Suspended timber subfloors can provide an excellent base for engineered and solid wood flooring, but they must be structurally sound, dry, flat and free from unacceptable movement before the new floor is installed.
The correct preparation depends on what is actually beneath you. Traditional floorboards, structural chipboard, plywood and floating acoustic chipboard decks all behave differently and should not automatically receive the same treatment.
This guide explains how we assess and prepare these floors professionally, including fixing loose boards, removing high spots, overboarding with flooring-grade plywood, using flexible smoothing compounds, floating engineered flooring and fully bonding timber floors.
What Is a Suspended Timber Subfloor?
A traditional suspended timber ground floor normally consists of timber floorboards or structural panels fixed over timber joists, with a ventilated void beneath.
Modern upper floors frequently use tongue-and-groove structural chipboard instead of individual boards.
You may also encounter acoustic or thermal constructions where tongue-and-groove chipboard forms a floating deck over resilient insulation.
Although all are timber-based, the preparation method can be very different.
Start by Checking the Structure
Do not begin by covering movement with plywood or levelling compound.
Check the existing floor for:
- Loose or squeaking boards.
- Broken or split floorboards.
- Movement at chipboard joints.
- Excessive deflection between joists.
- Unsupported board edges.
- Protruding nails or screws.
- Rot or moisture damage.
- Large localised high points.
- Rocking or unstable sections.
Check What Is Under the Floor Before Screwing
Traditional timber floors often contain heating pipes, water services and electrical cables immediately beneath the boards.
Never blindly drive long screws through an existing floor without establishing where services run.
Where the construction is accessible, inspect beneath the floor or carefully lift a board. Electronic cable and pipe detectors can also help, while a small inspection opening and borescope can be useful where tongue-and-groove chipboard prevents easy access.
Ground-Floor Suspended Timber Needs Ventilation
Suspended ground-floor timbers rely on ventilation beneath the floor to control moisture.
Before covering an older floor, check for obvious dampness, decay or blocked external air bricks.
Do not block the subfloor ventilation path with insulation, debris or alterations to the floor construction.
Preparing Traditional Floorboards
Traditional floorboards are common in older UK properties and can make an excellent subfloor once properly stabilised and flattened.
Secure Loose Boards
Refix loose boards wherever the joist position and services beneath can be safely identified.
Punch Down Old Nails
Any proud nail heads should be safely driven below the timber surface before sanding or planing begins.
Repair Damaged Boards
Replace broken, rotten or badly split sections rather than expecting the new flooring to bridge them.
Check Flatness
Use a long straightedge across several directions and identify high boards, ridges and hollows.
Remove High Points
Sand or carefully plane high areas until the subfloor falls within the flooring manufacturer's required flatness tolerance.
Vacuum Thoroughly
Remove dust and debris before underlay, plywood or adhesive is installed.
Which Screws Should Be Used for Loose Floorboards?
Purpose-designed flooring screws are preferable to repeatedly driving ordinary screws into an old floor.
Tongue-Tite screws provide a discreet fixing for suitable tongue-and-groove timber, while Floor-Tite anti-creep flooring screws are designed for fixing floorboards, chipboard, OSB and structural panels to timber joists.
Tongue-Tite Plus stainless-steel screws provide a corrosion-resistant option where that specification is appropriate.
Should Old Floorboards Be Sanded Flat?
Often, yes.
Our normal professional approach is to secure the boards first and then run suitable floor-sanding equipment across the subfloor to remove raised edges and high spots.
The objective is not to make an old house perfectly horizontal.
It is to create a sufficiently flat surface for the installation system.
For smaller DIY projects, a suitable belt or orbital sander may be practical. A planer can also remove a significant high spot where it can be used safely.
Can Engineered Flooring Be Floated Directly Over Floorboards?
Yes, where the engineered flooring manufacturer permits floating installation and the existing boards are sound, flat, dry and secure.
This is often the simplest route for smaller domestic rooms.
Floating Installation
Lay the specified wood-flooring underlay over the prepared timber subfloor and join the engineered planks according to the flooring manufacturer's instructions.
Fully Bonded Installation
Adhesive fixes the new flooring across its complete underside. For traditional floorboards, our professional preference is normally to create a suitable plywood receiving surface first rather than bond directly to individual moving boards.
Duralay Timbermate Excel and Duralay Silentfloor Gold are both suitable underlays for compatible engineered floating-floor installations over suspended timber substrates.
Do Floating Floors Have a Maximum Room Size?
There is no single universal maximum area that applies to every engineered floor.
Flooring manufacturers specify their own requirements for:
- Maximum continuous floor dimensions.
- Expansion gaps.
- Expansion breaks.
- Doorways.
- Large open-plan spaces.
- Heavy fixed furniture or kitchen islands.
Floating Tongue-and-Groove Engineered Flooring
Where a conventional tongue-and-groove engineered plank is approved for floating installation, a suitable D3 PVAc adhesive may be required through the joints.
Lecol PVAC Tongue & Groove Adhesive is designed specifically for this type of floating installation.
It is not a full-surface wood-floor adhesive.
Herringbone & Chevron Are Different
Do not assume that because an engineered plank from a manufacturer can be floated, the manufacturer's herringbone or chevron can also be floated.
Patterned engineered floors are commonly specified for full bonding.
When Should Floorboards Be Overboarded with Plywood?
Plywood is particularly useful where:
- The floorboards remain too uneven after practical sanding.
- A smoothing compound is required.
- A fully bonded engineered or solid floor is planned.
- The existing boards contain too many joints or local irregularities for reliable adhesive transfer.
- A more continuous receiving surface is required.
The plywood does not repair failed joists or rotten floorboards. Those problems must be dealt with first.
What Type of Plywood Should Be Used?
Use genuine flooring-grade plywood rather than an unidentified general-purpose sheet.
A useful professional benchmark is plywood manufactured to the requirements used for flooring subfloor preparation, including BS 8203 Annex A, EN 636-3 classification and a Class 3 glue bond.
For a typical overlay over existing sound floorboards, approximately 5.5–6mm flooring plywood is commonly used where that thickness is sufficient for the construction.
How Should Flooring Plywood Be Fixed?
Lay the sheets so joints are staggered and avoid lining the plywood joints directly over obvious joints within the floorboards where practical.
Industry flooring-plywood guidance typically uses close fixing centres, with approximately:
- Maximum 100mm centres around the perimeter of each sheet.
- Maximum 150mm centres through the body of the sheet.
All screw heads must finish flush or slightly below the plywood surface.
Screws, Coil Nails or Staples?
For general plywood overlay work, suitable countersunk flooring screws or correctly specified ring-shank/coil nails may be used where permitted by the plywood and flooring system.
We would not rely on lightweight staples as the primary fixing for a plywood overlay intended to control movement beneath a wood floor.
Fully Bonding the Plywood Overlay
On some professional wood-floor installations, we fully bed the flooring-grade plywood into a compatible flexible wood-floor adhesive as well as mechanically fixing it.
Products commonly used within our wood-flooring systems include:
- Bona R848 for many standard engineered flooring installations.
- Bona Quantum for wider boards, solid flooring and more demanding applications.
- Osmo MS Trade as an alternative elastic wood-floor adhesive.
When bedding plywood into adhesive, the sheet needs full and consistent support while the adhesive cures. Weighting or loading may be required to keep the plywood flat and prevent springing.
This is a professional preparation method rather than a substitute for the plywood manufacturer's fixing instructions, and substrate/adhesive compatibility should always be confirmed.
Why Ply Before Fully Bonding over Floorboards?
In theory, some adhesives and flooring constructions may permit wood flooring to be bonded to certain timber substrates.
In practice, traditional floorboards contain many individual pieces of timber, joints, historic movement and varying surface levels.
Our professional preference for a fully bonded engineered or solid floor is therefore normally to create a continuous plywood receiving surface first.
This provides:
- A more uniform surface for the adhesive.
- More consistent adhesive transfer.
- Reduced influence from individual board joints.
- A suitable base for further smoothing where required.
- A better foundation for herringbone and other precision layouts.
What If the Plywood Is Still Not Flat Enough?
Once the timber structure is secure and the plywood overlay is correctly installed, a suitable flexible smoothing compound can be used where further correction is required.
Our preferred product for this situation is F. Ball Stopgap 700 Superflex.
Stopgap 700 is fibre reinforced and specifically designed to tolerate the limited normal movement associated with correctly installed plywood and other suitable flexible substrates.
Prime the Plywood Correctly
Where the selected F. Ball smoothing system requires it, Stopgap P131 is suitable for absorbent timber-based surfaces including plywood and chipboard.
Always follow the current F. Ball Technical Data Sheet for the correct dilution and application process rather than assuming one primer mix suits every substrate.
What About Stopgap 1200 Pro?
Stopgap 1200 Pro is an extremely useful refurbishment compound, particularly over suitable well-bonded adhesive residues and other approved substrates.
It is not our default choice over a flexible timber floor.
Where the receiving base is plywood or another substrate with limited normal movement, we generally prefer the fibre-reinforced Stopgap 700 system.
Preparing Structural Chipboard
Structural tongue-and-groove chipboard is common in modern homes and new-build construction.
The first job is to establish whether it is genuinely structural and stable.
Check for:
- Movement at joints.
- Loose boards.
- Unsupported edges.
- Squeaking.
- Water-damaged or swollen areas.
- Contamination from plaster, paint or building work.
- Localised high joints.
Refixing Creaking Chipboard
Where joists and services can be positively identified, loose structural chipboard can often be refixed using Floor-Tite anti-creep screws.
The difficulty with tongue-and-groove chipboard is that pipes and electrical services are often hidden completely beneath it.
If access is restricted, use suitable detection equipment, investigate from below where possible or make a controlled inspection rather than guessing.
Can Wood Flooring Be Fully Bonded Directly to Chipboard?
Yes, suitable structural chipboard can form an excellent direct-bond substrate when it is dry, sound, stable, clean and accepted by the adhesive and flooring manufacturer.
Current Bona guidance includes suitable tightly screwed structural chipboard within approved timber substrates for its silane adhesive systems.
This is one major difference between a sound structural chipboard deck and old individual floorboards.
Can Chipboard Be Smoothed?
Suitable timber-based substrates can receive specialist smoothing systems, but product selection must be based on the exact board and construction.
Stopgap P131 is approved by F. Ball for suitable chipboard as a primer within compatible preparation systems.
Because chipboard constructions vary, particularly between structural joisted floors and floating decks, confirm the selected smoothing compound with the manufacturer's current technical guidance before application.
Floating Chipboard Floors over Insulation
Another construction found in apartments, acoustic upgrades and some modern buildings is a tongue-and-groove chipboard deck floating over acoustic or thermal insulation.
The chipboard panels are normally bonded together at their tongue-and-groove joints and deliberately separated from the structural slab or floor beneath.
This means some movement under load can be normal for the system, but excessive rocking is not.
Can You Float Engineered Flooring over a Floating Chipboard Deck?
Where both the chipboard system and wood-floor manufacturer permit it, a flat and stable floating chipboard deck can receive an engineered floating floor over the correct underlay.
For smaller straightforward areas this can be a practical solution.
However, the final construction then contains one floating chipboard deck with another floating timber floor above it.
Why We Often Prefer Full Bonding to a Stable Floating Deck
Where the chipboard deck manufacturer and flooring specification allow it, our preference is often to fully bond the engineered flooring to the chipboard.
The timber and chipboard then behave together as one flooring assembly rather than having two independently floating layers.
This can provide a firmer feel underfoot and is particularly attractive for larger plank areas and fully bonded formats.
What If the Floating Chipboard Is Unstable?
The correct response depends on the amount and cause of the movement.
Where limited movement remains but the underlying deck is otherwise sound, a thicker plywood overlay may sometimes be used to distribute load and stiffen the receiving surface.
Depending on the construction, this could mean plywood considerably thicker than a normal 6mm floorboard overlay — potentially 12mm or 18mm where the assessment justifies it.
The plywood would need to be installed as part of a properly designed build-up rather than simply laid loose over a rocking floor.
When the Correct Answer Is to Lift the Chipboard
If the deck rocks badly, feels unstable across large areas or has failed joints, additional layers may simply hide the real problem.
In severe cases, lift the chipboard and inspect the construction beneath it.
Common causes include:
- Uneven structural concrete beneath the insulation.
- Debris left beneath the floating deck.
- Insulation that is inappropriate or damaged.
- Poorly fitted or unsupported panel joints.
- Insulation of inconsistent thickness.
- Localised compression beneath heavily trafficked areas.
Floorboards vs Chipboard vs Plywood
| Subfloor | Typical preparation | Floating engineered floor | Fully bonded floor |
|---|---|---|---|
| Traditional floorboards | Secure, punch fixings, sand/plane high areas | Often suitable once flat and stable | WWFS normally prefers plywood overlay first |
| Structural chipboard | Secure joints, clean, repair damage, check flatness | Often suitable where flooring manufacturer allows | Can often be directly bonded where adhesive manufacturer approves |
| Flooring-grade plywood | Securely fixed, clean and suitably smooth | Suitable receiving surface where floor permits floating | Excellent receiving surface for compatible adhesives |
| Floating chipboard / acoustic deck | Confirm entire deck is stable and system remains compliant | Possible where both systems permit it | Often WWFS preference where deck manufacturer permits |
Recommended WWFS Products
Frequently Asked Questions
Can engineered flooring be laid directly over old floorboards?
Yes, particularly as a floating installation, provided the existing boards are structurally sound, secure, dry and sufficiently flat and the engineered flooring manufacturer permits the method.
Should floorboards be sanded before laying engineered flooring?
Where raised board edges or local high spots prevent the floor meeting the required flatness tolerance, sanding or carefully planing those areas is often the most practical preparation method.
Do I need plywood over floorboards?
Not always. A small floating plank installation may only require the boards to be secure and flat. Plywood becomes more useful where a full-bond installation, smoothing compound or more consistent receiving surface is required.
What thickness plywood should I use?
Approximately 5.5–6mm flooring-grade plywood is commonly used as an overlay over sound existing floorboards. Greater thickness may be required where additional stiffness is needed, but plywood should never be used to conceal structural failure.
Can I staple 6mm plywood to floorboards?
We do not recommend relying on lightweight staples for this type of wood-floor preparation. Suitable flooring screws or approved ring-shank fixings provide a more secure installation. If Stopgap 700 will be applied, F. Ball specifically requires the plywood to be screw-fixed and states that it should not be nailed or stapled.
How closely should plywood be fixed?
Published flooring-plywood guidance typically specifies a maximum of approximately 100mm centres around each sheet perimeter and 150mm through the main body of the sheet. Follow the exact plywood and flooring-system requirements.
Can I use levelling compound directly over floorboards?
We normally create a suitable flooring-grade plywood receiving surface first. The floorboards must already be structurally sound, and the chosen smoothing system must be approved for the final substrate.
Which levelling compound is best over plywood?
Stopgap 700 Superflex is our preferred F. Ball option for correctly installed plywood because it is fibre reinforced and specifically designed for suitable flexible substrates.
Can Stopgap 1200 be used over a timber floor?
It should not be treated as the default timber-floor compound. Stopgap 1200 Pro is primarily an excellent refurbishment product for a range of approved substrates and residues. For correctly prepared plywood where limited normal movement must be accommodated, WWFS generally prefers Stopgap 700.
Can engineered wood be glued directly to chipboard?
Yes, where the chipboard is a suitable structural grade, firmly secured, clean, dry and sufficiently flat, and both the adhesive and flooring manufacturers approve direct bonding.
Should I screw down a creaking chipboard floor?
Where the supporting joists and hidden services can be positively identified, additional structural flooring screws can help control movement. Never blindly screw through chipboard where pipes or cables may be hidden underneath.
Can herringbone be floated over floorboards?
Many engineered herringbone systems require full bonding. Never assume the floating instructions for an engineered plank also apply to the manufacturer's herringbone or chevron range.
Can I lay a floating engineered floor over a floating chipboard floor?
It can be possible where both systems permit it and the chipboard deck is flat and stable. WWFS generally prefers full bonding to a suitable stable chipboard deck where permitted, avoiding two independently floating floor layers.
What if the chipboard floor rocks badly?
Investigate the original floor construction. Severe movement can be caused by uneven support, debris, damaged insulation or failed joints. Adding more layers should not replace correcting a defective base.
WWFS Final Recommendation
Timber subfloor construction, plywood thickness, fixing requirements, floating-floor limits, adhesive suitability and smoothing-compound specifications vary by manufacturer and project. Always follow the latest flooring, plywood, adhesive and preparation-product Technical Data Sheets and investigate structural movement or hidden services before drilling, screwing or overboarding.