Subfloor vs. Underlayment: Purpose | Types | Differences

By Samuel N •  Updated: 02/19/23 • 

Subfloor and underlayment are used in flooring and refer to completely different layers. A general floor has different layers, including joists, subfloor, underlayment, and floor covering. All these different layers build up to a sturdy and practical floor that handles different loads and forces.

In this post, I’ll talk about subfloor and underlayment. If you’re thinking about remodeling your floors, this is a good post to get you started with the right information. Enjoy.

What is a Subfloor?

The subfloor is the structural layer that rests on joists and can be made of OSB, plywood, or other materials. The main purpose of the subfloor is to offer structural support to the other floor layers as well as loads and forces on the floor. It also offers a flat surface for the underlayment and the floor finish. In the basement or homes with slab-on-grade foundations, the concrete slab may be considered a subfloor.

Constructing a Subfloor

Types of Subfloors

There are different types of subfloors. The most common ones are plywood, OSB (Oriented Strand Board), and concrete.

Plywood Subfloor

Plywood is one of the most preferred subflooring materials around. Plywood is a composite building material made by bonding several thin sheets of wood. The popularity of plywood as a subflooring material is because it is straight, flat, and relatively cheap. While the standard plywood can be used as subflooring, usually, plywood for subflooring comes with a tongue and grooves for interlocking with each other.

Plywood subfloor installed

Plywood subfloor installed

The thickness of plywood for subflooring is generally between 1/2 and 3/4 inches. The joist spacing will determine the thickness of the plywood you choose. The size of the plywood sheets is usually four-by-eight or four-by-twelve.

OSB Subfloor (Oriented Strand Board)

OSB, short for Oriented Strand Board, is a type of synthetic wood made of compressing under high-pressure chipped/strands wood in different layers. An OSB subfloor is a great base for hardwood, engineered wood, vinyl, and other floorings. The advantages of OSB subfloor over plywood are that it is cheaper, denser, and, when new, comes flat and knot-free. OSB is also a green product because it reduces the need to log forests and makes good use of waste products.

Installing Advantech OSB Subfloor

Installing Advantech OSB Subfloor

One downside of the OSB subfloor is that it is heavier than plywood but less durable or stronger than plywood. It can easily warp and crack when subjected to heavier loads. This makes it less ideal for heavier flooring like stone or ceramic tiles. Another disadvantage of OSB subflooring is that once it gets wet, it stays that way for a longer period. It also gets swollen after moisture damage and cannot revert to its original size.

Concrete Subfloor

Concrete as a subfloor makes for a hard, smooth, and durable type of subfloor. It is great for installing heavier flooring like stone or ceramic tiles without any structural problems. However, like plywood or OSB flooring, other types of flooring materials will require underlayment or moisture barrier. If you have a concrete subfloor in your basement, a moisture barrier underlayment is necessary to keep moisture from rising through the subfloor and flooring.

The major disadvantage of concrete subflooring is its hardness and coldness. It’s near impossible to nail into concrete, and it conducts cold from the ground below, which can lead to damp floors. For concrete subflooring that is below grade or in basements, it is recommended you do not install solid hardwood flooring.

What is a Flooring Underlayment?

A flooring underlayment or underlay is a layer of thin material beneath the top floor. The underlayment is sandwiched between the subfloor and the top floor. Underlayment provides a number of benefits depending on the type of underlayment and its application. The main purpose of a flooring underlayment is to provide a smooth surface for better installation on the top floor.

The subfloor is sometimes flat but could be smoother. The underlayment works to smoothen out small imperfections on the subfloor. Apart from that, the underlayment can also work as a sound insulator and moisture barrier and offer some soft cushioning.

Types of Underlayment Materials

The underlayment can come in different materials depending on your needs and floor covering choice. Some common underlayment options include plywood, cement boards, and thin foam padding.

Plywood Underlay

Plywood underlayment comes in about 1/4 inches thick, but depending on the situation, you can use 1/2 inch plywood underlayment. Plywood underlayment sheets come in 4-by-8-foot AC-grade pieces. The face of the plywood is usually smooth, while the rough side sits on the subfloor. Plywood underlay works best for resilient, vinyl tile, carpet, ceramic tile, wood, and engineered hardwood flooring.

Plywood Underlayment

Plywood underlayment should be distinct from the subfloor. The plywood used in the subfloor is two different layers, where one is the subfloor, and one is the underlay.

Cement Backer Board Underlay

A cement backer board underlayment, also called a cementitious backer unit or CBU, is best for only mortar floorings such as stone, ceramic, and porcelain tiles. Because a cement backer board is porous, mortar, grout, and thinset adhere very well to it for a lasting bond. Cement backer boards can be screwed or nailed to the subfloor, studs, and concrete.

cement backer board, attached to the subfloor with special screws

Cement Backer Board Attached to the Subfloor with Special Screws

Sheets of Foam

Foam underlay is one of the most basic types of underlayment around. It is cheaper and comes in plenty of options, from basic foam to foam underlayment with a vapor barrier. Foam underlayment works with various floorings, such as laminate, vinyl, and wood. Apart from its cheap price, foam underlayment offers plenty of sound reduction when used.

foam underlayment

Foam Underlayment

Foam underlayment is also easier to install. Some foam underlays even come with adhesive strips, which makes it easier to stick them to the subfloor.

Apart from plywood, cement backer board, and foam underlayment, other types of underlayment are also available. These include rubber, felt, cork, and carpet pads. Check out our different posts on choosing the base underlayment for each type of flooring.

Samuel N

Samuel N is the founder of Improve Floor and has been in the flooring industry since 2005. Since then, his mission has been to make flooring easier for everyone. He helps countless people with flooring installation, finishing, maintenance, and repairs each year.