What should go behind tile around a bathtub?

Tile and grout are not waterproof. The Schluter installation handbook says ceramic and stone tile are durable and hygienic but are not inherently waterproof, so they must be installed with a waterproofing system that manages moisture and protects moisture-sensitive building materials while controlling mold growth. Water gets through grout lines, cracks and caulk, and it ends up in the wall.

The U.S. Department of Energy's Building America Solution Center states the rule directly. It says to install cement board or an equivalent moisture-resistant backing on walls behind tub and shower enclosures made of tile or panel assemblies with caulked joints, and not to use paper-faced backer board, meaning paper-faced drywall, behind seamed tub and shower enclosures. Its longer description adds that paper-faced gypsum board will not prevent moisture penetration and should not be used in wet areas, and that non-paper-faced fiber-reinforced gypsum board, moisture-resistant gypsum board and "green board" products are also not recommended behind tub surrounds.

One exception applies to seamless enclosures. The same source says that if a monolithic tub and shower enclosure with no seams, such as fiberglass, is used, a paper-faced backer board can be acceptable if it meets the ASTM mold-resistant standards named for ENERGY STAR. Tile is not a monolithic surface, so that exception does not help a tiled tub wall.

### What backer materials are approved?

The Building America guide names several materials that suit the job:

• Cement board, which has a cementitious core and glass mats on both sides to strengthen it.
• Coated glass mat, which has a gypsum core with glass fibers between fiberglass surface mats.
• Fiber cement, which is reinforced with wood fibers.

Any of these is an "alternate approved product" in the guide's wording, along with fiber-reinforced gypsum and fiber mat-reinforced cementitious backer panels. The guide also notes that cement board comes in standard 5 foot wide sheets, which is the width of a standard tub surround. Schluter takes a different route. Its handbook covers a foam-core waterproof building panel and a sheet membrane, both for bathtub surrounds, and both are meant to be applied over solid backing or framing.

No, and this is the misunderstanding that causes most tub-wall failures. The Building America guide says that, unless it is specifically manufactured as a waterproof product, cement board is water resistant but not waterproof. It says you must coat it with a fluid-applied waterproofing or apply a water-resistant barrier behind it that drains. It also says to apply the fluid-applied waterproofing to the cut edges of the board when you trim it.

The James Hardie guide leaves the choice partly to your local building code. For interior installations it says that a waterproof membrane, vapor barrier or vapor retarder is optional unless the local building code requires it, and that if the code requires one, you install a code compliant product and follow that product's manufacturer instructions. So ask your city whether it requires a membrane behind tub tile, and plan for waterproofing either way.

The James Hardie guide gives the framing and fastening rules for its cement board in tub and shower enclosures:

1. 1Framing. Use at least 2 by 4 wood studs or 20-gauge metal studs, straight and aligned, at a maximum of 16 inches on center. In tub and shower enclosures, the framing should be adequately reinforced at the corners.
2. 2Board height above the tub. Install boards 1/4 inch above the floor, tub or shower pan and caulk the gap. This keeps the board from touching the tub and wicking water.
3. 3Fasteners. Use the specified nails or screws at a maximum of 8 inches on center at all supports, keep them 3/8 inch from board edges and 2 inches in from sheet corners, and set the heads flush without overdriving.
4. 4Joints. Fill all joints with mortar and embed 2 inch wide alkali-resistant glass fiber tape in the mortar and level it.

The Building America guide adds two practical points. Check that the wall framing is plumb before you start, so the board and the tile line up. And cement board needs different cutting tools from drywall: an abrasive saw blade or carbide-tipped cutters.

These two spots leak first, so treat them as part of the plumbing job.

The joint at the tub. Schluter's tub surround instructions tell the installer to fill the 1/4 inch gap between the solid backing and the tub with a sealant. Where the backing is installed above the tub flange, the handbook says to apply sealant to the tub flange. The waterproofing strip or membrane is then embedded over that joint so it bridges the tub and the wall.

Holes for plumbing. The handbook says that any penetration through the membrane, such as the mixing valve or the shower head, must be sealed with a manufactured seal, sealant or other suitable product. The idea applies to any waterproofing system: every hole you cut for a valve, spout or shower arm is a place for water to enter unless it is sealed. The membrane also has rules for seams. Schluter says to make seams by overlapping the edges by 2 inches, or by butting adjacent sheets and centering a waterproofing band over the joint.

Because the valve and spout stubs come through the wall, the plumber's rough-in should be finished and tested before the backer goes up. If you are changing the valve, see our page on what is involved in converting a tub to a shower, and if you suspect the wall is already wet, a leaking shower valve behind the wall shows what to look for.

### What about exterior walls?

If the tub sits on an exterior wall, the Building America guide says to air seal and insulate the wall cavity behind the tub enclosure. Otherwise a thermal bypass can let moisture into the walls and leave you with a cold tub. It says any piping or wiring penetrations through the walls behind the tub should be air sealed and the cavity insulated before the cement board goes on. Check this during the tear-out, because it is far easier before the wall is closed.

If the wall is being rebuilt, the valve depth has to match the new wall finish so the trim fits. Our page on how to replace shower valve trim covers what happens when the valve sits too far back.

Call before the wall is closed. A plumber can check the supply and drain rough-in, pressure test the lines, and set the valve at the right depth for the backer and tile you chose. The tile setter then handles the backer and waterproofing. If you are replacing a tub as well, our page on whether you need a permit to remodel a bathroom in Phoenix explains when the work needs one. For the shower version of this question, see shower pan liner code.

HQ Plumbing and Air is licensed and insured, takes emergency calls 24/7, and gives free estimates with an upfront price before work starts. See our plumbing services.