A floor screed is the levelling layer between your concrete base and the tiles, vinyl, or boards on top. When it fails, the screed stops holding firm, and the floor covering above it cracks, lifts, or bubbles as a direct result of what is happening in the layer below.
Not every crack or mark means the screed has failed, and knowing the difference saves you from either ignoring something serious or ripping up a floor that was perfectly sound. Pro-Mix Concrete has supplied ready-mixed concrete and floor screed across London and the UK for more than 20 years, and these are the signs we are asked about most.
1. Cracks That Are Wide, Deep, or Still Moving
Most floor screed cracks are harmless, and it is worth knowing that before you start worrying. UK screed specialists put 80 to 90% of screed cracks down to harmless shrinkage as the floor dries, and the British Standard does not treat hairline cracks under about a credit-card width as needing repair. The ones that point to a real failure behave differently from shrinkage cracks, and once you know what to look for, the distinction is clear.
Harmless Cracks
Fine, random hairlines that formed during the drying period and have not changed since are almost always due to shrinkage. They do not affect the floor’s strength or performance and do not need filling unless a floor covering requires a perfectly flat surface.
Cracks That Point To Failure
- Wider than a credit card or deep enough to catch a fingernail
- Running in a straight line between two walls, across a bay, or out from a corner
- Following the line of buried pipes or services underneath
- Still lengthening or widening weeks after the pour
Quick Note:
Cracks appearing within hours of laying are usually shallow plastic shrinkage, while cracks that open over days or weeks point to a deeper drying or structural movement issue. The timing of when a crack appeared is often the clearest clue you have about whether it matters.
2. A Hollow & Drummy Sound When You Tap It
A properly bonded screed sounds solid and dull when you knock it because it is locked tight to the base underneath. When the bond breaks and the screed lifts away from the base, an air gap forms beneath the surface and changes the sound completely. A simple tap test can identify every hollow patch without removing a single tile or floorboard.
Debonding is the single most common way a bonded screed fails, and it never recovers on its own. Once foot traffic, furniture, or a floor covering starts loading a hollow section, it cracks and breaks apart quickly because nothing underneath is supporting it.
How to carry out a tap test
- Use a coin or the handle of a screwdriver
- Tap firmly across the whole floor, working in a grid every 30 to 40 cm
- A dull, solid knock means the screed is still bonded to the base
- A hollow, echoing ring means it has lifted away below
- Mark every hollow patch with chalk so you can map exactly how much of the floor is affected
3. A Surface That Dusts, Crumbles, or Scratches Away
Drag a coin firmly across the surface, and a smooth surface will resist without leaving a mark. If the surface powders when swept, scratches or gouges away with a key, or feels gritty and loose underfoot, the top layer has not gained the strength it needs, and sealing over it will not solve the problem because the weakness runs through the material itself.
A weak surface often starts with mistakes made during the screed installation process. Identifying the cause is essential because the same issue can reappear if those mistakes are repeated during replacement.
What Causes A Weak & Dusty Surface
- Incorrect water ratio leaving cement particles unable to hydrate and harden properly
- Rapid drying in heat, wind, or draughts before the mix had time to set
- Frost on the day of laying, which freezes the water and leaves a friable, crumbly top
- Insufficient compaction, particularly in corners, along edges, and around buried services
4. Edges or Corners That Have Lifted
Unlike bonded screeds, which are fixed directly to the concrete base, floating and unbonded screeds are installed over a membrane or insulation layer. Because they are not attached to the base, they rely on consistent drying and proper installation to maintain a level surface.
As the screed dries, the top layer loses moisture faster than the material below. This difference in drying rates causes the screed to shrink unevenly, lifting the edges and corners in a process known as curling.
Curled edges may appear to be a minor surface issue, but they can lead to significant damage over time. The raised sections lack adequate support underneath, causing movement when people walk across them. Repeated movement can eventually crack both the screed and the floor covering above it.
What To Look For Across The Floor
- Edges or corners sitting visibly higher than the middle of the room
- A raised lip running along a joint line or around the perimeter of a bay
- Corners that rock, click, or flex when stepped on directly
- Tiles or boards cracking or lifting first at the walls rather than in the centre of the room
5. Soft or Springy Spots Underfoot
A finished screed that has properly hardened feels identical underfoot wherever you walk: completely firm, solid, and unyielding. When part of the floor flexes or gives slightly under weight, it usually indicates that the screed beneath the surface has deteriorated, even if the visible finish appears intact. In many cases, the damaged area extends beyond the section where movement can be felt.
What Causes A Screed To Go Soft Beneath The Surface
The two most common causes are early loading and an unstable sub-base. Walking on the screed, placing equipment on it, or allowing traffic before it reaches its design strength can permanently damage the material before it has fully cured.
An unstable sub-base can also lead to failure. If the insulation beneath the screed shifts, has gaps at the joints, or lacks even support, the screed cannot cure against a stable surface. As the insulation moves under load, soft spots can develop in the screed above.
Where Soft Spots Tend To Appear First
- Along busy routes through the room and near doorways
- Around columns, drains, or pipework where compaction was interrupted
- In areas that were trafficked or loaded during the construction phase
Quick Check:
Walk the floor carefully in socks and pay close attention to what you feel underfoot rather than what you can see on the surface. Soft areas almost always extend further beneath than the patch you detect from above, so mark every spot that gives, feels springy, or sounds different as you cross it.
6. Cracked Tiles, Lifting Boards, or Bubbling Vinyl
The screed often appears completely sound, while the floor covering is the first part to show signs of failure. Tiles may crack in a straight line without an obvious cause, vinyl may bubble or lift at the seams, and laminate boards may rise or sound hollow underfoot. These issues are often blamed on the floor covering or the adhesive, but the underlying problem usually lies beneath the surface.
In many cases, the screed has shifted, lost its bond with the base, or retained excess moisture that weakens the adhesive layer. The floor covering simply makes the problem visible. Replacing the covering without assessing the condition of the screed often leads to the same failure within a few months.
What Each Type of Covering Failure Usually Indicates
- Tiles cracking in a line or across a bay: The screed beneath has cracked, and the tile has followed it
- Tiles tenting upward: The screed has expanded, or the bond beneath the tile has broken
- Vinyl bubbling or lifting at seams: Moisture trapped in the screed is migrating up through the adhesive
- Boards lifting or feeling hollow: The screed below has debonded or softened beneath the board
7. Damp Patches, Efflorescence, or a Floor That Will Not Dry
A screed that retains moisture long after it was laid, or one that was covered before it had time to dry, causes failures that can take weeks or months to show on the surface. Damp patches that keep returning, a white powdery deposit called efflorescence blooming on the surface, or a floor covering that lifts and bubbles without any obvious mechanical cause, all moisture points trapped in or beneath the screed working its way upward and breaking down the adhesive and the finish above it.
The most common cause is a missing or damaged damp-proof membrane, allowing ground moisture to rise through the base. The second is covering the screed before it has dried. Screed dries at roughly 1mm a day, which means a 50mm floor needs around five to six weeks in stable conditions before any covering goes down. Sealing a wet screed traps the moisture inside, breaks the adhesive bond, and ruins the finish above.
How To Check For Moisture Before It Becomes A Problem
- Use a calibrated moisture meter across the whole floor, not just one spot
- Run a plastic sheet test: tape a sheet flat to the floor for 24 hours and check for condensation underneath
- Always compare the reading against the threshold set by your flooring manufacturer before laying any adhesive or covering
How To Put A Failed Screed Right
Noticing one of these warning signs does not usually mean the entire floor needs to be removed. Targeted repairs are often enough once the underlying issue has been identified.
The most effective solution depends on the specific problem, but one principle remains consistent: fix the source of the damage before addressing the visible effects. If the underlying issue is left unresolved, the problem is likely to return even after the work is completed.
| The Fault | The Usual Fix |
| Stable, non-moving cracks | Filled with resin or stitched |
| Random cracks where movement is expected | Cut and sealed into controlled joints |
| Crumbly, dusty, or weak surface | Ground back and sealed, or cut out and rebuilt |
| Hollow or debonded sections | Broken out and re-laid |
| Soft or crushed areas | Cut out and rebuilt with repair mortar |
| Trapped moisture | Source identified, membrane repaired, screed allowed to fully dry |
Wrap Up
A failed screed is rarely the disaster it first looks like, but it does get worse the longer it is left. A hollow patch that could be repaired in an afternoon becomes a full bay re-lay once traffic has broken it up, and a moisture problem that needed a week’s extra drying time becomes a failed adhesive job, a ruined covering, and the cost of doing both again.
Catching the signs early, dealing with the cause, and putting back the right screed in the right strength and depth is what keeps a floor sound for decades rather than months, and that starts with a quality mix, batched correctly and delivered when you need it.
Pro-Mix Concrete has supplied high-quality Concrete Mix, floor screed, and ready-mixed concrete across London and the UK for more than 20 years. Every order is accurately batched for your project, delivered the same day or the next day, with concrete pump hire available to reach tight or upstairs spaces.
Call us on 0800 772 3808 to discuss your project and get the right concrete solution for your needs.
- Dennis Broderick is the founder and owner of Pro-Mix Concrete Company, a trusted name in ready-mix concrete solutions across the UK. With over 20 years of hands-on experience in the construction and concrete industry, Dennis brings unmatched expertise, practical insights, and a commitment to quality on every project - from residential driveways to large-scale commercial developments.
