1. Screeding - what do you need to know?

Screeding is a thin base that provides a strong foundation for the top floor. It is applied over the concrete subfloor or other structural material and is several centimetres thick. Screed is a mixture of cement or asphalt and anhydrite, and the different types of screed get their name from the binder used. Thus, there are cement, self-levelling, and calcium sulphate-based screeds, as well as polymer-cement coatings based on polymers, and other similar ones. A screed creates an even, smooth base in preparation for the placement of the final floor layer. In addition, screeding provides highly effective soundproofing and a fire barrier. Screed is applied manually or by machine. All that is important is that it is uniform and level, so that it dries properly, can easily be prepared for the next layer, and to prevent cracking.

2. Screeding - contractors

Screeding a floor is one of the more demanding construction jobs, which is why it is very important to find experienced and professionally trained contractors. A floor that is poorly screeded can easily become damaged later on so it needs doing properly. You can do it yourself if you read up on it and follow the instructions.

3. Types of screed

3.1. Types of screed according to the installation method

3.1.1. Bonded screed

This is the simplest type of screed; it is placed directly on the subfloor and has one or more layers. In order to ensure it is bonded to the substrate, special agents or emulsions are used for better adhesion.

3.1.2. Floating screed

This is the most complex type; it is separated from the substrate by a layer of insulation. It is applied in such a way that wide gaps (at least 4 mm, but at least 10 mm wherever they are installations) are left between construction elements (walls, pillars, chimneys, etc.).

3.1.3. Unbonded screed

It is separated from the substrate by a protective layer (for example, a PVC membrane) and there must be gaps of at least 4 mm to the other construction elements, but these must not touch each other. An unbonded screed is used for areas that will have to endure a heavier load, but where insulation and soundproofing is less important.

3.2. Types of screed according to composition and how they are laid

3.2.1. Standard

The contractor applies and levels a sand-cement mix to obtain a smooth surface.

3.2.2. Dry screed floor panels

These are actually chipboard or fibre cement panels that are laid as a floating floor. The main advantage of such a screed is that it can be installed at whatever time of year since it does not need to dry. In addition, installation is relatively simple and fast, the screed is lighter and thinner, and the final flooring material (parquet, tiles, etc.) is simply and quickly laid over it.

3.2.3. Manually-applied screed

This type of screed is usually used for small surface areas, such as bathrooms, toilets, and storage rooms, but also where the use of machines is not possible due to the confined space. It can also be used to fill in any gaps or chases resulting from replacing wires or pipes.

3.2.4. Machine-assisted screed

Machine-assisted screed consists of several layers of insulation and then the cement-based screed. The contractor pipes the screed over the layers of insulation, thus fixing the pipes, or over the concrete substrate as a floating floor separated from the construction elements, thus preventing sound being transmitted throughout the building.

3.3. Advantages and disadvantages of standard screeding

Since the powder mix for standard screeding is factory-produced, it is uniform. Once applied, it requires some minor levelling. This type of screed is recommended for larger areas or rooms where there will be underfloor heating. It is very strong and resistant to pressure and warping; due to its compact nature and the fact it can be poured evenly, it means that even on larger surfaces there is no need for expansion joints. On the other hand, its biggest drawback is the drying time - a 1 cm thick screed must be left to dry for a week, and the drying time increases as the screed thickness increases. This is why it has to be laid during dry season when the outside temperatures are high. During raining season, drying will take several months - due as much to the high humidity as the lower temperatures. Rainy season, meanwhile, should be avoided at all costs due to delayed drying and cracking. In other words, traditional screeding is unsuitable if deadlines are short. Accelerating the drying process using various dryers only increases the installation costs.

3.4. Advantages and disadvantages of dry screed panels

Dry screed panels are an excellent base for all types of final floors. They can be installed quickly, do not need to dry, and the final flooring can be laid over them immediately. Unlike traditional screeding, they are much lighter and therefore ideal for old wooden stairs or mezzanine floors, for example. Their downside is they cost more so people usually opt for them when it is the only option.

4. Screeding process

4.1. Correct installation procedure for standard screed

Standard screed is divided into three types: traditional cement-based, fast-setting/quick-drying, and self-levelling. For the traditional type, the mix is first prepared in a cement mixer and is then piped onto a specific surface. The contractor then levels off the surface using a screeding level and finally polishes it with a special machine. Fast-setting or quick-drying screed is used when the deadline is short and when the final flooring has to be installed just a few days later. Instead of ordinary cement, fast-setting cement is added to the mix in the cement mixer. This type of screed is very expensive and the installation price is further increased by the dehumidification equipment that must be placed in the room in order to prevent the formation of a high concentration of moisture. Humidity levels can’t rise above 70%.

Self-levelling screed is a factory-made mix that is brought to the site in a cement mixer truck. Water is added to this mix at the construction site and it is then poured onto the surface up to the required height. In this process, expansion joints are often forgotten. In order to prevent the formation of cracks, the screed must be separated from walls and pillars, and there must be appropriate expansion joints, determined according to the thickness, size, shape and other characteristics of the screed. Another common mistake, especially when it comes to traditional screeding, is that the surface is not levelled out and smoothed down properly, with the resulting unevenness creating a problem when installing the final flooring.

4.2. Correct installation procedure for dry screed panelling

Dry screed panelling usually consists of three layers: a levelling course, a layer of insulation, and a supporting layer, which in fact consists of a double layer of crossed-laid panels. The finest sand, or a mixed sand, is using for the levelling course. It is spread across the surface and levelled using a screed board. Before that, insulation tape needs applying around the edges of the space which is being screeded, i.e. along the walls. This will later prevent the transmission of sound. Also, any piping must be covered by at least 1 cm of sand. Insulation boards and panels are then laid, starting from the doorway, and cut to fit as necessary. If the levelling course is under 4 cm thick, it can be compacted by walking over it; however, when it is thicker, a plate compactor or hand tamper must be used. When the base is completely level and compact, the top layer can be laid. Two layers of panels are usually laid, one layer at right angles to the other.

Glue is applied between the edge panels, with their own weight enough for them to stick together. Before laying the final flooring, the joins between the panels should be filled and screwheads covered. This method of installation requires the screed to then be left to stand and harden for 12 hours, before the final flooring can be laid. The most common mistake is that the levelling course is of insufficient thickness to cover any unevenness in the base, which will mean that the final flooring will be difficult or even impossible to lay. Another issue is that the insulation tape along the edge of the walls is often forgotten, so sounds are transmitted to other parts of the building through the walls and floor.