Preparation guide
Does my subfloor need levelling?
Most flooring manufacturers want the subfloor flat to within 3mm over a 2 metre span. Anything outside that and the floor you lay on top will show it.
Most flooring manufacturers want a subfloor flat to within 3mm over a 2 metre span. That single figure is what decides whether your floor needs levelling, and you can check it yourself in about ten minutes with a straightedge.
We check every subfloor against it before we lay anything, because the condition underneath is the most common reason a finished floor disappoints.
At a glance
- Most manufacturers specify 3mm over 2m. Check the guide for your exact product, because it varies
- A 2 metre straightedge and a torch is the whole test
- Dips and humps underfoot, creaking in one spot and a door that has started catching are all signs
- Luxury vinyl shows unevenness first. Click systems fail at the joints
- Moisture is a separate test from flatness, and a floor can pass one and fail the other
How To Check Your Subfloor Flatness
The test is simple and the tools are cheap. A 2 metre straightedge, an aluminium rule or a long spirit level will do, plus a torch.
- Clear the floor back to the subfloor. Old covering, underlay, gripper and adhesive residue all sit proud and will give you a false reading.
- Lay the straightedge across the floor and shine the torch along the underside of it.
- Look for daylight. Any gap you can see is a dip. Any point where the straightedge rocks is a hump.
- Measure the worst gap. Slide something of known thickness under it. A 3mm gap is roughly two pound coins stacked.
- Move the straightedge around. Walk it across the room in both directions and diagonally. A floor can pass along the boards and fail across them.
- Check the doorways and the perimeter. Thresholds and edges are where levels change and where most problems start.
Anything over 3mm across that 2 metre span needs attention before a new floor goes down.
The Signs You Can Feel Before You Own A Straightedge
Most people notice a subfloor problem before they measure one. The common signs:
- A dip or a hump you can feel underfoot, usually in the middle of a room or in a doorway
- A patch that feels soft or springy when you walk on it
- Creaking that happens in one particular spot rather than generally
- A door that used to swing clear and has started catching
- Furniture that rocks in one position and sits flat a metre away
- Visible gaps under the skirting that vary along one wall
None of these on its own proves the subfloor needs levelling. Together they are worth ten minutes with a straightedge.
What Happens If You Skip It, By Floor Type
Unevenness shows differently depending on what goes on top. This is the part that decides how much the preparation matters for your job.
| Floor type | How an uneven subfloor shows |
|---|---|
| Luxury vinyl tile and sheet vinyl | Telegraphs the subfloor. Every dip and hump reads through the surface, usually within months |
| Click laminate | Uneven load on the joints. Locking systems separate, click apart or break along a run |
| Engineered wood | Movement and hollow spots underfoot. Joints open over dips |
| Solid hardwood | Squeaking and cupping. Fixings work loose over a hump |
| Carpet | Wears in a line over a high spot. The dip shows once the pile flattens |
| Carpet tiles | Edges lift at the corners over an uneven screed |
Luxury vinyl is the least forgiving, which surprises people, because it is thin and flexible and feels like it should cope. It conforms to whatever is underneath it rather than bridging it.
The Options, And What Each One Involves
There is more than one way to correct a subfloor, and the right one depends on how far out it is and what the base is made of.
| Method | What it is | When it suits |
|---|---|---|
| Self-levelling compound | A flowing cement-based mix poured over the base and left to find its own level | Shallow undulations across a broadly sound floor |
| Latex screed | A latex and cement mix, trowel applied, for a smooth finished surface | Uneven screeds and floors needing a fine finish before vinyl or LVT |
| Plywood overboarding | Sheets of ply fixed over the existing floor to create a new flat deck | Timber floors with movement, or boards too uneven to skim |
| Localised repair | Cutting out and replacing a defined area | A single failed patch in an otherwise sound floor |
| Full subfloor replacement | Taking the deck up and relaying it | Structural failure, rot, or a floor beyond correction |
We survey the subfloor and price the preparation before the job starts, rather than raising it as a variation once the old floor is up. It is stated in our terms and it is deliberate. Preparation that arrives as a surprise halfway through is how flooring jobs go wrong.
Timelines, The Half Most Guides Leave Out
Levelling takes time as well as money, and the time is the part that catches people out when they have booked a fitting date.
- Self-levelling compound needs to cure before anything goes on top. Drying time depends on depth, the product and the conditions in the room
- Latex screed is similar, and a deeper application takes longer
- Plywood overboarding is mechanical, so there is no drying time, but it raises the floor level and usually means doors need easing
- Moisture-related work is the slowest. A damp subfloor has to be dealt with properly before anything is laid over it
The practical point is that preparation and fitting are often not the same visit. Where they can be combined we will say so at survey.
Moisture Is A Separate Test
A subfloor can be perfectly flat and still be unfit to lay on. Flatness and moisture are two different checks and passing one says nothing about the other.
Concrete and screed hold moisture, and a floor laid over a base that is still drying will fail regardless of how flat it is. Timber subfloors in older properties can sit over unventilated voids.
Two things worth separating, because they are often used interchangeably:
- A damp-proof membrane is a physical barrier installed to stop moisture reaching the floor covering
- A moisture barrier or surface DPM is applied to the subfloor surface, usually as a liquid, where a test shows residual moisture above the level the covering will tolerate
Which one applies is decided by testing the subfloor, not by looking at it. On commercial work we test as standard, and on domestic work we test wherever the base is concrete or screed and the age or history of the slab is unclear.
When It Is A Repair And When It Is A Whole Floor
Not every uneven floor needs taking up. A single soft patch in an otherwise sound timber deck is a repair. A ground floor screed that is out by 15mm across a room is a levelling job. A floor with rot, structural movement or a failed slab is a replacement.
The honest answer is that you cannot always tell which until the old covering comes up, and anyone who quotes a subfloor without seeing it is guessing. What we can do is tell you at survey which of the three we expect, and price the difference before we continue if the floor turns out to hide something the survey could not reasonably have seen.
Before You Book A Fitting Date
Subfloor condition is the single biggest variable in a flooring quote, and it is the reason two quotes for the same room can differ. It is also the most common thing left out of a low headline number, which we cover in our carpet fitting cost guide.
If you have run the straightedge test and found a gap over 3mm, or you have any of the signs above, it is worth having the subfloor surveyed before you commit to a date. Details of what we check are on our flooring services page.
Questions
Common questions
How to tell if your subfloor is level?
Lay a 2 metre straightedge across the floor and look for daylight underneath it. Most manufacturers want the subfloor flat to within 3mm over that 2 metre span. Move the straightedge around the room, including diagonally and across doorways, because a floor can pass in one direction and fail in another. Mark any gap wider than 3mm.
Is it expensive to level a subfloor?
It depends on the area, how far out of flat it is, and which method suits the floor. A shallow skim over a small room is a different job from a deep latex screed across an open-plan ground floor. We survey the subfloor and price the preparation before work starts, rather than raising it as a variation once the floor is up. That way you see the whole figure before you commit.
What happens if the subfloor is not level?
The floor you lay on top shows it, and how it shows depends on the covering. Luxury vinyl telegraphs every dip and hump. Click laminate and engineered boards put uneven load on the joints, which can separate or break. Carpet wears in a line over a high spot. In most cases the covering is doing exactly what it was always going to do over an uneven base, rather than being faulty.
What is the typical lifespan of a subfloor?
A sound subfloor lasts the life of the building. What fails is usually the covering above it, and an uneven or damp subfloor shortens that life considerably. A floor that starts lifting, creaking or separating at the joints within a few years is more often a preparation problem than a product problem.
Can I just lay flooring over an uneven floor?
You can, and most manufacturers will decline a warranty claim if you do. Installation guidance almost always specifies a flatness tolerance, and laying outside it is one of the most common reasons a claim is refused. It is worth reading the tolerance in the guide for the exact product you have chosen, because it varies by floor type.
Not sure what your subfloor needs?
A free survey first, then one price that does not move. Tell us about the floor and we will come and look at it properly.
Or call 01279 942302