What is floor screed?
Floor screed is a layer placed over a structural base, membrane or insulation to establish level or falls, carry a floor finish, or form a wearing surface. It is not automatically the structural concrete slab.
Specification unit: m² · verify the complete floor system
Floor screed, screed concrete and concrete screeding
Floor screed is the installed layer described on this page. It commonly uses cement and sharp sand or a proprietary cementitious or calcium-sulphate system, but its role is normally to create a suitable level, fall or receiving surface above the structural base.
Concrete screeding is different: it means striking off and levelling freshly placed concrete with a straightedge, vibrating screed, laser screed or similar equipment. A search for a power screed or concrete screed tool therefore has equipment intent, not floor-material intent.
The Concrete Society defines screed by its functions: obtaining a defined level, carrying the final flooring or providing a wearing surface. It also separates bonded, unbonded and floating construction.
Bonded, unbonded and floating screed
| Construction | Position in the floor | What must be specified |
|---|---|---|
| Bonded screed | Bonded directly to a prepared structural base with the specified bonding system. | Substrate strength and preparation, bonding product, layer range, loads and finish. |
| Unbonded screed | Separated from the base, commonly by a membrane. | Membrane continuity, perimeter isolation, thickness, reinforcement if required and movement joints. |
| Floating screed | Placed over thermal or acoustic insulation rather than bonded to the slab. | Insulation stiffness, loads, screed strength and thickness, reinforcement, joints and heating layout. |
| Proprietary flowing or rapid screed | Installed only within the tested system and declared application range. | Binder type, primer or separation layer, minimum and maximum depth, curing, moisture test and covering compatibility. |
Floor screed thickness and underfloor heating
There is no safe universal screed thickness. Conventional sand-cement guidance and proprietary products can give materially different limits. As one example, the Concrete Society gives 25–40 mm as the optimum range for conventional bonded sand-cement screed, at least 50 mm for unbonded screed, and more than 65 mm for a lightly loaded floating floor. A specific Sika product instead declares its own 20–60 mm application range and separate minima by construction.
Those figures are examples, not a design table to copy. The selected product, bond condition, substrate, loads, insulation stiffness, reinforcement, movement joints and local standard determine the build-up. With underfloor heating, also coordinate pipe or cable cover, thermal movement, commissioning and the heating manufacturer's start-up procedure.
Compare the general guidance with the exact product data: SikaScreed-1 product data sheet. Do not transfer one manufacturer's thickness or drying claim to another screed.
How to specify a floor screed
- Define the function. Record whether the layer creates level, falls, a base for flooring or the final wearing surface.
- Choose the construction. State bonded, unbonded or floating and show every membrane, insulation and perimeter layer.
- Set performance and depth. Use the governing standard, manufacturer data, loads and substrate rather than a generic catalogue value.
- Coordinate joints and heating. Show bay limits, movement joints, reinforcement where specified, heating zones and commissioning sequence.
- Define readiness for covering. Name the curing or drying conditions, moisture-test method and acceptance value required by the screed and floor finish.
- Plan verification. Check substrate preparation, finished level or falls, surface regularity, soundness and moisture before covering.
Drying time is not a fixed calendar promise. Thickness, binder, water content, temperature, humidity and ventilation change the result. Follow the current screed data sheet and the flooring manufacturer's moisture requirement; use the specified test rather than appearance alone.
Floor screed questions
Is screed the same as concrete?
No. The structural slab and floor screed have different roles. Also, “screeding concrete” can mean levelling fresh concrete with a tool, which is a process rather than a separate floor layer.
How thick should floor screed be?
It depends on the construction, product, load, base, insulation and heating system. Use the project specification and exact manufacturer's declared range; do not copy one generic minimum.
Can floor screed go over underfloor heating?
Yes, when the complete screed and heating systems are designed together. Pipe or cable cover, insulation, joints, reinforcement if required, drying and controlled commissioning all need to match the chosen systems.
When can flooring be laid?
After the screed has reached the condition required by both manufacturers and passed the specified moisture or readiness check. A nominal number of days is not enough on its own.
