Grout for Granite Tile: Polished, Flamed and Resin-Filled Stone

Polished granite tile usually takes a 1/16 to 1/8 inch joint with unsanded, fine high-performance or epoxy grout. Flamed granite takes sanded grout in wider joints, with a grout release on its rough face. Granite tolerates most cleaners, but light, porous varieties can absorb grout pigment, so check absorption with a water test and seal before grouting.

For a comparison of granite with calcite stones like marble and travertine, see the natural stone overview.

What makes granite different

Geologically, granite is an igneous rock of quartz (about 7 on the Mohs scale), feldspar (about 6) and smaller amounts of mica and dark minerals. It is a siliceous stone: acid-resistant, hard and, in most varieties, dense. ASTM C615, the specification for granite dimension stone, limits absorption to 0.40 percent by weight, which is comparable to porcelain's 0.5 percent threshold under ANSI A137.1.

The stone trade uses "granite" more loosely. Black "granites" are often gabbro or diabase, many patterned "granites" are gneiss, and some exotic varieties are partly marble or contain calcite veins. A few light-colored stones sold as granite absorb far more water than the ASTM limit. For grouting, this means granite is usually forgiving, but the specific stone has to be checked, not assumed.

Polished granite: tight joints and resin

Polished granite tile is usually calibrated to an even thickness and cut square, often with a small microbevel. Size consistency is good, so narrow joints are realistic. Many installers use 1/16 inch; 1/8 inch hides slight lippage and is easier to grout well. The rules for setting tight joints are in narrow grout joints.

Resin-filled granite

Most granite tiles and slabs are treated with resin (typically epoxy-based) before polishing. Resin fills fissures, pits and the spaces between crystals, giving a uniform gloss and reducing absorption. Two practical consequences for grout:

• Resin is softer than quartz and feldspar. The stone itself resists scratching by sanded grout, but resin-filled pits and fissures can be dulled. Seen under a raking light, this looks like small hazy patches within the shine. Unsanded or fine-aggregate grout avoids the risk on tight joints anyway.
• Resin can react to strong chemicals. Some solvent-based cleaners and strippers can soften or cloud factory resin. Some dark stones are also treated with color enhancers that a strong solvent can lift. Follow the stone supplier's cleaning guidance, and do not use aggressive solvents to remove grout or epoxy residue without testing.

Mica-rich granites (with visible glittery flakes) also have soft spots, since mica is around 2.5 to 3 on the Mohs scale. Polished faces with large mica areas deserve the same care as resin.

Flamed and textured granite

Flaming (thermal finishing) passes a high-temperature torch over the stone, making surface crystals pop off and leaving a rough, slip-resistant face. It is common for outdoor paving, pool surrounds and steps. The process also opens the surface, so flamed granite is more absorbent than the same stone polished.

Grouting flamed granite is closer to grouting slate: grout catches in the texture and can leave a gray film that is very difficult to scrub out. Apply a penetrating sealer as a grout release to the face first, work small areas, and clean with a barely damp sponge followed by a soft brush. Wider joints (3/16 to 3/8 inch) are typical because flamed tile is often thicker and less precise. Outdoors, the grout must also handle freeze-thaw and drainage; see outdoor grout.

Pigment staining on porous granites

Dense dark granites rarely absorb grout pigment. Light granites are another story. Some white, cream and pale gray varieties with large feldspar crystals and open grain absorb water readily. On these, fresh cement grout can push pigment and moisture into the edges of the tile, leaving a dark border along every joint (picture framing). Water from the setting bed can also darken the stone temporarily, as it can with light marble.

Testing absorption before you grout

Place a few drops of water on the face of a spare tile and watch for 5 to 10 minutes. If the drops stay beaded and the stone underneath does not darken, the stone is dense enough that pigment staining is unlikely. If the stone darkens within a few minutes, it is absorbent: pre-seal it and test grout on a sample before using a contrasting color. The method is explained in more detail in the water drop test.

Choosing grout color

Granites are speckled, so grout is usually chosen to match the dominant background tone rather than a single mineral color. Black and dark granites take charcoal or black grout; salt-and-pepper grays take a mid gray; warm granites with pink or gold feldspar take a sand or warm gray. Dark grout on light granite is the highest staining risk. More guidance is in the grout color guide.

Grout type for granite

• Unsanded grout suits polished granite with 1/16 inch joints. Basic unsanded grout is fairly absorbent, so it benefits from sealing in kitchens and baths.
• High-performance cement grout (ANSI A118.7) covers narrow and wide joints with fine aggregate, lower absorption and better color consistency. It is a good default for most granite floors.
• Sanded grout suits flamed, bush-hammered and wide-joint granite.
• Epoxy grout (ANSI A118.3) pairs well with dense polished granite: it cleans off the low-porosity surface much as it does from porcelain, and it resists the oils, acids and stains of kitchens. It is a common choice for granite tile countertops; see countertop grout. On porous light granite or flamed surfaces, test first for darkening and cleanup.

Sealing granite and its grout

Sealer is only useful where it can penetrate. Many dense, resin-treated polished granites absorb almost no sealer, and sealer left on the face dries into a sticky or streaky film. The rule is to apply, allow the dwell time on the label, then wipe off every bit of surface excess. If the water test showed no absorption, the stone may not need sealing at all, but the cement grout still might; see do you need to seal grout.

• Before grouting: seal absorbent and textured granites as a grout release. Keep sealer off the joint edges.
• After grouting: seal cement grout once cured (timing in how long before sealing grout). Epoxy grout generally needs no sealer.
• Residue: if sealer haze appears on polished granite, follow sealer haze and sticky sealer rather than scrubbing hard.

Cleanup and haze removal

Granite tolerates the acidic haze removers used on porcelain far better than marble does, but caution still applies. Calcite-bearing "granites" etch, factory resin and color treatments can be affected, and strong acids can dull the polish of some minerals over time. A stone-safe haze remover is the safer first choice. If you use an acidic remover, test it on a spare tile, wait the full grout cure time on the label, and rinse thoroughly. Never use products containing hydrofluoric acid or ammonium bifluoride on any stone: they attack silicate minerals. Daily care is best done with pH-neutral cleaners; see grout cleaners by pH. The full haze routine is in how to remove grout haze.