Laminate on an uneven floor: how flat is flat enough, and how to fix it

You can lay laminate on an uneven floor only when the unevenness stays within about 3 mm over 1 m. Beyond that, the floor needs levelling first, because there is no shortcut: underlay does not fill dips, and boards laid over a hollow flex until the click joints work loose. That is why a bad subfloor sits at number 1 on our list of laminate mistakes, and it is the one mistake you cannot fix without lifting the whole floor. The check itself costs nothing, and the fix is usually a bag of compound or an afternoon with a sander. The same rule and the same fixes apply to vinyl plank and engineered wood, since every floating floor shares the weakness.

How flat does a floor need to be for laminate?

Flat to roughly 3 mm of deviation over 1 m, which is the tolerance most manufacturers print in the fitting instructions and the point past which warranties quietly stop applying. Flat is the word that matters: a floor can slope gently and still be fine, because a consistent slope does not make boards bridge or rock. Checking takes 10 minutes:

• Use a straight edge and a torch. A long spirit level or a known straight board, laid across the floor in several directions, with a light behind it. Light under the edge marks a dip.
• Rock test the high spots. Where the straight edge pivots like a seesaw, you have a ridge. High spots cause rocking boards, which feel worse underfoot than dips do.
• Measure the worst gap. Slide a rule or a stack of spacers into the gap under the edge. Under 3 mm, carry on. Over it, mark the spot with chalk and keep mapping.
• Check the predictable places. Concrete dips mid-room and ridges where pours met; floorboards cup along their edges and drop where joists have settled; door thresholds and old hearth positions fail more than anywhere else.

What happens if you lay laminate over the dips anyway?

The floor fails slowly, and it starts the day you finish laying. A laminate board is stiff, so it bridges a hollow rather than following it, and every step on the bridge flexes the click joints at each end. Those joints are a few millimetres of pressed fibreboard, and they are the only thing holding the floor together:

Gaps from an uneven subfloor are the stubborn kind: unlike the seasonal sort covered in the gaps opening up guide, they cannot be tapped closed for long, because the cause is still under the floor flexing the joint back open.

How do you level a concrete floor for laminate?

With self levelling compound for the dips and a grinder or bolster chisel for the high spots, in that order of discovery but the opposite order of work. Knock the high spots down first, because they set the level the compound has to meet:

1. Grind or chisel the ridges where pours met and around old fixings, then vacuum the slab clean. Skim coats of old adhesive count as high spots too.
2. Prime the slab with the compound manufacturer's primer so the pour cures evenly instead of losing its water into the concrete.
3. Mix and pour the compound into the marked dips and feather the edges flat. A 20 kg bag covers roughly 4 square metres at 3 mm depth, so one bag handles the low spots in a typical room.
4. Let it set overnight, then let it dry. Walkable the next morning, but a fresh pour holds moisture, so it needs a few days and then the same damp test as the slab before anything goes over it.

The rest of the concrete rules, the moisture testing and the damp proof membrane, are their own subject and the difference between a flat floor and a dry one: the concrete guide covers them in full.

How do you level a timber floor for laminate?

Mostly with screws and a sander, because timber floors are rarely uneven in the way concrete is. Floorboards fail as ridges at cupped edges, springy loose boards and settled joists, and each has its own fix:

Standard self levelling compound does not belong on bare floorboards, since it cracks as the boards move; the plywood overlay is the timber equivalent and costs about 17 mm of height. The full prep sequence, including the moisture checks and which way the new floor should run, is in the floorboards guide.

Can underlay hide an uneven floor?

No. This is the most common way people try to buy their way past the straight edge test, and it makes the floor worse rather than better:

• Underlay compresses. At 2 to 3 mm thick it squashes flat under a board, so it cannot fill a dip; the board still bridges, it just bridges on a softer bed that lets it flex further.
• Thicker underlay means more flex, not less. Anything much past 3 mm of soft foam lets the boards move enough to work the click joints loose on a floor that was flat to begin with.
• Rigid fibreboard is the one partial exception. It is dense enough to bridge slight unevenness, a touch under the 3 mm line, but it is a finish for a nearly flat floor, not a cure for a failed one.

The underlay guide compares the types in full. The short version: level the floor, then buy a normal 2 to 3 mm underlay, rather than paying extra for thickness the floor will punish you for.

What if the laminate is already down on an uneven floor?

Then the floor has probably already told you, and the honest answer depends on what it said. A click floor comes apart the way it went together, which makes this recoverable without buying a new floor, if the boards' joints have not been ground down by months of flexing:

1. A soft spot with no gaps yet is a warning, not yet damage. It will not improve on its own, but you can schedule the fix rather than doing it tonight.
2. Gaps or creaks over one area near a wall: lift the beading or skirting, disassemble rows back from the nearest wall to the fault, level the spot with compound or sanding, and relay the same boards. An afternoon, not a weekend.
3. Faults in the middle of the room mean lifting everything back to that point. Tedious, but still cheaper than the alternative, which is the joints failing one row at a time in the same direction.
4. Boards with crushed or worn locking edges need replacing rather than relaying, which is one more reason spare boards from the original batch are worth keeping.

Whichever floor you are dealing with, planned or laid, the subfloor check belongs at the start: 10 minutes with a straight edge against a weekend of relaying. Once the floor passes, measure up and plan the layout properly. The laminate calculator turns the room size into whole packs with a waste allowance, and the layout planner gives you the full layout and cut list before you buy, with the expansion gap rules already built into the plan. Level floors are not glamorous, but they are the only kind laminate stays flat on.