Shower Floor Grout: Mosaics, Slope, Drains and the Best Grout Type

The best grout for a shower floor is usually an ANSI A118.3 epoxy, or a high-performance cement grout (ANSI A118.7) that is sealed and maintained. Shower floor grout lasts when the floor beneath it drains: a 1/4 inch per foot slope, a working pan or bonded membrane, and tile small enough to follow the slope.

This page covers the floor only. For the full shower, including walls, niches and benches, see the zone-by-zone shower grout guide.

Why the shower floor is the hardest grout job in the house

Shower walls shed water. The floor collects it. Every gallon that leaves the shower head crosses the floor, and the last film of water sits there until it evaporates or drains. That film carries soap, body oil, shampoo residue, skin cells and, if the water is hard, dissolved calcium and magnesium. When it dries, it leaves those behind in the grout joints, which sit slightly below the tile face and act as tiny channels.

Add wet, soapy foot traffic, cleaners ranging from acidic scale removers to alkaline degreasers, and constant wet-dry cycling, and cement grout that would last decades on a bedroom floor can erode, stain and grow mildew within a few years on a poorly drained shower floor.

How much of the floor is grout

Small tile means more joints, and the share climbs quickly as tile size drops. The figures below are straightforward geometry for square tile, rounded.

A 1 x 1 inch mosaic floor with 1/8 inch joints is roughly one-fifth grout. That is why a premium grout on a mosaic shower floor is money well spent, and why the mosaic tile grout guide treats joint filling and cleanup as the main event.

Slope, tile size and why mosaics are standard

Plumbing codes and industry guidance call for the finished shower floor to slope toward the drain at a minimum of 1/4 inch per foot (about 2 percent). Many setters aim for 1/4 to 1/2 inch per foot. Less than that and water lingers in low spots; much more and the floor feels uneven underfoot.

A center drain creates four planes that meet at the drain like a shallow funnel. Each plane is flat, but the lines where planes meet are creases, and large flat tile cannot bend over a crease. Small tile can. A 2 x 2 inch sheet follows the planes with joints opening and closing slightly, while a 12 x 12 inch tile bridges the crease and leaves a low corner, or lippage, or both. That is the real reason mosaics dominate center-drain floors: they let the floor drain properly, and they also add traction because of all the joints.

Larger tile is fine when the floor is a single plane. Linear (trench) drains placed along one wall or at the entry let the whole floor slope one direction, so 12 x 24 inch porcelain can work. Check the tile maker's slip rating for wet barefoot areas, and see large format tile grout for joint width with bigger tile.

Pan or membrane: what is under the grout

Grout is not waterproofing (see does grout waterproof tile). Cement grout absorbs water, and even epoxy joints let some water past at hairline gaps and tile edges. The shower floor stays sound because of what lies underneath, and the two main systems behave very differently for the grout.

Traditional mortar bed over a liner

A sloped mortar preslope sits on the subfloor, a PVC or CPE liner goes over it, and a second mortar bed (often 1 1/4 inch or more) goes on top, with tile set on that. The top bed is meant to get wet. Water that passes the grout soaks into the mortar, runs down to the liner, and drains through weep holes in the clamping-ring drain. If the preslope is missing (liner laid flat on the subfloor) or the weep holes are blocked with thinset or debris, the mortar bed stays saturated. The grout then stays dark, smells musty and grows mildew from below.

Bonded waterproof membrane

A sheet or liquid membrane meeting ANSI A118.10 is applied over a sloped mortar bed or foam tray, and tile is set directly on it with thinset. Only the thinset layer and grout hold water, so the floor dries far faster. The drain uses a bonding flange that the membrane laps onto. This design is kinder to grout because there is very little reservoir of water below it.

Either system should be flood tested before tile, typically by plugging the drain and holding water for 24 hours, or as long as the local code or membrane maker requires. A pan that leaks will ruin any grout.

Choosing grout for a shower floor

The shortlist for shower floors is short. Selection depends mainly on how much maintenance the owner will accept and how confident the installer is with epoxy.

Epoxy earns its place because it does not absorb soap film or body oils and shrugs off most cleaners. It is harder to install on mosaics because the cleanup must be thorough before the resin sets; read how to apply epoxy grout first. Cement grouts are easier to install and can be very durable, but they wear faster under soapy water.

Some premixed and urethane grouts are labeled for showers but need a long cure before the floor sees water, sometimes 7 days or more for wet-area use. Check the TDS, and do not use any premixed product on a shower floor unless the label explicitly allows submerged or continually wet floors.

Color

Mid-tone grays, tans and charcoals hide soap film and mineral deposits. Bright white cement grout on a shower floor shows every bit of residue and is best reserved for epoxy, where the surface does not absorb stains. Very dark cement grout on a hard-water floor shows white mineral haze instead. The grout color guide covers this tradeoff.

Grouting around drains

The drain is where the grout works hardest, because every drop passes it and it sits at the lowest point.

• Square or round point drains: Cut the mosaic so the tile finishes just short of the drain grate frame, leaving a joint about the same width as the field joints. Grout this joint, or use sealant if the drain body can move independently of the tile. Never leave a wide, thin sliver of grout around the grate, since it cracks.
• Adjustable drain tops: Set the grate height so it sits flush with or just below the tile face. A grate that sits proud of the tile creates a ring of standing water around it.
• Linear drains: The tile edge along the channel is a straight cut. Most systems want a sealant joint where tile meets the channel frame, because it is a junction of two different materials. Follow the drain maker's details.
• Tile-in grates: Tile set into the grate tray is grouted like the field but must stay removable. Keep grout off the grate edges so it can lift for cleaning.

Grout around the drain tends to stain first. Look closely at it during cleaning; dark rings there are often the first sign of trapped water below.

The curb, the perimeter and what gets caulked

A shower floor meets other surfaces in several places, and each is a change of plane. Per TCNA EJ171, changes of plane get a flexible sealant, not grout. Grout in these joints cracks as the walls and floor move slightly with temperature, moisture and use.

• Floor-to-wall joint: Sealant, all the way around the shower.
• Curb to wall: Sealant at the vertical corners where the curb ends meet the walls.
• Curb faces and top: Grout the joints within each face. Use sealant at the inside corner where the curb meets the shower floor.
• Bench and floor: Sealant where the bench face meets the floor.

Use a 100 percent silicone or a sealant the grout maker sells in matching color. Sanded siliconized acrylics wear quickly in a shower floor corner. See caulk vs grout in movement joints and grout vs caulk vs silicone for the reasoning.

The curb top deserves a slight inward slope (about 1/8 to 1/4 inch across its width) so water runs back into the shower instead of sitting on the grout joints.

Keeping shower floor grout clean

Most shower floor grout looks bad because of residue, not damage. A squeegee or a quick rinse with clean water after showering removes most soap film before it dries. Weekly cleaning with a pH-neutral tile cleaner keeps residue from building.

• Use acidic cleaners only for mineral scale, and only occasionally on cement grout, because acid dissolves cement paste. The grout cleaners by pH guide explains which to use when.
• Mildew in the corners and around the drain is a moisture problem first. See grout mold and mildew.
• Epoxy floors tolerate most cleaners but can build a soap film that looks like dirty grout.
• For step-by-step cleaning of the whole shower, see cleaning shower grout.

Sealed cement grout on a shower floor usually needs resealing more often than wall grout. A water drop test near the drain tells you when.

Signs the floor needs more than cleaning

Cracked grout along the floor-to-wall joint means grout was used where sealant belongs. Cracks through the field, loose mosaics or tiles that sound hollow suggest movement or a failed bed. Joints that stay dark for a day or more after use, a musty smell and white crust in the joints point to trapped water. Regrouting fixes surface wear; it does not fix drainage. Before planning work, read regrouting a shower and when regrouting won't work.