Curbless Shower on a Concrete Slab: Recess, Slope and Drain

On a wood-framed floor, a zero-threshold shower is mostly a matter of lowering joists or adding sloped foam. Concrete changes everything. Building a curbless shower on a concrete slab means you either cut into the slab to create a recessed shower floor or raise the entire bathroom floor so the shower can slope down to a drain, and both paths start with confirming what is inside and under the concrete.

Part of our Bathroom Remodeling & Flooring guide — explore the full guide →

The payoff is real: a barrier-free entry that works for aging in place, wheelchairs and walkers, plus a clean, open look. The risks are also real. Slabs can hide plumbing, radiant heating tubing, electrical conduit and, in many newer homes, post-tensioned steel cables that must never be cut without an engineer. Silica dust from cutting concrete is a serious health hazard.

Plan the structural check, drain location, slope, waterproofing and permit before any saw touches the floor, and most of the pitfalls go away.

Planning a Curbless Shower on a Concrete Slab

A curbless shower needs height to work with. Water has to flow from the entry toward the drain, and the floor at the entry must sit level with or slightly higher than the adjacent bathroom floor. With a typical slope of 1/4 inch per foot, plus mortar or foam, membrane, thinset and tile, you usually need roughly 1 1/2 to 3 inches of depth across the shower area, depending on shower size and drain type.

There are two basic ways to get that depth on a slab:

• Recess the slab: Cut and remove concrete in the shower footprint, dig out the fill, route the drain and pour a new, lower sub-slab. This keeps the bathroom floor at its current height.
• Raise the bathroom floor: Build up the rest of the bathroom with a thicker mortar bed, foam boards or a raised floor system so the shower floor can slope down from that new height without cutting the slab. A raised shower base on a concrete floor avoids demolition but creates a step at the bathroom door unless the transition is handled with a ramp or threshold.

A hybrid approach is also common: a shallow recess of an inch or so combined with a thin build-up in the rest of the room. The choice depends on what the structural check reveals, whether the drain must move and how the bathroom meets the hallway.

Step One: Structural Check and Slab Scanning

Before planning a cut, find out what kind of slab you have and what runs through it. This step decides whether recessing is even possible.

Post-Tension Slabs

Many homes built on expansive soils, particularly in the South and West, sit on post-tensioned slabs. These contain high-strength steel cables stretched under enormous force. Cutting one can cause it to release violently, injure anyone nearby and compromise the slab. Warning signs include a stamped notice near the garage or in a closet, cable ends patched along the slab edge and construction documents mentioning tendons. Do not cut, core or trench a post-tension slab without a structural engineer’s review and approval. In many cases the engineer will steer you toward raising the floor instead.

Locating Plumbing, Radiant and Conduit

Conventional slabs often have drain and supply lines, radiant floor tubing or electrical conduit embedded or running just beneath. Hire a concrete scanning company using ground-penetrating radar to map the area before cutting. Radiant heating tubing is easy to nick and expensive to repair. Review original plans if you have them, and check where existing drains, the toilet and supply lines run.

Engineer Review

Even without cables, check for thickened footings, grade beams or load-bearing walls near the shower. A structural engineer can confirm how much concrete can be removed, how the new pour ties into the old slab and what reinforcement to add. Plan on a building permit for slab cutting, drain relocation and shower waterproofing.

Cutting and Trenching the Slab Safely

Concrete contains crystalline silica. Cutting, grinding or breaking it releases fine respirable dust that can cause lung disease. Treat dust control as a core part of the job, not an afterthought.

• Cut wet: Use a walk-behind or handheld saw with water feed, or a saw with an integrated dust shroud connected to a HEPA vacuum.
• Protect your lungs: Wear at minimum a well-fitted N95, and preferably a half-face respirator with P100 filters, plus eye and hearing protection.
• Contain the area: Seal doorways with plastic sheeting, cover HVAC returns and turn the system off while cutting.
• Clean correctly: Never dry-sweep or use compressed air. Clean with a HEPA-filtered vacuum and wet wiping.

Given the dust, weight and hidden hazards, many homeowners hire a concrete cutting contractor for the saw work. A typical recess sequence looks like this:

1. Mark the shower footprint and the drain trench on the slab after scanning.
2. Score the perimeter with a wet saw to a controlled depth.
3. Break out the concrete inside the cuts with a rotary hammer or jackhammer.
4. Excavate fill to the planned depth, leaving room for a new sub-slab and vapor barrier.
5. Have a licensed plumber install or relocate the drain, trap and vent, then inspect.
6. Compact the fill, add vapor barrier and any reinforcement or dowels specified by the engineer, and pour the new recessed slab.

Drain Choice and Relocation

Drain type drives the slope layout. A linear drain is the most popular choice for curbless showers on slabs because it lets the floor slope in a single plane toward one wall or the entry. That single slope is easier to build and allows large-format tile. A center point drain requires four-way slope, which needs more depth and forces smaller tiles or envelope cuts.

Moving a drain in a slab means trenching to the existing drain line, maintaining proper pipe slope and adding or modifying a trap and vent. This is licensed plumber work in nearly every jurisdiction and usually requires a permit and inspection before the trench is backfilled. A linear shower drain can often be positioned at the back wall, reducing how far the trench must run.

Building the Slope

The shower floor needs a consistent slope of about 1/4 inch per foot toward the drain. Some codes and designers allow up to about 1/2 inch per foot. For a 4-foot-deep shower with a linear drain along the back wall, that means the entry sits about 1 inch higher than the drain.

• Mortar bed: A sand and cement dry-pack mortar bed is floated to screed guides set to the correct pitch. It is inexpensive and flexible for odd shapes.
• Pre-sloped foam panels: Lightweight trays and wedges built for curbless installations speed things up and guarantee the pitch.
• Self-leveling products: Used for flattening the rest of the bathroom, not for sloping the shower itself.

Extend the slope or a gentle transition a little beyond the shower opening in small bathrooms, so water sprayed toward the entry flows back. A glass panel or door sweep can help contain spray.

Waterproofing on Grade

Slabs on grade can wick ground moisture upward, and curbless showers have no curb to hold water back. Both facts make waterproofing more demanding.

• Install a vapor barrier under any new recessed pour.
• Use a surface-applied waterproofing membrane, either liquid or sheet, over the sloped bed, bonded to the drain flange and carried up the walls.
• Extend the membrane beyond the shower entry, often across the entire bathroom floor, since there is no curb.
• Reinforce inside corners, seams and pipe penetrations with the membrane maker’s corners, bands and collars.
• Flood test the finished membrane for at least 24 hours before tiling.

Mold follows trapped moisture, so fix any existing leaks or slab dampness before building. Run a properly sized exhaust fan vented outdoors. Hire a remediation pro if you uncover mold covering more than about 10 square feet during demolition.

Costs and Timeline

Costs vary widely with slab type and region. As rough ranges, concrete scanning often runs a few hundred dollars, slab cutting and removal for a shower recess can run from about $1,000 to $3,000 or more, and plumbing relocation typically adds $1,000 to $3,500. Engineering review, permits, waterproofing, drain hardware and tile are extra. A complete curbless shower conversion on a slab commonly lands in the $8,000 to $20,000 range or beyond, with raised-floor approaches sometimes cheaper when no concrete is removed. Expect the project to take two to four weeks, including inspections and concrete curing.

Troubleshooting Common Problems

• Water escaping the shower: Slope is too shallow near the entry or the drain is too far from the opening. Add a glass panel or reposition the drain during planning.
• Puddles on the shower floor: The pitch is inconsistent. Check it with a level before membrane and tile.
• Cracked tile over the patch: The new pour was not tied into the old slab, or an uncoupling layer was skipped. Follow the engineer’s doweling detail.
• Damp or musty floor: Missing vapor barrier or a membrane gap. Test before tiling.

When to Call a Licensed Professional

Bring in a structural engineer before cutting any slab, and always for post-tension slabs. A licensed plumber should handle drain relocation, venting and trap work, and a permit will commonly be required for the slab work, plumbing and shower. Concrete cutting contractors have the wet saws and dust controls for the demolition, and an experienced tile contractor familiar with curbless systems can build the slope and waterproofing correctly. Homeowners can often handle design choices, tile selection and finish work while the pros tackle the concrete and plumbing.