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🔨 MARYLAND TILE BOSS — MYTH BUSTING

“Nobody Uses Plywood Under Tile Anymore. You Need Cement Board.”

I had a homeowner bring me a question recently after talking with a “tile expert” at a tile store.

She was told:

“Nobody uses plywood as a tile underlayment anymore. You should use cement board or an uncoupling membrane.”

That statement needs some context.

After 45+ years in the tile trade, I've watched installation methods and products change dramatically. Newer doesn't automatically mean better for every application, and different underlayments solve different problems.

First: Cement Board Doesn't Make Your Floor Stronger

This is probably the biggest misconception.

Cement backer board was developed as a durable tile substrate, particularly valuable as an alternative to moisture-sensitive gypsum products in wet-area wall applications. It eventually became extremely common over wood-framed floors as well.

But here's the important part:

Cement board is not there to structurally stiffen your floor.

It provides a compatible surface for bonding tile, but manufacturers generally do not consider cement backer board a structural panel. If your wood floor system has excessive movement or deflection, putting cement board over it doesn't magically fix the structure underneath.

That's why I have no problem using an appropriate backer board in the right bathroom application—but on a larger kitchen, sunroom, or similar framed floor, I want to pay very close attention to the structure underneath.

What About Plywood?

Plywood is different.

Plywood is structural.

Properly selected and installed plywood can increase the stiffness of the wood subfloor assembly and, importantly for tile, help reduce between-joist deflection.

And this is where decades-old advice about “never bonding tile to plywood” can also be misleading.

Modern polymer-modified mortars have come a long way. There are mortar systems specifically rated by their manufacturers for bonding tile to suitable exterior-glue plywood substrates when all of the required installation conditions are met.

So the statement:

“You can't install tile over plywood.”

is simply too broad.

You can have successful tile installations involving plywood. But the plywood type, thickness, fastening, joist/subfloor construction, mortar, tile type, installation method, and manufacturer requirements all matter.

And there are important limitations—especially with natural stone and certain assemblies—so this is not permission to stick tile over any piece of plywood you happen to find on a floor.

And What About Uncoupling Membranes?

I'm a fan of uncoupling systems when the application calls for them.

They can provide valuable benefits such as uncoupling, waterproofing when installed as a complete approved system, vapor management, and easier installation compared with some traditional assemblies.

But an uncoupling membrane isn't a substitute for correcting an inadequate structure either.

You have to solve the structural problem first.

Here's the Real Question

When I'm looking at a floor, I don't start with:

“Plywood or cement board?”

I start with:

What is underneath this floor?

What are the joists?

What's the joist spacing?

What's the span?

What's the existing subfloor thickness and condition?

Ceramic/porcelain or natural stone?

How much movement is in the floor?

What's going on in the room?

Then we determine the appropriate tile assembly.

That's the difference between choosing a product and designing an installation system.

One More Thing About “Experts”

The homeowner told me the advice came from the “tile expert” at the store.

There are some extremely knowledgeable people working behind tile counters, and I wouldn't dismiss someone's advice simply because they work in sales.

But homeowners should understand the distinction between a product recommendation and an installation recommendation based on the actual structure of their house.

Someone standing behind a counter usually hasn't inspected your joists, measured the span, evaluated your existing subfloor, or seen the conditions we're going to encounter once demolition begins.

And yes, some retail salespeople work under sales incentives. That doesn't automatically make their recommendation wrong—but it is another reason homeowners should ask:

“Why is this the right system for MY floor?”

Not simply:

“What product should I buy?”

🦀 TILE BOSS BOTTOM LINE

Cement board has a purpose.
Plywood has a purpose.
Uncoupling membranes have a purpose.

They're not interchangeable, and there isn't one underlayment that's automatically right for every tile floor.

A good tile installer isn't asking:

“What's everybody using now?”

He's asking:

“What does this particular floor need to support a tile installation for the long haul?”

That's the conversation homeowners should be having before the first tile goes down.

— Maryland Tile Boss
45+ Years of Tile Experience — Old-School Knowledge. Modern Installation Methods. Done Right the First Time.