Interior and finishes · Flooring Materials
Anhydrite Flowing Screed (Calcium Sulfate)
Sets out how a sulfate bound flowing screed differs from a sand and cement screed in chemistry, placing and drying, and why the interface with any bonded covering becomes a subject in itself.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What anhydrite screed is
Anhydrite flowing screed is a pumpable floor screed bound with calcium sulfate rather than with Portland cement. It arrives wet, is poured over a separating layer, insulation or heating pipework, and is dispersed with a levelling bar so that it finds its own level. There is no compaction and no trowelling in the sense a traditional screed needs, which is why large areas can be placed in a single operation.
Against a sand and cement screed the differences begin with the binder. A cement screed is a semi-dry mix laid and compacted by hand, gains its strength through cement hydration, and copes far better with damp exposure. A sulfate bound screed is a liquid that levels itself, needs no compaction, and softens under sustained moisture. It is also commonly laid in thinner sections and is often used to encase underfloor heating pipework, though the regime for any specific product comes from that product's documentation.
Two consequences dominate site practice. Fine material rises as the screed flows and dries as a weak surface skin, which has to be removed and the surface prepared and primed before anything is bonded to it. And because the substrate is a sulfate rather than a cement, the compatibility of adhesives, levelling compounds and coverings with that chemistry is a question to settle with the suppliers rather than a matter of habit.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- calcium sulfate screed
- gypsum screed
- liquid screed
- flowing screed
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Floor build-ups where underfloor heating pipework is being encased and a flowing material suits the geometry.
- Large open areas where a single pour is preferable to laying and compacting a screed by hand.
- New build floors over insulation where a separating layer allows the screed to float above the structure.
- Refurbishment where a level base is needed across an irregular existing floor before coverings are laid.
- Projects where the programme favours a rapid pour over a slower hand laid screeding operation.
- Areas where hand laying would be awkward because of pipework, service runs or an irregular plan shape.
Consideration
Where it is commonly considered
- Where a level base is wanted over a large area and hand laying would be slow or inconsistent.
- Where the floor build-up encases heating pipework and the material has to flow around it.
- Where the internal environment can be kept dry throughout construction and afterwards.
- Where the programme allows for drying to be verified before coverings are laid over it.
- Where preparation of the surface, including removal of the surface skin, is planned into the works.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- In locations subject to sustained moisture, since a sulfate binder softens under those conditions.
- Where a covering is to be bonded and the adhesive has not been confirmed as compatible with a sulfate substrate.
- Where the programme cannot allow for drying to be verified by test before an impermeable covering goes down.
- Where the surface will be left exposed as a wearing floor, which is not what a screed of this kind is for.
- Where the perimeter, isolation and any bay layout have not been designed for a floating pour.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The binder is sensitive to sustained moisture, and drying before an impermeable covering is laid has to be verified by testing rather than judged by time or appearance. Both those points separate it sharply from a cement screed.
- Substrate dependency
- This is a base, not a finish. It has to be prepared before anything bonds to it: the weak surface skin removed, the surface primed as the supplier directs, and the result confirmed sound before a covering is ordered.
- Installation dependency
- Placing is a pumping operation, so access for the pump, protection of adjoining areas and the sequence of the works all change. Perimeter isolation and bay layout are set out before the pour, not adjusted during it.
- Thermal behaviour
- Flowing material encases heating pipework closely and is commonly used for that reason. The heating regime, including how and when a heated floor is first brought into use, comes from the product documentation and the system designer.
- Documentation
- Which adhesives, primers and levelling compounds are compatible with a sulfate substrate is a supplier question, and the answers should be obtained in writing before the covering is chosen rather than after the screed is down.
- Repairability
- Local repairs mean matching a poured material to an existing surface, which is more awkward than patching a hand laid screed. Damage during following trades is a common reason repairs are needed at all.
Exposure
Exposure and environmental conditions
- Will this floor ever be exposed to sustained moisture, either during construction or afterwards?
- Is the building weathertight and able to be ventilated while the screed dries?
- What covering is planned on top, and has its adhesive been confirmed against a sulfate substrate?
- Is there heating within the floor, and who is specifying how it is first brought into use?
- How will the screed be protected from following trades before coverings are laid?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- A separating layer or insulation below determines whether the screed floats or bonds, and that is decided first.
- Perimeter isolation strips allow the poured field to move and must survive until the coverings are complete.
- Any damp proof provision below the floor is central here because of the binder's response to moisture.
- Heating pipework has to be fixed down so it stays in position while a liquid material is poured around it.
- The interface with the covering above needs a primer and preparation regime agreed by both suppliers.
Appearance
Appearance and finish considerations
- The surface is a base and is not intended to be seen, so its appearance matters only as a bonding surface.
- The dried surface skin looks acceptable and is not, which is why it is removed rather than left in place.
- Surface flatness is generally good from a flowing pour, but it is checked rather than assumed.
- Bay joints and perimeter details are the visible geometry of the pour and are set out in advance.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- How will this screed behave if the building is exposed to weather before it is enclosed?
- How will it be protected from following trades while it dries?
- How is the surface being tested for soundness after the skin has been removed?
- What happens if drying is slower than the programme assumed?
- Is the covering above compatible with a sulfate substrate over the long term as well as at installation?
- Is there sufficient cover over the pipework for the build-up proposed, and who has confirmed that?
Upkeep
Maintenance and repair considerations
- How would damage caused during construction be repaired before coverings are laid?
- Who is responsible for preparing and priming the surface, the screeding contractor or the floor layer?
- What happens if the covering later has to be lifted, and how is the screed protected during that?
- How is a localised soft or damaged area identified before it is covered over?
- Is any record being kept of the screed type for whoever works on this floor in future?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Access for a pump and hoses governs where and when the pour can happen on a live site.
- Perimeter isolation, bay layout and any movement provision are set out before the material arrives.
- Heating pipework and any insulation must be secured so they do not move or float during the pour.
- Drying conditions, including ventilation and heating, are managed rather than left to the weather.
- Surface preparation, including removal of the weak skin, is a separate operation with its own programme slot.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The supplier's product data covering the specific screed being poured on this project.
- Guidance on surface preparation, including removal of surface laitance and priming before coverings.
- Drying and testing guidance, including the test method the supplier expects to be used.
- Confirmation from the covering and adhesive suppliers that their products suit a sulfate substrate.
- Guidance for first use of any heating within the floor, agreed with the heating system designer.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Why is a flowing screed being proposed here rather than a sand and cement screed?
- How will drying be verified before the covering goes down, and what test will you use?
- Who removes the surface skin, and how will the surface be checked afterwards?
- Has the adhesive for the chosen covering been confirmed against a sulfate substrate in writing?
- How is the screed being protected from following trades and from water on site?
- Where do the bay joints and perimeter isolation fall, and how do they relate to the finished floor?
- Who is specifying how the underfloor heating is first brought into use over this screed?
- What happens to the programme if the floor is not dry when the covering is due to be laid?
Blind spots
Commonly overlooked points
- The speed of the pour is used to shorten the programme, and the drying time it does not shorten is then squeezed.
- The weak surface skin looks like a sound floor and gets covered over, taking the new covering with it when it fails.
- Adhesive compatibility with a sulfate substrate is assumed from experience with cement screeds.
- Water left standing on the floor during construction does more harm here than it would to a cement screed.
- Nobody records which screed was used, so the next contractor to work on the floor treats it as cement based.
What this page does not do
- Drying, testing and covering compatibility are matters for the suppliers involved and for a qualified professional, and no timings or test criteria are given here.
- How and when a heated floor is first brought into use over this screed comes from the product's documentation and from the heating system designer.
- Exposure to water is a question for the whole floor design and its drainage, resolved by a qualified professional and not by the screed alone.
- Build Design Hub publishes this entry as general reference and does not test, approve or recommend screeds, primers or adhesives.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Flooring Materials
Reference entries for floor coverings and floor surface systems — wood and engineered wood, laminate, vinyl and LVT, linoleum, rubber, cork, bamboo, carpet, terrazzo and resin.
Browse all flooring materials entries →