How to Level a Subfloor for New Flooring

How to Level a New Sub-Floor for New Flooring Such as LVT Flooring?

A beautiful new floor can still feel disappointing if it follows every hollow, ridge and loose board beneath it. Knowing how to level a subfloor before fitting your chosen surface is what helps laminate click together properly, keeps LVT looking smooth and gives carpet a more secure, comfortable finish. It is also the point at which small fitting issues can be dealt with before they become costly ones.

The right method depends on what the subfloor is made from, how uneven it is and the flooring you plan to install. A shallow dip in a concrete floor needs a different solution from a squeaky timber floor with raised joints. Start with the condition below, rather than choosing a levelling product first.

Why a level subfloor matters

Most floor coverings need a base that is flat, sound, clean and dry. Flat does not always mean perfectly level from one side of the room to the other. Older properties, in particular, may have a gradual fall that has been there for decades without causing a problem. What matters most is that there are no local high spots, dips or abrupt changes that affect the finished floor.

Floating floors such as laminate, click LVT and engineered wood are especially sensitive to uneven areas. Unsupported sections can flex underfoot, placing stress on click joints and causing movement, gaps or broken locking edges over time. Glue-down LVT can telegraph imperfections through the surface, while sheet vinyl is particularly unforgiving of rough patches and poorly filled joints.

Always check the installation guidance for your specific product. Manufacturers set their own flatness tolerances, often measured over a two-metre straightedge. If your floor is outside that allowance, levelling the subfloor is part of preparing for a lasting installation, not an optional extra.

Check the subfloor before you level it

Clear the room completely and remove the existing floor covering, underlay, adhesive residue and loose debris. Vacuum thoroughly. A levelling compound will not bond reliably to dusty concrete, flaky paint or traces of old adhesive, and a plywood overlay cannot cure movement in loose floorboards.

Use a long straightedge or a two-metre level across the room in several directions. Mark high areas with a pencil or chalk, then identify low areas by measuring the gap beneath the straightedge. Pay close attention to doorways, the centre of the room, old hearth locations and where different floor finishes meet.

Before choosing a repair method, establish whether the issue is cosmetic or structural. Investigate cracks that are wide, spreading or uneven on either side. Check timber floors for damp, rot, woodworm, loose boards and excessive bounce. Levelling over an active problem simply hides it temporarily.

You will usually need a straightedge, spirit level, tape measure and vacuum, plus suitable personal protective equipment. For larger repairs, keep a mixing bucket, drill paddle, trowel and smoothing tool ready. Accurate preparation is quicker than trying to correct a rushed pour once it has started to set.

How to level a concrete subfloor

Concrete is commonly levelled using a compatible smoothing or self-levelling compound. Despite the name, these products still need to be mixed correctly, poured in a controlled way and guided into place. They flow more easily than traditional mortar, but they do not automatically fix every deep dip or poorly prepared edge.

Deal with high spots first

Grinding down proud ridges and removing lumps of old screed can reduce the amount of levelling compound required. This saves material and helps avoid raising the finished floor height unnecessarily. Be cautious around underfloor heating pipes, and use suitable dust control when mechanically abrading concrete.

Prepare, prime and fill

Repair cracks and holes with an appropriate repair product before levelling. Then apply the primer recommended for both the subfloor and the smoothing compound. Primer improves adhesion and controls how quickly a porous surface draws moisture from the mix.

For deeper hollows, check the product's stated minimum and maximum thickness. Some compounds are designed for thin feathering, while others can be laid deeper or bulk-filled. Applying a thin-only product too deeply can lead to cracking or weak spots. If a substantial build-up is needed, a screed or a product specifically designed for deeper application may be the better choice.

Mix only the amount you can spread within the working time. Pour from the lowest point where practical, use a trowel or smoothing rake to encourage the compound into corners, and maintain a wet edge between mixes. Do not keep adding water as the compound begins to stiffen, as this compromises its performance.

Allow the floor to cure for the period stated by the manufacturer before laying flooring. Drying time varies with depth, room temperature and humidity. A surface that feels dry may not yet be ready for moisture-sensitive wood or adhesive-based installations.

How to level a timber subfloor

Timber subfloors require a little more investigation because movement is often the real issue. Start by securing loose boards or sheets to the joists with suitable screws. Replace damaged sections, sink protruding fixings and plane or sand high board edges where necessary. If the floor flexes significantly, assess the joists and supporting structure before progressing.

Choose the right levelling approach

A thin, localised dip may be corrected with a flexible, fibre-reinforced smoothing compound approved for timber substrates. The floor must be clean, stable and primed exactly as the product instructions require. In many cases, joints and gaps need reinforcing or filling first to stop compound escaping below or cracking as the timber moves.

For widespread minor unevenness, flooring-grade plywood can provide a smoother, more consistent surface. It should be acclimatised where required, laid with staggered joints and securely fixed at appropriate intervals. Keep in mind that plywood raises the floor level, which can affect doors, skirting boards, thresholds, stairs and kitchen plinths.

Plywood is not a solution for a bouncy or damp floor. Nor should it be used to bridge major voids without correcting the cause beneath. Where floorboards are badly uneven, lifting and repairing them, packing joists correctly or replacing damaged sections may be more reliable than building up layers above them.

Do not confuse levelling with moisture protection

A damp-proof membrane and a levelling compound perform different jobs. A liquid membrane can control residual moisture where it is approved to do so, but it will not flatten a floor. Equally, a beautifully smooth levelling layer does not guarantee that a concrete subfloor is dry enough for engineered wood, glue-down LVT or other moisture-sensitive finishes.

Follow the flooring and adhesive manufacturer's moisture requirements, particularly on ground floors, new concrete and areas with a history of damp. In a kitchen, utility room or bathroom, choose flooring, underlay and installation products that suit the room's expected moisture exposure. Waterproof surface flooring does not remove the need for a sound, dry base.

Check finished heights before fitting

Levelling can add anything from a few millimetres to much more, so plan the full floor build-up before you begin. Include the finished flooring, underlay where applicable, adhesive, levelling layer and any membrane. Compare this with adjoining rooms and door clearances.

This is especially useful when choosing between laminate, LVT, engineered wood and carpet. A thin glue-down LVT may suit a room where door height is tight, while a click floor with built-in underlay can simplify installation but still needs a subfloor within tolerance. Matching trims, thresholds and stair profiles help create a neat transition once the new floor is down.

Common subfloor levelling mistakes

The most expensive mistakes usually happen before the compound is mixed. Avoid laying over loose boards, skipping primer, guessing the depth of dips or assuming underlay will hide unevenness. Standard underlay can improve comfort and reduce sound, but it is not designed to fill hollows or support a floating floor over significant low points.

It is also tempting to pour levelling compound only where the floor looks low. A straightedge may reveal that the surrounding area needs feathering too, otherwise you create a new ridge at the repair edge. Take time to recheck the floor once the repair has cured and before opening flooring packs.

For large rooms, extensive damage, suspected damp or a floor that continues to move, professional assessment is often money well spent. The cost of correcting the base is usually far less than replacing a newly fitted floor that fails early.

When planning your flooring order, include the preparation materials as part of the job rather than an afterthought. Floor Land can help you select floor coverings alongside compatible underlay, adhesives, trims and levelling products, so the installation is considered from subfloor to finished edge.

A level subfloor may never be seen once the room is complete, but it is what lets the colour, texture and finish of your new flooring do the talking for years to come.