Who this guide is for
- Owners considering a seamless floor and unsure what the base has to be like first
- Anyone weighing a formed floor against a covering that arrives as a product
- People with an existing slab who have been told it could be ground and polished
- Readers who want to understand why preparation dominates the price of these floors
- Anyone planning a programme where a curing period sits on the critical path
The substrate is the specification
Almost every failure in this family turns out to be a substrate story rather than a material story. A jointless system depends entirely on its bond, and laitance, contamination, weak surface layers and old coatings all fail at the bond line rather than within the finish. Preparation is mechanical, and the amount required is usually the largest single item in the work.
Thin applied finishes inherit the behaviour of what is under them. A screed that flexes, a board that deflects, hollow bedding under old tiling or a joint that opens and closes all tend to register at the surface, and existing cracks can telegraph through. The base decides, and the coating does not.
Polished concrete is the extreme case: the slab is the finish
Grinding a slab reveals it rather than hiding it. Trowel marks, colour variation, patch repairs, old drainage runs and separate pours all become part of the finish, and grinding deeper exposes more aggregate. An applied coating hides a substrate; polishing displays it.
Because material removed cannot be put back, the level of exposure is agreed before work starts, and a trial area is the usual way of finding out what the slab actually contains. Where the result is planned from the outset, the pour and the polishing specialist are coordinated before the concrete is placed.
Moisture beneath has nowhere to go
Moisture rising through a slab with no effective barrier beneath it can lift a cured film from below, and once a continuous layer has been bonded over the top there is no route out. Establishing the moisture condition of the base before any system is chosen is not a formality here.
The same applies to a thin trowelled coating, where moisture arriving from below, from a leaking detail or from construction water still leaving a slab disrupts both bond and appearance. Whether an existing floor has effective moisture control is settled before work starts rather than afterwards.
Joints have to be carried through, whatever the finish looks like
A seamless appearance is exactly what makes people forget the joints. Structural and movement joints have to be carried through the finish rather than covered over, and a hard finish carried continuously across a moving joint will crack along it. In terrazzo, divider strips are read as decoration when their main job is deciding where the floor will crack.
Granolithic joint positions are set to follow those in the base rather than chosen for convenience, and a joint layout is therefore something to ask about before the finish is chosen for its appearance.
The site has to be given up while it happens
These are wet or dusty trades that take a space out of use. Grinding generates noise and either slurry or dust and usually means other trades leave the room. Resin work needs the space closed off and kept clean throughout application and curing. Asphalt brings hot works, melting plant and site agreements that have to be arranged well ahead of the pour.
Curing is an activity, not waiting. A granolithic topping dried too quickly loses surface strength and dusts, and traffic returning early can mark a resin surface permanently. Protecting the finished floor from following trades is a genuine issue and is frequently underestimated.
Conditions on the day are part of what is bought
A floor formed on site is made from components in a room, so temperature, humidity, airflow and cleanliness during application and cure all form part of the finished result. Trowelled work in particular depends on the applicator's skill and on conditions during curing, which is why sample panels prepared by the actual applicator in the room's own light are the only reliable preview.
Asphalt adds temperature in service as well as during laying: the binder stays thermoplastic, so warm rooms, solar gain and any heat within the floor change how it behaves and have to be raised with the specialist early.
Repair means matching a one-off
Local damage in a jointless floor is repaired by preparing and recoating an area, and the repair usually shows as a difference in sheen even when the colour matches. In a trowelled finish a patch is a fresh area within an older one. In granolithic a repair reads differently in colour and texture for a long time.
There are exceptions worth knowing. Asphalt is cut out and re-poured so the new material fuses with the old. Polished concrete can be reground. Terrazzo can be cut out, refilled and reground, though matching the aggregate distribution is difficult. How a repair will be approached is worth agreeing in advance rather than after the event.
Before committing to a floor formed in place
- 1Establish what the existing base is, including previous pours, patches and repairs
- 2Have the moisture condition of the base investigated before choosing a system
- 3Ask what preparation is included and resist trimming it when the price is negotiated
- 4Ask whether the substrate moves, deflects or cracks, and whether that has been assessed
- 5Identify every structural and movement joint and confirm it is carried through
- 6For a ground slab, agree the depth of exposure and ask for a trial area
- 7Ask for sample panels prepared by the actual applicator, in the room's own light
- 8Confirm what conditions the system needs during application and curing
- 9Agree how long the space is out of use and who is kept out of it
- 10Establish the curing period from the manufacturer, not from the programme
- 11Agree how the finished floor is protected from following trades
- 12Ask what a realistic repair looks like, and agree the approach in advance
- 13Where sealers are involved, agree who renews them and on what basis
Common mistakes to avoid
- Choosing a seamless finish for its appearance and discovering the joint layout afterwards
- Trimming the preparation when the price is negotiated, which is the item that decides the result
- Bonding a continuous film over a slab whose moisture condition was never established
- Treating curing as waiting rather than as an activity with its own requirements
- Grinding deeper to chase a look, when material removed cannot be put back
- Applying a thin coating over a base that moves and expecting the coating to bridge it
- Judging a hand-trowelled finish from a workshop sample rather than a panel in the room
- Underestimating how disruptive on-site grinding, hot works and wet trades really are
When to involve a professional
- Whether a substrate is sound, stable and appropriate for a bonded formed finish is assessed by a qualified professional and the system manufacturer
- Moisture condition of a slab, and whether an effective barrier exists beneath it, is investigated by someone qualified to interpret the result
- Whether a structure can take the additional weight of a terrazzo or similar topping is an engineering matter
- Where deep grinding could reduce cover over reinforcement or reach heating pipes, that is a matter for a qualified professional
- Slip behaviour where a surface will regularly be wet is discussed with a specialist rather than assumed
- Hot works, where asphalt is used, require site agreements arranged with the contractor in advance
Frequently asked questions
Questions readers ask about this topic
Why is preparation such a large part of the price?
Because it is what the floor depends on. A jointless system is only as good as its bond, and laitance, contamination, weak surface layers and old coatings all fail at the bond line rather than within the finish. It is also the item most often trimmed when a price is negotiated.
Can an existing slab be polished?
Often, but the result is inherited rather than chosen. What is in the slab decides what appears, including trowel marks, patch repairs, old services and separate pours. A trial area is the usual way of finding out what is actually there before committing to the look.
Do these floors still need movement joints?
Yes, and the seamless appearance is exactly why they get forgotten. A hard finish carried continuously across a moving joint will crack along it, so joints in the construction have to be carried through the finish rather than covered over by it.
How long does the floor take to cure?
That belongs to the manufacturer of the specific system rather than to the programme, and it is not something a general page can state. What is worth establishing early is who owns that period, what the space cannot be used for during it, and what returning early would cost.
Can a formed floor be repaired invisibly?
Rarely. A local repair generally shows as a difference in sheen, colour or texture even when carefully done. Asphalt is the unusual case, because fresh material fuses into the old rather than sitting on it, and polished and ground floors can be refinished across an area.
Keep reading