Choosing a screed comes down to one question. Should the layer bond to the concrete base, or float free of it above a membrane? That is the heart of the bonded vs unbonded screed decision, and the answer changes how thick the floor must be, how well it holds off damp, and how likely it is to crack. Bonded screed fixes to the slab and stays thin for strength. Unbonded screed floats over a membrane and protects against damp and movement.
Get the choice wrong and a finished floor can crack, curl or take on rising damp, and correcting that costs far more than getting it right the first time. Government figures recorded “1.4 million dwellings (5%)” in England with a damp problem in 2024, which is one reason the base and the membrane deserve real thought before the pour.
Both options use the same basic material, usually sharp sand and cement, or a flowing liquid screed. The only real difference is how the layer relates to the concrete base underneath. That one factor decides how thick the screed must be, how well it resists damp, and how likely it is to crack once the floor is in use.
What Is Bonded Screed
Bonded screed is laid straight onto the concrete base and fixed down with a bonding agent or primer. The screed and slab then work together as one solid layer.
The Bond Between Screed and Slab
The bonding agent locks the new screed to the prepared surface of the slab, which turns the two into a single working layer. This shared action is what gives the floor its strength. Because the bond does the heavy lifting, the screed can be laid thin and still carry a real load. The base must be clean, sound and properly primed first, or the layer risks lifting away later.
Typical Thickness of Bonded Screed
Bonded screed is the thinnest of the three arrangements. Sand and cement versions usually run between 25mm and 40mm, while some liquid screeds go thinner still, down to around 12mm to 25mm. That shallow depth makes bonded screed useful where floor height is tight and every millimetre counts against door thresholds and existing levels.
Best Uses for Bonded Screed
Bonded screed earns its place on floors that take real punishment and on jobs where depth is limited. It works best in these situations:
- Heavy traffic and high load floors such as workshops and commercial spaces
- Projects where the finished floor height cannot rise much
- Sound, clean slabs in good condition that hold a bond reliably
Beyond those cases, a bonded layer suits any solid base where strength matters more than moisture protection. The trade-off is preparation, since the slab has to be cleaned and primed with care before the screed goes down.
What Is Unbonded Screed
Unbonded screed is laid over a damp-proof membrane rather than straight onto the slab. The membrane keeps the screed separate, so the layer floats free of the base and moves independently of it.
The Membrane That Separates the Layers
The damp-proof membrane or polythene sheet forms a clean break between the screed and the concrete below. This separation blocks moisture rising from the slab and stops any movement in the base from reaching the screed. The floor gains flexibility that a bonded layer cannot offer. That freedom also means the screed relies on its own thickness for strength, since nothing anchors it down.
Typical Thickness of Unbonded Screed
Unbonded screed needs more depth to stay sound without a bond. Sand and cement versions call for a minimum of around 50mm, while calcium sulphate liquid screeds can go thinner, often to about 30mm. The extra material adds height to the floor, which is worth checking against your levels before the pour.
Best Uses for Unbonded Screed
Unbonded screed suits floors where the base is less than perfect or where damp is a concern. The situations that point toward this arrangement include:
- Older or weaker slabs
- Uneven surfaces
- Ground floors with a moisture risk
- Bases that may settle or shift
Where a base moves over time, the floating layer absorbs that movement instead of cracking through. Reinforcement with steel mesh or fibres is common here to control shrinkage as the screed dries.
Quick Comparison Between Bonded and Unbonded Screed
The bonded vs unbonded screed comparison comes down to a handful of practical factors. The table below sets the two side by side before we look closer at the ones that matter most.
| Feature | Bonded Screed | Unbonded Screed |
| Bond to base | Fixed to the slab with a bonding agent | Floats over a damp-proof membrane |
| Minimum thickness (sand and cement) | 25mm to 40mm | 50mm and above |
| Floor height added | Least | More |
| Crack resistance | Lower, slab cracks can transfer up | Higher, isolated from slab movement |
| Damp protection | None built in | Membrane blocks rising damp |
| Substrate preparation | Clean, sound and primed base | Less preparation needed |
| Curling risk | Low | Higher, needs controlled drying |
| Best for | Heavy loads and tight floor heights | Damp risk and older or weaker slabs |
Three of these differences deserve a closer look before you decide.
Thickness and Floor Height
Bonded screed adds the least depth to a floor. Unbonded screed needs more, and that extra height matters where thresholds or existing levels are tight. On a new slab with room to spare the difference is minor, but on a refurbishment it can decide the whole approach.
Crack Resistance and Movement
Bonded screed works as one with the slab, which means any crack or movement in the base can travel up through the finished layer. Unbonded screed avoids that problem because the membrane keeps it separate from the slab below. The floating action gives better crack resistance, though the same freedom raises the risk of curling at the edges if the screed dries too quickly.
Damp and Moisture Protection
Moisture protection is where the two arrangements differ most clearly. Compare them on this single point:
- Bonded screed has no barrier, so it offers no defence against damp rising from the slab
- Unbonded screed carries a damp-proof membrane that blocks rising moisture before it reaches the floor finish
For a ground floor or any base with a moisture risk, that membrane is often the deciding factor. Where the slab stays dry and the load is heavy, the stronger bond usually wins instead.
Not sure which build-up your base can take?
Tell our team at Pro-Mix Concrete about the slab and the floor you are laying, and we will match the screed type and depth before anything is ordered. We supply screed across London, delivered ready-mixed concrete or mixed fresh on site, batched to the exact mix your floor needs.
Where Does Floating Screed Fit In
Floating screed is the third option, and it works differently from the other two. The layer is laid over a bed of insulation instead of being bonded to the slab or floated over a membrane on its own. That insulation is what gives the floor its thermal and sound separation.
Floating screed is the usual pick for a few specific jobs:
- Underfloor heating installations
- Floors that need acoustic insulation between levels
- New builds working to modern energy standards
Because it covers insulation and often houses heating pipes, floating screed is poured thicker than the others, usually 65mm to 75mm. Demand is climbing with underfloor heating, a market “worth £213 million” in 2024 and led by water-based systems in new builds. For most projects, the real choice still comes down to a bonded or an unbonded layer.
How to Choose Between Bonded and Unbonded Screed
The right choice depends on the base you are working with and the floor you are building on top. A few practical points settle it.
Weigh these factors before you commit to a screed type:
- Condition of the existing slab: Sound, clean and level bases suit a bonded layer. Weak, old or uneven slabs are better isolated with an unbonded build-up.
- Available floor height: Bonded screed adds the least depth to the floor. Unbonded layers need more, and that matters where thresholds or floor levels are tight.
- Damp risk: Wherever moisture could rise from the slab or the ground, the membrane under an unbonded screed gives a built in barrier.
- Load and traffic: Heavy point loads and constant use favour a bonded floor that acts as one with the slab. Lighter domestic floors take either type.
- Expected movement: Where a base may settle or shift, an unbonded layer absorbs that movement instead of cracking through.
- Budget and programme: Bonded uses less material but needs careful surface preparation. Unbonded uses more screed with less groundwork.
For a straightforward slab in good condition, a bonded layer keeps the floor thin and strong. Where damp, movement or a poor base are concerns, an unbonded screed is the safer choice.
Is bonded screed stronger than unbonded screed?
Bonded screed carries heavy point loads better because it works as one layer with the slab. Unbonded screed still performs well for domestic floors, and its greater thickness compensates for the lack of a direct bond to the base.
How thick should unbonded screed be?
Unbonded sand and cement screed needs a minimum of around 50mm to stay sound without a bond to the base. Calcium sulphate liquid screeds can go thinner, often to about 30mm depending on the product used.
Can bonded and unbonded screed be used with underfloor heating?
Both can be used, though heated floors usually use a floating build-up over insulation. Bonded and unbonded layers suit pipes fixed to the slab, while a floating layer gives the best thermal spread across the finished floor.
Does unbonded screed need reinforcement?
Steel mesh or added fibres help unbonded and floating screeds by controlling shrinkage and curling as the layer dries. Reinforcement matters more here because the screed is not held down by the base beneath it.
How long before flooring can be laid on screed?
Traditional sand and cement screed dries at roughly one millimetre per day. A 50mm layer can therefore take around two months, while liquid screeds often dry faster. Test the moisture level before laying any floor finish.
Bottom Line
The bonded route keeps a floor thin and strong on a solid base. The unbonded route protects against damp and movement where the slab is older or less reliable. The bonded vs unbonded screed decision depends on the base and the floor finish, and matching the screed to both keeps the layer performing for years.
Whichever build-up your floor needs, whether a thin bonded layer or a thicker unbonded one, the mix has to be right before it reaches the ground. At Pro-Mix Concrete, we supply screed across London and the surrounding areas, delivered ready-mixed or batched fresh on site, and pumped straight to where it is being laid, even on upper floors and awkward access.
Size your pour in minutes with our free online concrete calculator, then order, and we will have the mix at your door ready to go.
- 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.
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