When installing tile, it’s easy to focus on your layout, spacing, and grout. But tile subflooring and substrates are just as important. Because tile is prone to cracking if it’s not flat and level, tile is only as good as the surface it’s installed on. Whether you’re tiling a floor, backsplash, wall, or shower, the surface underneath is often just as important as the tile itself.
Let’s explore how to prepare a surface so your tile performs as good as it looks for years to come.
Understanding the Surfaces Beneath Tile & Why They Matter
While it may seem easier to lump everything underneath tile into one category, there are actually a few different layers involved. Each performs an important role.
A tile subfloor is the structural layer beneath the finished tile. It’s usually made from plywood, OSB, or concrete. It needs to support the weight of your tile and, for flooring, the room’s furniture and fixtures.
A tile substrate is the surface that you install tile onto. It is the layer between the subfloor and the tile. Substrate materials may include cement board, waterproof boards, or drywall (for dry areas only). It can also include extra membranes, such as acoustic underlayments to prevent echoing or uncoupling membranes to isolate movement.
If the surface beneath your tile isn’t prepared correctly, it can cause a wide range of problems, such as:
- Cracked tile or grout
- Uneven tile edges (lippage)
- Loose or hollow-sounding tiles.
- Unseen water damage
- Mold and mildew
- Premature replacement costs for damaged tile
Expert Tip: Drywall is acceptable to use under tile in dry areas, but the Tile Council of North America (TCNA) strictly prohibits using drywall or greenboard in showers or other wet areas. This is because grout is not 100% waterproof. Even if tiled correctly, moisture can slowly seep through and cause drywall damage.
Many homeowners assume a kitchen backsplash is a “dry area,” and to the TCNA, it technically is. But the section behind the sink is splashed constantly. If water seeps through the grout lines, the drywall will soften over time. It’s best to use cement backer board for the section behind the sink to avoid this.
Evaluating a Surface Before Installing Tile
Tile is inflexible, so it can’t be used to hide imperfections in your surfaces. In fact, it can often amplify the appearance of uneven surfaces. Likewise, covering up existing problem areas with tile usually just means more expensive replacements down the road.
It’s helpful to evaluate your surfaces before and after removing the existing material where you’re going to install tile. Some issues only affect the surface material, while others are invisible until you peel back the layers.
For example, wood laminate might swell and peel when it gets water damage, but moisture may not have reached the subfloor, where it causes structural damage. On the other hand, you might find hidden mildew behind a shower wall that was tiled incorrectly decades ago.
With that in mind, here’s how to check a surface before investing in tile installation:
- Check the structural stability.
- Does the floor bounce or squeak when you walk on it?
- Are there any areas that feel spongy or soft when you walk on them?
- Are there any cracks in the existing tile or grout?
- Visually check for flatness and use a level.
- Are there obvious signs of bowing or unevenness?
- Are the transitions between rooms smooth and even?
- Do you see dips near doorways?
- Are the walls plumb?
- Identify moisture issues.
- Is there an odor of mildew in the area?
- Do you see staining?
- Have materials expanded or peeled?
Want a quick confirmation that your floors are stable? Try the “heel drop test,” where you stand in the center of the room and drop heavily onto your heels. If you hear the kitchen cabinets rattle, your subfloor probably has too much flex for tile and needs reinforcement.
Flat Doesn’t Equal Level
Flat and level are not the same when it comes to tile. In some situations, tile is specifically installed so that it’s not level. For example, tile may angle slightly towards a drain so that water doesn’t pool in a utility room or shower. In these situations, the floor actually needs to have a slight angle. However, it still needs to be flat so that the tile doesn’t crack.
Design Insight: If you’re in love with large-format tiles or heavy natural stone like marble, your substrate prep must be flawless. Larger tiles have zero tolerance for dips or bows. Even a 1/8-inch dip in the floor can cause a toe-stubbing lip on a large-format porcelain plank.
Repairing Damage Before Installation
If you identify any of these issues, make sure you remedy them before installing new tile. Otherwise, you risk using tile as an expensive band-aid solution for an underlying problem.
- Water damage or rot: Start by identifying the source of the damage to make sure it can’t happen again. This may involve addressing plumbing or foundation issues before proceeding. After diagnosing the damage, you can replace affected subflooring or drywall with new materials.
- Squeaky or bouncy subfloors: Identify why the subfloor panels are loose in the first place. The solution may be as simple as securing panels with additional fasteners or brackets.
- Uneven or out-of-plumb surfaces: Low spots may be remedied with self-leveling flooring compounds, but you want to make sure that there’s not an underlying foundation issue that needs to be assessed first. For uneven walls (especially in older homes), you will need to identify whether the drywall is uneven or if the wall itself needs framing adjustments.
- Cracked concrete slabs: Before installing tile over concrete, diagnose why cracks exist in the first place. Concrete is prone to cracking even without damage, but it’s important to make sure there’s not a deeper issue before investing in tile. Some cracks can be addressed with repair kits, but it’s best to consult an expert if you think the cracks might be from structural movement.
Choosing the Right Tile Substrate Materials for Installation
Once your subfloor or underlying walls are structurally sound, dry, properly fastened, flat, and level, you can select a substrate for installation.
Not all substrates are created equally, and not all substrates are appropriate for every area. You will need to think about factors such as moisture, cracking risks, and echoes.
- Cement backer board is commonly used for bathroom floors, tub surrounds, shower walls, and laundry rooms. While it’s resistant to moisture damage, it is not waterproof. If you want a truly waterproof solution, you will need to use an additional membrane specifically designed to keep moisture out.
- Waterproof foam tile boards are waterproof by design, making them a good all-in-one solution for high-moisture environments. They are generally more expensive than standard cement backer board. However, they’re faster and easier to install because you don’t need a secondary waterproofing layer.
- Uncoupling membranes are commonly used for installing tile over wood subfloors. Because they isolate vibrations, they help reduce minor movements that can cause tiles to crack over time. Keep in mind, they don’t correct structural deficiencies that might cause subfloor movement in the first place.
- Crack isolation membranes can be used on concrete slabs that have minor existing cracks, preventing the damage from affecting the tile surface above. However, they aren’t designed to address significant structural cracking.
- Acoustic underlayments are an additional layer you can use between tile and the substrate material to reduce noise transmission. As a secondary benefit, they can have a similar effect to crack isolation membranes. However, you can’t use just an acoustic underlay; you need to use it in addition to an appropriate substrate.
- Concrete acts as a subfloor and a substrate for some tile applications. For basements and ground-level slabs, tile can be applied directly onto concrete as long as it is free from dust, paint, sealants, and adhesive residue.
The Perfect Foundation for a Flawless Finish
Taking the time to evaluate, repair, and select the right substrate might feel like extra homework, but it is the single most important step in the tiling process. Whether you are tackling a complex DIY bathroom remodel or working with a professional installer to build your dream space, you don’t have to navigate the technical details alone.
At Tile X Design, we offer more than just world-class ceramic, porcelain, natural stone, and mosaic tiles. Our team is here to help you choose the ideal materials, substrates, and installation systems tailored specifically to your project’s unique demands.
Ready to get started on a beautiful space built to last? Visit our showroom or contact our team to explore our collections and get expert guidance for every layer of your next project.