How to Prepare a Subfloor for Tile Installation
Time Required: Over 1 day
- A subfloor is the bottom-most structural level of your floor. It supports your decorative floor finish, whether it’s carpet, hardwood, vinyl, laminated wood or tile. For more information, download this PDF.
- Preparation for installing floor tile depends on the type of subfloor in the room you are tiling. Concrete subfloors are prepared differently than wood subfloors.
- The subfloor should be able to hold the weight of the tile and preparation materials required to complete the installation. And if it doesn't have underlayment, your subfloor isn't suitable for tile until it does.
- Check for dips in the floor by sliding a 4 to 6-foot straight edge in different directions all around the room. If you find hollows and valleys, you must fill them with a self-leveling underlayment.
- Fill seams in the subfloor with caulk. Tape off any affected vents with painter’s tape.
- Roll on a layer of latex primer/sealer. This prepares and seals the surface for the self-leveling underlayment and improves its bond.
- Pour the liquid underlayment into the dip in the floor. It will seek its own level, and once dry will create a flat and even surface.
Tip: Make sure the subfloor will not deteriorate when it gets wet. Particleboard is not an acceptable subfloor material for tiling projects. Oriented strand board, or OSB, and exterior-grade plywood will work if first covered with a latex-modified mortar and a tiling backerboard.
Tips: When you’re working on any tile subfloor project, you may find the terms thin-set and mortar used interchangeably. Don’t get confused; both refer to the same material.
Take care not to mix too large a batch of thin-set. It has a limited working life, and if you’ve mixed too much, it will start to harden before you’ve had a chance to use all of it. If this happens, you’ll need to discard it and mix more.
- Install a cement-based backerboard following the manufacturer’s instructions. This provides a solid surface under the tile that prevents flexing that could lead to cracking.
- Use a polymer-modified thin-set to attach the backerboard to the subfloor and to fill in for any potential gaps that might exist between the backerboard and the subfloor. The polymer-modified thin-set provides additional strength and eliminates the possibility of any movement between the backerboard and the subfloor.
- Follow the directions on the packaging to properly mix the thin-set mortar. Wear safety glasses. Use a large bucket and a drill with a mixing paddle, and clean the mixing paddle promptly so the mortar does not harden on it. Some mortars may require a latex polymer admixture, which will improve the performance of the mortar and increase bond strength. Some installation material manufacturers require the latex admix to be used at full strength as a replacement to water. Do not dilute if this is the case.
- With a trowel, scoop out and spread enough mortar on the wood subfloor to cover an area slightly larger than your first piece of backerboard.
- Comb the mortar with the notched edge of the trowel at a 45-degree angle to the floor, taking care to make sure the trowel lines go in the same direction. Work quickly while the mortar is wet.
- Place the first sheet of backerboard on the mortar.
- Walk gently on the panels as you begin to screw the backerboard into the subfloor using special backerboard screws. Use 1-5/8-inch screws for 1/2-inch backerboard, and use 1-1/4-inch screws for 1/4-inch backerboard.
- Drive screws every 8 inches over the entire surface, 1/2 inch from the edges, and 2 inches from the corners. Some backerboard manufactures have identified the locations on the backerboard where the screws need to be placed.
- Expect to use 54 to 60 screws per sheet of backerboard. Every screw head should be flush with the panel. Use a drill with a clutch to speed up driving the backerboard screws.
- Continue this process to lay the rest of your backerboard panels, leaving a 1/4-inch gap at the walls. Use 16d common nails to create a gap of 1/8 inch between the sheets.
- Arrange the sheets so that the ends are staggered and don’t align with each other. In other words, be sure that no four corners of the backerboard touch. Also, if your subfloor runs east to west, lay your backerboards in the opposite direction, north to south.
- Leave a 1/4-inch gap between the edges of the backerboard and the walls to allow for subfloor expansion and contraction.
- Backerboard panels are traditionally 3 by 5 feet but can be up to 4 by 8 feet.
- If you find that you need to cut the backerboard, measure and mark where you need the cut and score along your line with a carbide backerboard cutter and a straight edge. Depending upon the manufacturer, you may need to score both sides. Press down with your hand and knee on the edge of one side of the line and lift the opposite edge to snap the panel.
- After all backerboard sheets are in position, remove the 16d common nails. Then reinforce the joints with self-adhesive alkaline-resistant glass fiber tape specially made for backerboard.
- Cover the tape with thin-set mortar and level it with a straight edge. If you use non-adhesive tape, you’ll want to first fill the gaps with mortar before setting the tape in the mortar. Then cover the tape with another layer of thin-set and level.
- Allow the thin-set to dry before proceeding, usually a day. Check the package for specific drying times.
- To prepare your concrete subfloor, make sure that it can accept water penetration. Test by sprinkling water on various areas of the subfloor. If water penetrates and leaves a wet spot, then a good bond can be achieved. If water beads, surface contaminants are present, and loss of bonding may occur. Contaminants should be removed before installation.
- The concrete must be free of efflorescence or salts on the surface and not subject to hydrostatic pressure or moisture penetrating the slab from below. Concrete subfloors should have a coarse finish to form the bond with the thin-set. Smooth concrete slabs must be etched or roughed up using chemical or mechanical methods to achieve a good bond.
- Check for dips by sliding a 4 to 6-foot straight edge in different directions all around the room. If there are hollows and valleys, fill with a self-leveling underlayment made for this purpose.
- Your concrete floor should be clean, dry, flat and free of adhesives. Residue from strippers or cleaners should also be cleaned and rinsed with water.
- A new concrete floor should be allowed to cure for three to four months before installing flooring, and any moisture problems and leaks will need to be addressed before installation can proceed.
- A self-leveling compound can be poured over the entire surface if the surface has several high and low spots. If you have only a few uneven spots, you can treat just those areas.
- Mix the leveler in a bucket according to the manufacturer’s instructions. Wear rubber gloves and safety glasses. If you’ve made any adjustments to your floor, you must allow them to completely dry before installing the new floor. Follow the manufacturer’s instructions, but this usually takes several days.
- If there are cracks larger than 1/8 inch in your concrete subfloor, those should be patched and filled with a cement patching compound.
- Concrete subfloors move, shift and crack over time. To prevent that movement from creating cracks in your grout and tile, once your concrete subfloor is clean, flat, and free of cracks larger than 1/8 inch, you will need to install a crack prevention membrane. You can either roll on a liquid latex membrane, or put down a peel-and-stick crack prevention mat over the entire concrete subfloor.