Not the finish floor, not the subfloor--flooring underlayment occupies a middle position in the anatomy of the entire flooring structure.
What Is Flooring Underlayment?
Flooring--the entire structure--is composed of many different layers. Flooring underlayment is a thin, hard layer of wood or cement board upon which the top, or finish, flooring can rest. Soft underlayments of foam or cork will be discussed in brief, but do not fall into the same category.
Here is one typical composition of flooring layers:
Top Layer - Finish Floor: The finish floor, such as hardwood, laminate, or tile.
Second Layer Down - Underlayment: Plywood, fiberboard, or cementboard, and is usually either 1/4" or 1/2".
Third Layer Down - Subfloor: Subfloor will already be in place (unless this home is under construction). The subfloor is typically made of OSB and ranges from 19/32" to 1 1/8" thick.
Fourth Layer Down - Joists: The wooden members, often microlam, which support all of the previously-mentioned floorings.
3 Reasons For Flooring Underlayment
On occasion, builders and homeowners remodeling their homes do without flooring underlayment. While not always essential for flooring, the underlayment does help make for better overall flooring.
- Provides a smoother surface for the finish floor than the subfloor.
- Gives entire floor better stability by running boards perpendicular to the subfloor and by simply adding more material to the entire structure.
- Provides a better place for ceramic tile to stick (and other types of flooring requiring mortar or adhesive) than just the subfloor.
When Underlayment Is Not Needed
Flooring underlayment can be a confusing thing. Sometimes it's needed, sometimes it's not. And sometimes the subfloor is really the same thing as the underlayment.
So, let's boil it down to a few ideas you can count on.
If you are remodeling, you will typically be laying down underlayment before installing the finish floor. Often, the existing finish floor or subfloor are in such bad condition that installing underlayment is necessary.
If you are building a new house, the subfloor itself may be adequate for installation of the finish floor--eliminating the need for underlayment. If the finish floor is hardwood or engineered wood, all you need is a layer of rosin paper and you are good to go. But for all other types of applications, read on:
Types of Flooring Underlayment
- Plywood: Sheets of 4' x 8' AC grade plywood cut to size.
- Cement Board: Cement board like Wonderboard or fiber-cement board like DuRock.
- OSB: Orient-strand board is usually not used but can be, if needed.
- Underlayment Panels: Underlayment panels are interlocking and come in 2' x 2' tiles.
- Foam and Cork: Soft "underlayment" materials of cork or foam that come in rolls and are "floating," rather than attached.
What's the Best Flooring Underlayment?
It depends on your situation, but 1/4" to 1/2" AC grade plywood tends to be the best flooring underlayment for all dry applications (hardwood, laminate, and engineered wood).
The A-graded side is smooth enough even for the thinnest vinyl flooring.
For mortared flooring, such as stone and porcelain or ceramic tile, I like DuRock. It's smooth, easy to cut, and mold-resistant.
Can You Use Foam Underlayment Instead?
Plywood underlayment is such a hassle--hauling the materials to your home plus the labor of cutting the sheets to size. Can't you just roll out foam or cork underlayment instead?
Because these do not provide the same kind of structure as wood or cement board, we consider these to be a different type of underlayment altogether. In addition, these are floating underlayments, which means that they are not attached to the subfloor.
Foam and cork "underlayment" do two things. First, they provide slightly--but only the very slightest--buffer between the finish floor and imperfections on the subfloor (or existing finish floor, if you choose to install on top of that).
Foam and cork underlayments will account for imperfections like barely-protruding screw heads and knot-holes of no more than 1/8".
A rule of thumb with foam or cork underlayment: Could you reasonably install the flooring without the foam/cork underlayment and get roughly the same results? In other words, look at this type of underlayment as a bonus rather than a cure for any subfloor issues.
Do You Need Shiplapped or T&G Underlayment?
No. You can use plywood underlayment with square edges. But if tongue-and-groove or shiplap plywood is available for about the same price, it's nice to have.
Keep your underlayment as thin as possible. Thicker underlayments will begin to pose problems. Room ceilings begin to "lower" as you install thicker underlayment surfaces.
Are you installing underlayment to provide structural stability to your floor? While underlayment can help with this to some degree, that is not the purpose of underlayment. Underlayment is there to help out with the flooring above, not the subfloor or joists below. If you have subfloor or joist problems, address those issues before dealing with underlayment.