Home Improvement Interior Remodel Flooring & Stairs

Subflooring vs Joists vs Underlayment: What's the Difference?

Learn the key differences

Layered flooring systems are flexible, sturdy, inexpensive, and easy to build. The flooring system found in most homes starts with a floor covering (laminate, vinyl, tile, or other) on top of an optional buffering underlayment. The floor covering and underlayment rest on a thick, solid subfloor, which is supported by sturdy joists.

Learn the differences between subflooring, joists, and underlayment, and their roles below floor coverings such as wood, tile, laminate, or vinyl.

Subfloor with wooden joists closeup

The Spruce / Margot Cavin

Material Position Purpose Required?
Floor covering 1 Top visual/walking layer Yes
Underlayment Buffer No
Subfloor 3 Solid flat base Yes
Joists 4 Main superstructure Yes
Finished floor covering with black and wooden bench next to woven basket

The Spruce / Margot Cavin

Subfloor

The subfloor is the flat, stable, level surface below the floor covering. The subfloor rests on top of the joists and is nailed to the joists.

Most all houses have subfloors. The subfloor is made of plywood or OSB typically in thickness from 19/32 inches to 1-1/8 inches. If there is a concrete slab floor, the slab itself may be considered the subfloor. Subfloor plywood or OSB sheets typically have tongue-and-groove edges to reduce floor sagging and bounce.

Floor Covering

Floor covering is an industry term for flooring: the topmost layer that you see and walk on such as ceramic tile, solid wood, laminate, engineered wood, luxury vinyl, or carpeting. Floor covering is not structural.


Plywood subfloor installed with with electric drill

The Spruce / Margot Cavin

Joists

Joists form the bottom-most superstructure of the flooring system. Made of engineered wood, laminated wood, or dimensional lumber, joists are arranged parallel to each other and typically 16 inches apart, though this spacing may vary. If the home has a concrete slab floor, it will not have joists.

Wooden joists supporting ceiling

The Spruce / Margot Cavin

Underlayment

Underlayment is a thin buffer between the floor covering and the subflooring. Underlayment is a fluid term and it can mean many things. Underlayment is the one flooring system material that is optional:

  • Wood: When installing solid hardwood or engineered wood flooring, thin luan or plywood specifically engineered as an underlayment smooths out uneven subflooring.
  • Tile: For wet, mortared applications, such as tile and stone flooring, the underlayment may be a cement backer board.
  • Laminate (Separate): One type of underlayment for laminate flooring is a thin foam that comes in rolls and is then taped together.
  • Laminate (Attached): A more common type of laminate underlayment is pre-attached to the bottoms of the laminate planks.
Underlayment of plywood

The Spruce / Margot Cavin

Tip

An older home that has been extensively remodeled may have many layers of old and new flooring. Homeowners may have added layer upon layer of flooring rather than stripping away lower layers. Keeping layers in place is valuable when one of those layers contains asbestos. When it is kept in place and undisturbed, asbestos is safe. But sanding, grinding, or scraping asbestos floor layers is hazardous.

How to Determine Flooring Type and Thickness

You may need to find out how many layers of flooring materials the floor system has and the system's total thickness. You will need a straight-edge ruler or a tape measure.

  • Floor heating vent: Removing a floor HVAC vent is the best way to check for total floor thickness. If the home has central heat and/or air conditioning with floor vents, it is easy to remove a vent by lifting it straight up. With the vent removed, you have a wide, clear view of a cross-section of your floor.
  • Top of stairs: Sometimes, the top of a staircase has an unfinished opening that permits a cross-section view of the entire floor. Even if the stairs are fully finished, you can get an approximate reading of the thickness by measuring the distance from the top of the riser to the top of the floor covering.
  • Basement and crawlspaces: This method might give you a rough idea of the layers if the basement's ceiling and joists are exposed. Reach up and try inserting a straight edge or tape measure up through any space or gap you find between the subfloor panels and take note of the measurement.
  • In a closet or pantry: This method only measures the thickness of the floor covering. Often, closets or pantries do not have baseboard molding. By placing your straight edge along the wall, you can insert it between the floor covering and the wall, sliding it down until it touches the subfloor.
Exposed floor HVAC vent in wood flooring

The Spruce / Margot Cavin