Underlayment Plan

The room

Date

Room

Length

Width

AREA

Waste %

NEEDED

Roll nominal

Overlap loss %

REAL per roll

ROLLS

Spare

Product

Moisture barrier?

Rules that matter more than the maths

⚠ Layers

ONE. Never double up

Why

spongy floor → load onto the plank JOINTS

And

voids most flooring warranties

⚠ Pad already on the plank

add NOTHING underneath

Want more cushion?

one THICKER underlayment, rated for it

Over concrete slab

vapour barrier REQUIRED

Over ventilated wood subfloor

usually none — can trap moisture

Typical overlap/trim loss

about 5%

Common roll sizes

100, 200, 630 sq ft

Direction

same as the planks, seams staggered

Slab damp test

taped plastic square, 24 hours

Laying it

• Checked whether the planks already have a pad attached
• Subfloor swept absolutely clean — grit telegraphs through
• Subfloor flatness checked with a long straightedge
• High spots ground down, low spots filled
• Vapour barrier decided by what is UNDER the floor
• ONE layer, no exceptions
• Running the same direction as the planks
• Seams butted or overlapped as the product specifies
• Seams taped if required
• Only rolled out one or two runs ahead of the flooring
• Trimmed at the wall, not bunched up the skirting

Never two layers

It feels like more cushioning and is not. A floating floor that flexes transfers load into the planks' locking joints, which work loose, gap and break — and it voids most warranties. Want more cushion? One thicker underlayment.

Check the plank first

Many laminate and vinyl planks ship with underlayment already bonded on. Adding more beneath is the same doubling-up error, and neither package mentions the other. Check before buying anything.

The barrier is about the subfloor

Over a concrete slab you need a vapour barrier. Over ventilated timber you usually do not, and adding one can trap moisture on the wrong side. What is under the floor decides this, not what the floor is.