Subfloor Preparation and Levelling in London
The Foundation Every Floor Restoration Depends On
An uneven, unstable, or damp subfloor is responsible for more failed wood floor projects than almost any other factor. Problems such as creaking boards, cupping, excessive movement, gaps, and premature finish failure often originate beneath the floor rather than on its surface.
With more than 30 years of experience, we provide subfloor preparation and levelling in London for solid timber floors, engineered flooring systems, and concrete subfloors before sanding, refinishing, or new floor installation. Unlike screeding carried out for vinyl or LVT, wood floor preparation must account for joist condition, timber movement, structural stability, and moisture behaviour, not simply surface flatness.
We offer free surveys, professional moisture assessments, and structural preparation services from only £6/m².
Timber Subfloor vs Concrete Subfloor
Why the Levelling Method Is Completely Different
Not every uneven floor is corrected in the same way. Timber and concrete subfloors behave very differently under load, respond differently to moisture, and require completely different preparation techniques before wood flooring can be sanded or installed.
We assess the construction of the existing subfloor before recommending any levelling method. Choosing the wrong approach can shorten the lifespan of a timber floor, reduce adhesive performance, or fail to solve the underlying cause of unevenness altogether. Some common methods we use:
Levelling a Timber Subfloor or Existing Floorboards
Proper timber subfloor levelling in London relies on correcting the timber itself rather than covering it with levelling compounds. Suspended timber floors naturally move throughout the year, making self-levelling products unsuitable for most traditional joisted floors.
Minor unevenness is corrected by uneven floor sanding off high spots in London using a professional drum sander, beginning with coarse 24–40 grit abrasives before progressing through finer grits for a smooth finish. Boards that are excessively cupped, bowed, or twisted are assessed individually. Some can be re-levelled through sanding, while others require lifting, refitting, or replacement where distortion is too severe.
Levelling a Concrete Subfloor Before a Wood Floor Is Fitted
Preparing a concrete subfloor preparation wood floor in London project involves an entirely different process. Where deviations exceed approximately ±3mm over three metres, self-levelling compounds are used to create a suitable surface before timber flooring is installed.
Before any levelling begins, we carry out subfloor moisture testing on the wood floor using an insulated hygrometer or calcium chloride testing to confirm the slab is sufficiently dry. Where moisture readings remain too high, an appropriate damp-proof membrane is installed before levelling proceeds. Once cured, a plywood overlay may also be required depending on the flooring system and adhesive specification.
Plywood Overlay Specification for Wood Floors
Where the existing subfloor remains structurally sound but lacks the flatness required for timber flooring, a plywood overlay often provides the most reliable solution. Weather and boil-proof plywood, typically between 12mm and 18mm thick, is fixed using staggered joints and mechanical fixings at approximately 300mm centres. It creates a stable, uniform substrate for engineered or solid timber flooring while avoiding unnecessary structural alterations.
When the Problem Is Structural, Not Surface
Joist Deflection and Subfloor Instability
Surface unevenness is not always the root cause of an uneven floor. A floor that bounces underfoot, dips across the length of a room, or visibly sags when viewed along the skirting usually indicates a structural issue rather than a surface irregularity. Attempting cosmetic levelling without addressing the supporting structure simply masks the symptoms while allowing the movement to continue beneath the finished floor.
During every subfloor structural assessment, we investigate whether unevenness results from joist deflection, uneven floor in London, undersized joists, excessive service notching, historical water damage, or general structural movement. Where necessary, we recommend sagging floor joist repair in London before any levelling or flooring work proceeds. If structural concerns extend beyond the scope of flooring repairs, we advise consultation with a structural engineer before cosmetic work begins.
For localised board repairs, please see our Wood Floor Repairs service, while movement-related squeaks are covered in our Squeaky Wood Floor Repair service.
Moisture Testing Before Any Levelling or Floor Fitting Begins
Wood flooring is considerably more sensitive to moisture than vinyl, laminate, carpet, or ceramic tiles. Even when a subfloor appears dry, excessive moisture trapped beneath the surface can cause timber to expand, cup, lift, or break down adhesive bonds months after installation. For this reason, every project includes a thorough moisture assessment before levelling compounds, plywood overlays, or new flooring are specified.
Where we identify a damp subfloor wood floor, the moisture source must be addressed before any restoration or installation work proceeds. Concrete subfloors are tested using calibrated hygrometers or calcium chloride testing, while suspended timber floors are assessed at both surface and joist level. If readings exceed the manufacturer’s recommended limits, we specify an appropriate DPM wood floor subfloor solution before continuing with the programme.
Moisture Testing & Preparation Guide
| Subfloor Type | Assessment Method | What We Check | Recommended Action |
|---|---|---|---|
| Concrete Subfloor | Calcium chloride test or insulated hygrometer | Relative humidity, curing condition, and moisture migration | Wood flooring is generally installed only when readings fall below approximately 75% RH or the adhesive manufacturer's specified limit. |
| New Concrete Slab | Moisture and curing assessment | Drying progress through slab depth | New concrete typically dries at around 1mm per day initially, with thicker slabs often requiring 10 weeks or longer before timber flooring can safely be installed. |
| Suspended Timber Subfloor | Surface and joist moisture readings | Moisture within floorboards, joists, and subfloor void | We inspect ventilation, rising damp, timber condition, and hidden moisture before any restoration work begins. |
| High Moisture Readings | Confirmation testing | Source of damp and moisture movement | Installation is delayed until the cause is resolved, after which a suitable liquid or sheet damp-proof membrane may be specified where appropriate. |
| Before Levelling or Installation | Written assessment | Moisture compliance for the flooring system | Every recommendation is documented before the final programme is agreed with the client. |
Suspended timber subfloors that move seasonally are also the most common cause of wide gapping between boards, see our gap filling with timber strips service → for the correct treatment of seasonal gaps once subfloor moisture is confirmed stable.
Subfloor Preparation and Levelling Cost in London
The average subfloor preparation cost in London ranges from £6 to £40 per m², although some services, such as moisture testing or board repairs, are priced separately. The final cost depends on the type of subfloor (timber or concrete), the extent of levelling required, moisture levels, structural condition, materials required, and whether additional work, such as plywood overlays, damp-proof membranes, or joist repairs, is necessary before the floor can be sanded or new timber flooring installed.
Timber subfloor – sanding high spots & structural preparation
Sanding uneven boards, structural assessment, localised levelling, and preparation ready for sanding or flooring installation.
Board lifting and refitting (distorted boards)
Careful lifting, correction of distorted boards, refitting, fixing, and levelling with surrounding flooring.
Concrete subfloor – self-levelling compound
Supply and installation of latex/self-levelling compound at standard depth following moisture testing and preparation.
Plywood overlay supplied and fitted
WBP plywood, staggered fixing pattern, mechanical fixings, and preparation suitable for engineered or solid wood flooring.
Liquid or sheet damp-proof membrane
Supply and installation of an appropriate damp-proof membrane before timber flooring installation where moisture levels require protection.
Moisture testing with written assessment
Hygrometer or moisture testing, structural observations, and a written report outlining preparation recommendations.
Actual costs vary depending on floor condition, moisture readings, structural repairs, access, and the specification required for the chosen flooring system. Following our survey, we provide a detailed quotation outlining every stage of the preparation work before installation begins.
For period properties where subfloor condition is frequently linked to original Victorian or Edwardian construction, see our Victorian period floorboard restoration service → For a complete breakdown of restoration and finishing costs, see our full wood floor price guide →
Our Recent Work
Our Other Services
Wood Floor Sand & Seal
Varnish Treatment
Floor Finishes and Seal
Stair Stripping and Sanding
Commercial Floor Sanding
Hardwood Floor Sanding
Maintenance and Care
Parquet Floor Sanding
See What Our Happy Clients Have to Say!
Sarah
Maidstone, Kent
★★★★★
"I recently hired Sanding Wood Floors to restore the original floorboards in my Maidstone home, and I couldn't be happier with the results! I highly recommend their services!"
James
Tunbridge Wells, Kent
★★★★★
"As a homeowner in Tunbridge Wells,kent I was amazed by the skill and efficiency of the Sanding Wood Floors team. They brought my worn-out wooden floors back to life, leaving them looking brand new."
Olivia
Ramsgate, Kent
★★★★★
"I'm extremely satisfied with the results. The finished parquet floors are absolutely beautiful and have added so much value to my home. I highly recommend them to anyone in need of wood floor restoration."
Frequently Asked Questions
Can an uneven timber floor be levelled without lifting all the boards?
Yes, in many cases. Minor unevenness caused by normal wear, slight cupping, or raised board edges can often be corrected by careful sanding of the high spots. Where only a few boards are distorted, they can usually be lifted and refitted individually rather than removing the entire floor. A survey determines whether sanding alone is sufficient or whether structural repairs are required.
Do I need a damp-proof membrane before fitting a wood floor on concrete?
Not always, but every concrete subfloor should be moisture tested before timber flooring is installed. If moisture readings exceed the flooring or adhesive manufacturer’s recommended limits, a suitable liquid or sheet damp-proof membrane is installed to prevent moisture from migrating into the timber. Skipping this step can lead to cupping, adhesive failure, and premature floor movement.
My floor bounces or feels springy. Is that a subfloor levelling problem?
Usually not. A springy or bouncing floor typically indicates movement within the supporting joists rather than an uneven surface. Levelling compounds or sanding will not eliminate this movement because the underlying structure continues to flex. The joists must be inspected first to determine whether repairs or strengthening are needed before cosmetic work is carried out.
Is plywood or self-levelling compound the right choice for my subfloor?
The correct solution depends on the existing floor construction. Concrete subfloors often require a self-levelling compound to achieve the required flatness before timber flooring is installed. Structurally sound timber floors are more commonly improved using a plywood overlay, while suspended timber floors generally rely on board repairs and sanding rather than levelling compounds. We recommend the most suitable method following a site inspection.
How much does subfloor preparation and levelling cost in London?
Cost varies depending on whether the subfloor is timber or concrete, the amount of levelling required, moisture conditions, and whether structural repairs are necessary. Timber subfloor sanding and structural preparation: from £6–£12/m². Concrete self-levelling compound: from £18–£30/m². Plywood overlay: from £22–£40/m². A typical 25m² room on a timber subfloor: £220–£440. The same room on concrete with a plywood overlay: £625–£950. Following a detailed survey and moisture assessment, we provide a written quotation outlining every stage of the preparation work.