A subfloor is the structural layer that sits directly on your floor joists or foundation and supports everything above it, including any underlayment and the hardwood, LVP, laminate or carpet you actually walk on. In most wood-framed homes the subfloor is plywood or OSB panels, and in slab-foundation homes and most basements the concrete itself serves as the subfloor. Its flatness, firmness and moisture level decide which flooring can be installed and how well that floor holds up over the years.
What a Subfloor Does
The subfloor has two jobs. It carries weight, spreading the load of people, furniture and appliances across the joists or foundation below so no single spot takes all of it. It also gives the finished floor a solid, flat surface to attach to, whether that floor is nailed, glued or floating on top.
You never see the subfloor once a project is finished, but you feel it. A floor that flexes when you walk past the refrigerator, a squeak in the same spot in the hallway every morning, or a dip that makes a dining chair rock are usually subfloor issues showing through the finished surface. Replacing the top layer without addressing what is underneath tends to carry those problems straight into the new floor.
Subfloor vs. Underlayment vs. Finished Flooring
Homeowners often use these terms interchangeably, but they are different layers. From the bottom up, a typical floor assembly looks like this:
- Floor joists or a concrete foundation, the structure of the house
- The subfloor, plywood or OSB panels fastened to the joists, or the concrete slab itself
- Underlayment, a thin layer such as a foam pad or moisture barrier, when the flooring calls for one
- The finished floor, the hardwood, LVP, laminate or carpet you see
Not every floor has underlayment. Solid hardwood nailed to plywood and engineered hardwood glued directly to concrete both sit right on the subfloor. Carpet uses a pad, which plays a similar role. The subfloor is always there, though, and it is the one layer that cannot be easily changed once the finished floor is in.
Common Types of Subfloor
Plywood
Plywood is made from thin wood veneers glued together in layers, with the grain of each layer running a different direction. That construction makes the sheets strong and resistant to warping, and it holds nails well, which is why plywood is a natural base for nail-down solid hardwood.
OSB (Oriented Strand Board)
OSB is made from wood strands pressed together with resin into large panels. It does the same structural job as plywood, and like plywood it works under hardwood, LVP, laminate and carpet when it is flat, dry and firmly attached to the joists.
Concrete Slab
In homes built on a slab, and in most basements, the concrete is the subfloor. Concrete is strong and stable, but it holds moisture differently than wood does. That single fact rules out some flooring products and changes how others are installed, which is why we treat slab projects differently from the first conversation.
How Your Subfloor Affects Your Flooring Options
The subfloor is often the deciding factor in what goes down, more than style or color.
Solid hardwood needs a wood subfloor. It is nailed down, and it should not be installed directly over concrete because it expands and contracts too much as moisture levels change. If you want real wood over a slab, engineered hardwood is the right product, installed either glue-down or floating, and direct glue is our preferred method on concrete. On a plywood or OSB subfloor, both solid and engineered hardwood are strong choices, and the decision comes down to species, plank width and finish. Our hardwood flooring page covers those tradeoffs in more detail.
Luxury vinyl plank comes in two main constructions, and the subfloor helps decide between them. SPC (stone plastic composite) has a rigid core that bridges minor subfloor imperfections, which makes it our usual recommendation for slabs and basements. WPC (wood plastic composite) is softer and quieter underfoot, but it does not bridge flaws as well, so it belongs on a flat plywood or OSB subfloor. On concrete, an LVP installation should include a moisture barrier. Our luxury vinyl plank page compares SPC and WPC side by side.
Laminate flooring and other floating floors need a flat base, since the planks lock together and rest on the subfloor rather than being fastened to it. Humps and dips show up as joints that flex or open over time. Carpet and pad hide small dips better than hard surfaces do, though a soft or squeaky subfloor still comes through underfoot.
Signs Your Subfloor Needs Attention
Most subfloor problems show up before the old flooring ever comes out. Watch for:
- Soft or spongy spots that give when you step on them
- Squeaks or creaks that repeat in the same places
- Dips, humps or a slope you can feel underfoot or see with a level
- Hard-surface joints that lift, open or flex in one area
- Stains, swelling or a musty smell that point to past water exposure
A squeak by itself is usually a fastening issue. Soft spots and water damage deserve a closer look, because putting a new floor over a compromised subfloor shortens the life of the new floor.
What Happens to the Subfloor During a Flooring Project
On our projects, the subfloor gets assessed before anyone talks numbers. During the free in-home consultation, your Project Manager measures the rooms and checks the subfloor, which is why we provide written quotes only after that visit and never over the phone. Subfloor prep then appears as its own itemized line in your written estimate, so you can see exactly what is being done beneath the new floor. Our flooring installation process lays out each step.
The questions we work through at that visit include:
- Is the subfloor flat enough for the product you have chosen?
- On a slab, what moisture protection does the installation need?
- How will the new floor's height line up at doorways, stairs and transitions to other rooms?
- Can the existing flooring stay, or does it need to come out?
On one laminate project, the home had a double-layered subfloor that was out of level. The crew leveled it, built a clean new transition at the stairwell and installed the laminate, with the whole job wrapped in about two days. On a large glue-down LVP job over concrete, the slab was patched and smoothed throughout before a single plank went down. Neither prep step is visible in the finished room, and both are the reason those floors sit flat.
Can New Flooring Go Over an Old Floor?
Sometimes. Floating LVP can often go over existing tile or hardwood if that surface is flat, level and structurally sound, which can save the cost of removal. Tile with heavy texture, broken pieces or uneven grout lines usually needs to be leveled or removed first, and hardwood that is cupping, peeling or damaged is not a stable enough base. We advise against installing LVP over engineered wood that is glued to a slab. In every case, the old floor is acting as the subfloor, so it gets judged by the same standard.
When you schedule a free consultation, your Project Manager checks the subfloor while you look at samples, so your estimate covers what goes under the new floor as well as what goes on top of it.
