You’ve picked out your perfect hard flooring – whether it’s solid wood, engineered wood, laminate, or luxury vinyl tiles (LVT). The finish, the colour, the texture – everything is just right, and you’re eager to see it down and transforming your space. But before you get to that satisfying final step, there’s a crucial phase that’s too often overlooked: preparing your subfloor.
One of the most common questions we hear is:
“Can I lay my new flooring directly onto my old, slightly uneven floor?”
The simple answer? No – not if you want your new floor to last.
Laying hard flooring on an uneven surface is a shortcut to disappointment. It can lead to visible imperfections, annoying movement underfoot, excessive wear on joints, and in many cases, void your flooring warranty altogether.
In this guide, we’ll explain:
Why uneven subfloors cause problems
What level of unevenness is acceptable
How to check your subfloor properly
The best ways to fix common issues
Let’s make sure your beautiful new floor has the solid, smooth foundation it deserves—so it looks stunning and performs perfectly for years to come.
Why a Perfect Subfloor is Non-Negotiable for Hard Flooring
Hard flooring—whether it’s solid wood, engineered wood, laminate, or LVT—is inherently rigid. Unlike carpet, it doesn’t bend or mould to the surface below. Instead, it mirrors every imperfection in the subfloor beneath it. And that’s where the problems begin.
Aesthetic Disaster
Even slight dips, humps, or slopes in the subfloor will show through your finished floor. The result?
Ripples or waves that break up the smooth appearance
Visible gaps between boards or tiles
Soft spots or movement underfoot
These flaws can completely undermine the visual impact of your new flooring—and often become more pronounced over time.
Compromised Durability & Longevity
Click-System Floors (Laminate, Engineered Wood, Some LVT)
Click flooring is designed to float and lock tightly together. But on an uneven base, the joints bear excessive strain as the boards flex and shift. This leads to:
Creaking and noise
Broken or worn-down click joints
Gapping or board separation
Lifting or instability
Ultimately, this shortens the life of your floor significantly.
Glue-Down Floors (Certain Engineered Wood, LVT, Parquet)
Glue needs consistent, full contact to work properly. Dips and hollows in the subfloor create:
Poor adhesion
Hollow-sounding spots
Gradual lifting of boards
Failure of the adhesive bond over time
Nail-Down Floors (Solid Wood)
Nailing down solid wood onto an uneven base means the boards won’t sit flush. This results in:
Squeaking and shifting
Insecure fixings
Gaps or lifted boards
It can also make installation significantly more difficult and time-consuming.
Voided Warranty
Most manufacturers set clear tolerances for subfloor flatness—often no more than 3mm deviation over 2 linear metres. If your subfloor doesn’t meet these specs, your warranty will likely be void, meaning any future issues will be your responsibility, not the supplier’s.
Poor Feel Underfoot
Beyond appearance and durability, the feel of your floor matters. An uneven subfloor often causes:
A bouncy or spongy sensation underfoot
Creaking or cracking sounds with every step
An overall unstable feel that detracts from your space
Understanding “Uneven”: Level vs. Flat
Before you reach for a spirit level, it’s important to understand that “level” and “flat” are not the same thing—and when it comes to laying hard flooring, flatness matters more.
Level
A level floor is one that is perfectly horizontal—like a billiard table. This is useful in some situations, such as large open-plan spaces or rooms where fixtures and fittings rely on level surfaces. However, for most standard rooms, achieving a perfectly level floor is not essential for hard flooring to function correctly.
Flat
Flatness refers to a floor that is free from noticeable humps, dips, slopes, or inconsistencies across its surface. It might not be perfectly level from one end to the other, but the surface needs to be even and consistent. This is crucial because:
Gaps and height differences between boards can occur if the floor isn’t flat
Stress is placed on locking or adhesive systems when boards rock or flex over uneven spots
The floor may feel unstable or wear unevenly
In short, you don’t need a perfectly level subfloor—but you absolutely need a flat one. Flatness ensures your flooring lies evenly, connects properly, and performs reliably over time.
How to Assess Your Subfloor’s Flatness
Before you even think about laying your new hard flooring, you need to understand the condition of your subfloor. Assessing flatness properly is the first step to a successful, long-lasting installation.
1. Clear the Area
Remove all existing flooring, rugs, furniture, and any obstructions. You need a completely bare floor to get an accurate reading.
2. Clean Thoroughly
Dust, dirt, and debris can give you false readings. Sweep and vacuum the entire subfloor so you’re assessing the actual surface—not what’s sitting on top of it.
3. Use a Long Straight Edge
A 2-metre straight edge (such as a long spirit level, metal rule, or flat timber batten) is ideal. This will help you visually and physically detect any unevenness.
Lay it across the floor in multiple directions:
Lengthwise (with the room)
Crosswise (across the room)
Diagonally (corner to corner)
Look underneath the straight edge:
If you see gaps, you’re dealing with dips.
If it rocks, that means there are humps.
4. Mark Problem Areas
Use chalk or a pencil to mark every spot where the floor is out of tolerance. This will guide you when it’s time to level or smooth the surface.
5. Check for Moisture (Concrete Subfloors Only)
If your subfloor is concrete, don’t skip a moisture test. High moisture content can:
Lead to mould and mildew
Cause wood or laminate boards to warp
Break down adhesives used in glue-down installations
You can hire a moisture meter or ask a professional to perform the test. For certain products, this step is required to maintain the manufacturer’s warranty.
Addressing Unevenness: Solutions for Common UK Subfloor Types
The right method for fixing unevenness depends entirely on the type of subfloor you’re working with. In the UK, that usually means either concrete (on ground floors) or timber joist/floorboard construction (on upper floors or in older properties). Here’s how to deal with both:
1. Concrete Subfloors (Common in Ground Floors)
High Spots
Grinding: For significant humps or raised areas, hire a concrete grinder. This can be a dusty process, so wear safety gear and isolate the area if possible.
Chiselling: For smaller, isolated bumps, a hammer and chisel may be sufficient to level them out.
Low Spots / Dips / General Unevenness
Self-Levelling Compound (Screed)
A cement-based mix that flows into low spots and dries to a perfectly flat finish.Preparation: Subfloor must be clean, dust-free, dry, and primed with a product recommended by the levelling compound manufacturer.
Application: Pour and spread using a float or spiked roller.
Drying Time: Typically 24–48 hours, but always check the product label. Good ventilation is essential.
Patching Compounds
Use a fast-setting repair compound for small dips, holes, or cracks. These are ideal for minor corrections where full levelling isn’t required.Crack Repair
Minor cracks (non-structural): Use an epoxy filler or specialist crack-repair product.
Structural cracks: These should be assessed and repaired by a professional before proceeding with floor installation.
Damp Proof Membrane (DPM)
Even if the floor is flat, concrete subfloors on ground levels typically require a DPM to prevent moisture ingress.Options: Liquid-applied membranes or underlayments with an integrated DPM layer.
2. Wooden Joist / Floorboard Subfloors (Common in Upper Floors & Older Properties)
Loose or Squeaky Floorboards
Secure Floorboards: Screw any loose boards down into the joists using appropriate wood screws. Loose boards will creak and compromise the stability of your new floor.
High Spots (Cupped or Crowned Boards)
Sanding: Minor cupping or crowning can often be fixed with a floor sander to level the surface.
Replacement: Severely warped or damaged boards should be removed and replaced with new timber cut to fit.
Low Spots / Dips / General Unevenness
Plywood or OSB Overlay
For broad or consistent unevenness, overlaying with plywood or OSB creates a smooth and stable base.Materials: Use exterior-grade plywood (usually 6mm or 9mm) or OSB sheets.
Installation:
Fix securely every 15–20cm with wood screws.
Aim to screw into joists, but also fix between them to minimise movement.
Sand or plane edges to ensure a flush, continuous surface.
Expansion Gaps: Leave a small gap between sheets and around the room’s edge, just as you would for your finished flooring.
Shims or Felt Pads
For localised dips beneath individual floorboards, install shims or felt pads on top of the joists before re-securing the boards. This is a more structural adjustment than a surface one, but useful in isolated problem areas.
The Role of Underlayment (Crucial, But Not a Leveller!)
Underlayment is an essential component in many hard flooring installations, especially for laminate, engineered wood, and some LVT click systems. However, a common misconception needs to be addressed:
Underlayment is not a substitute for subfloor levelling.
What Underlayment Does
Adds cushioning for improved comfort underfoot
Reduces sound transmission between floors
Acts as a moisture barrier, especially when fitted with a vapour-resistant backing or combined with a DPM
Bridges extremely minor imperfections, such as slight texture or hairline surface inconsistencies—usually less than 1mm over a short distance
What Underlayment Doesn’t Do
Level a floor
Fix dips, humps, or slopes
Prevent movement on significantly uneven subfloors
Protect your click or adhesive joints from stress caused by uneven support
If your subfloor has noticeable unevenness, underlayment will compress unevenly, resulting in:
A spongy or unstable feel underfoot
Click joint failure due to flexing
Gapping, lifting, and creaking over time
The Bottom Line
Underlayment is a support layer, not a correction layer. It plays an important role in the overall performance of your floor—but only when installed on a properly prepared, flat surface.
When to Call a Professional
While many subfloor preparation tasks—like screwing down loose boards or applying a small patching compound—are entirely DIY-friendly, there are times when it’s best to bring in the experts. Knowing your limits can save time, money, and a lot of frustration later on.
Consider calling a flooring or subfloor professional if:
🔧 You have significant structural issues
If your floor shows signs of serious problems—such as rotting joists, sagging floorboards, or major settling or heaving—these need to be properly assessed and repaired before any flooring can be safely laid.
📏 The unevenness is extreme or widespread
If the dips or humps are severe, cover a large surface area, or involve both high and low spots, levelling becomes more complex. A professional will have the right tools and experience to bring the floor within acceptable tolerances.
🧰 You’re not confident using grinders, sanders, or self-levelling compound
These tools and materials require precision, patience, and a good understanding of technique. Improper use can lead to more unevenness—or even damage your home.
💧 You have ongoing or unexplained moisture problems
Persistent damp or moisture in a concrete subfloor, especially on ground level, needs proper testing and possibly damp proofing. If you’re unsure, it’s wise to get a professional moisture assessment to avoid future mould, warping, or adhesive failure.
Frequently Asked Questions (FAQ)
Q1: What is ‘acclimatisation’ and why is it so important for my new flooring?
A1: Acclimatisation is the process of allowing your new wood (solid or engineered) or laminate flooring to adjust to the temperature and humidity of its environment before installation. Wood is a natural material and needs time to stabilise, or it may expand or contract after fitting—leading to gaps, buckling, or cupping.
How to do it: Leave the unopened packs of flooring in the room where they will be laid for at least 48–72 hours (or as per the manufacturer’s advice). Lay them flat (not upright), with good air circulation around each pack.
Q2: Can I install hard flooring over underfloor heating?
A2: Yes, but only specific types of hard flooring are suitable, and you must follow the correct installation process.
Best choice: Engineered wood (due to its stable construction)
Also suitable: Certain laminate and LVT products (check product specs)
Not recommended: Solid wood flooring—it’s more prone to shrinking or warping
Key considerations:
Check the manufacturer’s guarantee for compatibility
Never exceed surface temperatures (usually 27–28°C)
Follow the correct heat-up and cool-down schedule
Q3: How important is moisture testing for my subfloor, especially on concrete?
A3: Extremely important. Moisture is one of the most common causes of flooring failure. Even if your concrete feels dry, it can still emit moisture that damages wood, adhesive, or underlay over time.
Use a professional moisture meter for accurate readings
If levels are too high, install a Damp Proof Membrane (DPM)—either liquid or sheet
Wooden subfloors may also require inspection for leaks or poor ventilation
Q4: How should I properly maintain and clean my new hard wood or laminate floor?
A4: Good care extends the life of your floor. Follow these tips:
Daily/weekly: Vacuum or sweep to remove dust and grit
Mopping: Use a lightly damp mop with pH-neutral cleaner
Avoid: Excessive water, steam mops, or harsh chemicals
Protect:
Felt pads on furniture
Rugs in high-traffic zones
No high heels or sharp footwear on the surface
Q5: Can I install hard flooring in bathrooms or other high-moisture areas?
A5: It depends on the product:
Solid/Engineered Wood: Generally not suitable for bathrooms
Laminate: Standard laminate isn’t ideal, but waterproof versions exist
LVT: Excellent for bathrooms and kitchens—100% waterproof
Always check the manufacturer’s moisture tolerance before installation in wet areas.
Q6: You mentioned engineered wood can be refinished. How many times, and how does it compare to solid wood?
A6:
Solid wood can be sanded 5–7+ times across its lifespan
Engineered wood can typically be refinished 1–3 times, depending on the thickness of its real wood wear layer
A wear layer of 4–6mm allows more refinishing than thinner ones
While solid wood has greater longevity for refinishing, engineered wood still offers excellent durability for most homes.
Q7: My house is quite old, and I’m worried about asbestos in the subfloor. What should I do?
A7: If your property was built before 2000, there’s a chance older materials—like vinyl tiles, bitumen adhesives, or certain underlays—could contain asbestos. Do not disturb these yourself.
Instead:
Contact a licensed asbestos professional to inspect and, if necessary, remove or encapsulate the material safely.
Your health and safety must always come first.
Q8: Can I lay my new hard flooring directly over existing old hard flooring (e.g., old floorboards or laminate)?
A8: Not always—here’s a breakdown:
Over floorboards: Possible if they’re flat and sound, but a plywood/OSB overlay is recommended to improve flatness and avoid telegraphing
Over laminate or floating floors: Not recommended—this creates instability and voids most warranties
Over glued-down LVT/vinyl: Sometimes allowed if the surface is perfectly flat and secure—check manufacturer guidance
When in doubt, it’s usually best to remove the old flooring and start from a properly prepared subfloor.
The Foundation of a Beautiful Floor
Subfloor preparation might not be the most glamorous or exciting part of laying new flooring—but make no mistake: it is the most important.
It’s tempting to rush ahead and get your stunning new boards down, but skipping or skimping on proper subfloor prep is a shortcut to future regret. Unevenness that seems minor now can lead to visible flaws, performance issues, and expensive repairs later on.
Taking the time to ensure your subfloor is flat, clean, stable, and dry lays the groundwork for success. It allows your hard flooring—whether solid wood, engineered wood, laminate, or LVT—to shine both visually and functionally.
In short, a well-prepared subfloor is the silent hero beneath every beautiful, long-lasting floor. Get it right, and your flooring will thank you for years to come.
