A basement shower has unique waterproofing demands: it sits on a concrete slab (often slightly damp), has typically lower drains running into the slab, and any leak goes directly into the foundation. Get the waterproofing right and the shower is bulletproof. Get it wrong and you're tearing it apart in 2 years to fix water damage.

This guide covers waterproofing a basement shower from slab to ceiling, using modern Schluter Kerdi or RedGard liquid membrane systems.

Before you start

Three things to settle:

1. Drain location and slope. Basement showers typically use a recessed drain in the slab. Slab must slope toward drain (1/4 inch per foot minimum). If existing slab is flat: cut and re-pour a sloped pre-pan, or use a foam pre-formed pan (Wedi, Kerdi-Shower-Tray).

2. Membrane system: Schluter Kerdi vs RedGard vs traditional. Schluter Kerdi: orange membrane fabric that bonds with thinset, forms continuous waterproofing. Reliable, used by pros. RedGard: liquid-applied membrane, paint on with roller. Easier DIY. Traditional CPE liner: under tile, requires careful installation. For first-time DIYers, RedGard or Kerdi are most reliable.

3. Vapour barrier or no?. Behind tile in a shower: do not install polyethylene vapour barrier behind cement board. Modern best practice: water-resistant cement board + topical waterproofing membrane on the front face. Old method (poly vapour barrier behind cement board) creates double moisture barrier and rot risk.

What you'll need

Tools

Trowel (notched, 1/4 inch typical)
Mixing paddle and bucket
Margin trowel (small)
Roller and brush (for liquid membranes)
Tape measure
Utility knife
Caulk gun

Materials

Cement board (HardieBacker, Durock — 1/2 inch typical)
Cement board screws
Schluter Kerdi membrane + Kerdi-Band + Kerdi-Mixing Valve seal OR RedGard liquid membrane + alkali-resistant fibreglass mesh
Modified thinset (or unmodified per Kerdi spec)
Pre-sloped shower pan or membrane pan
Drain matched to system (Kerdi-Drain or compatible)
Silicone sealant (100% silicone)
Backerod (foam rod for sealant joints)

Waterproofing system must be continuous

Membrane must extend from inside the shower up to ~6 inches above shower head, around all walls.

All seams (corners, drain, mixing valve, tub edge) must be sealed.

One gap in the system = leak. Treat waterproofing as one continuous skin.

Step-by-step instructions

Verify slab is dry and sloped to drain

Concrete slab must be dry — no efflorescence, no active leaks.

Drain at low point. Slope: 1/4 inch per foot toward drain.

If slab is flat or sloped wrong: pour a pre-pan with deck mud (sand-cement mix), shaped to slope to drain.

Or use foam pre-formed pan (Wedi, Kerdi-Shower-Tray) over flat slab.

Install drain matched to membrane system

Kerdi-Drain (for Kerdi system): clamps Kerdi membrane to drain.

Standard tile drain (for RedGard or CPE liner): drain has flange that liner clamps to.

Mortar drain in place; verify level and tight.

Build pre-pan if needed

If slab is flat: pour a deck mud pre-pan sloped to drain.

Mix deck mud (sand:cement 5:1, water to consistency).

Spread on slab, slope to drain at 1/4 inch per foot.

Trowel smooth.

Let cure 24 hours.

Install membrane pan over pre-pan

Schluter Kerdi-Shower-Tray: foam pan with built-in slope; replaces deck mud pre-pan.

Or: liquid membrane (RedGard) painted over deck mud pre-pan.

Or: CPE liner over deck mud pre-pan, then second mortar bed.

Continue membrane up walls 6+ inches.

Install cement board on walls

1/2 inch cement board (HardieBacker, Durock) over framing.

Cement board screws every 8 inches at edges, 12 inches in field.

Tape and thinset cement board joints.

Cement board extends from pan up to ceiling height.

Apply waterproofing membrane on walls

Schluter Kerdi: trowel thinset on cement board, embed Kerdi membrane sheet, pressure-roll to bond.

RedGard: paint two coats of liquid membrane on cement board with roller. Each coat dries 4-6 hours; final wait 24 hours.

Continuous coverage from pan up over the wall, including corners.

Verify pinhole-free coverage (RedGard shows visible colour change when applied properly).

Reinforce corners with mesh and seal

Schluter Kerdi: pre-formed Kerdi-Kereck (corners) embedded in thinset over Kerdi sheets.

RedGard: alkali-resistant fibreglass mesh in corners, embedded between RedGard coats.

Critical: corners are the most leak-prone.

Seal at drain, mixing valve, and any penetrations

Drain: clamp Kerdi-Drain or seal CPE liner per system.

Mixing valve: Kerdi-Seal-MV (Kerdi system) or careful RedGard around the valve.

Pipe penetrations: gasket or careful sealing.

All penetrations must be watertight.

Test for leaks (flood test)

Block drain (with Kerdi-Drain test plug or temporary).

Fill shower pan with 2-3 inches of water.

Wait 24 hours.

Check water level — should not drop more than 1/8 inch.

Check below for any leaks.

Pass = waterproofing is sound. Fail = find and fix leak before tiling.

Tile over waterproofing

Standard tile installation over the membrane.

Modified thinset for tile-to-Kerdi (or per system spec).

Tile, grout, seal tile/grout per finish phase.

Common mistakes that cost you

Polyethylene vapour barrier behind cement board. Old method, creates moisture trap. Use topical membrane on cement board face.
No pre-pan slope. Water pools, leaks. Always sloped pan.
Skipping flood test. Find leaks after tile = expensive. Always test before tiling.
Membrane gaps at corners. Corners leak first. Reinforce with mesh and pre-formed corners.
Wrong drain for system. Membrane can't bond to incompatible drain. Match drain to membrane.

Frequently asked questions

Schluter Kerdi vs RedGard for DIY?

Kerdi: more reliable for first-timers — sheet membrane is easier to verify coverage.

RedGard: cheaper, simpler tools needed, but easy to apply too thin.

If first DIY shower waterproofing: Kerdi is the safer choice.

How long should the membrane extend up the wall?

Minimum: 6 inches above the shower head height.

Better: full ceiling-height for steam showers.

For basement showers without steam: 6 ft from floor is typical.

Do I need a permit for a basement shower?

Yes — basement plumbing additions require Ontario plumbing permit.

Drain into existing main + water supply tap-in: a plumbing permit applies, submitted through MyServiceOttawa.

If you're cutting through slab to add drain: more involved permit and rough-in inspection.

When to call us instead

Basement showers require expert waterproofing. Our team handles the full scope: drain, framing, waterproofing, tile, fixtures.

Book a free 60-minute site visit →

Related Ottawa services

If you're adding a basement bathroom, our basement renovation team handles plumbing, waterproofing, and finishing. Book a free 60-minute consultation.