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newSchoolWorks.com › VoTechWorks › Building & Construction › Plumbing & Pipefitting › Plumbing Fixtures: Selection, Rough-In, and Trim-Out › Lesson 6
BLD-PLUM06-05 · Lesson 6 · 15 clock hours · lab

Bathtub and Shower Receptor Installation

Objective. Students will set a one-piece bathtub or shower receptor, level and secure it to framing, install an integral drain and overflow assembly, and connect the fixture to the rough-in drain and supply, passing a standing-water leak test.

Bathtub and Shower Receptor Installation

Why This Work Matters

A bathtub or shower receptor that is installed even slightly out of level will drain poorly, stress joints over time, and fail inspections. More importantly, a poorly seated unit can flex, crack its surround, and create hidden water leaks inside finished walls. Getting the installation right the first time protects the homeowner and demonstrates the craftsmanship that journeyman-level work demands.

Safety First

Before entering the work area, put on safety glasses — mortar mixing and fastener driving both create projectile hazards. Wear knee pads throughout; you will be kneeling on subfloor and concrete board repeatedly. Tubs and receptors are heavy and awkward — always use a second person or a dolly when moving a cast-iron or acrylic unit. Keep the work area dry: mortar dust and wet mortar on a sloped floor are a slip hazard. When using a framing nailer, confirm it is set to sequential (not bump-fire) mode and keep fingers clear of the nailing flange edge. Dispose of excess plumber's putty and mortar in a trash bag, never in drains.

Types of Units

Alcove bathtubs fit into a three-wall recess and are by far the most common residential installation. They are manufactured in acrylic, fiberglass, enameled steel, and cast iron. Acrylic and fiberglass units are light enough for one person to maneuver with care; cast-iron units require a crew and a dolly.

Shower receptors (sometimes called shower pans or bases) are one-piece molded units that replace the traditional mud-bed and liner system. They come in acrylic, fiberglass, and solid surface materials and must be supported uniformly across their floor.

Preparing the Subfloor and Rough Opening

Confirm the drain rough-in location matches the manufacturer's template. The center of the drain opening in the subfloor must align with the drain outlet on the underside of the unit. Check the rough opening dimensions: the alcove should be no more than 1/4 inch wider than the unit so the nailing flanges reach the studs on both side walls.

Sweep or vacuum the subfloor. Look for high spots, fastener heads, or debris — any of these can crack an acrylic shell.

Mortar Bed and Foam Support

Acrylic and fiberglass tubs flex when filled with water. Without continuous support beneath the floor of the tub, repeated flexing fatigues the shell and the drain joint. Two methods are standard:

Cast-iron tubs require no mortar bed — their weight alone provides stability — but the subfloor must be flat.

Leveling the Unit

Place your 4-foot level along the long axis of the tub floor, then across the short axis. The tub must be level within 1/8 inch over its full length. If the subfloor is out of level, shim the low end with composite shims before the mortar sets, or use adjustable legs on receptors that include them. A properly leveled unit will also show the tub floor sloping slightly toward the drain — this is built into the unit by the manufacturer and is not something you adjust.

Waste-and-Overflow Assembly

The waste-and-overflow (W&O) assembly connects the tub drain at the bottom (the drain shoe) to the overflow port near the top of the tub end wall, then exits to the trap below the floor. Most assemblies use a trip-lever or toe-touch stopper mechanism mounted in the overflow faceplate.

Installation sequence:

1. Press a rope of plumber's putty around the drain flange (unless the manufacturer specifies a gasket instead — read the instruction sheet).

2. Thread the drain shoe up through the tub floor from below, engaging the drain strainer from above. Tighten with a drain wrench or flat-blade screwdriver until the putty squeezes out evenly — do not overtighten acrylic.

3. Attach the overflow pipe to the overflow port on the tub, using the rubber gasket supplied. The gasket must seat flat against the tub wall — no gaps.

4. Connect the waste arm (horizontal pipe) from the drain shoe to the sanitary tee or P-trap below.

5. Install the stopper mechanism and faceplate per the manufacturer's instructions.

Nailing Flanges and Waterproofing

Alcove tubs have a nailing flange — a lip that runs along the back and side walls at the top of the tub. Drive corrosion-resistant screws or nails through the flange into the wall studs. Do not puncture the tub field (the flat interior surfaces). The wall surround material (cement board, tile backer) should overlap the flange, not butt to it. Apply silicone at the tub-to-surround joint — never caulk over a screw head.

Leak Test Procedure

Once all connections are made and the mortar bed (if used) has cured sufficiently:

1. Install a drain plug.

2. Fill the tub to the overflow inlet level.

3. Wait five minutes with no additional water entering.

4. Remove the plug and watch every joint — drain shoe, overflow gasket, slip-joint nuts — as water drains.

5. Record all observations on your lab log. Any moisture at a joint is a failure and requires disassembly, re-sealing, and a repeat test.

Key terms8

Waste-and-overflow (W&O) assemblyThe combined fitting that connects the tub drain at the bottom to the overflow port near the tub rim, routing water to the trap and drain system.
Drain shoeThe fitting at the base of the tub drain opening that threads onto the drain strainer and connects to the waste arm leading to the trap.
Nailing flangeA perforated lip around the top perimeter of an alcove tub that is fastened to wall studs to hold the unit in position.
Mortar bedA layer of dry-pack mortar spread on the subfloor beneath a tub or receptor to provide continuous, rigid support and prevent flexing.
Plumber's puttyA soft, oil-based sealing compound pressed around drain flanges and strainers to form a watertight seal between the fitting and the fixture surface.
Overflow portThe opening near the top of the tub end wall through which water exits if the tub fills above the normal water line, preventing overflow onto the floor.
Shower receptorA one-piece, factory-molded shower base that replaces a field-built mud-bed and liner pan system.
Standing-water leak testA quality-control procedure in which the fixture is filled with water and held for a set time before draining, allowing inspection of all joints under pressure.

Worked examples4

1. You place a 4-foot level on the long axis of a 60-inch alcove tub. The bubble reads 3/16 inch off center. Does the installation pass the level check?
  1. Step 1 — Identify the tolerance: the acceptance criterion is level within 1/8 inch (0.125 in) over the length of the tub.
  2. Step 2 — Convert the reading to a comparable value: 3/16 inch = 0.1875 inch.
  3. Step 3 — Compare: 0.1875 in > 0.125 in, so the unit exceeds the allowable deviation.
Answer: No — the tub fails the level check. You must add shims at the low end and re-check before proceeding.
2. The tub rough-in alcove measures 60 3/8 inches wide. The tub unit is 60 inches wide. Will the nailing flanges reach the studs on both side walls?
  1. Step 1 — Find the gap on each side: (60 3/8 in − 60 in) ÷ 2 = 3/8 in ÷ 2 = 3/16 inch per side.
  2. Step 2 — Evaluate: the flange only needs to overlap the face of the stud, not span a large gap. A 3/16-inch gap per side is small enough that the flange will still contact the stud face and can be fastened.
  3. Step 3 — However, confirm the maximum gap allowed by the manufacturer's installation instructions — most allow up to 1/4 inch per side before extra blocking is needed.
Answer: Yes — 3/16 inch per side is within typical tolerances, so the flanges reach the studs. Always verify against the specific product instructions.
3. You are mixing a mortar bed. The bag directions say to use 3 parts sand to 1 part Portland cement by volume. You have a 1-gallon bucket as your measure. How many buckets of each material do you need to fill a volume of 4 gallons of dry-pack mortar?
  1. Step 1 — Total ratio parts: 3 parts sand + 1 part cement = 4 parts total.
  2. Step 2 — Each part equals 4 gallons ÷ 4 = 1 gallon.
  3. Step 3 — Sand needed: 3 × 1 gallon = 3 gallons (3 bucket-fills).
  4. Step 4 — Cement needed: 1 × 1 gallon = 1 gallon (1 bucket-fill).
Answer: 3 gallons (buckets) of sand and 1 gallon (bucket) of Portland cement.
4. During the five-minute standing-water test, water drains out of the tub before the five minutes are up. The drain plug appears to be fully seated. What should you check first?
  1. Step 1 — The overflow faceplate houses the trip-lever or stopper linkage. If the linkage is adjusted too short, the stopper may not fully close the drain, or the overflow plate itself may not be sealing the overflow port completely.
  2. Step 2 — Inspect the overflow gasket: if it is pinched, torn, or not seated flat against the tub wall, water can bypass the stopper and travel down the overflow tube even when the stopper is 'closed'.
  3. Step 3 — Check stopper adjustment: remove the overflow cover and adjust the length of the linkage rod per the manufacturer's directions so the stopper seats firmly.
Answer: Check the overflow gasket for proper seating and the trip-lever linkage adjustment — a misaligned linkage or leaking overflow gasket is the most common cause of this symptom.

The math in this lesson2

Learners measure out a 3:1 sand-to-cement mortar mix by volume.
6.RP.37.NS.3
In class: Instructor demo: mortar-bed a bathtub shell on a mock framed platform, set the drain and waste-and-overflow assembly with gaskets and drain shoe, explain nailing flange requirements, and review waterproofing at the surround transition. · Students set and level their assigned tub or shower receptor on a mortar bed or support legs, install the drain and overflow, secure the nailing flange, and connect to the stubbed-out drain — instructor checks level and drain slope before students plug and fill. · Students fill the tub to the overflow, wait five minutes, then drain while inspecting the waste-and-overflow connections and drain shoe for leaks; findings recorded on lab log.
Materials: Alcove bathtub or shower receptor units, mortar or foam bath support, waste-and-overflow assemblies, drain shoes, plumber's putty, screwdriver set, channel-lock pliers, level (4 ft), framing nailer or screws, safety glasses, knee pads, lab log sheets
Check: Tub or receptor level within 1/8 inch over its length; no leaks at drain or overflow joints after a five-minute fill test — both criteria required to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Plumbing — Fixtures and Trim
1. What is the maximum allowable deviation from level for a bathtub or shower receptor installation to pass inspection in this program?
NCCER Plumbing — Fixtures and Trim
2. Why is continuous mortar-bed or foam support required under the floor of an acrylic or fiberglass bathtub?
NCCER Plumbing — Fixtures and Trim
3. Which component seals the joint between the overflow pipe and the tub wall at the overflow port?
State Journeyman Plumber — Code Compliance and Inspection
4. During the standing-water leak test, a small drip appears at the slip-joint nut on the waste arm. What is the correct first action?
NCCER Plumbing — Fixtures and Trim
5. The nailing flange on an alcove tub should be covered by the wall surround material, not left exposed. What is the primary reason for this?
NCCER Plumbing — Safety and Health
6. Which of the following is the safest practice when moving a cast-iron alcove bathtub into position?
NCCER Plumbing — Fixtures and Trim
7. Plumber's putty is pressed around the tub drain flange before installing the strainer. What should you observe when the strainer is properly tightened?
State Journeyman Plumber — Code Compliance and Inspection
8. A student connects the drain shoe to the P-trap with a slip-joint using only hand pressure and skips testing because no mortar was used. Why is skipping the leak test a problem from both a licensing and a trade quality standpoint?
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