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How to Prepare Subfloor for Tile?

Beneath the visible flooring of hardwood, laminate, tile, or carpet lies the often-overlooked subfloor. It’s a critical structural element typically hidden from view. It only comes to attention during major renovations or when serious problems arise.

Unfortunately, early warning signs of subfloor issues are frequently missed. When the problem is noticeable, it might already be too late for the subfloor.

The subfloor, spanning across floor joists, is the foundational layer for any flooring material. Depending on the age of a home, it could be made of softwood boards like pine or fir, plywood, or OSB (oriented strand board). OSB consists of glue and wood scrap strips, available in 4-feet-wide sheets and lengths of 8 or 12 feet. Due to its quick installation and cost-effectiveness, OSB is a popular choice for modern subfloors.

Prepare Subfloor for Tile

Under normal conditions, a well-maintained subfloor can last as long as the house. However, if the subfloor is damaged or flawed, its lifespan may be reduced to 20 to 30 years. In such cases, homeowners may need to remove the top flooring layers for repair or replacement. 

Preparing a subfloor is crucial in installing floor tile, as it ensures a level surface for the tiles to adhere to and stay in place. This guide will walk you through preparing your subfloor for a tile installation project, focusing on creating an ideal base for supporting ceramic or porcelain tile.

By following these steps, you’ll ensure that your tile installation is successful and long-lasting.

Related: Underlayment and Subfloor Options for Ceramic Tile Flooring  |  Which tile backer board is the best? | What Type Of Mortar Is Used For Exterior Wall And Floor Tiles?

Table of Contents

How to Prepare Subfloor for Tile?

To prepare a wood subfloor for tile installation, follow these steps:

Step 1: Prepare a Wood Subfloor for Tile

Prepare a Wood Subfloor for Tile
  1. Check for Level: Use a yellow level or a 4-foot to 6-foot straight edge to find dips in the floor. Slide the straight edge in various directions across the room and mark any uneven areas for leveling.
  1. Fill Seams: Apply caulk to fill any subfloor seams. If there are any vents in the area, cover them with painter’s tape to protect them during the process.
  1. Sand Uneven Spots: Sand down any high or uneven spots on the subfloor. Since you will be applying backer board, a primer product is optional for wood subfloors.
  1. Apply Liquid Underlayment: Pour liquid underlayment into the dips you identified. This material will level itself out as it dries, creating a flat and even surface.

Tip:

Ensure the subfloor material won’t deteriorate when wet. Particleboard is not suitable for tiling projects. Oriented strand board (OSB) or exterior-grade plywood can be used. Still, they should first be covered with latex-modified mortar and a tiling backer board.

Step 2: Prepare and Install the Backer Board

  1. Install the Backer Board: Begin by placing a piece of backer board on the thin set spread on the wood subfloor. Follow the manufacturer’s instructions for installing a cement-based backer board to create a solid surface under the tile, preventing flexing and cracking.
  1. Cut the Panels: Backer board panels usually measure 3 feet by 5 feet and can be trimmed as needed. Measure and mark the size, then cut and score along the line with a carbide backer board cutter and straight edge. Depending on the manufacturer, scoring both sides of the board might be necessary. Snap the panel by pressing down on one side of the score line while lifting the opposite edge.
cutting-hardie-backer-board
  1. Apply Mortar: Use polymer-modified thin-set mortar to attach the backer board to the subfloor. This mortar provides additional strength and reduces movement between the backer board and the subfloor. Properly mix the thin-set mortar as instructed, wearing safety glasses and using a large bucket and a drill with a mixing paddle. Clean the mixing paddle immediately to prevent the mortar from hardening.
  1. Spread Mortar: With a trowel, spread enough mortar on the wood subfloor to cover an area slightly larger than your first piece of backer board. Comb the mortar at a 45-degree angle using the notched edge of the trowel, ensuring the lines go in the same direction.
  1. Set the First Panel: Place the first sheet of backer board on the mortar. Walk gently on the panel to set it into the thin set.

Step 3: Secure and Finish the Backer Board Installation

  1. Secure the Board: Screw the backer board into the subfloor using special backer board screws. Use 1 5/8-inch screws for 1/2-inch backer board and 1 1/4-inch screws for 1/4-inch backer board. Drive screws every 8 inches around each board’s perimeter, keeping them 1/2-inch from the edges and 2 inches from the corners. Follow the backer board manufacturer’s guidelines for screw placement.
Secure and Finish the Backer Board Installation
  1. Continue the Installation: Continue laying the rest of the backer board panels, leaving a 1/4-inch gap at the walls and placing 16d common nails between sheets to create a 1/8-inch gap. Stagger the panel ends and lay them perpendicular to the direction of the subfloor.
  1. Allow for Movement: Maintain a 1/4-inch gap between the backer board edges and the walls for subfloor expansion and contraction.
  1. Reinforce Joints: After positioning all backer board sheets, remove the spacer nails and reinforce the joints with self-adhesive alkaline-resistant fiberglass tape. Cover the tape with thin-set mortar, leveling it with a straight edge. If using non-adhesive tape, fill the gaps with mortar before setting the tape, then apply another layer of thin-set.
  1. Smooth the Surface: Smooth out any ridges or high spots and feather the edges of the thin-set across each board.
  1. Drying Time: Allow the thin set to dry, typically between 24 and 48 hours, checking the package for specific times.

Step 4: Prepare a Concrete Subfloor

Like a wooden subfloor, ensuring a concrete subfloor is level is vital for a successful tile installation. Additionally, it’s essential to guarantee a solid bond between the concrete and the tile.

Prepare a Concrete Subfloor for Tile
  1. Clean and Prepare the Surface: Start by ensuring your concrete floor is clean, dry, flat, and adhesive-free. Clean away any residue from strippers or cleaners, and rinse thoroughly with water. The absence of adhesives is crucial for proper tile bonding.
  1. Water Penetration Test: Check the concrete’s ability to absorb water. Sprinkle water on various areas of the subfloor. If the water soaks in, leaving a wet spot, the surface is ready for a good bond. If the water beads up, it indicates the presence of contaminants like adhesives, which can hinder bonding. These contaminants must be removed before tile installation.
  1. Check the Efflorescence and Hydrostatic Pressure: Ensure the concrete is free of efflorescence (salts on the surface) and not subject to hydrostatic pressure or moisture from below. A coarse finish on the concrete subfloor helps form a bond with the thin set. If the concrete is smooth, it must be etched or mechanically roughened to achieve a suitable bonding surface.
  1. Level the Floor: Use a 4- to 6-foot straight edge to check the levelness of the floor in different directions around the room. Fill any dips or hollows with a self-leveling underlayment designed for this purpose.
  1. Cure New Concrete: Allow new concrete floors to be cured for three to four months before installing tile. Address any moisture issues or leaks before beginning the installation process.

Step 5: Level a Concrete Subfloor & Install Crack Prevention

Crack Prevention
  1. Use Self-Leveling Compound: If your subfloor has many high and low spots, pour a self-leveling compound over the entire surface. For just a few uneven areas, treat only those specific spots.
  1. Mix and Apply the Leveler: Follow the manufacturer’s instructions to mix the leveler in a bucket. Always wear rubber gloves and safety glasses for protection. After adjusting your floor, allow sufficient time for them to dry completely before laying the new floor. Drying usually takes several days, but always refer to the manufacturer’s guidelines.
  1. Patch Cracks: If your concrete subfloor has cracks larger than 1/8-inch, patch them with a cement patching compound. Correctly filling these cracks is essential to ensure a smooth surface for tiling.
  1. Install a Crack Prevention Membrane: Over time, concrete subfloors can move, shift, and develop new cracks. To prevent this movement from affecting your tile and grout, install a crack-prevention membrane. Once the concrete subfloor is clean, flat, and free of significant cracks, apply the membrane.

You have two options: roll on a liquid latex membrane or use a peel-and-stick crack prevention mat. Cover the entire concrete subfloor with your chosen membrane.

By leveling the subfloor and installing crack prevention, you create a more stable and durable base for your tile, reducing the risk of future damage.

How to Recognize A Damaged Subfloor?

Squeaky Floors

A little squeak in flooring is quite normal and often goes unnoticed. But if your floor starts sounding like a creaky door, it’s time to look closer. Squeaks usually happen when the wood in your subfloor rubs against a nail. Usually, nails keep the subfloor tightly attached to the joists and prevent squeaks.

However, suppose the subfloor, particularly softwood ones, begins to warp or twist. In that case, nails may start loosening from the joists, leading to more noticeable squeaking.

Sinking Section

When walking across a room feels like traversing rolling hills, it’s a clear indicator of subfloor problems. Floors that start to sink between the joists are a definite sign of subfloor failure. This often results from moisture damage to the wood, but improper installation can also be a culprit.

During subfloor installation, the ends of boards or sheets should align directly over a floor joist. If these joints are even slightly misaligned, the continued pressure from foot traffic over time can cause the end of the sheet to sink gradually.

Moisture

A well-kept, polished hardwood floor is a sight to behold, but even a few cupped floorboards can mar its beauty. Cupping happens when hardwood boards start to curl along their length, similar to a taco shell but less extreme. This issue is typically due to moisture.

While the moisture might not originate from the subfloor, if the hardwood above shows signs of cupping, likely, that the subfloor is also being affected.

Cracked and Popping Tiles

Tiles that crack and pop are clear indicators of a problem. Usually, a well-adhered tile should remain intact for years. If you find a loose tile with its back fully covered in dried thinset, without visible trowel marks, it’s likely a subfloor issue. While moisture is a common culprit, incorrect subfloor material is also a frequent cause.

For tile projects, laying cement board—a combination of lightweight concrete and fiberglass mesh—over the subfloor is essential. This adds strength, reduces floor bounce, and offers a better surface for tiles to adhere to. Regardless of the cause, cracking or popping tiles suggest an underlying problem with the subfloor.

Potential Water Damage

Water poses a severe threat to flooring. If water has been trapped between the finished flooring and the subfloor, some damage will likely occur. Sources such as a leaking drain in the wall, a nearby leaky sink, or a roof leak during a recent storm could lead to the subfloor absorbing water. In such cases, it’s important to assess the extent of the damage to understand the necessary steps for repair.

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