The different types of screeds explained
Apart from those installed in buildings like warehouses where they may be left entirely exposed, it is common practice to cover concrete floors with a layer of screed. This material is typically formed from a combination of sand and cement with additional fibres and other additives. The purpose of the screed layer is to create a smooth and completely level flooring surface. Once the screed has dried, tiles, carpet, timber or other floor coverings can be fitted into place.
There are many kinds of screed available to purchase suited for a variety of applications. However, with multiple options available, selecting the correct solution isn’t always easy. In the following sections, we’ll take an in-depth look at the different types of screeds offered, including their individual properties, their thicknesses and the kinds of applications each is best suited for. Read on to find out more.
A closer look at screed composition
Most screeds are commonly fabricated from a 1:3 to around 1:4.5 ratio of part cement to sand. Screeds are also available that are enhanced screeds with specific additives that improve the innate properties of their natural ingredients. Enhanced screeds offer certain benefits that are not available with standard screeds. For instance, screeds that include additives may deliver extra strength or dry far faster than a standard solution.
Manufacturers also provide self-compacting screeds. These options can be pumped directly through a delivery hose before being levelled out with a dedicated dappling bar. Most of these screeds are anhydrite compounds that are based on a binder made from calcium sulphate. This approach allows large surface areas to be covered swiftly, however, caution must be exercised to make certain that this screed type completely dries.
A screed made from calcium sulphate must be laid with a slight slope to stop standing water. As a result, this type of screed is unsuitable for areas that will be permanently wet, such as those that surround swimming pools, and in steam rooms, saunas and other wet rooms.
All screeds will contract and expand to some degree. Consequently, large areas must have either crack inducer cuts or expansion joints to facilitate movement that won’t cause cracking. Depending on certain requirements, the specification of a screed may vary. Such variables include durability, load-bearing capacity, and whether they need to create a level surface, provide a base for the floor covering or offer a hard-wearing surface.
Bonded screed
In this option, the screed layer is completely bonded onto the substrate using either a bonding agent or a primer. This approach is commonly employed for thinner screeds in environments where heavy loading is anticipated and where insufficient space is available for the unbonded screed to be laid.
The optimum thickness for a cement and sand bonded screed is usually around 25 to 40 mm. However, some calcium sulphate-based flowing screeds can also be laid with a minimum depth of as little as 12 mm. Caution must be taken in the laying process to make sure de-bonding never occurs as this can potentially result in instability and the screed can ultimately fail.
Ideal uses for bonded screed include car parks, driveways and other heavy loading applications. However, bonded screed can also be used in underfloor heating systems that involve cast-in water pipes.
Unbonded screed
Rather than being bonded to the base, an unbonded screed is applied on top of a damp proof membrane, or DPM for short, that is laid directly over the concrete base. For an unbonded cement and sand screed, the minimum thickness possible is usually around 50 mm. However, calcium sulphate screeds can also be laid unbonded with a minimum depth of about 30 mm. The benefit of unbonded screeds is that the flooring is never in contact with the main concrete structure. As a result, the possible impacts of shrinkage or settlement can be less of an issue. The DPM presents a barrier that stops any damp rising up from the substrate below.
It's worth mentioning that an unbonded screed is more prone to issues like curling if it is dried quickly. Caused by moisture content or temperature differences within the thickness of the screed, curling is effectively a vertical distortion of its edges. However, this issue is preventable by adhering to the specified minimum thicknesses and allowing for a slower drying process when necessary. Calcium sulphate screed is not prone to this problem due to its lower shrinkage rates compared to cement and sand screeds.
Ideal uses for unbonded screed are similar to those of bonded screed.
Liquid or flow screed
This type of screed is fast becoming recognised as the industry standard for floor screeds. This kind of floor is sometimes referred to as calcium sulphate screed or self-compacting screed. Exceptionally simple in terms of preparation, flow screed is suited for light pedestrian traffic within 24 to 48 hours. Additionally, partitions may be installed and the screed can effectively be loaded out seven days after installation.
It’s worth remembering that not every flowable screed is manufactured from calcium sulphate. Many of these screeds are based on cement but use a copolymer to create a hardwearing and robust surface akin to concrete. This type of screed is often used in conjunction with underfloor heating installations.
Floating screed
This method involves the screed being laid on top of the insulation layer to create flooring that is thermally efficient. Floating screeds are often used with underfloor heating systems where acoustic or thermal insulation is needed. Floating cement and sand screeds are usually around 65 mm thick for lightly loaded flooring and 75 mm for heavily loaded flooring. Floating screeds made with calcium sulphate can be installed at 35 mm depth in domestic settings or at 40 mm at commercial sites. Floating screed is ideal for improving insulation and heating efficiency for buildings.
Underfloor heating screed
This approach sees the floating screed layer installed directly over insulation or underfloor heating pipes. The screed works to evenly conduct heat across the floor surface. As a result, it can retain heat for a longer time period and cold and hot spots are avoided. These elements are always inserted above the insulating panels to ensure the heat only propagates in the direction required. Cement and sand screeds need a minimum thickness of between 65 to 75 mm.
When fibres have been added the minimum thickness can be reduced to as low as 50 mm and some calcium sulphate screeds can be laid with just a 20 mm cover to heating pipes, providing screed depths of below 40 mm.
Cutting-edge screeds
Cement-based screeds that use advanced technology are now available and can incorporate all the advantages of a semi-dry screed. This includes creating steps within doorways to facilitate different floor coverings whilst incorporating the benefits of a flow-applied screed.
For example, Retanol Xtreme never requires a curing period under polythene and can dry to under 75 per cent Relative Humidity (RH) within three to four days. It can be laid from 35mm thick floating and 15mm thick bonded and is suitable for dry or wet external or internal conditions.
It can also dry uniformly across the entire body of the screed instead of from the top down. As a result, it experiences no curling or cracking and so it doesn’t need any fibre reinforcement. It is an ideal option for underfloor heating systems. This is because you can apply it with only 30mm coverage while commissioning can be undertaken after 24 hours. It also offers a 60 per cent improvement in heat conductivity over standard screed systems. Heavy-duty versions are also available when required.
Fast screed drying services
At Rapid Screed Drying, we specialise in supplying effective hire equipment for the construction sector that efficiently speeds up how long it takes to dry screed on site. Our robust electric boilers can plug directly into your building's manifold, enabling the underfloor heating system which can in turn assist with drying the screed.
For further information, and a free no-obligation quote, get in touch with our expert team today.
