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

Floor Covering

Floor covering is 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, so it will always have a subfloor under it.

Underlayment

Underlayment is the optional thin buffer between the floor covering and the subflooring.

Underlayment is not required in all installations, and it can have different meanings, such as:

• Wood: Thin luan or plywood that smooths uneven subflooring for solid hardwood or engineered wood flooring.
• Backer board: Cement backer board for tile and stone flooring.
• Laminate underlayment: Thin foam or felt installed under laminate flooring.

Subfloor

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

Most houses have subfloors. The subfloor is typically made of plywood or OSB, ranging from 19/32 inches to 1-1/8 inches thick. Subfloor plywood or OSB sheets typically have tongue-and-groove edges to reduce floor sagging and bounce.

If the bottom-most floor is a concrete slab, its definition as a subfloor becomes more fluid. The concrete can be painted or left uncoated and used as-is. Or a subfloor system can be built on top of it to separate a floor covering (like carpeting or laminate) from the concrete. A third option is to install an uncoupling membrane like Schluter Ditra on the concrete and then install ceramic tile on top of the membrane.

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.

How to Determine Flooring Type and Thickness

You may need to determine the number of layers of flooring materials and the floor’s total thickness. You will need a straight-edge ruler or a tape measure to do this.

• Floor heating vent: Remove a floor HVAC 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 thickness reading 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.
• Unfinished areas: If an unfinished area like a closet or pantry lacks baseboard molding, place the straight edge along the wall. Slide it down until it touches the subfloor to measure the thickness of the floor covering and any underlayments.