Floor screeding: What you need to know

Floor screeding might not be the most glamorous part of a building project, but it's definitely one of the most important. It is a thin layer of material that sits between your concrete base and the final floor covering, creating a smooth and level surface. Without it, tiles may crack, laminate might warp, and carpets can develop uneven ridges.

Our guide takes you through everything you need to know about screed: what it is, what it's made of, the different types available, how it's mixed and applied, and how long it takes to dry. Along the way, we'll also highlight best practices.

What is floor screed?

Floor screed is a mixture of cement, sand, and water that is applied as a finishing layer over a concrete subfloor. Its primary role is to create a level base for final flooring materials, making sure they look good and perform well over time.

Unlike concrete, which contains larger aggregates for strength and bulk, screed is finer and smoother. This makes it unsuitable as a structural material but ideal for creating the perfect foundation for tiles, laminate, vinyl, wood, or carpet.

Why screed matters:

Provides a level and stable surface by filling in any dips and bumps so that the final flooring is flat and stable.
Reduces the risk of cracks or gaps in flooring by distributing weight evenly across the floor, minimising any movement between the subfloor and the top flooring layer.
Enhances durability and load-bearing performance by providing a dense, compact surface that supports flooring materials and daily foot traffic.
Works effectively with underfloor heating systems due to its smooth, continuous layer that allows heat to spread evenly across the floor, and good thermal conductivity providing consistent warmth without hot or cold spots.

What is screed made of?

At its simplest, screed is a mixture of three basic ingredients: cement, aggregates, and water. But depending on the application, more components may be added to enhance performance. These extra additives can improve properties such as strength, flexibility, drying time, or resistance to cracking, helping to make sure the screed meets the demands you need for the project.

Core Ingredients

Each ingredient plays a role in creating screed. Here's how they do:

Cement: The binder that holds everything together.
Aggregates: Usually sharp sand, sometimes small stones. The type and quality of aggregate influence the finished surface.
Water: Added sparingly to create a semi-dry mix. Clean, potable water is always recommended.

Optional Additives

Additives are used to tailor screed performance for specific applications:

Plasticisers: improve workability and reduce water demand.
Accelerators: speed up drying time.
Air-entraining agents: increase frost resistance.
Fibres: strengthen the screed and reduce cracking.

Because additives alter the properties of screed, it's always essential to follow the manufacturer's instructions to achieve the best results.

Types of floor screed

Not all projects require the same type of screed. Choosing the right one depends on factors like floor loading (how much weight the floor needs to support), drying time (how quickly the screed needs to cure before flooring can be installed), and whether underfloor heating is involved, since some screeds conduct heat more efficiently or are easier to pour over heating systems.

You might also think about the size of the area, the level of surface smoothness needed, and the type of final flooring material.

By function or structural placement

Screeds can be classified based on their structural placement and function within a floor system. This can help determine how the screed interacts with the subfloor, insulation, and any heating systems in place. The main categories, which are bonded, unbonded, floating, and underfloor heating screeds, all have specific applications and thickness requirements to provide the optimal base for your flooring.

Bonded screed: Applied directly onto a concrete subfloor, bonded screed provides maximum strength and stability. It is ideal where a durable, long-lasting surface is required.

Thickness: 25-40mm
Benefits: Maximum strength and stability & minimal movement or cracking

Unbonded screed: Laid over a separating layer such as plastic sheeting, unbonded screed reduces the risk of cracks caused by subfloor shrinkage or movement.

Thickness: ≥50mm
Benefits: Reduces shrinkage-related cracks and allows slight movement

Floating screed: Laid over insulation boards, often used with underfloor heating, floating screeds "float" above the subfloor, allowing for expansion and contraction.

Thickness: 65–75mm (75mm when used with underfloor heating)
Benefits: Reduces thermal bridging and sound transmission

Underfloor heating screed: Specifically formulated for use with heating systems, these screeds are usually flowing or liquid for better heat transfer.

Thickness: Dependent on the heating system and the manufacturer's requirements for the materials you're using
Benefits: Good for heat conductivity and is self-levelling

By material or composition

Screeds can also be classified according to their material composition and additives, which influence how they behave during installation, curing, and when in use. The choice of material affects factors such as flow, drying time, strength, and suitability for underfloor heating or heavy loads. Common types include traditional sand and cement screeds, liquid or flowing screeds, anhydrite screeds, self-levelling screeds, and fast-drying screeds.

Traditional sand and cement screed: This is the most commonly used screed, offering reliability and versatility for most residential and commercial projects. It is strong, easy to mix and apply, and compatible with most floor coverings.

Ratio: Typically 1:3 to 1:4.5 (cement to sand)
Benefits: Supports a variety of flooring types and is suitable for residential and light commercial use

Liquid / Flowing screed: Sometimes made with calcium sulphate (Anhydrite screed), liquid screeds are self-levelling, making them ideal for large areas or over underfloor heating systems.

Benefits: Excellent for large surfaces and underfloor heating, and reduces labour time due to flow properties

Self-levelling screed: Contains additives that allow it to spread evenly without manual levelling. Perfect for thin layers or correcting uneven surfaces.

Benefits: Excellent for thin applications and creates a consistently level surface

Fast-drying screed: Modified with additives (e.g., calcium chloride accelerators or superplasticisers) to speed up curing, fast-drying screeds allow flooring to be installed in days rather than weeks.

Benefits: Ideal for time-sensitive projects, for example, when fitting our shops where downtime should be minimised, or home renovations where new flooring needs to be laid quickly.

Preparing to screed a floor

Preparation is often the difference between a flawless floor and one that causes problems in the future. There are a few key considerations to keep in mind regarding areas of installation, so let's review them.

Subfloor assessment

The concrete base must be stable, clean, and free from dust, grease, and debris. Any cracks or holes should be repaired, and uneven areas levelled before you start screeding. This is so the screed bonds properly, sets evenly, and provides a strong, durable, and level surface for the final flooring.

Membranes and barriers

Depending on your project, protective layers may be necessary to manage moisture and thermal expansion:

Damp-proof membrane (DPM): Prevents rising moisture
Insulation boards: Often used in floating screeds or with underfloor heating
Edge insulation: Allows screed to expand and contract without cracking

Underfloor heating

If heating is being installed, it must be laid and pressure-tested before the screed is applied. You should securely fix any pipes or cables so they don't move when the screed is poured.

Levelling guides

Timber battens or screed rails can help create uniform sections and keep the same thickness across the floor. These guides are removed when the screed sets to ensure smooth transitions between sections.

How to screed a floor

The screeding process involves mixing, applying, levelling, finishing, and curing. Each step requires attention to detail to achieve a high-quality result, so common issues like cracking, uneven surfaces, and poor adhesion are avoided. This helps to make sure that the floor is ready to support the final flooring material, whether it's tiles, laminate, wood, or carpet.

Step 1: Mix the screed

Mixing involves combining cement and aggregates with water to achieve a semi-dry consistency. The "snowball test" is commonly used to confirm correct moisture content.

Combine cement and sharp sand in the correct ratio
Add just enough water to achieve a semi-dry consistency
Perform the "snowball test" to check: the mix should clump when squeezed but not leak water

Step 2: Apply the screed

Screed should be applied starting from the farthest corner. Spread evenly, compact firmly, and ensure the mixture fills all gaps and voids.

Start in the farthest corner of the room
Spread the screed evenly using a shovel or trowel (if not liquid screed)
Compact it firmly to remove voids

Step 3: Level the screed

A straightedge or screed board is used with a sawing motion to level the surface if the surface is not self-levelling. Work section by section, removing battens once adjacent strips are complete, and fill any gaps immediately.

Use a straightedge or screeding board with a sawing motion
Work section by section, removing levelling battens once adjacent strips are complete
Fill gaps immediately

Step 4: Finish the surface

Bull floating smooths imperfections and produces an even surface. Re-floating after initial setting can correct some minor defects, such as small ridges left by the screeding board or shallow depressions where the mix wasn’t fully compacted.

Float the screed to remove imperfections
Bull floating can be repeated after the initial set if required

Step 5: Cure the screed

Curing is vital for strength. Cover the screed with polyethylene sheeting to retain moisture, and leave it for at least seven days. Extend curing if temperatures drop below 10°C or if environmental conditions are suboptimal.

Cover with polyethylene sheeting to retain moisture
Leave in place for at least 7 days
Extend curing if temperatures drop below 10°C

How to mix screed properly

If you want your screed to be consistent, strong, and easy to work with, then it's important that it is mixed properly. Using the correct ratios of cement, sand, and water, following the proper mixing sequence, and utilising suitable equipment all make a significant difference in the final result. Well-mixed screed reduces the risk of shrinkage, cracking, and uneven surfaces.

Standard mix ratios:

Traditional screed: 1 cement : 4 sand (semi-dry)
Heavy-duty areas: 1 cement : 3 sand
Underfloor heating: 1 cement : 4 sand (with flexibility additives)

Mixing equipment:

Hand mixing or free-fall mixers aren't suitable. Instead, use:

Forced action mixers (paddle mixers)
Screed pumps for mixing and pumping large quantities

Mixing sequence:

Load half the sand into the mixer
Add cement
Add the remaining sand
Add water gradually, testing the consistency constantly

Quality checks:

Snowball test – ensures correct moisture content
Consistency – should be semi-dry, not sloppy

Common mistakes:

Adding too much water, leading to shrinkage and dusting
Using the wrong sand (avoid fine building sand or contaminated sand)
Over-mixing, which can cause balling (when clumps of cement and sand form instead of blending smoothly, resulting in weak spots and an uneven finish)

How long does screed take to dry?

Drying time depends on thickness, screed type, and environmental conditions. You must make sure that your screed fully dries so you prevent issues like warping or adhesive failure.

Traditional sand and cement screed

Walkable after 24–48 hours
Full drying: ~1mm per day up to 40mm
Beyond 40mm: slows to ~0.5mm per day

Example:

40mm screed: ~40 days
75mm screed: ~110 days

Liquid/anhydrite screed

Dries at roughly 1 week per 10mm of screed thickness
Minimum 28 days before final coverings
Must be sanded before adhesive is applied

Fast-drying screed

Walkable within 24–48 hours
Can be ready for flooring in as little as 5–7 days

Factors that affect drying

Temperature: Around 20°C is ideal
Humidity: High humidity slows drying
Ventilation: Good airflow speeds evaporation
Thickness: The thicker the screed, the slower it dries

Testing for readiness

Time estimates are guidelines. The only reliable way to confirm readiness is to test for residual moisture, and some of the methods used include:

Hair hygrometer: Measures the relative humidity above the screed surface inside a sealed chamber to indicate whether moisture levels are low enough for floor coverings.
Carbide bomb test: A small screed sample is mixed with calcium carbide in a sealed vessel, producing gas that reveals the exact moisture content.
Protimeter readings: An electronic moisture meter that gives a quick, non-destructive surface reading of moisture levels.

Ready to get started on your flooring project?

Choosing the right materials is just as important as proper floor screeding. Whether you're planning a full renovation or a small update, having the right products at your fingertips makes the process smoother and more efficient.

Explore our wide range of flooring products to find exactly what you need for your project. For a closer look at our selection of high-quality flooring screed, browse our collection today. If you have any questions or need advice on which products are best for your project, then contact us today.

By Katie Taylor