How To Lay Floor Tiles

Subfloor Preparation Is Where Most People Fail

The way you lay floor tiles is almost entirely determined by the condition of the subfloor underneath them. Get this wrong and you will spend the rest of your life dealing with cracked tiles and loose grout lines. Start by checking that your subfloor is flat within a quarter inch over ten feet. Use a long aluminum straightedge or a laser level to find the high and low spots. If your existing concrete slab has depressions deeper than a half inch, you need a self-leveling compound. Polyurethane-based compounds work better on older slabs because they do not shrink as they cure. On wooden subfloofs, two layers of 3/4-inch exterior-grade plywood with staggered seams is standard. The top layer should be perpendicular to the bottom layer. Screw everything down. Construction adhesive between the layers and the joists reduces deflection. Before you even open a bag of thinset, figure out your layout. Measure the room in both directions and find the center point. Snap chalk lines at those center points. Dry lay a few rows of tiles along both lines without adhesive to see how the cuts will fall at the walls. If your edge cuts end up narrower than two inches, shift your starting line by half a tile width. Nobody wants a sliver of tile barely holding on at the doorway. I learned this the hard way on a bathroom renovation where my first layout left me with a three-quarter inch cut tile along an eight-foot wall. The installer who came in after me had to pull three rows out and start over. It cost me about four hours and a lot of wasted thinset. Now you are ready for the actual installation. Mix your thinset to a consistency that holds a peak when you lift the trowel. Not soupy, not dry. ANot a rubbery paste that slumps the moment it touches the floor. Use a 3/8-inch square-notched trowel for most 12x12 inch tiles and a 1/2-inch square-notched trowel for larger format tiles up to 24x24 inches. Larger tiles sometimes need a 1/2-inch by 3/4-inch U-notched trowel for adequate coverage. Back-butter every large format tile. That means spreading a thin layer of thinset on the back of the tile itself in addition to the troweled bed on the subfloor. This alone gets you well above 95 percent coverage, which is what most manufacturers require for their warranty.

Place the tile, give it a slight twisting motion to embed it, and check the lippage with your fingers running across the adjacent tile edges. If one corner is higher than its neighbor, use tile leveling clips and wedges. They are cheap and save you from grinding down high spots later. Maintain your chosen grout joint width with spacers. Two millimeter spacers for a tight look, 3/16 inch for standard ceramic, 1/8 inch for large format porcelain where you want an almost seamless appearance. Keep in mind that wider joints are more forgiving on uneven tiles. If your tiles have wavy edges, which most cheaper porcelain does, narrow joints will highlight every imperfection. I once installed 24x24 inch porcelain tiles in a kitchen with 1/16 inch spacers because I thought it looked cleaner. The wavy edges were so visible it looked like a disaster. Switching to 3/16 inch spacers masked most of the issue. Took thirty minutes to go back and fix it. Lesson learned. Working in sections is important. Do not cover more floor than you can tile within the thinset open time. This varies by product but usually falls between ten and twenty minutes in normal conditions. If the thinset skin forms over before you set your tile, you have lost bond. Scrape it off and start fresh. Pressure washing will not help at that point. Check coverage frequently during the first few rows by lifting a tile. You should see full, even transfer of thinset across the entire. If coverage is patchy, your notch size is wrong or you are not using enough thinset. Let the thinset cure fully before grouting. Twenty-four hours is the minimum for most polymer-modified thinsets under normal temperature and humidity conditions. Walk on it sooner and you risk shifting tiles slightly, which creates micro-movements in the bond. Once cured, mix your grout according to the package instructions. Sanded grout for joints 1/8 inch and wider. Unsanded for tighter joints and epoxy grout if you are working with porous natural stone or expect heavy staining. Apply grout with a rubber float held at a 45-degree angle, forcing it into the joints. Work diagonally across the tiles for better fill. Remove excess grout with a damp sponge before it skins over. Three or four passes with a clean damp sponge usually gets the tile surface clear without pulling grout out of the joints.

Caulk the perimeter and any movement joints instead of grouting them. Silicone caulk in a matching color works well for inside corners and where the tile meets a bathtub or shower curb. These areas experience different amounts of movement than the field of tile, and rigid grout will crack there eventually. Use a polyurethane or silicone movement joint compound at expansion joints in large floors, typically every 20 to 30 feet in each direction. Seal natural stone before grouting and again after the grout cures. Marble, travertine, and limestone are all porous enough that grout haze will soak in and stain if you do not. Penetrating sealers work for most installations and last between one and three years depending on traffic. Impregnating sealers are better than surface coatings because they do not change the appearance of the stone. Test the seal by dripping water on a small area. If it darkens within a few minutes, you need more sealer. If it beads up, you are good. Curing time before foot traffic depends on the product. Light foot traffic in 24 hours is typical. Wait 72 hours before moving furniture back onto the floor. Full cure for grout is around seven days, though this varies with humidity and temperature. Use underfloor heating only after the thinset and grout have fully cured, and raise the temperature gradually no more than five degrees per day to avoid thermal shock. I've seen installers skip this step and crack entire floors within the first winter.

A few things this process does not solve. Vaulted ceilings and exterior floors with standing water will never work with standard tile installation methods. Exterior installations require frost-rated porcelain, a sloped substrate for drainage, and a waterproof membrane rated for outside use. Interior showers need a proper waterproofing system, not just a membrane glued over cement board. Kerdi, sheet membranes, and liquid-applied waterproofers all work, but they must be installed on structurally sound substrates with sealed seams and proper flashings at drains and corners. The membrane is not optional. Tile and grout are not waterproof. Water will get through them if you give it enough time and pressure. That is the reality of it. Skip the waterproofing layer and you will have a mold problem underneath your floor in a couple of years. I have torn out more rotted subfloors than I care to remember, all from the same mistake.