Subfloor Preparation: What It Involves and Why It Matters
What proper subfloor preparation involves before new flooring goes down, why it matters, and what our fitters check on survey day.
Almost every flooring failure we're called out to investigate — lifting seams, hollow-sounding laminate, carpet that ripples within months — traces back to a subfloor that wasn't properly prepared. It's the least glamorous part of any flooring job, and the part that determines whether the finished floor lasts ten years or ten months.
Subfloor preparation covers everything done to the existing base — concrete slab, sand-cement screed, or timber boards — before the new floorcovering is laid. That can mean levelling, cleaning, moisture testing, or replacing sections that have failed. Skipping it to save time is the single most common cause of expensive comebacks in our trade.
A survey visit isn't just a formality before quoting a price — it's the point where a fitter physically gets down on the floor, runs a straightedge across it, scrapes at suspicious patches with a bolster, and works out what the base is actually made of underneath any existing covering. Old houses in particular can hide surprises: a quarry tile floor under carpet, a step in level between an extension and the original house, or a patch of screed that was never properly compacted when it went in decades ago.
What counts as an unsuitable subfloor
Subfloors fail preparation checks for a handful of recurring reasons: uneven or dished areas beyond the tolerance of the new covering, loose or bouncy floorboards, old adhesive residue or paint that won't bond with new adhesive, and damp readings above the safe threshold for the chosen product.
- Dips or humps exceeding roughly 3mm over a 2m straightedge
- Cracked or friable screed that crumbles underfoot
- Old tile adhesive, carpet gripper pins or underlay fragments left in place
- Squeaking or springy timber boards indicating loose fixings
- Residual sheet vinyl or lino glued down so firmly that scraping it back risks damaging the base beneath
Concrete slabs versus timber suspended floors
Concrete and timber subfloors need different treatment. A concrete slab that's structurally sound but uneven is usually levelled with a self-smoothing compound, whereas a timber suspended floor is more often over-boarded with plywood to create a stable, flat working surface and to stop movement transferring into the new floor.
Timber floors also need checking for ventilation beneath — poor airflow under suspended timber is a common cause of damp and rot that has nothing to do with the flooring itself.
Air bricks blocked by patio paving, flower beds banked up against the wall, or insulation stuffed into the void can all starve a suspended floor of the airflow it was designed around, and a fitter who spots this during survey will flag it rather than simply boarding straight over the top.
Tools a fitter uses to assess the base
- A 2m aluminium straightedge and feeler gauges to measure dips and humps precisely rather than by eye
- A bolster and hammer to test for hollow-sounding or delaminating screed
- A moisture meter for a quick relative reading before deciding whether further testing is warranted
- A damp probe or torch to check ventilation gaps and skirting for signs of past water ingress
Why preparation is specified, not assumed
We don't quote preparation as a blanket add-on because every subfloor is different. A survey lets us measure levels, test moisture and inspect the base so the specification matches what's actually there, rather than a generic allowance that either overcharges you or under-prepares the floor.
What happens if preparation is skipped
- LVT and vinyl telegraph every bump and crack through to the surface over time
- Laminate and engineered wood locking joints can separate on uneven ground
- Carpet wears unevenly and grippers can lift if boards are loose
- Adhesive bond failure on porous or contaminated surfaces
How preparation is priced
Preparation costs vary hugely depending on the extent of the work — a light sand and vacuum is very different from screeding an entire floor. As a guide only, minor levelling compound work is typically priced per square metre, while full re-screeding or over-boarding is priced as a separate job element on your quote, itemised so you can see exactly what's included.
Sequencing preparation around the rest of the job
Preparation almost always has to happen in a set order: strip-out and disposal first, then structural repairs, then moisture checks, then levelling or boarding, and only then the new flooring itself. Trying to skip a step to save a visit — levelling over debris, for instance — tends to store up problems that surface weeks later rather than on the day.
Where more than one trade is involved, such as a plumber needing access under floorboards before they're overlaid, this sequencing needs agreeing well before fitting day so nobody is waiting on someone else halfway through the job.
Signs a previous floor was fitted onto poor preparation
- Skirting that sits proud of the floor with an obvious gap suggesting the floor level has dropped or shifted
- Faint ridging visible under LVT or vinyl that follows the line of old floorboard joints
- Carpet that has worn through in patches corresponding to high spots underneath
- A hollow, drumming sound when you knock on certain areas of a laminate or engineered floor
Frequently asked questions
- Can I prepare the subfloor myself before fitters arrive?
- You can clear the room and remove old floorcoverings if you're able to, which helps keep costs down. Levelling compound, moisture testing and structural checks are best left to the fitting team, since getting them wrong can affect the whole installation.
- Does every job need levelling compound?
- No. Many subfloors are flat enough for direct fitting, particularly for carpet. Levelling is only specified where our survey identifies dips, humps or an uneven surface that would show through or affect the new flooring.
- How long does subfloor preparation add to the job?
- Minor prep can often be done on the same day as fitting. Self-levelling compound needs curing time, usually a matter of hours before it can be walked on, but full re-screeds need longer to dry properly and are scheduled as a separate visit.
- Will you tell me if preparation is needed before I commit?
- Yes — our survey identifies any preparation work needed and it's itemised clearly on your quote, so there are no surprises once fitting starts.
Keep reading
How latex self-levelling compound works, when it's needed, and how it differs from full sand-cement screed before new flooring.
Why subfloor moisture testing matters before new flooring is laid, the methods used, and what happens if readings are too high.
A realistic timeline of what to expect on the day your new flooring is fitted, and how to prepare your home beforehand.
See how we survey, prepare and fit LVT across Kent.
See how we survey, prepare and fit carpet across Kent.
Talk it through with a fitter, not a call centre
Book a free, no-obligation survey with Kent Flooring Warehouse and our in-house fitting team will check your subfloor and confirm the right subfloor preparation before you commit to anything — call 07702 737267.