How much underlay do I need?

Underlay covers the same area as the flooring, with the same extra for cuts, and it is bought in whole rolls: divide that area by the m² printed on the roll and round up. A 4 m × 3 m floor is 12 m², 13.2 m² with the example 10% extra, so it needs one roll that covers 15 m², or two rolls that cover 10 m².

The four steps

1. Floor area = length × width, in metres. For an L-shaped or awkward room, split the floor into rectangles and add them.
2. Area to cover = floor area × 1.1, for 10% extra. It is the same area the flooring is bought for.
3. Rolls = area to cover ÷ the m² one roll covers.
4. Round up to a whole roll.

The 10% is BuildBatch’s labelled example for the flooring, and the underlay follows whatever figure the flooring uses. Change one and the other changes with it.

Where the roll coverage comes from

It is printed on the roll, in m². Roll sizes vary a lot: thin foam commonly comes on rolls of 15 to 20 m², while denser acoustic underlay and underlay for vinyl planks comes on smaller rolls, often 6.5 to 10 m². The planner does not assume a roll size. It shows the area to cover, marked Check first, and works the rolls out once you enter the figure off your roll.

Worked example

A floor 4 m × 3 m, with the example 10% extra.

• Floor area: 4 × 3 = 12 m²
• Plus 10%: 12 × 1.1 = 13.2 m²
• On rolls of 15 m²: 13.2 ÷ 15 = 0.88, so 1 roll, about 1.8 m² over
• On rolls of 10 m²: 13.2 ÷ 10 = 1.32, so 2 rolls, 20 m², about 6.8 m² over
• On rolls of 7 m²: 13.2 ÷ 7 = 1.89, so 2 rolls, 14 m², about 0.8 m² over

The roll sizes are figures typed in as examples, not products. Same floor, three roll sizes, three different amounts left over.

A quick table

At the example 10% extra, with roll sizes typed in as examples:

Two rows are close. The 3 m × 3 m floor on one 10 m² roll leaves about 0.1 m² over, and the 6 m × 4.5 m floor on two 15 m² rolls, or three 10 m² rolls, leaves about 0.3 m². The sum takes the m² on the roll as it is, so a count that close leaves almost nothing spare: check the roll’s figure and the room’s measurements before you rely on it.

The 4.2 m × 3.6 m floor shows the other side of whole rolls. It is 1.632 m² more than one 15 m² roll covers, so it takes two, and about 13.4 m² is left over.

When you need none

Boards with underlay attached need none. A glued-down floor takes none at all; it takes adhesive instead. The flooring’s instructions decide, so read them before you buy a roll.

When a membrane is a separate line

On a concrete or ground floor, a moisture barrier usually goes under the underlay unless the underlay has one built in. Check the underlay first. If it has none, a separate membrane goes on the list, covering the same area: 13.2 m² in the example. The membrane’s roll says what it covers; divide and round up in the same way, and tape the overlaps as its instructions say.

A suspended timber floor above a ventilated void does not usually need one. New concrete must be dry enough before any floor goes down, and the flooring’s instructions give the test. Which floors need what is in underlay and damp-proof membranes.

The joins between underlay strips are sealed with tape, and so are a membrane’s overlaps. One roll of tape usually does a room; the roll length and the number of joins decide.

Carpet underlay is counted differently

Under carpet, BuildBatch counts the underlay from the measured floor with no extra. A 4.2 m × 3.6 m room is 15.12 m², and at the planner’s example of a roll or pack covering 15.07 m², a generic figure and not a product’s, that is 2 rolls: the floor is 0.05 m² more than one covers. See how much carpet do I need? and the carpet calculator.

What to check first

• Whether the floor needs underlay at all: the flooring’s instructions say.
• Whether the underlay has a moisture barrier built in, if the floor is concrete or on the ground floor.
• Underfloor heating. Both the flooring and the underlay say whether they suit it.
• The m² on the roll. It changes from one underlay to the next.

Do it with BuildBatch

Open the flooring calculator, type the room, say you need to buy underlay and enter the m² from the roll. It works out the rolls and what is left over, and lists the membrane, the joining tape and the rest of the floor’s lines with them. What you need to lay laminate flooring goes through that list line by line.

What this guide does not cover

Which underlay suits which floor. Sound insulation rules for flats. Finding the cause of damp. Stairs. Adhesives for glued floors.

Related guides

• Underlay and damp-proof membranes: what to check first
• How many packs of flooring do I need?
• What you need to lay laminate flooring: the full list