Most floating floors need underlayment. It's a thin layer rolled out under the planks to add sound dampening, a little cushion, and moisture protection. The main exception: if your planks come with an attached pad already on the back, you skip separate underlayment entirely. Over a concrete subfloor, you also need to add a moisture barrier.
Underlayment is cheap insurance against a noisy, hard, or moisture-damaged floor. Knowing when you need it, and how much, helps you avoid buying too little or doubling up by mistake. You can estimate quantities alongside your planks using the flooring calculator.
What Underlayment Actually Does
A roll of underlayment quietly handles three jobs at once:
- Sound. It dampens the hollow click of footsteps on a floating floor, the difference between a floor that sounds solid and one that sounds like a drum.
- Cushion. It gives planks a hair of give, smoothing minor subfloor imperfections so the floor feels better underfoot and the locking joints aren't stressed.
- Moisture. It slows water vapor rising from below, which protects moisture-sensitive cores like the HDF in laminate.
When You Need It (and When You Don't)
The decision comes down to two questions: how is the floor installed, and what's on the back of the plank?
- Floating floors: these click together and "float" over the subfloor rather than being nailed or glued down. They need underlayment for the sound, cushion, and moisture benefits above. Most laminate and many LVP and engineered-wood floors install this way.
- Planks with an attached pad: many modern LVP and laminate products come with foam or cork already bonded to the back. In that case, do not add separate underlayment. Stacking a pad on a pad makes the floor too spongy, can void the warranty, and stresses the locking joints. Check the carton.
- Nailed or glued-down floors: solid hardwood is typically nailed down, and some floors are glued. These follow different prep rules and usually don't use floating-floor underlayment.
If you're still choosing a material and install method, our guide to hardwood vs. laminate vs. vinyl explains which floors typically float and which get fastened.
The Concrete Exception
Concrete is porous and wicks moisture from the ground, which is bad news for any plank floor. So there's a firm rule:
Over concrete, add a separate moisture barrier under the underlayment, even if the underlayment claims some moisture resistance.
This applies to basements and slab-on-grade floors especially. Some products combine the pad and a vapor barrier into one roll, which simplifies things, but confirm it on the label before you count on it. When in doubt over concrete, add the barrier.
How Much to Buy
Underlayment is sold in rolls, and one roll covers about 100 sq ft. The math mirrors your flooring estimate:
- Find your room's square footage (see how to measure a room for flooring).
- Divide by 100 to get the number of rolls.
- Round up to a whole roll.
A 168 sq ft room needs 168 ÷ 100 = 1.68 rolls, which rounds up to 2 rolls. Unlike flooring, underlayment doesn't need a big waste factor, since you butt the seams edge to edge rather than cutting around a pattern. A small overage covers the edges and any overlap at seams.
You can grab compatible underlayment in the right thickness for your floor, and over concrete, look for a roll that pairs the pad with a vapor barrier.
Quick Install Notes
- Roll out underlayment perpendicular to your planks, seams taped, with no overlapping pad layers.
- Don't compress it under heavy underlayment plus a separate barrier where one combo roll would do.
- Acclimate your flooring boxes in the room about 48 hours before installing, regardless of underlayment.
Wrapping Up
If it's a floating floor, you almost certainly need underlayment, unless the planks already carry an attached pad. Add a moisture barrier any time you're over concrete. Buy by the roll at roughly 100 sq ft each, round up, and you should be in good shape. Run your room through the flooring calculator to size both the flooring and the underlayment in one pass.