Estimate how much underlayment to order for a flooring job. Accounts for seam overlap and a waste factor for cuts.
Usage Tip
Check the flooring spec first; some products have underlayment attached and need none, while others require a specific type.
order = area × overlap factor × (1 + waste ÷ 100)
Rolls overlap at the seams, so you order more than the bare floor area.
The overlap factor assumes seams roughly every 4 feet.
What Is Underlayment?
Underlayment is the thin layer that goes between your subfloor and a floating floor. It cushions the floor, smooths minor subfloor imperfections, quiets footsteps, and – with a vapor barrier – protects the floor from moisture rising out of the subfloor. It is not the same as the subfloor (the structural plywood/OSB) and not the same as the finished floor; it is the layer in between.
Underlayment Comparison
| Type | Purpose | Best for |
|---|---|---|
| Foam | Basic cushion | Laminate, budget jobs |
| Cork | Sound reduction | Condos, upstairs rooms |
| Rubber | Premium sound control | Maximum quiet, gyms |
| Vapor barrier | Moisture protection | Over concrete or below grade |
| Combo (foam + film) | Cushion + moisture | Laminate over concrete |
Flooring Compatibility
| Flooring | Typical underlayment |
|---|---|
| Laminate | Foam, or foam + vapor (combo) over concrete |
| Vinyl plank (LVP) | Thin foam if not pre-attached; vapor over concrete |
| Engineered hardwood | Cork or foam; vapor over concrete |
| Solid hardwood (nail-down) | Rosin/felt paper – not foam roll |
| Tile | Uncoupling membrane or backer board – not foam roll |
Many LVP planks come with underlayment already attached – adding more can void the warranty, so check first. Solid hardwood and tile use entirely different systems (paper, membrane, backer), not rolled foam.
Moisture Barrier Explanation
Concrete is never truly dry – water vapor moves up through a slab and will damage a wood-based floor without protection. Over concrete or below grade, use a vapor barrier (6-mil poly or a combo underlayment with a film). Over a wood subfloor on an upper floor, a vapor barrier is usually unnecessary and can even trap moisture. Examples: laminate over a basement slab – combo underlayment with a taped film; engineered wood over a ground-floor slab – vapor barrier under the underlayment.
Sound Reduction Guide
If quiet matters – a condo, an apartment, a bedroom over a living room – underlayment is your cheapest acoustic upgrade. Foam helps a little; cork is a solid step up; rubber gives the best impact-sound control. Look for IIC (impact) and STC (airborne) ratings, and check your HOA or building requirement – many condos mandate a minimum IIC. Examples: upstairs bedroom – cork; condo over a neighbor – rubber to meet the IIC rule.
Roll Coverage
Underlayment is sold by the roll; coverage varies, but a common roll is about 100 sq ft. Find your area, add a little waste for overlap and trimming, and divide by the roll coverage. Quick reference at 100 sq ft per roll:
| Area | Rolls (100 sq ft roll) |
|---|---|
| 100 sq ft | 1 |
| 250 sq ft | 3 |
| 500 sq ft | 5 |
| 1000 sq ft | 10 |
Installation Tips & Mistakes
- Roll it out perpendicular to the planks, seams butted (not overlapped) and taped per the maker.
- Over concrete, lap the vapor film up the wall slightly and tape the seams – this is what keeps moisture out.
- Do not double up if your LVP already has attached underlayment.
- Do not use a thick, soft pad under click-lock floors – too much give breaks the joints.
- Acclimate the flooring and keep the expansion gap at the walls and transitions.
- Match the product to the warranty – the wrong underlayment can void floor coverage.
Frequently Asked Questions
How much underlayment do I need?
Your floor area plus a small waste allowance, divided by the roll coverage. A 500 sq ft room needs about five 100 sq ft rolls.
Which underlayment should I buy – foam, cork, or rubber?
Foam for basic cushion, cork for good sound reduction, rubber for the best sound control. Add a vapor barrier (or combo) over concrete.
Do I need a moisture barrier?
Yes over concrete or below grade. Over a wood subfloor upstairs, usually no.
Does vinyl plank need underlayment?
Often not – many LVP planks have it attached. If not, use a thin foam, plus a vapor barrier over concrete.
Can I use underlayment under solid hardwood or tile?
Not rolled foam – nail-down hardwood uses rosin/felt paper and tile uses a membrane or backer board.
Will the wrong underlayment void my warranty?
It can – flooring makers specify the type and sometimes the brand. Check the floor warranty before buying.
Related Flooring Calculators
Note: underlayment recommendations and quantities are general guidance and vary by product, flooring, and subfloor. Manufacturer requirements differ – always confirm the specified underlayment type, moisture barrier, and thickness for your exact flooring and follow them, especially over concrete and below grade, since the wrong choice can cause moisture damage or void the floor warranty. General DIY guidance, not a professional specification.