Underlayment Calculator
Calculate rolls of flooring underlayment needed for laminate, vinyl plank, hardwood, and tile installations.
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How to Calculate Flooring Underlayment
Calculating underlayment follows the same principle as flooring itself: measure the room area and add waste for overlaps. The formula is: Room Length x Room Width x 1.10 (for 10% overlap waste) / Roll Coverage = Number of Rolls. Unlike flooring, underlayment waste comes from overlaps rather than cutting patterns, so 10% is sufficient for all room shapes. Round up to whole rolls since partial rolls aren't available.
For multiple rooms on the same floor, calculate each room separately and add the totals. Don't try to run a continuous sheet through doorways — cut and restart at each threshold. This makes installation easier and allows for slight height differences between rooms if you're using different flooring types.
Underlayment Types and When to Use Each
Basic polyethylene foam (2 mm) is the cheapest option and works for budget laminate installations over plywood subfloors. Foam with built-in vapour barrier (2-3 mm) is the standard choice for laminate and engineered hardwood over any subfloor — the vapour barrier protects against moisture migration from concrete or crawl spaces. Cork underlayment (3-6 mm) provides excellent sound dampening and thermal insulation, ideal for upper floors in multi-storey homes or condos with noise requirements.
Rubber underlayment (2-5 mm) offers the best acoustic performance and is specified in many condo HOA requirements (minimum IIC 60 or STC 50 ratings). It costs more but prevents impact noise transmission between floors. For tile installations, cement backer board (HardieBacker, Durock) acts as the underlayment — it provides a dimensionally stable, moisture-resistant surface that won't flex or degrade under thinset mortar.
Underlayment for Vinyl Plank Flooring
Vinyl plank (LVP) is the most common flooring installed today, and underlayment requirements are different from laminate or hardwood. If your vinyl plank has an attached cork or foam backing (check the underside of a plank — most products priced above $3/sq ft include this), do NOT install additional underlayment. The double cushion effect causes the planks to flex too much under foot traffic, eventually breaking the click-lock connections and creating gaps.
For vinyl plank without attached backing, use only underlayment specifically rated for LVP — typically 1-1.5 mm thickness. Standard 3 mm laminate underlayment is too thick and too soft for vinyl. On concrete subfloors, use a combination underlayment with moisture barrier built in, or install a separate 6-mil poly vapour barrier beneath the LVP underlayment. Always follow the flooring manufacturer's warranty requirements for underlayment type and thickness.
Do I Need a Vapour Barrier?
A vapour barrier (moisture barrier) is required when installing floating floors over concrete subfloors at or below grade level. Concrete wicks moisture from the ground — even slabs that appear dry can release enough moisture to damage flooring over time. The barrier prevents this moisture from reaching the underside of your flooring where it causes warping, mold, and adhesive failure.
You typically need a vapour barrier: over concrete slabs (ground floor or basement), over crawl spaces, and in rooms below grade. You typically don't need one: over plywood subfloors on upper levels, or when the underlayment already includes a built-in vapour barrier (check the product spec — most quality foam underlayments include one). Never install two vapour barriers — moisture trapped between them has nowhere to escape and causes more problems than it prevents.
Frequently Asked Questions
How much underlayment do I need?
Measure the room area (length x width) and add 10% for overlap and waste. Standard underlayment rolls cover 100-200 sq ft (9-19 m²). For a 12x14 room (168 sq ft), you need about 185 sq ft of underlayment with overlap, which is typically 1 roll of 200 sq ft coverage. Always check the specific product's coverage listed on packaging.
Do I need underlayment for vinyl plank flooring?
It depends. Vinyl plank with attached backing (built-in pad) does NOT need separate underlayment — adding more can make the floor too soft and damage the click-lock joints. Vinyl plank without attached backing needs a 1-1.5 mm underlayment designed for LVP. Never use thick foam underlayment meant for laminate under vinyl plank — it's too cushioned and causes joint failure.
What type of underlayment do I need?
Laminate flooring: 2-3 mm foam or cork underlayment with vapour barrier. Engineered hardwood: 2-3 mm foam, cork, or felt. Vinyl plank (no attached pad): 1-1.5 mm LVP-specific underlayment. Solid hardwood (nail-down): 15 lb felt paper or rosin paper. Tile: cement backer board (1/4 inch for floors, 1/2 inch for walls). Over concrete subfloors, always use underlayment with a built-in moisture barrier.
How much does flooring underlayment cost?
Basic foam underlayment: $0.15-$0.35 per sq ft. Standard foam with vapour barrier: $0.25-$0.50 per sq ft. Cork underlayment: $0.50-$1.50 per sq ft. Rubber underlayment (sound-reducing): $0.75-$2.00 per sq ft. For a 200 sq ft room, budget $30-$70 for standard foam, $100-$300 for cork, or $150-$400 for premium rubber.
How much overlap do I need for underlayment?
Most manufacturers require 2-4 inches (5-10 cm) of overlap where rolls meet, taped with the manufacturer's recommended tape. Foam rolls typically overlap 3-4 inches. Against walls, run the underlayment up 2 inches (it will be hidden behind baseboard). This wall overlap acts as a moisture barrier at the perimeter. Never leave gaps between underlayment sheets.
Can I put new underlayment over old underlayment?
No. Remove old underlayment before installing new flooring. Double-layering underlayment makes the floor too soft, voids warranties, and can cause click-lock joints to fail over time. The one exception: if the old underlayment is a thin vapour barrier film (less than 1 mm) on concrete, you can leave it and install new underlayment over it.
Do I need underlayment on the second floor?
Yes, but for different reasons. On ground floors over concrete, underlayment provides moisture protection and thermal insulation. On upper floors over plywood subfloors, underlayment primarily provides sound dampening (reduces footstep noise to rooms below) and smooths minor subfloor imperfections. Cork and rubber underlayment offer the best sound reduction (IIC/STC ratings of 60+).
How many rolls of underlayment do I need?
Divide your room area (with 10% waste) by the roll coverage listed on the packaging. Common roll sizes: 100 sq ft (economy), 150 sq ft (standard), 200 sq ft (contractor size). For a 300 sq ft room: 300 x 1.10 = 330 sq ft needed. With 200 sq ft rolls: 330 / 200 = 1.65, rounded up to 2 rolls. Always round up — you cannot buy partial rolls.