An essential element in many construction and renovation projects, the concrete screed is the part that covers the slab. It consists of a cement and binder-based mortar and serves as a support for the floor covering. The concrete screed can also be used to integrate underfloor heating or to carry out levelling in some cases.
Liquid screed, floating or bonded screed, discover the different types of concrete screeds and our advice for carrying out your masonry work.

What is the difference between a screed and a concrete slab?
Often confused, screeds and slabs actually play two very different but complementary roles.
The concrete slab is a load-bearing and structural element serving as the base for laying a screed. It is thicker than a screed and reinforced with metallic reinforcements (reinforced concrete slab).
The concrete screed is a thinner layer used to level the floor surface and to accommodate finishing coverings (tiles, parquet, etc.). Some screeds also serve to integrate thermal insulation (floating screed on insulating panels) or a heating system (screed on underfloor heating) into the floor.

Depending on the specific needs and technical constraints of construction sites, several types of concrete screeds can be made:

• Bonded screed

• Unbonded screed

• Floating screed

• Screed on underfloor heating

• Liquid screed (or fluid screed)

• Levelling screed.

Each has different characteristics and implementation methods.

Relatively thin , the bonded screed is poured directly onto the concrete slab . Adhesion between the two, however, requires the application of a primer or a cement grout (mixture of water and cement).
This type of concrete screed offers excellent mechanical resistance  and is used in areas where surface stability is prioritised (floors of professional and industrial premises, workshops, etc.).

As its name suggests, an unbonded screed is separated from the substrate on which it is applied. This technique insulates and protects the substrate against humidity. The unbonding is achieved by laying an intermediate waterproof layer (a plastic film, polythene, or a membrane).
An unbonded concrete screed is recommended for renovating old floors, in wet rooms (kitchens, bathrooms), or for waterproofing under tiling.

Floating screeds are very common in new house construction and in projects requiring enhanced thermal insulation for floors and concrete slabs.
Independent of the substrate, a floating screed rests directly on an insulator (mainly expanded or extruded polystyrene panels, or polyurethane boards).
This concrete screed can also be combined with acoustic insulation such as rock wool to create acoustic floors between the stories of collective buildings (apartment buildings, offices, etc.).

This type of concrete screed is applied in houses and dwellings equipped with underfloor heating (electric or water-circulating underfloor heating). The screed under underfloor heating covers and protects the heating system elements (cables, coils or pipes).
It also allows heat to be distributed in the floor to ensure a homogeneous temperature and greater comfort in the dwelling.

A fluid screed is also called a liquid screed, self-levelling screed, or self-smoothing screed. Quick to install, the fluidity of this concrete screed allows for perfect levelling of the floor. This flatness is essential for premises and establishments where floor coverings will be laid directly on the concrete screed (mainly flexible flooring such as vinyl, linoleum, or carpet). A fluid screed also offers a high level of finish for creating a concrete floor suitable for large format tiles in spacious living areas.

Levelling is a process widely used for renovation work. It makes it possible to correct irregularities and flatness defects of the existing substrate before laying the new covering.
A levelling screed can, for example, be used to cover old tiling in a kitchen or a raw concrete slab in a garage. Like a fluid screed, levelling can be quite liquid and act as a self-levelling compound.

As mentioned earlier, screed plays a fundamental role in construction.
In summary, a concrete screed allows for:

• Ensuring the flatness and finish of the floor

• Improving the thermal and acoustic insulation of the slab

• Protecting insulation or underfloor heating systems

• Forming a solid and homogeneous base for laying floor coverings

To ensure these different functions, a concrete screed must be laid by carefully following these steps.

Preparation is a crucial step to guarantee the adhesion and quality of the concrete screed. Make sure to thoroughly clean the substrate by removing dust and any material residues. Depending on the type of screed to be poured, apply an intermediate layer (primer or polythene).
Define the thickness of the screed and mark this height around the perimeter of the room, starting from a "zero point." Then install a peripheral strip to protect the walls and absorb concrete expansion.

What mixture for a concrete screed? A screed mortar is composed of cement, sand, and water. Admixtures or fibres can also be incorporated into the mixture.
The dosage recommended by DTU 26.2 for a screed in areas with low stress is between 250 and 350 kg/m3.
The consistency of the mortar should be homogeneous and not too fluid so that it can "hold its shape." Apply the mixture in small areas using a mason's rule.

After spreading the mortar with a straightedge, check the level and flatness of the concrete screed. For finishes, use a float to lightly rub the concrete surface to smooth it out.

The drying time of a screed depends on its thickness and the method used. For a traditional screed, the indicative time is one week per centimeter of thickness (e.g., 4 weeks of drying for a 4 cm screed). A flowing screed will be dry enough to walk on in 24 hours, but drying will be complete within 7 to 10 days.