How Much Underlayment Do I Need?

The short answer

A 15 × 12 room is 180 sq ft. With 10% waste you need 198 sq ft laid.

A roll labelled 100 sq ft really covers about 95 once you allow for overlaps and trimming — so you need 3 rolls, or 300 sq ft nominal.

That sounds like a lot for a 180 sq ft room. It is. The rounding is in whole rolls and the rolls are big.

A roll doesn't cover what it says

The number on the packaging is the nominal area of the material. You lose some of it in the act of laying it:

• Seams either overlap or carry a taped flap that sits over the neighbouring run
• Every run gets trimmed at the wall

About 5% is realistic for a plain rectangular room. A room with alcoves, a chimney breast or several doorways loses more.

The calculator reports what one roll really covers next to what it claims, because a 5 sq ft loss per roll is exactly the size of gap that leaves you short on the last run.

Now the important part: never two layers

This is the mistake worth publishing a calculator to prevent.

Two layers of underlayment feel like better cushioning and better sound insulation. They deliver neither.

What they actually produce is a spongy floor. And a floating floor that flexes underfoot transfers the load into the locking joints of the planks instead of down into the subfloor.

Those joints then:

1. work loose
2. open into visible gaps
3. eventually break

It also voids most flooring warranties outright — which means the failure you caused is also the failure you pay for.

If you want more cushion, buy one thicker underlayment that's rated for it. That's a product decision, not a stacking decision.

Check your planks before you buy anything

A great many laminate and luxury vinyl planks ship with underlayment already bonded to the back.

Putting a separate underlayment beneath those is exactly the same doubling-up problem — same sponginess, same joint failure, same voided warranty.

It's a common way to waste an entire purchase, and the reason it happens is mundane: the underlayment packaging says nothing about the planks, and the plank packaging says nothing about the underlayment. Nobody tells you. Check the back of a plank before you're standing at the till.

The moisture barrier is a different question

And it's decided by what's under the floor, not by what the floor is.

• Over concrete — any below-grade or on-grade slab — you need a vapour barrier. Either a combined underlayment that includes one, or a separate 6-mil sheet
• Over a wooden subfloor with a ventilated space beneath — usually not, and adding one can trap moisture on the wrong side where it does more harm than good

Not sure about a slab? Tape a square of plastic sheet down tight for 24 hours and look for condensation underneath. Free, and it answers the question properly.

Thicker isn't better past what the plank specifies

This is where the doubling-up instinct comes from, so it's worth stating directly.

Underlayment has a job: smooth out minor subfloor variation, reduce sound, sometimes block moisture. Beyond the thickness the flooring manufacturer specifies, more of it makes the floor less stable rather than more comfortable.

The plank's locking system was engineered against a particular amount of give. Check what your flooring specifies, and buy that, once.

Buy the roll size that suits the room

Underlayment rolls vary far more than people expect — 100 sq ft is common for foam, but 200 and 630 sq ft rolls exist.

A big roll in a small room is mostly offcut. Match the roll to the room rather than buying whatever's stacked at the end of the aisle.

Laying it

• Run it the same direction as the planks
• Stagger the seams rather than lining them up
• Tape the seams if the product calls for it
• Don't roll the whole room out first. Lay one or two runs ahead of the flooring as you go

That last one matters more than it sounds. Underlayment that's been walked on, knelt on and dragged across for an afternoon arrives at the plank stage torn, bunched and compressed — and every one of those faults telegraphs up into the finished floor.