How to Lay Engineered Hardwood Flooring Over an Uneven Subfloor

There's a particular kind of frustration that comes with laying engineered hardwood flooring. You've spent good money on the boards, you've watched the YouTube tutorials, and then you kneel down and realise the subfloor isn't remotely level. It undulates. There are humps near the door, a dip by the radiator, and a suspicious creak under the carpet you've just pulled up. This is the reality of most British homes, particularly anything built before 1960.

The good news is that an uneven subfloor doesn't have to derail your project. With the right approach, you can create a perfectly flat surface to install over — and the finished floor will look as though a professional did it. Here's how.

Understanding Your Subfloor

Before you can fix an uneven subfloor, you need to understand what you're dealing with. In UK homes, you'll typically encounter one of three types:

• Concrete slab — common in post-war houses and extensions. Often uneven due to settling or poor original pouring.
• Suspended timber boards — found in Victorian and Edwardian properties. Boards may have shifted, cupped, or be suffering from rot.
• Chipboard or plywood sheeting — common in 1970s–90s construction. Often the quickest to level if in good condition.

Use a long spirit level (at least 1.8m) or a straightedge to identify high and low spots. The general rule for engineered flooring is that the subfloor should be flat to within 3mm over any 1.8m radius. Anything more than that and you'll get movement, squeaks, and potentially gapping in the installed floor.

Levelling a Concrete Subfloor

For concrete, self-levelling compound is your best friend. Products such as Mapei Ultraplan, Ardex K15, or Tilemaster Rapid Prime are widely available from Screwfix, B&Q, or Travis Perkins and will do an excellent job. Here's the process:

1. Clean thoroughly. Remove all debris, old adhesive, and loose material. Vacuum up the dust. Any contamination will prevent the compound from bonding.
2. Prime the surface. Most self-levelling compounds require a primer to ensure adhesion and control absorbency. Apply by roller and allow to dry fully — typically 30–60 minutes.
3. Mix the compound. Add the powder to water (not the other way round) according to the manufacturer's ratio. Mix with a drill and paddle attachment until smooth and lump-free, then allow to rest for 2 minutes before using.
4. Pour and spread. Pour from the furthest point and work back toward the door. Use a gauging trowel or smoother to guide the compound into low spots. It will find its own level to a degree, but you'll need to help it in corners.
5. Allow to cure. Most compounds are walkable in 2–4 hours but shouldn't receive flooring for 24–48 hours. Check the packaging. In winter or in poorly ventilated rooms, curing takes longer.

For high spots on concrete, you have two options: grind them down with an angle grinder fitted with a diamond cup wheel (hire from HSS or Speedy Hire for around £40–60 per day), or feather them out with the levelling compound applied at a slight angle.

How Much Compound Do You Need?

A 25kg bag of self-levelling compound typically covers about 3.5m² at 5mm depth, or 7m² at 2.5mm. For a standard 15m² living room with an average 3mm unevenness, budget for 3–4 bags. At roughly £15–20 per bag from Screwfix, that's £45–80 in materials — a worthwhile investment before laying expensive flooring.

Levelling a Timber Subfloor

Timber subfloors require a different approach. The main issues you'll face are:

• Springy, bouncy boards — usually loose or inadequately fixed to joists. Fix by screwing into joists with 50mm decking screws, pre-drilling to avoid splitting.
• Cupped or warped boards — boards that have absorbed moisture and bowed. If the cupping is severe, they may need to be replaced. Minor cupping can sometimes be corrected by screwing down.
• High spots at board edges — use a hand plane or belt sander to bring these flush. Mark the high spots with chalk first so you know where to work.
• Low spots and gaps between boards — fill with floor-levelling compound or a flexible floor filler such as Everbuild Floor Leveller.

Once the boards are secure and any major unevenness is addressed, overlay with 6mm or 9mm exterior-grade plywood, screwed at 200mm intervals. This creates a new, flat, stable surface. It will raise the floor height by 6–9mm, so check this doesn't cause issues with door clearances or thresholds before you commit.

Choosing the Right Installation Method

Engineered hardwood can be installed in three ways over a levelled subfloor:

• Floating — boards clip together with no adhesive to the subfloor. Fast, DIY-friendly, and the boards can move slightly with temperature and humidity changes. Suitable for most domestic applications. Requires underlay (use one with good sound-dampening properties over concrete, and a suitable damp-proof membrane in ground-floor rooms).
• Glue-down — boards bonded directly to the subfloor with wood flooring adhesive. More stable, better for underfloor heating, but harder to remove later. Products like Bona R848 or Mapei Ultrabond are widely used by UK flooring contractors.
• Secret nailing — used over timber subfloors, nails are driven at 45° through the tongue of each board. Requires a timber subfloor and either a flooring nailer (hire from HSS) or a lot of patience with a hammer and nail punch.

For most DIYers in the UK working over a levelled concrete slab, the floating method is the most practical choice. Over timber, floating or secret nailing both work well.

Acclimatisation: Don't Skip This Step

Engineered hardwood is more dimensionally stable than solid wood, but it still needs to acclimatise to the room conditions before installation. Leave the sealed packs flat in the room (not standing upright) for at least 48 hours — 72 hours is better. The room should be at normal living temperature and humidity. Never store the boards in a garage, unheated outbuilding, or next to a radiator.

The Installation Process

1. Lay the first row with the tongue facing into the room and leave an expansion gap of 10–12mm around all perimeter walls, pipes, and fixed obstacles. Use expansion spacers to maintain this gap.
2. Stagger the joints — the end joints of adjacent rows should be at least 300mm apart. A random stagger of 40–60% board length looks most natural.
3. Cut boards with a mitre saw for cross cuts and a jigsaw for curves around pipes. Wear a dust mask — sanding dust from engineered boards is a health risk.
4. Knock boards together firmly with a tapping block (never hit the tongue directly) and a rubber mallet.
5. The last row will almost certainly need cutting lengthways. Measure carefully — the gap varies if the wall isn't perfectly straight.
6. Cover all expansion gaps with skirting boards, beading, or threshold strips. These should be pinned or glued to the wall, never to the floor itself.

Key Mistakes to Avoid

• Not checking for damp before starting — use a damp meter on concrete subfloors. Readings above 75% RH require a separate damp-proof membrane even with a floating floor.
• Skimping on levelling — a floor that squeaks or flexes was usually installed over an inadequate subfloor.
• Not leaving expansion gaps — engineered boards will expand and contract. A floor with no gap will buckle in warm weather.
• Rushing the self-levelling compound — if it isn't properly cured, it'll dust and crumble under the flooring.

Take your time with the prep and the installation itself becomes surprisingly straightforward. A 20m² room can reasonably be completed in a weekend once the subfloor is sorted — and a well-laid engineered hardwood floor will look outstanding for decades.