Liquid Screed – The How to Guide

A Guide To Liquid Screed and Underfloor Heating Screed

Liquid screed, or flowing screed, is a thin layer of materials that are applied over a concrete subfloor. It is made with a very fine mixture of small aggregates and water, which allows it to pour throughout the contained area and level itself naturally. It is used to provide a very smooth and flat surface that primary flooring materials are then laid on top of.

There are two types of screed commonly used today. One is a mixture of calcium sulphate that is also known as anhydrite screed. The alternative is a cement based screed. Anhydrite is more popular because it has improved thermal properties. Cement however, is faster drying but is limited in supply in the UK and only available in certain postcode areas at present, although, this is slowly changing and demand increases.

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What Makes Liquid Screed So Great For Underfloor Heating?

One of the main reasons that so many people are turning to flowing screed is because of the efficiency boost it provides to underfloor heating systems.

Most experts would agree that liquid screed is the best flooring option available if underfloor heating pipes are used. In the past, ready mix cement was the popular choice, but it had many different flaws.

The most significant problem with traditional sand and cement is the texture. The aggregates are much larger and that greatly reduces the flowability of the concrete mixture. When poured over the pipes, there are often a large number of air bubbles left between the pipes and the concrete mixture. The air bubbles reduce the efficiency of the heating system and are nearly impossible to remove.

The aggregates in flowing screed are much smaller and the overall texture is much smoother. It easily flows over the pipes as you would expect any smooth liquid to do. It eventually cures directly against the pipes without any air bubbles between the pipe and the screed. This makes the heat transfer from the pipes to the floor and into the room more efficient.

Whether you’re installing an underfloor heating system in a residential space or a commercial office; heating efficiency is a big deal. The removal of those air bubbles may seem like a small improvement, but the increased efficiency will save you a lot of money on wasted energy over the years.

An increase in efficiency also means that the heating system doesn’t need to work as hard to produce the desired temperatures. The heating unit will experience less wear and tear, require less maintenance, and enjoy a longer lifespan. All of these are factors that will save you a considerable amount of money over time.

The required depth of the screed is another way in which it can boost heating efficiency. In most cases, flowing screed used in conjunction with an underfloor heating system only requires a depth of 50mm. Traditional sand and concrete alternatives may have a depth of 150mm or more. This has a significant impact on the heating system because it takes longer for the heat to transfer to the concrete and for the concrete to reach an optimal temperature.

A reduced depth means reduced time from the moment you set a temperature until that temperature is reached. Once again, it also means the heating unit has less work to do to reach the desired temperature. If the unit is working three times as hard to heat three times as much concrete, then you can expect it to require more maintenance, to encounter more problems, and to require replacement ahead of schedule.

It’s difficult to gauge the boost in efficiency that any one place might experience when using flowing screed instead of traditional screed options. The quality of the screed, the specific materials used, and the work performed by the installation company will all impact the efficiency boost. Assuming a high-quality screed is used and the installation company does a great job, you can expect flowing screed to be at least 100 per cent more efficient than traditional sand and concrete.

These benefits relate specifically to its use with an underfloor heating system. But that’s certainly not the only time that liquid screed should be used. It’s believed to be a better flooring option in many different scenarios. Let’s take a closer look at some of the other benefits that liquid screed offers to help us understand why it’s becoming so popular in the UK.

Why Is Liquid Screed So Popular Throughout Europe?
The use of flowing screed in commercial and residential environments has spread like wildfire throughout the UK and all of Europe. It’s even become an extremely popular option in many places in Africa. And these aren’t just homes and businesses with underfloor heating systems. As a matter of fact, the majority of homes that use flowing floor screed do not have an underfloor heating system. Why has it become such a popular choice? It’s because it offers numerous advantages that are too big to ignore.

Many of the benefits associated with flowing screed boil down to efficiency. As mentioned earlier, it improves the efficiency of the heating system. But even the installation and materials are considered to be more efficient than traditional screed. It takes less time to install, it can dry much faster, and there is far less preparation required. Thus, in terms of time, workload, and expenses, flowing screed is a more efficient solution than traditional screed.

Some estimates claim that flowing screed can be installed ten times as fast as traditional screed. That’s good news for the installation company as well as the client. An entire house can have liquid screed installed in only a few short hours. Most professional companies are capable of covering at least 1000 square meters in a single day.

Of course, the experience and abilities of the installation company are going to have an impact on installation time. But this impact is less noticeable than with traditional screed installation. The major difference is the simplicity of the installation. There are very few opportunities for a worker to make a mistake and slow down the installation. In comparison, costly mistakes during the installation of traditional screed can add hours or even an extra days to the project.

Shrinking and curling are two of the biggest problems you can face when relying on traditional screed. Shrinkage, cracking, and curling all occur during the screed drying phase. This happens because the mixture naturally shrinks a small amount while drying. The shrinkage occurs according to how the screed dries, which is from top to bottom. When the top dries and shrinks before the bottom it causes the screed to curl up at the edges and to crack.

Several additional factors influence the rate and degree with which curling occurs. For example, a corner area in an irregular floor plan will cause a separation that is far more likely to lead to cracking. Certain additives added to semi-dry screed, the bond strength, the thickness of the screed, and the drying time can all impact the outcome as well.

However, these are not problems that are encountered when using flowing screed. Liquid screed experiences very minimal differential shrinkage and thus curling does not occur. It also means that corner areas are far less likely to experience cracks due to differential shrinkage. The lack of shrinkage easily translates to saving in money because you are less likely to suffer damages that require new screed.

The quality of the screed is another major advantage. Flowing screed pumped into a development will have a much smoother and more even surface compared to semi-dry screed that is flattened by hand. The liquid nature and flowability of the screed allow it to evenly distribute naturally in a perfectly uniform manner.

Flowing screed is also generally denser than semi-dry screed. This is part of the reason why it is more thermally conductive than semi-dry sand and concrete. That denseness also helps to control sounds in the room. Sounds are deadened against the dense surface of the screed. This allows for increased privacy in any room.

Some of the benefits of flowing screed are affected by the type of materials used in the screed. For example, anhydrite-based screed will be more thermally conductive than a flowing screed made with cement. It’s important to keep these differences in mind when choosing the right screed for your project.

Despite the materials used, the installation of liquid screed is generally the same all-around. It’s a much faster and cheaper installation process than traditional screed. Next up is a breakdown of that installation process as well as certain factors that could influence its time or cost.

How Is Liquid Screed Installation Handled?

The installation of liquid screed is incredibly fast and efficient when compared to traditional screed. The entire process can be broken down into three phases. The first phase is preparation, the second is the pumping, and the final phase is a dappling finish. All of these phases can be completed within a few hours at most locations.

The first part of the preparation phase is applying the substrate. Flowing screed should always be laid on a prepared substrate chosen specifically for the project. The screed may either be bonded directly to the substrate or poured over a layer of polythene separation. The additional separation layer is often used to add insulation for thermal and or acoustic purposes.

If underfloor heating is to be used, then it will be added during this phase. The heating pipes are installed above the polythene layer and clipped into place. Water should be in the pipes and pressure tested prior to pumping of the screed. Edging strips are attached to the corners and edges of the walls. Finally, a laser level and tripods are used to determine the correct screed height.

The pumping of the screed is the fastest and easiest part of the process. In most cases, the screed is brought onsite with a cement mixer truck, which pours its load into the screed pump hopper and is then pumped out into the installation area via a hose. The pump will handle most of the heavy lifting. The workers simply direct the hose to ensure an even pouring. Overall, this requires significantly less work than manually filling a room with screed and using hand tools to even the floor.

The surface of the floor is then finished by dappling. Afterwards, the screed will need at least 36-48 hours to set before the room can become accessible to foot traffic again. Even though they will be safe to walk on at this point, it will not be completely dry. The exact drying time will depend on the depth of the screed and the materials used. More information regarding the drying process will be further down this page.

Follow Up Steps For Your New Screed Floor
An additional step is required to ensure the integrity of the floor. It involves sanding the surface and the laitance removal. You will need to wait about a week after installing the screed before you can follow up with laitance removal via a sanding machine, equipped with a dust extraction vacuumed system. At this point, the floor will not be completely cured, but it will be solid enough for sanding and maintenance.

Laitance is a serious problem with any concrete-based screed floor. It can easily cause a failure in the flooring system if not attended to. It is essentially a very weak layer on the top of the floor that can easily crack or break.

Laitance occurs when very fine aggregate rises to the surface of the cement mixture due to the volume of water in the mixture. The formation of the laitance will usually occur within the first hours of installation. More laitance is likely to form should the fresh installation be subject to rain water in the first 24 hours. It’s important to note that surface laitance will always be present on a new screed, so we will arrange a date and time after initial installation to return to your property to carry out our laitance removal procedure.

How Long Does Liquid Screed Take To Dry?

The two factors that will influence the drying time of flowing screed are the type of screed used and the depth of the screed. An anhydrite based screed will take considerably longer to dry than a cement based screed. It is a good guideline to give the anhydrite screed 1 day of drying time per each millimetre of thickness up to 40 mm. Once exceeding 40 mm the dry time increases to 2 days per millimetre. A cement based screed can dry in 30 percent less of this time.

Liquid screed must be poured at least 30 mm above the pipes used in underfloor heating systems. The average pipes in these systems have a diameter of 20 mm. Thus, most homes have a screed thickness of exactly 50 mm. If that was an anhydrite formula, then it would require a total of 60 days to completely dry. A cement screed alternative would require roughly 30-40 days to dry. Cement liquid screed is not available to every postcode location in the UK, so you would need to check availability in your area, you can do that by contacting us via our contact page, or calling us on 01452 689449.

In either scenario, it is usually safe to begin walking on the floors after 48 hours has passed. But you will need to wait until moisture testing has confirmed that the floor has completely cured before installing the final floor finish. There are two types of moisture testing to consider. The calcium carbide moisture test as well as the more traditional moister test with a digital moisture meter.