What is a Subfloor? The Foundation Beneath the Beauty

Published: September 26, 2023

If you’re in the initial stages of deciding what kind of flooring you want, the first thing that probably comes to mind is comparing styles, like wood look vs. carpet. But while planning your new floors, an important question to consider isWhat’s beneath the surface?” Gorgeous new hardwood, vinyl, tile, and carpet will be what your family and guests see, but underneath that beauty is an important, often overlooked component: the subfloor providing integral support to the flooring surface above it. What is a subfloor? Let’s find out. 

Layers of Flooring

The floors in your home are made up of several layers, beginning with the joists below, the subfloor and underlayment in the middle, and the actual flooring material at the top. 


Joists are the first, lowest layer of flooring and support everything above. These are the beams that run from wall to wall, creating the “bones” of the floor. If you have a basement with an unfinished ceiling, you should be able to see the joists running the length of the home In most areas of a standard home, 2×8 or 2×10 lumber is used for joists. When more strength is needed—to span large distances without a supporting wall, for instance–larger joists made of engineered wood, laminated wood, or dimensional lumber will be used. All floors are supported by joists, except for concrete slabs. 


Directly on top of the joists, you’ll find the subfloor, which acts as the foundation for both the underlayment and surface levels of flooring. In a finished basement, the subfloor will often rest directly on the concrete slab, oftentimes with a thin moisture barrier between the two. The subfloor is what creates the stable, level surface you walk on and gives your flooring installers a material to which they can attach the new carpeting, laminate, hardwood, vinyl, or tile. 


Underlayment is the third layer, above the subfloor and immediately underneath whatever flooring you have chosen. While not an absolute necessity, underlayment is typically used because of the numerous benefits it can offer. Various materials can be used as underlayment, with each type offering different benefits. While the subfloor provides structural support, the underlayment serves as a finishing layer that can act as a sound-deadening barrier, provide extra insulation against temperature swings, or help protect your flooring against moisture. It’s often used simply to help smooth out any gaps or warps in the subfloor material. This prevents any irregularities from being felt through carpet and helps reduce any unevenness under hard surface floors that can lead to cracks or premature wear. 


This, of course, is the visible layerit’s your floors. The surface layer doesn’t add a structural purpose to your home but should still be chosen wisely based on your lifestyle and how the room is used. If you are expecting high traffic or have pets and/or kids, it’s best to get flooring that is highly durable. 

what is a subfloor

Different Types of Subfloors

Aside from concrete slabs, subfloors are almost always made of a wood product. The type of wood product will vary, and this can influence what type of underlayment is (or is not) needed and may even have an effect on how your floor feels and performs. 


Plywood is made from large, thin sheets of wood veneer that are stacked like a giant sandwich and bonded tightly bonded together with heat, pressure, and an industrial-strength adhesive. In addition to being dependably flat and strong, it’s much more affordable than solid wood sheets of the same size, making it a popular choice for subfloors in residential homes. It also resists expansion and warping better than solid wood. 

Oriented Strand Board

Competing in popularity with plywood is a relatively new material known as oriented strand board (OSB). OSB is fabricated with smaller pieces of chipped wood that are arranged in a crisscrossed pattern before being glued and pressed togetherBecause OSB compacts a large amount of smaller wood pieces into one sheet, it results in a denser and more consistent structure than plywood, with a surface that is less likely to have flaws or low spots. OSB is also more resistant to moisture than plywood, although plywood will usually dry out faster if it gets wet. But OSB is usually less expensive than plywood. It’s best to weigh your options and do the proper research to see if OSB subfloor is right for your home. 


Particleboard is made from sawdust and very small chips of wood that are bonded with resin and pressed into sheets. Though similar to OSB, it lacks the strength provided by the solid wood chunks in OSB or plywood. Because of its weak structure and because stronger, affordable options like OSB exist, particleboard is no longer recommended for use as a subfloor material. However, because it can be crafted with a very smooth, flat surface, particleboard makes a suitable underlayment material. 


Concrete subfloors are most commonly found in basements of single-family homes, and often on any floor level of high rises and condominiums. While incredibly durable, concrete floors are still susceptible to cracks or variations in surface smoothness. Some types of flooring can be installed directly onto concrete, but an underlayment is often suggested to help prevent seepage, regulate temperature, and provide an overall more comfortable surface. 

How to Identify a Damaged Subfloor

Years of wear and tear, especially from moisture, can have adverse effects on your subfloors. How can you tell a subfloor is damaged? Some signs include: 

  • Squeaking sound when you walk on your floors. This sound may be common if you have older wood floors. The problem arises when you haven’t heard this noise before. This means the noise may be beyond the surface and may be a sign of a damaged subfloor.
  • Sagging floors. If sagging is visible, or you feel it when you walk across your floors, this is a sure sign that your subfloor has worn out its welcome.
  • Foul Smell. Not every subfloor has visible signs of damage. Sometimes, you can detect subfloor problems by strange, unpleasant smells (often from mold accumulating in damaged areas).

If any part of your subfloor is damaged, you may need to have it removed and fully replaced or have a new subfloor installed on top of it. This can be the same material as the original, or maybe even something with a better structure. For instance, if you have a damaged particleboard, adding a layer of plywood above may be an option to consider before choosing new flooring. 

How Subfloors Can Make a Difference

Knowing the state of the subfloor is a necessity before having new floors installed. Providing a smooth, stable surface, subfloors are the structural base for your flooring. After all, a floor is only as nice as its foundation. The aesthetics play an important part in completing your home’s décor, but the subfloor is the most important factor in making sure your floors stay strong and beautiful for a long time. 

Subfloor issues can’t easily be diagnosed just by sight alone, so an inspection or conversation with a flooring professional is highly recommended. Knowing the status of the subfloors beneath you will also help you make an informed decision on your new flooring while allowing for a more accurate estimate before any work starts. Schedule a FREE In-Home Estimate today and ask a flooring professional about your options.

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