What Is a Subfloor?

While subfloors are hidden under visible flooring, they are critical to your home’s structural integrity. Failing subfloors can cause many problems, such as creaking floorboards. Our guide explains what a subfloor is, the best material options you have, signs your subfloor may be failing, and what to expect when it comes to repairing or replacing subfloors.

What Is a Subfloor?

A subfloor is the part of floor framing that sits above the joist but beneath the underlayment and the actual flooring. It ensures the structural stability of your floors and evenly distributes pressure onto the joists. In a finished basement, the subfloor will sit directly above the concrete slab with a thin moisture barrier in between.

Floor framing must have a subfloor layer so carpet, wooden floor, or tiles can be attached. Without the subfloor, you won’t have a stable, walkable surface, and your flooring material will sag around the joists.

Materials for Subfloor

Most subfloors are made of wood. There are several types of plywood subfloors used in modern homes:

• CDX: The most common option, this material is more affordable since it is rougher and contains more imperfections.

• Tongue-and-groove: Made with boards that fit snugly together, it prevents squeaks and provides a more durable structure.

• Marine plywood: A premium quality plywood made of hardwood, it’s highly moisture-resistant and is often used in bathrooms.

• Structural plywood or plywood sheathing: This is similar to CDX but doesn’t have the water-resistant glue between layers.

In addition to plywood, you may also find Oriented Strand Board (OSB) or chipboard subflooring and concrete subfloors.

How Much Does It Cost to Install a Subfloor?

Subfloors must be installed by a local flooring contractor. For new construction homes, subfloors are often installed alongside underlayment and flooring. For existing home remodeling, you will need to remove the flooring to access the subfloor, then reinstall the flooring.

The cost to install plywood subflooring for a 300-square-foot room is $620 on average, or $4.50 to $10.00 per square foot. The actual pricing will vary depending on the complexity of the project, your preferred subfloor material, and square footage.

Subfloor Repair and Replacement

Some signs that your subfloor might need repair or replacement include creaking floors, uneven floor boards, musty smells, or visible water damage. For bathroom subfloors, look for cracked or popping tiles. Sometimes failing subfloors also cause your toilet to wobble.

It is recommended that you hire a carpenter for subfloor replacement or repair due to the complexity of the project. A carpenter can also evaluate the structural integrity of your floor joists. If you are certain that the joists are in good condition, then a flooring contractor would be sufficient for subfloor and underlayment replacement.