A high-quality engineered wood floor is only as good as the surface beneath it. Subfloor preparation affects how solid the floor feels, how well the adhesive transfers, whether joints remain stable and whether moisture can damage the flooring after installation.
Unfortunately, the phrase āthe floor needs levellingā is often used far too loosely.
In most wood-flooring installations, the objective is not to make the entire room perfectly horizontal. The objective is to create a subfloor that is sound, dry, clean, sufficiently flat, sufficiently smooth and compatible with the chosen flooring system.
This guide explains how to assess concrete, screed, plywood, timber, existing adhesive residues and other common subfloors before installing engineered wood flooring, using the professional preparation products stocked by Wiltshire Wood Flooring Supplies.
Level, Flat and Smooth Are Not the Same Thing
Level
Level means horizontal relative to gravity.
A floor can slope gradually from one side of an old property to the other and still be perfectly usable for wood flooring if it remains sufficiently flat.
Flat
Flat describes local variation in the surface.
A flat floor has no unacceptable humps, hollows or abrupt changes beneath a straightedge.
Smooth
Smooth describes the texture and finish of the surface.
A surface can be flat but rough, or smooth but contain a significant dip.
What Does āSelf-Levelling Compoundā Actually Mean?
The term āself-levelling compoundā is commonly used throughout the building industry, but it can create the wrong impression.
Products such as Stopgap 300 HD and Stopgap 700 are more accurately described by F. Ball as self-smoothing compounds.
They flow sufficiently to create a smooth, consistent surface, but they do not magically determine the finished height of the room or correct every problem automatically.
The installer still controls:
- Where the product is poured.
- The amount used.
- The final application depth.
- High points that should be mechanically removed first.
- Transitions into adjoining areas.
- The finished floor height.
How Flat Does the Subfloor Need to Be?
Always use the flatness tolerance stated by the flooring manufacturer for the exact flooring being installed.
Professional wood-floor specifications commonly assess the surface using a long straightedge placed at multiple locations and directions across the floor.
A commonly encountered UK target is around 3mm variation beneath a 2m straightedge, but this should never replace the individual flooring manufacturer's installation requirement.
Different flooring formats can also respond differently. Long wide planks and herringbone tend to reveal poor preparation quickly because they require consistent support and adhesive transfer across a large number of joints.
Why flatness matters
A poorly prepared surface can cause:
- Boards rocking on high points.
- Hollow areas beneath fully bonded flooring.
- Insufficient adhesive contact.
- Movement and noise beneath floating floors.
- Stress through tongue-and-groove or locking joints.
- Poor alignment in herringbone installations.
- Visible lipping or uneven transitions.
How to Check Subfloor Flatness
Use a long straightedge
Place a straightedge across the floor in several directions rather than checking only one line through the room.
Look underneath it
Identify hollows and measure the largest gaps beneath the straightedge.
Check for rocking
If the straightedge rocks, there is probably a high spot beneath the centre.
Mark the floor
Mark high points, low areas and repairs directly on the subfloor so the preparation can be planned logically.
Recheck after preparation
Do not assume the floor is correct simply because smoothing compound has been applied. Test the finished surface again before installing the timber.
Should I Grind High Spots or Pour Levelling Compound?
Often both are used, but the first question should be whether unnecessary height can be removed before additional material is added.
If one localised high point causes the floor to fall outside tolerance, mechanically grinding that point may be much more sensible than raising the entire room with several millimetres of compound.
Grinding high areas can:
- Reduce compound consumption.
- Keep finished floor height lower.
- Improve door and threshold clearances.
- Reduce the height difference into adjoining rooms.
- Create a better base before final smoothing.
The Correct Order of Subfloor Preparation
Before reaching for a bag of smoothing compound, work through the subfloor systematically.
Identify the subfloor
Determine whether you have concrete, sand/cement screed, calcium sulphate screed, plywood, timber boards, tile, resin or another construction.
Check structural integrity
The subfloor must be stable, adequately supported and free from unacceptable deflection or movement.
Test moisture
Moisture should be measured using an approved method. Appearance and screed age alone are not reliable indicators.
Check surface strength
Weak laitance, loose screed and friable material must be removed. Adhesive and primer need to bond to the structural surfaceānot to a weak dusty layer.
Measure flatness
Mark high points and low areas using a long straightedge.
Mechanically prepare
Grind, scrape, sand or otherwise prepare contamination, laitance, coatings and high spots as required.
Repair local defects
Deal with deep holes, broken edges, cracks and small feather-edge defects before the final smoothing stage.
Prime or install moisture control
Select the correct product according to the substrate and measured conditions.
Smooth where necessary
Use the smoothing compound suited to the substrate and required application depth.
Final inspection
Confirm flatness, dryness, cleanliness and compatibility before adhesive or underlay is installed.
Preparing Concrete and Sand/Cement Screeds
Concrete and traditional cementitious screeds are common bases for fully bonded engineered wood flooring.
They should be:
- Structurally sound.
- Sufficiently dry.
- Free from laitance.
- Free from paint, grease, curing agents and contamination.
- Within the flooring manufacturer's flatness tolerance.
- Strong enough for the adhesive and timber stresses.
Power-floated concrete often needs mechanical preparation because the dense surface can prevent primers, smoothing compounds or adhesives from bonding properly.
Typical preparation route
Dry cementitious subfloor requiring smoothing
Stopgap 300 HD
Stopgap 300 HD is our main heavy-duty general-purpose smoothing compound for correctly prepared rigid substrates.
It can be applied from approximately 2ā20mm and covers around 5mĀ² at a 3mm thickness from a 25kg bag.
Suitable prepared surfaces include:
- Sand/cement screed.
- Concrete.
- Prepared calcium sulphate screeds.
- Sound ceramic and quarry tiles.
- Terrazzo.
- Compatible resin surfaces.
- Stopgap waterproof surface membranes.
- Minimal suitable adhesive residues.
When Speed Matters: Stopgap Fast-Track 30
Stopgap Fast-Track 30 is designed for projects where downtime must be reduced.
It is useful when the substrate is otherwise suitable but the programme demands rapid setting and drying.
Fast-setting products require efficient preparation and mixing. Everything should be ready before the first bag is mixed.
Preparing Calcium Sulphate / Anhydrite Screeds
Calcium sulphate screeds require different preparation from conventional sand/cement.
The surface should be mechanically prepared in accordance with the screed manufacturer's instructions to remove laitance and expose a suitable sound surface.
It must then be thoroughly vacuumed and allowed to dry to the appropriate moisture level before timber installation.
F. Ball specifies Stopgap P121 as its specialist acrylic primer for prepared calcium sulphate screeds.
Typical anhydrite preparation route
Dry calcium sulphate screed
Preparing Timber Floorboards
Existing timber floorboards need structural assessment before any smoothing compound is considered.
Check for:
- Loose boards.
- Broken or split boards.
- Excessive deflection.
- Inadequate support beneath joints.
- Protruding nails or screws.
- Rot or moisture damage.
- Large changes in height between adjacent boards.
Loose boards should be repaired and fixed securely before overboarding or smoothing.
When Should Timber Be Overboarded with Plywood?
Overboarding existing floorboards with suitable flooring-grade plywood can create a more stable and consistent surface for the flooring installation.
The plywood specification, thickness and fixing method should reflect the existing floor construction and final flooring system.
The sheets must be properly supported, securely fixed and free from vertical movement.
Overboarding should not be used to hide rotten boards, inadequate joists or structural problems.
Preparing Plywood
Plywood may look flat at first glance but joints, screw heads and slight sheet variation can still telegraph through a flooring system.
Where the plywood requires smoothing, use a compound designed to tolerate the small amount of movement associated with correctly constructed timber-based substrates.
Stopgap 700 Superflex
Stopgap 700 Superflex is a fibre-modified smoothing compound developed specifically for substrates such as flooring-grade plywood and steel.
The fibres help the cured compound tolerate limited normal movement within the properly constructed substrate.
Stopgap 700 can be applied from approximately 2ā10mm and provides around 4.8mĀ² coverage at 3mm from a 20kg bag.
Preparing Old Adhesive Residues
Refurbishment projects often reveal old adhesive once carpet, vinyl, parquet or another covering is removed.
Do not automatically smooth over it.
The residue should be assessed for:
- Bond to the original substrate.
- Thickness.
- Water sensitivity.
- Softness.
- Contamination.
- Compatibility with the proposed preparation system.
Loose, soft or excessive residue should be mechanically removed.
Stopgap 1200 Pro
Stopgap 1200 Pro is particularly useful in refurbishment because it can be applied over certain well-bonded residues, including suitable bitumen adhesive and carpet-tile tackifier residues.
It can be applied approximately 2ā10mm unfilled and extended up to around 30mm using the specified aggregate system.
The important phrase is well bonded and suitable. Not every old black adhesive or sticky residue should simply be buried.
Preparing Existing Tiles
Existing ceramic, quarry or other hard surfaces may sometimes remain if they are structurally sound, well bonded and accepted within the preparation system.
The surface must be cleaned thoroughly, contamination removed and the appropriate non-absorbent primer used before smoothing.
Stopgap P141 is specifically designed to create a textured key over suitable non-absorbent surfaces such as ceramic tiles, treated power-floated concrete and Stopgap waterproof surface membranes.
What About Cracks and Holes?
Local defects should be dealt with before the final smoothing stage.
Small surface defects
Use Stopgap Micro Rapid for suitable feather-edge repairs, screw heads and minor surface imperfections.
Deeper repairs
Use Stopgap 400 Repair for deeper holes, damaged screed, edges, ramps and larger localised repairs.
Subfloor Moisture: Never Guess
Concrete and screed can appear completely dry while still containing excessive moisture internally.
A new screed should therefore never be judged solely by:
- Colour.
- How long ago it was poured.
- Whether the heating has been switched on.
- Whether the surface feels dry.
The subfloor should be tested using a recognised moisture-testing method appropriate to the screed and flooring system.
Wood flooring is particularly moisture-sensitive, so moisture limits can be stricter than those used for some resilient coverings.
When Is a DPM Needed?
A liquid damp-proof or waterproof surface membrane is used where measured residual moisture is above the permissible level for the flooring system but remains within the DPM manufacturer's approved range.
It should not be applied simply because the installer thinks the floor ālooks a bit dampā.
Stopgap F77
Stopgap F77 is a two-component epoxy waterproof surface membrane for suitable cementitious substrates.
F. Ball states that it can control residual construction moisture up to 98% RH in approved applications, with a separate limit of up to 90% RH over suitable screeds incorporating underfloor heating.
F77 requires a sound, mechanically prepared and absorbent cementitious base.
After curing, most Stopgap smoothing systems require a compatible primer such as P141 before the smoothing compound is applied.
Bona R540
Bona R540 Primer/DPM is a one-component polyurethane primer for suitable absorbent and non-absorbent subfloors before Bona silane adhesives.
It can also provide residual-moisture control on suitable cementitious floors within Bona's specified limits and can help reinforce certain weak mineral substrates.
Where moisture control is not required and the subfloor simply needs priming or residual dust binding before a Bona silane adhesive, use Bona D501 where appropriate.
Do All Subfloors Need Primer?
No.
Primer is selected according to the substrate and the product going over it.
| Primer | Primary role | Typical prepared substrate |
|---|---|---|
| Stopgap P121 | Controls absorption on calcium sulphate screed | Anhydrite / calcium sulphate |
| Stopgap P131 | General-purpose primer for compatible absorbent and certain non-absorbent substrates | Concrete, sand/cement, plywood and other approved bases |
| Stopgap P141 | Creates a textured key on non-absorbent surfaces | Tiles, treated power-floated concrete, compatible waterproof membranes |
| Bona D501 | Adhesive primer and residual-dust binder | Compatible dry subfloors before Bona silane adhesive |
| Bona R540 | PU primer, reinforcement and approved residual-moisture control | Compatible cementitious and other approved subfloors |
Underfloor Heating Screeds
Underfloor heating adds another layer of responsibility because moisture, heat transfer and thermal cycling all affect the flooring system.
Before installing engineered flooring:
- The screed should be sufficiently cured.
- The UFH system should be commissioned in accordance with the screed and heating manufacturer's instructions.
- Moisture must be tested after the appropriate heating and cooling procedure.
- The floor should meet flatness requirements.
- Any DPM must specifically allow the particular UFH application.
- The finished engineered flooring must itself be approved for UFH.
Do not use the heating system simply as a shortcut to force-dry a new screed outside the screed manufacturer's commissioning programme.
Why Full Bonding Makes Subfloor Preparation Especially Important
With a fully bonded engineered floor, adhesive is spread across the subfloor and the board is bedded directly into it.
Good adhesive transfer depends on consistent contact.
If a wide board bridges a hollow, the adhesive ribs may never transfer properly to the underside of that section.
If the floor sits on a high spot, the board may rock or fail to bed evenly.
This is why adhesive should not be viewed as a replacement for proper smoothing.
Bona R848 or Bona Quantum?
Once the subfloor is correctly prepared, most engineered plank and herringbone installations can move into the adhesive stage.
Bona R848 is our starting recommendation for most engineered wood flooring installations.
Bona Quantum is the higher-specification choice for wider boards, solid timber and more demanding flooring formats.
Adhesive choice and trowel selection are covered in detail in our dedicated WWFS wood-floor adhesive guide.
Subfloor Preparation by Type
| Subfloor | Main checks | Typical WWFS preparation products |
|---|---|---|
| Concrete / sand-cement | Moisture, strength, laitance, contamination, flatness | P131, 300 HD, Fast-Track 30, F77, D501 or R540 depending on system |
| Calcium sulphate / anhydrite | Moisture, laitance removal, flatness, screed identification | P121 plus compatible smoothing system |
| Floorboards | Movement, joist support, loose boards, moisture | Structural repair / plywood overboarding before further preparation |
| Plywood | Grade, fixings, movement, joints, flatness | P131 where specified, Stopgap 700 |
| Old adhesive residue | Bond, contamination, thickness and compatibility | Stopgap 1200 Pro where suitable |
| Tiles / non-absorbent surface | Bond, contamination, cracks, flatness | P141 plus compatible smoothing compound |
| Damaged screed | Depth and cause of damage | 400 Repair, Micro Rapid, then smoothing where required |
Common Subfloor Preparation Mistakes
Assuming ālevelā means perfectly horizontal
The flooring usually needs a sufficiently flat supporting surface, not a room corrected to laser-level height from wall to wall.
Pouring before finding the high points
This can unnecessarily raise the complete room.
Skipping moisture testing
Screed age and appearance do not prove dryness.
Applying primer over dust
Primer needs to bond to the substrate. Heavy loose contamination must still be removed.
Putting smoothing compound over loose floorboards
Structural movement must be repaired first.
Ignoring laitance on screed
A weak dusty surface layer creates a weak point beneath the entire flooring installation.
Treating every screed as cementitious
Calcium sulphate screeds require different preparation and moisture considerations.
Assuming every black adhesive residue can stay
Old adhesive must be assessed for type, bond and compatibility.
Trying to flatten the floor with adhesive
Flooring adhesive should be applied at the correct trowel rate, not used as a repair mortar.
Not checking after smoothing
The compound may look beautiful and still contain an unacceptable ridge or local dip.
The Complete WWFS Preparation Workflow
WWFS Subfloor Preparation Products
Frequently Asked Questions
Does my subfloor need to be perfectly level?
No. It needs to meet the flooring manufacturer's flatness requirement. A gradual slope can be acceptable where there are no excessive local dips or humps.
What is the difference between level and flat?
Level describes whether the floor is horizontal. Flat describes whether the surface contains localised high or low areas. Flatness is normally the more important issue for wood-floor installation.
How do I check whether a subfloor is flat?
Use a long straightedge in several positions and directions, measure gaps beneath it and identify any rocking caused by high points.
Should I grind high spots before levelling?
Often yes. Removing localised high points first can prevent the need to raise the entire floor unnecessarily.
Can wood-floor adhesive fill dips?
Not significant ones. Adhesive accommodates normal surface texture but should not be used as a substitute for correct subfloor preparation.
Which levelling compound should I use on plywood?
Stopgap 700 Superflex is designed for properly constructed plywood and other suitable flexible substrates.
Which compound should I use over old adhesive?
Stopgap 1200 Pro can be used over certain well-bonded adhesive residues. The residue must first be assessed for bond, contamination and compatibility.
Which primer should I use on anhydrite?
Stopgap P121 is specifically designed for properly prepared calcium sulphate screeds within the compatible Stopgap system.
Can I use a liquid DPM over any screed?
No. DPM suitability depends on screed chemistry, moisture level, heating system and the manufacturer's approved substrate list.
Does a new screed automatically need a DPM?
No. Test it first. If it has naturally dried below the permitted moisture level and the structural DPM is satisfactory, an additional liquid membrane may not be needed.
Can I install engineered flooring over a sloping floor?
Potentially, yes. A consistent slope is not the same as poor flatness. The final floor must still meet the flooring manufacturer's requirements and transition safely into adjoining areas.
Can I lay engineered wood directly onto concrete?
Yes, where the concrete is sound, clean, sufficiently dry and within flatness tolerance, using an approved full-surface wood-floor adhesive system.
Can I install over existing tiles?
Sometimes. The tiles must be sound and well bonded, contamination removed, and the surface prepared using the compatible primer and smoothing system before wood-floor installation.
What is laitance?
Laitance is a weak, dusty or friable surface layer that can form on cementitious and calcium sulphate screeds. It should be mechanically removed where required because the flooring system needs to bond to the sound substrate below.
Why does subfloor preparation matter more with herringbone?
Herringbone contains many individual joints and must be set out accurately. Localised height variation can affect block alignment, adhesive transfer and the consistency of the pattern.
Our Final Recommendation
Subfloor preparation requirements vary according to the flooring, substrate, measured moisture, installation method and manufacturer. Flatness figures and moisture limits must always be checked against the current flooring, adhesive, screed and preparation-product Technical Data Sheets. This guide supports product selection but does not replace professional subfloor assessment.