Every fixture in the room sits in the spray zone. The toilet, the sink, the steam with nowhere to go, and twenty square feet of tile standing between your subfloor and rot. Skip one waterproofing layer and you will not discover the mistake for two years, when the floor goes soft underfoot.
Order of operations is everything in a wet bath, and this walkthrough gives you the right sequence from subfloor to finished tile. You will learn membrane choices, drain slopes, fixture placement, and the detailing around every penetration, plus the common shortcuts that cause slow and expensive failures. Build it like a boat, because functionally it is one.
The Wettest Room in the Smallest House
The tiny house bathroom concentrates every moisture risk in 20–30 square feet: shower spray, steam, splashing, and a floor that must drain. Most tiny house bathrooms are wet baths — the whole room IS the shower. Waterproofing is not a finishing touch; it is the structural system that keeps the house from rotting. This is the complete protocol.
1. Wet Bath vs Dry Bath: The Layout Decision
Wet bath: the entire bathroom is the shower — waterproof floor, walls, and ceiling; the toilet and sink live inside the spray zone (marine/RV fixtures). Pros: simplest plumbing, smallest footprint (24–36 sq ft total), no shower door or curtain needed. Cons: everything gets wet (toilet paper needs a dry box; the room needs drying time). Dry bath: separate shower stall with the toilet/sink outside it. Pros: dry fixtures, familiar feel. Cons: needs 35–50 sq ft — a luxury in a tiny house — plus a shower pan and door. The verdict: most tiny houses go wet bath for the space savings. This guide assumes wet bath; the waterproofing principles apply to both.
2. Framing for Water: The Structure Beneath
Wet bath framing differs from dry framing: pressure-treated bottom plates (the plate sits in occasional standing water — PT lumber or a composite plate), framing layout for the drain (the shower drain location determines joist layout — frame the opening before the floor goes down, not after), sloped subfloor (the entire bathroom floor slopes 1/4″ per foot toward the drain — build the slope into the framing with tapered sleepers or a mortar bed, not with wishful thinking), and blocking for fixtures (grab bars, shower heads, and wall-hung fixtures need solid blocking — add it now; retrofitting through tile is miserable). Frame it like a shower, because it is one.
3. The Waterproofing Membrane: The Critical Layer
The membrane is the actual waterproofing — tile is decoration. Options: sheet membranes (Schluter Kerdi, NobleSeal — fleece-backed polyethylene, bonded with thinset; the pro standard — ~$2–4/sq ft), liquid membranes (RedGard, Hydro Ban — paint/roll on 2–3 coats; DIY-friendly, ~$1–2/sq ft — the most common tiny house choice), and foam shower systems (Schluter Kerdi-Shower kits — pre-sloped trays and curbs with integrated membrane — expensive but foolproof). The protocol: membrane the FLOOR entirely (wall to wall, up the walls 6″+), membrane the WALLS to full shower height (or the entire wall in a wet bath), seal every corner and penetration with membrane banding (inside corners, drain flange, shower arm, and valve penetrations — the corners are where failures start), and flood-test before tiling (plug the drain, fill 2″ deep, wait 24 hours — any leak now is a $10 fix; after tile it is a $1,000 fix).
4. Drains: The Exit Strategy
Drain type: a standard 2″ shower drain with a clamping ring (bonds to the membrane — the clamp is the waterproof connection) or a linear drain (sleek, allows single-slope floors — $200–400 vs $30 for standard). Drain placement: center or corner — corner placement allows sloping the whole floor in one plane (simpler); center needs four-way slope (harder in a tiny footprint). The trap: P-trap below the floor (protect from freezing — the trap is the most freeze-vulnerable plumbing in the house; insulate and heat-trace in cold climates). Hair management: a removable strainer ($10) — the cheapest insurance against the #1 drain clog.
5. Tiling a Wet Bath: Materials and Method
Tile choice: porcelain (not ceramic — lower water absorption), small format on the floor (2×2″ or smaller — conforms to the slope; large tiles bridge the slope and pond water), slip resistance (COF ≥ 0.42 — wet bare feet need grip). Setting: polymer-modified thinset (the membrane manufacturer’s approved mortar — warranty depends on it), full coverage on the floor (no voids under floor tile — voids collect water), and epoxy grout (not cement grout — epoxy is effectively waterproof and stain-proof; $50–80 for a bathroom vs $15 for cement — the best $60 in the bathroom). Movement: silicone (not grout) at every change of plane (wall-to-floor, wall-to-wall corners — grout cracks at corners; silicone flexes).
6. Walls and Ceiling: The Spray Zone
In a wet bath, walls and ceiling get direct spray: backing: cement board (not drywall, not green board — cement board or foam backer board like Kerdi-Board), membrane: full wall coverage (liquid membrane over cement board, or sheet membrane), ceiling: membrane + tile or a solid-surface panel (steam condenses on the ceiling — it must be waterproof, not just water-resistant), and fixtures: marine/RV toilet (porcelain RV toilets handle the spray zone; residential toilets corrode), wall-mounted or compact sink (pedestal or wall-hung — no vanity cabinet to rot). Everything in the room must survive direct water — if it cannot get wet, it does not belong in a wet bath.
7. Ventilation: Drying the Room
A wet bath that never dries grows mold between uses: exhaust fan (50+ CFM, on a humidity sensor or timer — run 20+ minutes after showering; the fan is as important as the membrane), makeup air (the fan needs replacement air — an undercut door or vent grille), and drying routine (squeegee the walls after showering — 60 seconds that prevents 90% of mold issues). The waterproofing keeps water out of the structure; the ventilation keeps water from lingering on surfaces. Both are mandatory.
8. Common Failures (and Prevention)
- Skipped flood test: the leak discovered after tiling — always flood-test the membrane.
- Grout at corners: cracks within months — silicone at every change of plane.
- Drywall backing: disintegrates behind tile — cement board or foam board only.
- Flat floor: ponds water at the edges — 1/4″ per foot slope, no exceptions.
- Unsealed penetrations: the shower arm and valve leak into the wall — membrane banding at every penetration.
- No ventilation: surface mold despite perfect waterproofing — fan + squeegee.
Final Considerations
A tiny house wet bath is a shower built like a boat: sloped floor, full membrane, flood-tested, epoxy-grouted, ventilated, and squeegeed. The materials cost $500–1,500; the labor is a careful weekend. The alternative — a slow leak rotting the floor framing of a house on wheels — is the most expensive bathroom remodel in tiny house history. Waterproof like everything depends on it, because it does.