Grout for Pebble Tile: How Much, How Deep and How to Clean It

Pebble tile takes sanded grout or a high-performance cement grout rated for wide joints, and far more of it than flat tile: often 1 to 2 lb per sq ft. Fill the gaps so grout reaches about half to two-thirds of the pebble height, then shape it with a damp sponge. Seal the pebbles first, start cleanup early, and consider epoxy only after a test panel.

Types of pebble tile and why they matter

Pebble tile is natural or manufactured stones glued to a mesh backing, usually in interlocking sheets about 12 inches square. The pebble shape determines the gap size, the grout depth and the cleanup effort.

• Round natural pebbles are whole stones, typically 1/2 to 3/4 inch tall. Gaps are irregular, from almost touching to 1/2 inch or more.
• Flat or sliced pebbles have the tops cut off to make a flat walking surface. They are thinner, have less gap volume and are much easier to grout and clean.
• Standing (on-edge) pebbles are thin stones set vertically, creating deep, narrow gaps that take a lot of grout.
• Coated, dyed or polished pebbles have a surface treatment that may react to sealers, acids or epoxy. Check the maker's instructions.

The stone itself varies too. Many pebbles are hard siliceous stones such as quartzite or basalt, but marble and limestone pebbles are common, and they etch in acid like any calcite stone. The natural stone overview explains how to identify acid-sensitive stone. For small flat mosaics, which share some of these issues, see grout for mosaic tile.

Why pebble tile uses so much grout

Grout volume is joint area times joint depth. Flat tile has thin, shallow joints covering a few percent of the surface. Pebble tile has wide, irregular gaps covering a large fraction of the surface, filled to a depth of 1/4 inch or more. The difference is not a few percent; it is an order of magnitude.

These figures are geometric estimates assuming roughly 120 lb of grout per cubic foot of filled joint, with no allowance for waste. Real use depends on pebble size, how tightly sheets are laid and how deep you fill. Many pebble makers publish their own coverage figure; use theirs first. For regular tile, the grout calculator and grout coverage table give precise numbers.

How deep to fill pebble grout

The finished grout surface should usually sit about halfway to two-thirds up the pebbles. That leaves the rounded tops exposed while locking each stone in place.

• Too shallow (grout well below half height): pebbles are held mostly by thinset at their base, deep pockets collect soap and dirt, and sharp ledges of grout chip. In a shower, water sits in the pockets.
• Too full (grout near the tops): the floor looks like pebbles cast in concrete, the stones lose their texture underfoot, and a thin grout skin over pebble shoulders flakes off over time.
• Consistent matters more than the exact level. Uneven depth across sheets is what makes sheet seams visible.

Depth starts at setting. Thinset that squeezes up between pebbles steals grout depth and shows through as a gray layer. Use a trowel notch that fully beds the stones without overfilling the gaps, and clean thinset that rises more than about halfway up the stones before it sets. For general joint depth principles, see grout joint depth.

Sanded, high-performance or epoxy

Sanded grout is the traditional choice. Its aggregate controls shrinkage in wide gaps and makes the grout firm enough to shape. Make sure the product is rated for the widest gaps in your pebbles, often 1/2 inch; some gaps exceed that and need a grout rated for wide joints. Polished or soft pebbles may be marked by sand; test if the pebbles are polished marble.

High-performance cement grouts (ANSI A118.7) offer lower absorption and better color consistency, valuable on a surface where grout is a third of what you see. Check that the joint range on the TDS covers your widest gaps.

Unsanded grout is the wrong choice for pebbles. It shrinks, cracks and wears in gaps this wide and deep.

Epoxy grout (ANSI A118.3) resists stains, needs no sealing and is popular for pebble shower floors. The tradeoffs are real: epoxy clings to rounded stone, the volume needed is large and pot life limits how much you can work. Grout areas of a few square feet at a time, and clean as you go with the maker's recommended method. Some pebble makers recommend against epoxy on certain coated stones; check before buying. See how to apply epoxy grout.

Urethane and premixed grouts often have narrower maximum joint widths and fill depths. Some are not suited to pebble gaps at all; read the TDS.

Pebble shower floors

Pebbles are popular on shower floors for grip and the feel underfoot. They also put more grout in direct contact with water than any other tile. That has consequences:

• Waterproofing is beneath the tile. All that grout is porous (unless epoxy), and none of it waterproofs the shower. The membrane does; see does grout waterproof tile.
• Slope has to survive the pebbles. The floor must slope to the drain under the pebbles, and grout depth must be consistent so water does not pond in low spots between stones.
• Drying matters. More grout means more water held after each shower. Ventilation and a squeegee reduce mildew. Cleaning approaches are in cleaning shower grout.
• Change of plane joints where the pebble floor meets the walls get flexible sealant, not grout.

The wider decision for shower floors, including slope, drains and grout types, is covered in shower floor grout.

Sealing pebbles before and after grouting

Natural pebbles range from dense basalt to porous limestone. Porous stones absorb grout pigment and haze, and grout residue sticks to an unsealed, textured pebble surface. A penetrating sealer applied to the pebbles before grouting acts as a grout release and makes cleanup much easier.

• Seal after the thinset has cured enough to walk on and before grouting. Pebble gaps are wide, so sealer will reach the sides of the stones; a light coat applied with a pad or sprayer and wiped of excess is fine, but do not flood it into the gaps.
• Let the sealer dry fully per the label before grouting.
• If you plan to use a color-enhancing sealer, test it on a spare sheet first. Enhancers darken both pebbles and any grout they touch.
• After the cement grout has cured, seal pebbles and grout together. Showers usually need resealing more often; see sealing shower grout.

Cleanup timing and sponge technique

On flat tile, the sponge removes residue. On pebble tile, the sponge also sets the grout depth and exposes the stones. That makes timing more critical. The basics are in grout sponge technique; for pebbles, work like this:

1. Grout small areas. Fill 5 to 10 sq ft at a time with cement grout, and less with epoxy. Push grout into the gaps from several directions with a rubber float, then scrape the excess off with the float held nearly upright.
2. Start the first pass early. As soon as the grout has firmed enough that a light sponge does not dig it out of the gaps (often 10 to 20 minutes depending on temperature), wipe lightly with a well-wrung sponge in small circles to clear the tops of the pebbles.
3. Shape with the second pass. After a few more minutes, use a damp sponge with slightly more pressure to lower the grout to its final depth, working the same amount everywhere so the depth stays consistent. Rinse the sponge constantly in clean water; a two-bucket setup helps.
4. Brush and buff. When the grout is firm, lift remaining film from the stones with a soft nylon brush, then buff with a dry microfiber cloth once the haze turns powdery.

Haze left on pebbles after cure can be removed with a haze remover suited to the stone. If any pebbles are marble or limestone, use only non-acidic, stone-safe products; procedures are in how to remove grout haze.