How to Prepare Your Floor for Tile Installation

New tile can change how a room looks and feels, in a kitchen, a bathroom or a commercial space. What it cannot do is make up for a subfloor nobody prepared. The best tile you can buy, set by someone who knows the trade, still sits on whatever is underneath it, which is why a tile installation really starts down there. First Choice Tile LLC works in Atlanta and the surrounding areas, and what a tile job comes down to is the preparation under it.

Skip that step or rush it and you can end up with cracked tile and grout, uneven surfaces and repairs that cost money. What follows is what the preparation actually involves, on a wood subfloor and on a concrete slab.

Why floor preparation is not negotiable

If the subfloor is uneven, dirty or unstable, the floor you lay on top of it suffers for that. These are the problems that come out of poor prep:

• Tile is rigid, so movement in the subfloor, even microscopic, can transfer stress into it and crack tiles and grout.
• If the subfloor is not perfectly flat, the tiles sit at different heights. That is lippage, and it is both a trip hazard and an eyesore.
• Voids under a tile, from poor adhesion or from unevenness, can produce a hollow sound underfoot.
• Moisture that nobody addressed can lead to mold, mildew and adhesive failure.
• Dirt, dust or other contaminants stop the thin-set mortar bonding properly, and the tiles work loose.
• Most tile and mortar manufacturers require proper subfloor preparation for their warranties to stay valid.

The prep, step by step

How much of this applies varies slightly with what the subfloor is made of, wood or concrete. Both are covered here.

1. Clear the room and assess the existing floor

Clear the room completely first: furniture, appliances, baseboards and the flooring that is on it now.

Carpet comes up in strips. Cut it, roll it, then scrape up whatever padding and staples are left. Vinyl and linoleum might peel up over a small area; over a larger or more stubborn one, a floor scraper or a rented power stripper may be necessary, and all of the adhesive residue has to come off. Laminate and hardwood are typically pried up plank by plank. Existing tile is often the most challenging: you will likely need a jackhammer with a chisel bit or a heavy-duty floor scraper, and there will be dust and debris.

With the old floor gone you are down to the subfloor, and that is where the real assessment starts.

2. Inspect and repair the subfloor

Go over it thoroughly. You are checking for stability, soundness, cleanliness and flatness.

Wood subfloors: plywood or OSB

Most residential homes in Atlanta have wood subfloors.

1. Walk across the whole floor and pay attention to squeaks and bouncy spots. A squeak is often a loose subfloor panel or a loose joist. Screw the loose sections down into the joists below with construction screws, not drywall screws, long enough to go at least 1 inch into the joist. If the bounce is significant, add blocking between the joists.
2. Check the thickness and the joist spacing. For ceramic or porcelain tile the subfloor should be at least 1-1/8 inches thick in total, which usually means a minimum 5/8-inch main subfloor with 1/2-inch exterior-grade plywood or cementitious backer board over it. Standard joist spacing is 16 inches on center, and wider spacing, 24 inches for example, may require additional reinforcement or a thicker subfloor system. Then there is the L/360 deflection rating, a technical way of describing how stiff the floor system is: the floor should not deflect, or bend, more than 1/360th of the span length when weighted. Structural engineers calculate it, but sufficient subfloor thickness and proper joist spacing usually meet the requirement in a house. When in doubt, ask a professional or err on the side of over-reinforcement.
3. Vacuum it thoroughly. All of the dust, debris and loose particles have to go.
4. Check it for flat, which is the part that matters most. Run a long straightedge over the floor, a 6-foot level works well, and find the high and low spots. The industry standard for tile installation is a maximum variation of 1/4 inch in 10 feet or 1/16 inch in 1 foot, and any deviation greater than that must be addressed. Sand or plane the high spots down. Fill the low spots with a Portland cement-based patching compound or a self-leveling underlayment (SLU), make sure the product is designed for wood subfloors, and follow the manufacturer instructions carefully.

Concrete slabs

Many commercial properties in Atlanta have concrete slabs, and so do some basements and first floors in houses.

1. Look for cracks. Hairline cracks are generally not a concern unless they are active, which is to say moving, so keep an eye on them. Larger or active cracks have to be repaired with an epoxy injection kit or a suitable crack-filling compound, and if you suspect a structural problem, get a professional to look at it. Control joints are a different matter. They are designed to allow movement, so do not tile directly over one. Honor them and treat them as expansion joints inside the tile layout.
2. Test for moisture. Concrete can hold a lot of it, and that can cause significant problems for tile. A concrete moisture meter will tell you whether the level is too high. For a more accurate reading, especially over a large area or on a commercial project, consider a calcium chloride test or a relative humidity test, both professional tests. If the level is too high you will need a moisture barrier or vapor reducer on the concrete before any tile goes down, and First Choice Tile LLC often does that in a damp basement or a commercial building.
3. Clean it. Dirt, grease, oil, paint and any other contaminant has to come off. Sweep and vacuum thoroughly. A stubborn stain might need a mild detergent or mechanical abrasion, meaning grinding.
4. Check it for flat, the same as wood, with a long straightedge. The tolerance is the same too: 1/4 inch in 10 feet or 1/16 inch in 1 foot. Grind the high spots down with a concrete grinder, which throws a lot of dust, so ventilation and PPE are essential. For the low spots, use a self-leveling underlayment, a cement-based product that flows and sets perfectly flat. Follow the manufacturer recommendations on application, thickness and drying time. Professional application is often recommended for the best result.

3. Install the underlayment

Over a wood subfloor an underlayment is almost always required. It puts a stable, smooth, uncoupling layer between the wood and the tile.

Cementitious backer board (CBB)

CBB is made from cement and reinforced with fiberglass mesh. It is impervious to water and gives the tile a stable, rigid surface.

1. Cut it to size: score it with a utility knife and snap it, or use a specialized backer board saw.
2. Spread a thin layer of modified thin-set mortar onto the subfloor with a 1/4" x 1/4" notched trowel. That is the bond coat, and it fills any minor irregularities.
3. Press the boards firmly into the wet mortar with the seams staggered. Leave a 1/8-inch gap between boards and a 1/4-inch gap from the walls.
4. Screw the boards down with specialized backer board screws, corrosion-resistant and wafer-head, every 6-8 inches along the seams and in the field of the board. The screw heads finish flush or slightly recessed.
5. Tape all the seams with alkali-resistant fiberglass mesh tape, embedded in a thin layer of modified thin-set mortar.

Uncoupling membranes

An uncoupling membrane, Schluter-DITRA and MAPEI Mapelastic CI for example, is designed to give the tile an uncoupling layer that protects it from stress and movement in the subfloor and prevents cracks. These membranes also offer waterproofing and vapor management properties.

1. Apply a modified thin-set suitable for the membrane, checking the manufacturer instructions, to the subfloor.
2. Press the membrane into the wet mortar so it makes full contact, then roll it out to remove air bubbles.
3. If waterproofing is required, in a shower or another wet area, overlap or seal the seams with the specific tapes and sealants the manufacturer guidelines call for.

They are excellent at preventing cracks, especially over a problematic subfloor, and they are often thinner than CBB, which can be an advantage where floor height is a concern.

On most wood subfloors First Choice Tile LLC recommends and uses uncoupling membranes, for the superior crack prevention and the waterproofing.

4. Final clean-up and a second check

With the repairs and the underlayment done, vacuum the newly prepared surface: all of the dust, debris and loose particles, since thin-set needs a clean surface to adhere properly. Walk it one more time with the straightedge, looking for high or low spots the underlayment left behind, and check that everything is secure. Then bring the tile and the thin-set mortar into the room at least 24-48 hours before work starts, so they acclimate to the ambient temperature and humidity and do not cause problems during or after the installation.

Beyond DIY: when to call a professional

A motivated homeowner can take on some aspects of floor preparation. In other situations a professional is worth having, and in some of them you need one.

• Extensive rot, severe unevenness or a suspected structural problem requires professional assessment and repair.
• Diagnosing moisture in a concrete slab and mitigating it is complex, and a lasting installation depends on it.
• Large or commercial projects often involve specialized equipment, precise measurements and specific building codes to adhere to.
• Tiling over existing vinyl, painted concrete or another non-standard surface may require particular primers or preparation methods.
• Many tile and mortar manufacturers specify that the installation, prep included, has to be done by a qualified professional for the warranty to be valid.

First Choice Tile LLC has experienced tile installers in Atlanta, and whatever the subfloor turns out to be, in a kitchen, a bathroom or a commercial space, it gets prepared before the first tile is set.

Before the first tile goes down

Preparing the floor is the fundamental step, the one that dictates whether the new floor succeeds and how long it lasts. Get the subfloor stable, clean, dry and perfectly flat and the money you put into the tile is protected.

First Choice Tile LLC works with homeowners and business owners in Atlanta on tile installation, LVP flooring, kitchen renovations, bathroom renovations and commercial flooring, from the preparation through to the final grout line. Contact us for a consultation.