wet wipes, no dry sweeping.
• The floor and walls. A tile floor or surround laid on a mortar bed releases respirable crystalline silica when it is broken out. On construction work that puts the employer under OSHA's silica standard (29 CFR 1926.1153): wet methods or tool-mounted extraction, and a respirator only under a written program per 29 CFR 1910.134. Sheet flooring, floor tile and adhesive from before about 1980 can contain asbestos; sample before you break it out, and if it is positive, it goes to a licensed abatement contractor, not your crew.

Step 1: Read the floor framing and find your vertical budget (carpenter, with the tile setter)

A low-threshold shower needs depth. The shower floor has to slope down to a drain while its entry edge meets the bathroom floor at nearly the same height, so the drain sits lower than the floor around it. That depth comes from somewhere, and finding where is the first decision, not the last.

Open the floor and record: joist size, spacing and direction; what the subfloor is; and how thick the existing finished floor is above the subfloor.

The depth can come from three places:

1. The old floor. A bathroom tiled on a mortar bed decades ago may sit an inch or more above its subfloor. Removing it gives that depth back for free.
2. Lowering the subfloor between joists on ledgers, in the shower area only. The joist tops still cross the shower, so this works only where the drop you need sits between joists or the slope can be arranged around them.
3. Cutting the joists. The IRC limits notches in the top or bottom of a joist to one sixth of its depth, and none in the middle third of the span (R502.8, as adopted by your jurisdiction). On a 2x8 that is 7.25 / 6 = about 1.2 inches, near the ends only. Anything deeper means heading off joists, and that is an engineer's detail, not a field cut.

What breaks if you skip it: the tile setter discovers on day four that the slope cannot fit, and the threshold becomes a 2-inch curb after all.

Step 2: Relocate or recess the drain (plumber)

The tub drain sits at one end of the old tub, usually a 1-1/2 inch line. The shower drain goes where the slope plan needs it.

• Drain size. The IPC allows a shower drain outlet as small as 1-1/2 inches (IPC 417.3, as adopted); the UPC requires a 2-inch trap for a shower. Which code your jurisdiction adopted decides whether the old tub line can be reused or must be upsized.
• Trap arm. The distance from the trap to its vent is capped by pipe size, at 6 feet for 1-1/2 inch and 8 feet for 2 inch at the standard slope (IPC Table 906.1, as adopted; the UPC sets its own table). Moving the drain across the room can push the arm past the limit and require a new vent.
• Holes through joists. If the new line has to cross joists, holes may not exceed one third of the joist depth or come within 2 inches of the top or bottom edge (IRC R502.8, as adopted). A 2-inch line needs about a 2-1/2 inch hole: that fits a 2x10 (one third of 9.25 is about 3.1 inches) and does not fit a 2x8 (one third of 7.25 is about 2.4).
• Linear or point drain. A point drain in the middle needs the floor sloped from four directions toward it. A linear drain (a long slot drain along one edge) lets the floor slope in a single plane, which is what makes a flat, low entry possible. For a wheelchair user it is almost always the right choice.

The handoff: the plumber sets the drain body at the height the tile setter's slope plan calls for, not at a height of the plumber's choosing. Agree the elevation of the drain's top flange relative to the subfloor in writing before the plumber rough-in.

Step 3: Slope the floor to the drain (tile setter)

The IRC requires a shower floor to slope to the drain at no less than 1/4 inch per foot and no more than 1/2 inch per foot (P2709.1, as adopted). For a wheelchair user, stay near 1/4: a steeper floor rolls a chair toward the drain and makes a shower chair tilt.

Put the drain on the wall opposite the entry. Then the entry edge is the high point of the shower and water has to run uphill to leave it. A drain at the entry looks tidy and makes the person roll across the grate.

Step 4: Waterproof past the new wet zone (tile setter)

A tub kept water in a 60-by-30 inch footprint. A curbless shower sprays, splashes and drips onto the floor beyond, and that floor is now part of the wet zone.

• Membrane. A bonded waterproof membrane, tested to ANSI A118.10 (a consensus standard that binds through the product's listing and the job specification; check whether your adopted IRC edition lists it for shower liners), installed under the tile on the whole shower floor and up the walls. The IRC requires shower walls to have a nonabsorbent finish to at least 6 feet above the floor (R307.2, as adopted); take the membrane at least that high.
• Past the entry. Continue the membrane onto the bathroom floor. A common minimum is a few feet beyond the entry; in a small bathroom, membrane the whole floor and turn it up the walls a few inches, so the room itself becomes the outer tub.
• Flood test. Plug the drain, dam the entry temporarily, flood to the height of the entry edge, mark the line and hold it for the time the membrane maker and your inspector require, commonly 24 hours. Check the ceiling below. The plumber is present at the start, because the drain flange's seal to the membrane is the most common leak and it is the joint between their two scopes.

Step 5: Choose the threshold (tile setter, with the person)

The 2010 ADA Standards use a threshold no higher than 1/2 inch for a roll-in shower, beveled (608.7), a reference rather than a requirement in a private home. In practice there are three choices:

• True zero: the entry edge flush with the bathroom floor. Best for a wheelchair, and it depends entirely on the slope and the membrane doing their job.
• A collapsible water dam: a soft rubber strip that a wheel flattens and water does not cross. Cheap insurance on a zero entry.
• A low beveled lip within the 1/2 inch. Better for a walker user who wants a cue underfoot.

Ask the person how they will enter: rolling forward, backing in on a shower chair, or stepping with a walker. A lip that suits a walker catches a shower chair's small casters.

Step 6: Protect the hallway floor

Water leaves a curbless bathroom at the door. Two things stop it: the bathroom floor's membrane turned up at the door opening under the threshold, and a door threshold slightly higher than the bathroom floor it protects. The 2010 ADA Standards use a maximum of 1/4 inch vertical, or 1/2 inch beveled at 1:2, for a change in level (303.2 and 303.3); see related, Thresholds and Changes in Level. A handheld showerhead mounted at the far end, spraying away from the entry, and a weighted curtain do the rest. See related, The Shower Conversion That Flooded the Hallway.

Worked example: a 60 by 30 alcove over 2x10 joists

A 1950s house, a 60-by-30 inch tub alcove, entry along the open 60-inch side. The user is moving to a shower chair and will roll in from the hall.

Framing and budget. 2x10 joists at 16 inches on center, running parallel to the back wall. The bathroom floor is ceramic tile on an old mortar bed, 1-1/2 inches above the subfloor; the hall hardwood sits 3/4 inch above the subfloor, so there is a 3/4 inch lip at the door today. Sampling found no asbestos in the adhesive; the mortar bed is removed under wet methods. That 1-1/2 inches is the budget, and no joists are cut.

Drain. A linear drain along the 60-inch back wall. The jurisdiction runs the IPC, so the 1-1/2 inch line would be allowed, but the plumber upsizes to 2 inches for the capacity of a handheld at full flow. Because the joists run parallel to the back wall, the trap arm runs along a joist bay to the existing stack, 5 feet away, inside the 8-foot limit for 2-inch pipe, with no holes through joists.

Slope. 30 inches of depth from entry to drain at 1/4 inch per foot: 30 / 12 x 1/4 = 0.625 inch. The entry edge sits 5/8 inch above the drain's grate. The tile setter's sloped system fits that inside the 1-1/2 inch budget, with the drain body dropping into the joist bay below the subfloor.

Hallway. The new bathroom floor is built back to 1 inch above the subfloor, with the shower entry flush to it. A saddle threshold at the door is crowned at 1-1/4 inches: 1/4 inch above the bathroom floor, so standing water has to climb it to leave, and 1-1/4 minus 3/4 = 1/2 inch above the hall, beveled at 1:2 on that side. Both sides sit inside the ADA reference's limits, and the old 3/4 inch lip is gone.

Membrane and test. The whole bathroom floor is membraned and turned up 4 inches at the walls, shower walls to 7 feet. Flood-tested 24 hours with the entry dammed; no drop in the line, ceiling below dry.

Threshold. True zero with a collapsible dam, because she backs in on a shower chair and the casters catch on a fixed lip.

Who owns what, in one place

References

• IRC Sections R307.2, R502.8 and P2709, as adopted by your jurisdiction; IPC 417.3 and Table 906.1, or your UPC equivalents, as adopted.
• OSHA silica in construction, 29 CFR 1926.1153; respiratory protection, 29 CFR 1910.134.
• ANSI A118.10, bonded waterproof membranes, through the product listing and specification.
• 2010 ADA Standards for Accessible Design, 303 and 608, as a dimensional reference only in a private home.
• See related: Curbless Showers and Where the Water Goes; Bathroom Layout for a Wheelchair User; Anti-Scald Protection for an Older or Disabled User; The Shower Conversion That Flooded the Hallway.