How to Grout a Tile Floor and Seal the Movement Joints
Why this matters
Grout is the part of a tile floor the customer touches with a mop every week for the next twenty years, so blotchy color, haze, pinholes and low joints are what they judge the whole job by. The movement joints are the part they never notice until one is missing: a perimeter or field joint filled with hard grout turns the tile into one rigid sheet, and the first winter heating cycle or slab shrinkage tents it or cracks it along a line. The grouting crew is the last person who can leave those joints open, and the most likely person to fill them by accident.
Picking the grout itself is 'Mortar + Grout Selection Reference'. Where the movement joints go and how wide they are is 'Movement Joints in Tile Floors'. This card is the doing, in line with ANSI A108.10, the installation standard for grout in tilework.
The job this card follows
Every step below runs on one floor so the reasoning stays attached to real numbers. A great room on a slab on grade, 24 ft by 30 ft, set with 12 by 24 in porcelain about 3/8 in thick at a 3/16 in joint. The joint plan from the movement joints card calls for:
- A field joint running the full 24 ft across the room, sitting directly over the slab's sawn control joint at 15 ft, at the grout-joint width of 3/16 in.
- Perimeter joints at every wall, which the baseboard will hide, and at every fixed object, which nothing will hide: a 16 ft base-cabinet run, an island base of 6 ft by 3 ft (18 ft around), and an 8 ft sliding-door threshold. Those exposed perimeter joints are 1/4 in wide.
So 66 ft of joint takes sealant: 24 ft of field joint plus 42 ft of exposed perimeter.
Step 1: Wait out the mortar, then judge it
Grout only after the setting mortar has reached the time its bag states before grouting, and treat that time as a minimum that cold stretches. Grouting early lets foot traffic and float pressure shift tile in a bed that has not set, and water from the grout can wash into soft mortar at the joint and leave it weak.
On the great room: the mortar bag gave 24 hours at 70 F and warned that cooler conditions extend it. The slab read 62 F with a surface thermometer, so the crew grouted on the second morning, and confirmed the bed by pressing a thumbnail into the mortar squeezed up at a cut edge: it did not dent.
Step 2: Clean out the joints and protect the movement joints
Pull every spacer and clip base. Rake out mortar that squeezed up into grout joints; most grout makers ask for the joint to be clear to about two-thirds of the tile thickness or more, and their sheet is the rule. Vacuum the joints, not just the face.
Then close the movement joints to grout. Press closed-cell backer rod or bond-breaker tape into each one now, or run painter's tape over it, so the float cannot fill it.
Why now. Raking grout out of a 3/16 in joint after it firms takes as long as grouting the floor, and what is left on the joint walls stops the sealant from bonding to clean tile edges later. A field joint that looks like every other grout line is the one most often grouted shut, because nothing about it stands out to a crew working fast.
On the great room: the field joint was a straight line through the field over a sawn joint you can no longer see, so before the clips came off the setter ran blue tape along it, end to end, and wrote "MJ" on the tape every 4 ft. 1/4 in backer rod went into the island and cabinet perimeter joints.
Step 3: Mix the grout to the maker, not to feel
Water first, in the amount printed on the bag for the batch size; powder second; mix at the paddle speed stated; slake for the stated time; remix. Mix only what the crew can place and wash before it firms. Never add water to a stiffening batch.
Why. Grout color and strength both depend on water. Two batches mixed to feel come out two shades, and the difference appears only when the floor dries, as a patchwork the customer sees from the doorway. Extra water also leaves a weak, dusty surface and more shrinkage pinholes.
Hazard. Wet cement grout is strongly alkaline and burns skin through a long day of contact, often without pain at the time. Wear nitrile or other waterproof gloves and eye protection at the bucket, and rinse a splash with clean water at once. Sanded grout powder carries quartz, so mix where the dust does not drift through the house, pour gently, and use respiratory protection under a written program per 29 CFR 1910.134 (applied to construction work by 29 CFR 1926.103) if the dust cannot be kept down.
On the great room: the grout maker's bag of a high-performance cementitious grout, rated for this joint width, was split into half-bag batches, sized against the maker's coverage chart for this tile and joint so each one could be placed and washed inside its working time, mixed from a measured jug, with one person mixing and two grouting.
Step 4: Pack the joints on the diagonal
Hold a hard rubber float at about 45 degrees to the floor and push grout diagonally across the joints, pressing it full depth, working one area at a time. Then hold the float nearly upright and drag it diagonally to strip the excess off the face.
Why diagonal. A float pulled along a joint drags grout back out of it and leaves a low line. Across the joint at an angle, the float rides on the tile edges on both sides and leaves the joint full.
On the great room: areas of about 36 sq ft, three rows of the tile's 12 in face (3 ft) across a 12 ft swing of the float, which was what two people could pack and the mixer could wash before it firmed. The taped field joint was floated over, not into.
Step 5: Shape, wash with as little water as you can
When the grout is firm enough to take a thumbprint without sticking to the thumb, shape the joints with a damp sponge edge or a jointer to a uniform finish, then wash: a hydrophilic sponge wrung out until it barely drips, wiped diagonally across the joints in one light pass, flipped to a clean face, then rinsed in a second bucket.
Why so dry. A wet sponge pulls cement and pigment out of the joint surface, leaving a sandy, light, porous top that stains first, and carries water into the joint that brings efflorescence to the surface as it dries. A floor washed wet can be the right color in one area and two shades lighter in the next.
Step 6: Clear the movement joints and buff the haze
Pull the tape off every movement joint and take out any grout that bridged it while the grout is still soft. When the haze has dried to a film, buff it off with a dry microfiber cloth. Haze that survives is removed later with the cleaner the grout maker names, after the cure time it states; an acid cleaner on young cement grout etches it.
Step 7: Seal the movement joints
After the grout has cured the time its maker states, the movement joints get sealant, not grout and not caulk.
- Clean and dry. Vacuum the joint. The tile edges must be free of grout, mortar and dust.
- Bond breaker. The sealant should bond to the two tile edges only. Backer rod or bond-breaker tape at the bottom stops it bonding to the substrate as well, and a sealant stuck on three sides tears down the middle the first time the joint opens.
- Mask both tile edges with tape for a clean line.
- Gun the sealant in one continuous bead, pushed ahead of the nozzle so it fills rather than bridges, then tool it slightly concave with a spatula or tooling stick and pull the tape before it skins.
The sealant is one that meets ASTM C920, which is what the tile standards and the movement joint card call for, in the color nearest the grout. A sanded acrylic caulk from the grout maker's color card is not a C920 sealant and will crack out of a working joint within a season. Silicone sealants give off acetic or other cure vapors, so ventilate the room while the beads cure.
On the great room, the quantity:
field joint 3/16 x 3/16 in section
0.1875 x 0.1875 x 12 in = 0.42 cu in per ft
x 24 ft = 10.1 cu in
exposed perimeter 1/4 x 1/4 in section
0.25 x 0.25 x 12 in = 0.75 cu in per ft
x 42 ft (16 + 18 + 8) = 31.5 cu in
total = 41.6 cu in
one 10.1 fl oz cartridge = about 18.2 cu in
41.6 / 18.2 = 2.3 cartridges
Three cartridges cover it with about seven-tenths of one to spare. The crew brought four, so a full, unopened cartridge in the right color could stay with the customer for a future touch-up.
Step 8: Seal cement grout only if its maker says to
Some cementitious grouts call for a penetrating sealer after a cure period; many high-performance grouts do not, and epoxy and most urethane grouts do not. Follow the grout bag. A sealer applied over a grout that does not want one can leave a film that hazes or yellows, and one applied before the stated cure time traps moisture in the joint.
On the great room: the grout maker did not call for a sealer, so none was applied, and the care sheet says so in writing so a cleaning service does not add one.
How to check you got it right
With the grout cured, stand at each doorway in normal room light, then walk the floor:
- Joints full and even: no low lines, no pinholes, no grout proud of the tile edges; a fingernail dragged across a joint does not drop into it.
- Color: one shade across the whole floor once dry, with no lighter patches marking an over-washed area or a different batch.
- Haze: the porcelain face reflects light evenly with no dull film when viewed standing.
- Every movement joint: take the joint plan and tick each joint off as it is walked. Each one carries flexible sealant that gives under a fingernail and recovers, with no grout bridging it anywhere along its length. On the great room that was all 66 ft: the 24 ft field joint and the 42 ft of exposed perimeter, plus a look behind the baseboard stock before it went on to confirm the wall joints were still open.
The last check is the one that matters in a year. A floor with one grouted field joint looks identical to a correct one on handover day.
References
- ANSI A108.10 (installation of grout in tilework) and the tile installation standards' movement joint provisions, as referenced by your contract and the tile and grout makers' warranties; check the current edition
- ASTM C920, standard specification for elastomeric joint sealants
- 29 CFR 1910.134, respiratory protection (29 CFR 1926.103 applies it to construction)
- See related: Movement Joints in Tile Floors; Mortar + Grout Selection Reference; How to Lay Out and Set a Tile Floor