A tub-to-shower conversion looks straightforward from the outside: pull the tub, frame the curb, install a shower base and walls. The work that determines whether it lasts is everything you can’t see when it’s done. Bad prep failures don’t show for 12-24 months, and when they do, the fix is two to three times the cost of doing it right the first time. Here’s what real prep looks like.

What Gets Exposed When You Pull the Tub

Removing a 30-50 year old bathtub almost always reveals one or more issues:

• Rotted subfloor. Slow drips around the drain or faucet stem rot the OSB or plywood beneath the tub. We see this on roughly 60% of older Rapid City tub removals.
• Damaged or absent flashing. Older installs often have no flashing between tub flange and wall, water that gets behind the surround has nothing stopping it.
• Deteriorated drain pipes. Galvanized drain lines from pre-1980 builds corrode from the inside. Replacing them now adds $400-1,200 but prevents future opens-up-the-wall repairs.
• Hidden mold. Behind tub surrounds is the most common hidden-mold location in older homes. Remediation isn’t optional once it’s found.

Subfloor Repair: The Foundation of Everything

If subfloor is soft, swollen, or visibly damaged, it gets cut out and replaced before any shower work proceeds. A shower built over compromised subfloor will crack tile within 6-12 months and develop water leakage within 18-24.

Replacement uses 3/4″ tongue-and-groove plywood or AdvanTech, fully fastened to the joists below, sealed at edges, and integrated with the surrounding floor before any waterproofing layers go down.

Waterproofing: Schluter Kerdi or Equivalent

This is where most cheap conversions fail. Tile and grout are NOT waterproof, they’re the decorative surface over the waterproofing layer. The waterproofing IS the system.

Two acceptable approaches:

• Schluter Kerdi membrane system: orange fabric membrane bonded to the substrate, all seams sealed with Kerdi tape, integrated with a Schluter shower pan. The industry gold standard.
• Liquid-applied membrane (RedGard, Hydro Ban): two coats of polymer-modified liquid that dries to a flexible waterproof film. Acceptable when applied to manufacturer spec.

Mortar bed alone is NOT waterproofing. Mortar is porous. Water passes through it slowly but reliably. A “tile and mortar” shower without a membrane will leak.

Pre-Slope and Drain Integration

The shower floor must slope toward the drain at minimum 1/4″ per foot (steeper for low-curb showers). Cheap conversions skip this step and rely on the drain to “find” the water; reality is water pools at low spots, mineral-stains the tile, and accelerates grout failure.

Proper pre-slope is built up in mortar or pre-formed foam (Schluter Kerdi-Shower-T) before the waterproof membrane goes on. The drain assembly (clamping ring drain, not a regular tub drain) gets integrated with the membrane to create a watertight transition.

Curb and Wall Framing for Glass

If the conversion includes frameless glass enclosure, the wall framing must be solid enough to hold it. Glass weighs 6-9 pounds per square foot. Frameless installations require:

• Solid backing (full-sheet plywood or 2x blocking) behind the glass attachment points
• Walls plumb to within 1/8″ over 6 feet (out-of-plumb walls = visible gaps in the glass)
• Curb height and pitch to manufacturer spec for the glass system you’re using

Skipping the prep means a custom $3,500 glass enclosure that doesn’t fit and has to be re-templated.

Why Bruce Won’t Skip the Prep

Black Hills Premier Interiors averages 4-6 days of labor on the prep alone for a typical tub-to-shower conversion. That’s not slow work; it’s the work that matters. The shower you see installed in three days on TikTok is missing the layers that make it last.

If you’re comparing quotes, ask each contractor to walk you through their waterproofing system step by step. The honest answer takes 5 minutes. The “we’ve been doing this for years, don’t worry about it” answer takes 5 seconds and means they’re skipping the work.

Frequently Asked Questions

How long does a tub-to-shower conversion take?

5-10 working days for a typical conversion. Cheap conversions advertise 1-2 days, which is usually possible only by skipping waterproofing and pre-slope. Bruce’s shop averages 6-8 days for prep + install.

Do I need a permit for a tub-to-shower conversion in Rapid City?

Yes, the City of Rapid City requires a plumbing permit for any change to drain or water supply lines, which includes tub-to-shower conversions. We pull all required permits as part of the project.

Can a tub-to-shower conversion be done in one day?

Acrylic prefab conversions can be installed in 1-2 days but skip the custom-tile + waterproofing work. They’re fine for budget-conscious renovations; they don’t deliver the same look or longevity as a properly-prepped tile conversion.

What’s the cost of fixing a leaking shower from bad waterproofing?

$8,000-25,000 typically, full demo, subfloor replacement, ceiling repair in the room below, mold remediation if discovered, and rebuild of the shower with proper waterproofing. Many times the original conversion cost.

How do I know if my contractor is doing waterproofing right?

Ask to see the membrane system before tile goes on. Schluter Kerdi shows as orange fabric. RedGard shows as red liquid coating. If neither is visible (only tile setter mortar), the waterproofing is missing.

Should I get a curbless shower?

Curbless (zero-threshold) showers are excellent for accessibility and look cleaner, but they’re harder to waterproof and require precise floor pitch. They cost 25-50% more than standard curbed showers because of the prep complexity. Worth it when done right.

Serving Rapid City and the Black Hills region of South Dakota. Learn more about our services, browse our project gallery, or read about Bruce and the team.