How do I waterproof a shower before tiling?

You waterproof a shower by covering everything that gets wet, the floor, the walls to above the shower head and every corner, seam, niche and pipe opening, with one continuous membrane made for showers. Tile and grout do not do this job: membrane makers state that mortars and grouts for tile are not replacements for a waterproofing membrane.^[2] The membrane is what keeps water out of the framing.

The most common homeowner method is a liquid-applied membrane rolled or brushed onto cement backer board. The work is not hard, but it is unforgiving, and the product's data sheet is the rulebook. The details below come from two widely sold liquid membranes, Custom Building Products' RedGard and LATICRETE's HYDRO BAN; if you use something else, its own sheet governs.

Start with the right surface

A membrane is only as good as what it is stuck to. RedGard lists drywall only for interior dry areas,^[1] and HYDRO BAN's data sheet limits gypsum wallboard to non-wet areas and says not to bond it to OSB, particle board or interior glue plywood.^[2] Both list cement backer board as a suitable substrate.^[1][2] If your shower walls are drywall or green board, they come out before you waterproof.

The floor has to drain before it is waterproofed. RedGard's instructions call for the substrate to be properly sloped toward the drain flange before the membrane goes on,^[1] and its data sheet gives a minimum pitch of 1/4 inch per linear foot for drainage areas.^[1] Low spots under the membrane become puddles under the tile.

The drain has to be one a membrane can bond to. HYDRO BAN's data sheet says the drain must be a bonding-flange or clamping-ring type with weep holes.^[2] If your existing drain is not that kind, changing it is plumbing work, not waterproofing.

Corners, seams and the drain come first

Water finds the joints, so they get extra material before the main coats. RedGard asks for corners and floor-to-wall intersections to be coated more than 2 inches on either side, and suggests embedding reinforcing tape for extra protection at changes of plane and over gaps wider than 1/8 inch.^[1] HYDRO BAN calls for a band about 8 inches wide over coves and floor-to-wall transitions.^[2] Treat board seams, the inside corners of a niche, a bench, the curb and the openings around the valve and shower arm the same way.

Coats, coverage and colour

Thickness is what makes a liquid membrane waterproof, and the coverage rate on the label is how you control it. RedGard's general rate is 110 square feet per gallon per coat, with at least two coats when you use a roller or brush.^[1] To meet the ANSI A118.10 waterproofing standard, the same data sheet calls for two coats at 80 square feet per gallon each.^[1] Measure the wet area, work out how much product that means, and check that you have used about that much when you finish.

Both products tell you when a coat is dry by changing colour. RedGard goes on pink and dries dark red, typically in 1 to 1.5 hours per coat;^[1] HYDRO BAN changes from light sage to olive green.^[2] Inspect the first coat in good light before the second goes on. Pinholes and thin, translucent patches are easy to see at that point and impossible to see once tile covers them.

Temperature matters. RedGard needs air and surface temperatures above 40°F during application and for 72 hours after,^[1] which can be an issue in an unheated Virginia addition or basement bathroom in winter.

Flood test before you tile

A flood test is the one way to know the floor holds water before it is buried. HYDRO BAN allows flood testing about 2 hours after final cure at 70°F and 50 percent humidity, and 24 hours when the room is between 50 and 69°F.^[2] Plug the drain, fill the floor to just below the top of the curb, mark the water line, and check it, and the ceiling below if there is one, before you drain it. HYDRO BAN's data sheet also tells installers to get approval from the local building code authority before using it in shower pan applications,^[2] so ask your building office if you are replacing a shower pan.

Tile with the right mortar

The membrane is a bonding surface, not an adhesive. RedGard's data sheet calls for a polymer-modified mortar meeting ANSI A118.4 or A118.15 and says not to use premixed adhesives over the membrane.^[1] Where tile meets the tub, the floor or another wall, finish with flexible caulk rather than grout; see how to recaulk a tub or shower.

Safety and permits

For RedGard, the maker says to wear impervious gloves such as nitrile and eye protection, and use it in a well-ventilated area.^[1]

Virginia's building code lists installation or replacement of floor finishes, and replacement of interior wall finishes in homes, among ordinary repairs that do not need a permit application.^[3] But work that alters the water supply, drainage or vent system does need a permit,^[3] and so does removing or disturbing asbestos-containing materials during construction.^[3] If you are tearing out tile and mastic in an older house, EPA says you generally cannot tell whether a material contains asbestos by looking at it,^[4] and remodeling is one of the reasons to have it inspected by an accredited professional.^[4]

Stop and call a pro if

• The drain, valve or supply pipes need to move or be replaced. That is permit work for a licensed plumber.
• Demolition turns up soft or rotted studs, subfloor or joists. Framing repair comes before any membrane.
• The floor has no slope to the drain, or you need a new mortar bed or shower pan. Many homeowners hire this part even when they waterproof the walls themselves.
• The flood test fails and you cannot find where the water is going.
• The old tile, mastic or wall material may contain asbestos.

If something has already gone wrong in an existing shower, start with why a tile shower leaks. You can browse shower tile installers, look up flooring and tile businesses around Blacksburg, or read the other flooring and tile answers. To have a licensed installer quote the whole job, describe your shower on Loclis.