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Underlayment and Subfloor Options for Ceramic Tile Flooring

While ceramic and porcelain tiles are known for their durability, they also have a delicate aspect. These tiles are strong and, if installed correctly, provide very stable floors that can endure for decades. However, their performance relies on the condition of the floor structure beneath them. To learn about what underlayment for tile floor, you should first know that the structure of floor often includes floor joists, a subfloor, and an underlayment.

Suppose this supporting structure is robust. In that case, the flexible tiles may move when stepped on, leading to cracks in the grout and potentially ruining the entire floor.

Therefore, despite their inherent strength, floors made of ceramic and porcelain tiles can be pretty fragile. The success of these tiles largely depends on compatible subfloor and underlayment systems. They may face significant issues with unmatched materials, especially in moisture, movement, and adhesion contexts.

Related: What Type Of Mortar Is Used For Exterior Wall And Floor Tiles?  | Which tile backer board is the best?

Table of Contents

Reasons for Considering Underlayment Options

When considering underlayment options for floor tiles, these factors are essential:

  • To provide more rigidity for tile installation.
  • To prevent subfloor cracks from transferring to the tile installation.
  • To provide soundproofing.
  • To prevent moisture from entering the subfloor and affecting the finished floor.
  • To reduce the risk of tile lippage.
  • To support the load.
  • To level the floor.
  • To smooth out uneven spots in a subfloor, create an unbending layer that prevents flexing underfoot.
  • To provide a waterproof barrier between the tile and the subfloor.

Layers of Flooring

Your home’s floors consist of several layers, each serving a unique purpose:

Joists

These foundational beams run from wall to wall, forming the lowest layer of flooring. They provide support for everything above. You can often see the joists in homes with basements and unfinished ceilings. Standard homes typically use 2×8 or 2×10 lumber for joists.

For greater strength, as needed for large spans without supporting walls, larger joists made of engineered wood, laminated wood, or dimensional lumber are used. The only exception is concrete slab floors, which do not have joists.

Layers of Flooring

Subfloor

Positioned directly on the joists, the subfloor forms the underlayment and surface flooring foundation. In finished basements, the subfloor often rests on the concrete slab, sometimes with a moisture barrier in between. The subfloor creates a stable, level surface and provides a base for attaching new carpeting, laminate, hardwood, vinyl, or tile.

Underlayment

This layer sits above the subfloor and right below the chosen flooring material. While only sometimes necessary, underlayment offers several benefits. It can vary in material, each providing different advantages.

While the subfloor gives structural support, underlayment acts as a finishing layer. It can serve as a sound barrier, offer insulation against temperature changes, or protect the flooring from moisture. It’s often used to smooth out gaps or warps in the subfloor material, preventing irregularities from affecting the final flooring.

Surface

This is the visible top layer of flooring. While it doesn’t provide structural support, choosing a surface layer that suits your lifestyle and the room’s usage is essential. Selecting durable flooring is advisable for areas with high traffic, pets, or children.

Note that not every flooring system contains all three components. In homes with a concrete slab foundation or in basements, ceramic tile is often installed directly on the concrete slab.

For ceramic tile floors, the quality of the subfloor and underlayment is crucial. There are six widely accepted subfloor/underlayment structures for ceramic tile installation, each catering to the unique demands of tile flooring. These structures ensure the tiles remain stable, level, and properly supported, reducing the risk of damage or unevenness over time.

Common Types of Underlayment

Cement-Based Backer Board

hardiebacker-cement-board-close-up

Cement-based backer board is now considered the top underlayment choice for ceramic tile, suitable for floors, walls, and countertops.

The composition of cement-based backer board is as its name implies: it’s a cementitious material reinforced with fiberglass mesh for stability. It typically comes in sheets of either 1/4- or 1/2-inch thickness, standard sizes of three-by-five feet or four-by-eight feet. Due to its weight, the smaller sheets are easier to handle and install, making them a preferred choice for DIY projects.

The introduction of cement-based backer board and thin-set mortar adhesives marked a significant advancement in the ceramic tile industry. It allowed for the creation of a structurally sound base for ceramic tile installations. This task previously required skilled professionals to lay a wet-mortar base layer.

First introduced around 1970, these materials expedited professional installations (reducing costs) and enabled everyday DIY enthusiasts to undertake high-quality ceramic tile projects themselves.

Concrete Slab

Concrete Slab for floor

Concrete slab is an excellent underlayment for ceramic tile. It’s one of the best choices, similar to the solid mortar bases traditionally used in tile installations. However, the concrete slab must be flat, level, and stable, without any issues of settling or heaving. Also, tiling over expansion joints in the slab should be avoided, as this can lead to fractures in the tile work.

To reduce the risk of cracking risks, an intervening material known as an uncoupling membrane is often used. This membrane separates the tile from the concrete subfloor, preventing any cracks in the slab from affecting the tile.

Available under brand names like Schluter Ditra or Redgard Uncoupling Mat, this product is also referred to as an anti-fracture membrane, crack isolation membrane, or crack suppression membrane. Its use ensures a more durable and crack-resistant tile installation.

Mortar Bed

Mortar Bed for floor

Mortar bed installation, once a common practice, involves laying a thin layer of cement mortar and setting the ceramic tile directly into it. This method includes a wire mesh reinforcement within the mortar layer. Its advantage lies in adjusting for small imperfections and unevenness in the subfloor.

However, installing a mortar bed demands much skill and time, so it is challenging for most DIY enthusiasts. Professional installation can be expensive due to the method’s time-intensive nature, and skilled installers are rare and sought after.

Additionally, a mortar bed adds substantial weight to the flooring structure. Nowadays, mortar bed installations are relatively rare, mostly encountered during the demolition of older floors. Despite this, mortar beds remain a common choice for setting ceramic tile bases in shower areas.

Common Types of Subfloor

Plywood

Plywood for subfloor

Plywood, made by gluing together layers of veneer, has been widely used as a subfloor material since the 1950s. It is of higher quality than oriented strand board (OSB), with more resilience under pressure over time. However, standard plywood can separate under extreme heat, which makes it less suitable for warmer climates.

For best results in using plywood as a subfloor, 3/4-inch tongue-and-groove plywood is recommended. This type should be both glued and either screwed or nailed to the floor joists.

In the installation process, focus on details to ensure longevity. Properly installing plywood means gluing all joints and firmly securing them into place. This method helps to prevent any warping or separation that might occur due to changes in temperature or moisture levels in the environment. Follow these steps to make a durable and stable subfloor with plywood.

OSB (Oriented Strand Board)

Another popular subfloor material. Layering wood strands make it 3 to 4 inches wide, in a cross pattern, then gluing and pressing them together. OSB’s flat and smooth surface, without the bumps some plywoods have, makes it a good choice for subfloors. This ensures an even base for flooring materials like bamboo or engineered wood.

When installing floors over OSB, the thickness of the flooring is an important consideration. For solid bamboo floors that are ¾ inch thick, they can be laid directly onto a ¾-inch thick OSB subfloor. These planks should be set at 90-degree angles to the joists.

For thinner floors, such as engineered, cork-backed, solid, or floating floors that are 1/2 inch or less in thickness, adding an extra layer of 3/8 – 1/2 inch plywood underlayment on top of the OSB is recommended. This extra layer provides more stability and support for the thinner flooring materials.

Particle Board

particle board for floor

Particle board is a manufactured wood product created from wood chips, sawdust, and other wood particles, all bonded with resin. This material is cost-effective for subflooring and is easily shaped into various sizes and forms to fit different spaces.

While providing good insulation against moisture and temperature shifts, particle board offers a different strength and durability than plywood or concrete slabs. It suits certain applications but may be better for more demanding uses.

One key issue with particle board is its tendency to warp and crack over time, particularly in high-humidity areas. To prevent this, installing a vapor barrier is crucial when using particle board in such environments. Also, ensuring proper installation is vital; all joints should be glued securely and fastened with screws or nails for lasting stability.

Concrete Slab

Concrete subfloors work well with wood floors at or above ground level. However, for installations below grade, engineered bamboo floors are often a better choice due to their moisture resistance and dimensional stability. Glue-down flooring methods are also suitable for concrete subfloors, but only after ensuring the subfloor is moisture-free.

Floated floors can be installed over concrete subfloors if the concrete is levelled correctly. The hardness, durability, steadiness, and often smoothness of concrete make it an ideal base for various types of flooring. In moisture-prone areas like basements, a moisture barrier is always recommended when installing over concrete.

Besides, you should avoid using solid hardwood flooring in these moisture-prone areas.

How to Recognize a Damaged Subfloor

Repair-Damaged-Concrete-Subfloor

Learn how to recognize a damaged subfloor to maintain the integrity of your floors. Here are some signs to watch out for:

  • Squeaking Sounds: If you walk on your floors and hear a squeaking sound, especially if this is a new occurrence, it could indicate a problem beneath the surface. While older wood floors might naturally squeak, new or intensifying noise often points to subfloor damage.
  • Sagging Floors: Visible or noticeable sagging when walking across the floor is a clear sign of a worn-out subfloor.
  • Foul Smell: Damage to a subfloor isn’t always visible. Sometimes, an unusual and unpleasant smell (often due to mold in damaged areas) can signal subfloor problems.

If you find any part of your subfloor damaged, it may require removal and replacement, or a new subfloor might need to be installed over it. The replacement material can be the same as the original or more structurally robust.

For example, if you have damaged particleboard, adding a layer of plywood above it could be an excellent option to consider before installing new flooring.

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