The plumbing work is a new drain position for a shower base or pan, a shower valve rated for the showerhead and limited to 120 degrees F, a waterproof floor and walls, and usually a permit because the drain and valve move. The size, drain and waterproofing rules decide the job.
What actually changes when a tub becomes a shower?
A tub drains at one end. A shower base normally drains in the middle or at a corner. That single difference drives most of the work, because the drain in the floor has to line up with the base you buy. Kohler's shower receptor guide tells the installer to locate the rough plumbing for the drain according to the roughing-in dimensions for the specific model, and to cut a 4.5 inch diameter hole around the drain location. It also says to verify the subfloor is flat and level, and to shim or level the floor if it is not.
The full job usually includes these steps:
- 1Remove the tub and the wall finish around it. A cast iron tub is heavy, so plan the removal.
- 2Move the drain and adjust the trap. The new drain location has to meet the trap and vent rules.
- 3Replace the valve. A tub spout and diverter valve becomes a shower-only valve, or a new combination.
- 4Set the base or build the pan. Either a prefabricated base or a lined pan.
- 5Waterproof the walls and finish them.
- 6Test and inspect.
What do the drain and valve rules say?
### The drain
The IPC sets a minimum waste outlet size for showers. Section 421.3 says waste outlets serving showers must be not less than 1.5 inches in diameter, and that shower outlets other than in bathtubs must have removable strainers not less than 3 inches in diameter with openings at least a quarter inch in the least dimension.
The fixture unit table shows why a bigger shower can need a bigger drain. Table 709.1 rates a shower by the total flow through showerheads and body sprays: 2 drainage fixture units at 5.7 gallons per minute or less, then 3 for more than 5.7 up to 12.3, then 5 up to 25.8, then 6 up to 55.6. A single showerhead falls in the first group, and a shower with several heads or body sprays can land in a higher one. A bathtub, with or without an overhead shower, is 2 fixture units. If you plan a multi-head shower, tell the plumber, because it changes the drain and trap sizing.
Kohler's instructions also insist on a watertight drain seal. Its receptor guide warns of property damage if a watertight seal on the drain is not obtained.
### The valve
Section 412.3 says individual shower and tub-shower combination valves must be balanced-pressure, thermostatic, or combination valves, installed at the point of use, and rated for the flow rate of the installed showerhead. They must also have a means to limit the maximum setting to 120 degrees F, adjusted in the field according to the manufacturer's instructions. The section says in-line thermostatic valves do not count.
Section 421.2 requires the water supply riser from the valve to the showerhead outlet to be attached to the structure, using support devices made for the piping material or fittings anchored with screws.
What are the waterproofing and size rules?
Three rules in Section 421 shape the finished shower.
- Size. Shower compartments must be not less than 900 square inches in interior cross-sectional area and not less than 30 inches in least dimension, with an exception for 25 inches when the area is at least 1,300 square inches. The access opening must be at least 22 inches wide. Our page on the minimum shower size by code explains why the UPC figure differs.
- Walls. Shower walls must be smooth, corrosion-resistant, nonabsorbent and waterproof, extending at least 6 feet above the room floor and not less than 70 inches above the drain. The walls have to form a watertight joint with each other and with the floor.
- Floor. Floors under shower compartments must be lined and made watertight unless a prefabricated receptor is used. The IPC says liners must turn up on all sides not less than 2 inches above the finished threshold. Our page on shower pan liner code covers the pan rules in detail.
Windows and doors within a shower enclosure must meet the safety glazing requirements of the International Building Code, per Section 421.
Do you need a permit?
In Phoenix, almost certainly. The city's permit exemptions cover clearing stoppages, repairing leaks, and replacing valves or fixtures with compliant new ones, but they say a permit is required to relocate any valves, pipes or fixtures. A tub-to-shower conversion moves the drain and replaces the valve, so it falls on the permit side. Our page on whether you need a permit to remodel a bathroom in Phoenix explains the like-for-like test.
If you are converting a tub for accessibility or safety, ask the city plan reviewer about accessible shower requirements, because Section 421.4 says accessible showers must follow Section 404.1.
If the goal is easier access rather than a shower, a different option is a tub with a side door, and are walk-in tubs worth it lists what that tub asks of your plumbing and electrical.
For the wall finish that goes over the new valve and pipe openings, see what goes behind tile around a bathtub, which covers the backer, waterproofing and sealed penetrations.
What should you decide before you start?
- Prefabricated base or built pan? Kohler's base instructions say to construct a level subfloor that will adequately support the shower, and to verify the base is level and fully supported, using shims as needed. A lined pan needs the pan test and more work.
- One showerhead or several? The flow rate affects the drain.
- Where does the valve sit? It moves from the tub wall to a height suited to a shower.
- Is the wall behind the tub in good shape? A plumber will see any leak damage when the tub comes out. Our page on a leaking shower valve behind the wall shows what that looks like.
### How is a prefabricated base set and sealed?
If you choose a prefabricated base, the manufacturer's steps are part of the job. Sterling's receptor guide says the finished wall goes over water-resistant wall material, with a 1/4 inch gap between that material and the receptor, and a 1/8 inch gap between the finished wall and the receptor. The joints are then sealed with silicone sealant, or with tile grout depending on the installation method, and if clear silicone is used the guide says to leave weep holes in each corner of the receptor. It also warns not to add silicone unless directed, because inappropriate use can trap moisture and cause concealed leakage, mold or mildew.
Kohler's base instructions say to install the drain with a watertight seal, check that the base is level and fully supported by the subfloor, use metal shims to level it as needed, and apply 100 percent silicone sealant where the base meets the finished wall and along the floor seam. Both manufacturers make the same point: a level, supported base and a proper seal are what keep water out of the wall, so this is not a step to rush.
HQ Plumbing and Air is licensed and insured, takes emergency calls 24/7, and gives free estimates with an upfront price before work starts. See our plumbing services.
Before you commit to a layout, read the plumbing questions that matter before a bathroom remodel, when the plumber should be involved in a remodel, whether a fixture can move to a new spot and how long a home can go without water during a remodel.
The pan itself is a separate job, covered in how to install a shower pan.