Learn the key differences
Layered flooring systems are flexible, sturdy, inexpensive, and easy to build.
The floor covering and underlayment rest on a thick, solid subfloor supported by sturdy joists.
Floor covering is not structural, so it will always have a subfloor under it.
The Spruce / Margot Cavin
Underlayment
Underlayment is the optional thin buffer between the floor covering and the subflooring.
The subfloor is nailed to the top of the joists.
Most houses have subfloors.
The Spruce / Margot Cavin
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 Spruce / Margot Cavin
Theconcrete can be paintedor left uncoated and used as-is.
Joists
Joists form the bottom-most superstructure of the flooring system.
If the home has aconcrete slab floor, it will not have joists.
The Spruce / Margot Cavin
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 layerscontains asbestos.
When it is kept in place and undisturbed, asbestos is safe.
The Spruce / Margot Cavin
But sanding, grinding, or scraping asbestos floor layers is hazardous.
You will need a straight-edge ruler or a tape measure to do this.
Plywood is a stronger subfloor material and performs better than OSB if it gets wet.
The Spruce / Margot Cavin
Underlayment is not a substitute for subfloor.
Underlayment, if required, is a thin buffer that rests on top of the subfloor.
Underlayment is never installed directly on the flooring joists.
The Spruce / Margot Cavin
OSB subfloor is the most budget-friendly subfloor material.
One sheet of 23/32-inch OSB subfloor costs about $20 per 4-foot by 8-foot panel.
A comparably sized sheet of plywood subfloor panel costs $40.
So, plywood costs twice as much as OSB subfloor.
