How do I waterproof a Calgary bathtub surround before installing tile on cement board?

Cement board alone is not waterproofing — it's a stable substrate, but water will pass straight through it and rot the framing behind your tub within a few years. You need a continuous waterproof membrane applied over the cement board before a single tile goes up.

Here's how to do it properly.

Prepare the Cement Board First

Before any membrane goes on, make sure your cement board is installed correctly. Use 1/2-inch cement board (Durock, HardieBacker, or equivalent) screwed to studs every 8 inches with corrosion-resistant screws. All seams between sheets should be taped with alkali-resistant fibreglass mesh tape and skim-coated with unmodified thinset — this bridges the joints and gives the membrane a continuous surface to bond to. Let the thinset dry fully before proceeding.

Keep the bottom edge of the cement board 1/4 inch above the tub deck. This gap prevents the board from wicking water from the tub ledge. You'll seal this gap later with silicone, not grout.

Apply the Waterproof Membrane

You have three solid options for a tub surround in a Calgary home:

RedGard (liquid-applied) is the most DIY-accessible option. It rolls and brushes on like thick paint, going on pink and drying to a deep red when cured. Apply two coats — the first coat fills the surface, the second coat creates the continuous film. Pay close attention to corners and the tub ledge transition, where you should embed fibreglass mesh tape into the first wet coat before the second coat goes over top. Full cure takes 24-48 hours. In Calgary's dry winter air, RedGard actually dries faster than the manufacturer expects — watch for the colour change rather than relying solely on time.

Schluter Kerdi is a sheet membrane bonded directly to the cement board with unmodified thinset. Kerdi is the industry standard for a reason — it's a consistent thickness, bonds reliably, and the Schluter system includes pre-formed corners, niches, and tub ledge strips that eliminate the weakest points in any waterproofing job. Tile goes directly over Kerdi with unmodified thinset. This is the method most Calgary tile setters default to for shower and tub surrounds.

Laticrete Hydro Ban is another liquid-applied option, similar to RedGard but with a faster cure time and strong performance in wet areas. It's popular with professional installers who want a quick turnaround between waterproofing and tiling.

The Spots That Fail First

The tub-to-wall joint at the bottom of the surround is where most Calgary tub surround failures begin. Never grout this joint — grout is rigid and will crack as the tub flexes under the weight of water and a person. This joint must be filled with colour-matched 100% silicone sealant after tiling is complete. The same rule applies to inside corners — silicone, not grout.

Calgary's dry winters cause wood framing to shrink and shift seasonally, which puts stress on every rigid joint in a bathroom. Silicone at all plane changes is not optional here.

Alberta-Specific Note

If your tub surround project involves moving or adding any plumbing — even just relocating a showerhead or adding a body spray — that work requires a plumbing permit and inspection under Alberta's Safety Codes Act. The tile installation itself doesn't need a permit, but the plumbing does, and it must be done by a licensed plumber.

When to Call a Pro

Applying RedGard is within reach of a careful DIYer. Installing Kerdi sheet membrane, particularly around inside corners, the tub ledge, and any niches, is more technical — a missed overlap or a pinhole in the membrane can allow water to track behind the tile for years before you notice damage. If you're not confident in your membrane work, this is the one place in a tile project where hiring a professional tile setter pays for itself many times over.

Need help finding a tile installer for your tub surround? Calgary Tiling can match you with a local contractor for free — browse the directory at calgaryconstructionnetwork.com/directory?trade=tiling.