The subfloor is the unsung hero of any new flooring installation. Get it right and your new floor looks great and lasts years. Get it wrong and the most expensive carpet or LVT in the world will creak, bubble, crack at the joints, or wear unevenly within months. It's the step most DIYers underestimate — and the first thing a professional fitter will check before putting anything down.
The subfloor is whatever surface your new flooring will sit on — usually either a concrete screed (solid floor) or a timber floor (floorboards or plywood). Each has different preparation requirements.
Almost every flooring manufacturer specifies the same tolerance: the subfloor must be flat to within 3mm over any 1.8m span. That's not perfectly level (level means horizontal) — it means no sudden lumps, dips, or ridges within that distance. A floor can slope gently across a room and still be fine; what causes problems is a localised bump or hollow that the flooring has to bridge.
A good way to check: lay a 1.8m straight edge (a long spirit level or a straight piece of timber) across the floor and look for gaps underneath. Anything more than 3mm needs addressing.
Concrete can harbour moisture, which destroys most flooring types from underneath. Before doing anything else, carry out a simple damp test: tape a piece of polythene sheeting (about 500mm square) to the concrete, seal all four edges with tape, and leave it for 24–48 hours. If moisture collects on the underside, you have a damp problem that needs a damp proof membrane (DPM) before any flooring goes down.
⚠️ Never lay vinyl, LVT or laminate over a damp concrete floor without a DPM. Moisture will cause the flooring to lift, bubble, or grow mould underneath — and it voids most manufacturer warranties.
Small dips and hollows in concrete are filled with a self-levelling compound (also called floor leveller or smoothing compound). You mix it to a pourable consistency and it flows into low spots, levels itself out, and dries hard. Most compounds are walkable within 2–4 hours and ready to floor over within 24 hours. For deep dips (more than about 5mm), apply in two thinner coats rather than one deep pour.
Lumps and ridges are harder to deal with than dips. A floor grinder or angle grinder with a grinding disc is the right tool — don't try to fill over a high spot. For very minor ridges (nail heads from old gripper rods, etc.), a bolster chisel and club hammer will knock them flat.
Walk the floor and note any boards that move, spring, or squeak. These need to be fixed before you lay anything over them — squeaks don't go away with a new floor on top, they just become more annoying. Screwing down loose boards (use screws, not nails) usually solves this. Check the floor joists beneath while you're at it — any signs of rot or woodworm need addressing properly.
Any raised nail heads need to be punched below the surface with a nail punch. Raised nails will telegraph through vinyl and LVT, creating visible bumps that will wear through quickly.
Old floorboards often have gaps between them. For carpet, small gaps are usually fine — the underlay bridges them. For vinyl, LVT or laminate, gaps wider than about 3mm should be filled with flexible floor filler or covered with a layer of 6mm plywood before laying.
For any hard flooring (LVT, vinyl, laminate, engineered wood) going over timber, the best practice is to lay a 6mm tongue-and-groove plywood overlay first. This creates a smooth, stable surface that eliminates any flex or movement in the boards underneath. It adds a little height but it's the most reliable preparation — and most professional fitters will recommend it.
The most forgiving flooring type for subfloor prep. Minor imperfections are masked by the underlay. Concrete: check for damp. Timber: fix loose boards, punch nail heads. Gripper rods are nailed or glued around the perimeter — the fitter will bring and fit these.
Needs a flat, smooth surface — any imperfection will show through over time. Concrete: fill all hollows, DPM if any damp. Timber: plywood overlay strongly recommended. No gaps, no raised nails, no grit.
The least forgiving of hard flooring types — lumps and dips telegraph through quickly. Must be flat to 3mm/1.8m. Concrete: DPM + levelling compound. Timber: 6mm plywood overlay. Glue-down LVT requires a clean, dust-free, grease-free surface — any contamination prevents the adhesive from bonding.
Floats over the subfloor on underlay so it's slightly more forgiving than glue-down LVT, but still needs a reasonably flat surface. Any movement in the subfloor below will cause the click joints to work loose over time. Concrete: check for damp (use a moisture barrier underlay). Timber: fix all squeaks and movement first.
Similar to laminate for floating installations. For glue-down engineered wood, the requirements are as strict as glue-down LVT. The wood needs to acclimatise in the room for 48 hours before fitting regardless of subfloor type.
Sometimes — but there are important caveats. Laying laminate or LVT over existing vinyl or tiles is often fine, provided the existing floor is firmly fixed, flat, and not damp. The extra height can cause problems at doorways and threshold strips, so check clearances first.
You should never lay new flooring over flooring that is already lifting, hollow, or soft. And if the existing floor contains vinyl tiles from before 1980, get them tested for asbestos before disturbing them — this is serious and the testing is cheap.
💡 If in doubt, strip it back. Removing old flooring adds time and cost, but a new floor on a properly prepared subfloor will last decades. A new floor over a dodgy old one often doesn't make it past the warranty period.
When we come to measure your rooms in Hastings, St Leonards or Bexhill, we'll check the subfloor condition and flag anything that needs attention before we give you a quote. Our fitters carry levelling compound, plywood, and all the tools needed to prepare most subfloors as part of the fitting job — you don't need to sort it separately.
Book a free home measuring visit and we'll check it for you at the same time. No charge, no obligation.
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