Underlay for laminate flooring: which type, what thickness, and why it matters

Yes, laminate flooring needs underlay, and the right one is decided almost entirely by what is underneath it. Underlay is the thin resilient layer between your subfloor and the boards, and it does three jobs at once: it supports the click joints so a floating floor does not flex and wear, it quietens the hollow tap of footsteps, and it evens out the minor texture of the subfloor. On concrete it does a fourth job, blocking moisture, which is why a combined underlay with a built-in vapour barrier is the usual choice there. Get the type and thickness right and the floor is quiet and solid for years. Get it wrong, usually by buying the thickest roll on the shelf to hide an uneven floor, and you loosen the very joints the underlay is meant to protect.

Do you need underlay for laminate flooring?

Almost always, yes. Laminate is a floating floor: the boards click to each other, not to the subfloor, so the whole floor is one sheet resting on top of whatever is below. That sheet needs a thin, even, resilient layer under it, or it fails in several ways at once:

• The joints wear. With nothing to cushion them, the click joints take the full load of every footstep against a hard subfloor and work loose over time.
• The floor is loud. Laminate over bare concrete or ply has a hard, hollow tap. Underlay absorbs that into a softer, quieter step.
• Small imperfections telegraph. Grit, trowel marks and the grain of floorboards read through thin laminate. A 2 to 3 mm underlay smooths them out.

The one exception is laminate sold with underlay already bonded to the back of each board. That board is laid straight onto the subfloor, and adding a second underlay under it makes the floor too soft and voids most warranties. Check the back of a plank before buying anything separate.

What are the types of laminate underlay?

Four types cover almost every job. The choice is not about which is best in the abstract, it is about matching the underlay to the subfloor and the room:

Most domestic laminate floors use one of the first two. Reach for fibreboard when the subfloor is a touch uneven or cold, and pay for acoustic underlay only when noise travelling to the room below matters, such as a first-floor flat.

Which underlay suits which subfloor?

The subfloor decides more than any other factor, because the biggest risk, moisture, comes from below. Match the underlay to what you are laying over:

Two subfloors have guides of their own because they carry the most risk. Concrete is all about moisture and drying time, covered in the laying laminate on concrete guide, where the underlay must include or sit over a damp proof membrane. Old floorboards need to be flat, screwed down and gap-free before any underlay goes on, so the board edges do not read through, which the floorboards guide works through step by step. If the subfloor itself is out of tolerance, no underlay saves it: level it first, as the mistakes guide spells out, because a floating floor over a dip flexes until the joints fail.

How thick should laminate underlay be?

Between 2 and 3 mm for almost every laminate floor, and thicker is a trap. It feels intuitive that a plusher underlay means a better floor, but laminate joints need firm support, not a soft bed:

• Under 2 mm gives too little cushioning, so the floor stays loud and the joints take more load than they should.
• 2 to 3 mm is the sweet spot: enough to quieten the floor and support the joints, firm enough that the boards do not move under foot.
• Over 3 mm of soft foam lets the boards flex into the give every time they are stepped on, which slowly works the click joints apart. This is the single most common underlay mistake.

Fibreboard is the deliberate exception: it comes in thicker panels of 4 to 7 mm but is dense and rigid, so it bridges a slightly uneven subfloor without the flex that soft foam would add at the same thickness. Firmness is what counts, not raw thickness, so never stack two underlays or use a thick soft one to make up for a subfloor that should have been levelled.

What is a tog rating, and does it matter for underfloor heating?

The tog rating measures how much the underlay resists heat flow, and it only matters when there is underfloor heating below. For a normal floor a higher tog adds a little warmth underfoot. Over underfloor heating the logic flips: you want the heat to reach the room, so you need a low-tog underlay, ideally under about 1.0 tog for the whole build-up of underlay plus boards.

If you have underfloor heating, also confirm the laminate itself is approved for it, keep the surface temperature under 27 degrees, and raise the heat gradually over several days after laying rather than switching straight to full output. On concrete you still keep the vapour barrier, so look for a low-tog combined underlay made specifically for heated screeds.

How do you fit laminate underlay?

Once the subfloor is flat and clean, the underlay itself takes minutes per room. The order matters more than the skill:

1. Check the subfloor is flat to about 3 mm over 1 m and swept clean. Level any dips with compound and let it dry before the underlay goes down.
2. Lay the vapour barrier first on concrete. A damp proof membrane across the whole slab, joints overlapped 200 mm and taped, edges turned up at the walls. Skip this if the underlay has a barrier built in.
3. Roll out in the board direction, starting at the wall where the first row will run, and cut each length to the room.
4. Butt the edges, never overlap them. Overlaps make ridges that telegraph through the boards. Tape the seams so they cannot creep apart as you work.
5. Only cover what you can floor that day, so the underlay stays flat and clean, and trim it to the wall leaving room for the boards' expansion gap.

The expansion gap belongs to the boards, not the underlay, and it is a separate rule worth getting right; the expansion gap guide covers the 8 to 12 mm you leave on every wall. With the underlay down, laying starts the same as any floor, so plan the starting wall and the last row before the first click using the where to start guide.

What does laminate underlay cost, and how much do you need?

Underlay is one of the cheaper parts of the job, and you buy it to the same area as the floor plus a little overlap. Standard foam runs a couple of pounds per square metre, combined barrier underlay a little more, and fibreboard or acoustic underlay more again, but even the dearest is a small share of a laminate floor's total. Buy the floor area plus roughly 5 percent for trimming and butting, and round up to whole rolls or packs.

Because the underlay matches the floor area, the same measurement does both jobs: measure the room, then run it through the laminate calculator to get the floor area and whole packs of boards, and buy the same square metres of underlay alongside. If you would rather see the layout, cuts and waste before you order, drop the room into the floor planner and it turns your measurements into a board count and a cut list you can take to the shop.