Over the last 20 years or so, anhydrite screeds have grown in popularity. They have several benefits over the traditional sand and cement screeds, being easy to lay, as they are pumped into position, low cost, and fast drying. They do not curl or crack, and you can cover large areas with them easily. They also produce minimal shrinkage, and they are perfect for both domestic and commercial applications. In addition, they are ideal for use with underfloor heating, especially when using water-based heating pipes, as they fully cover the pipes leaving no gaps or voids.

Instead of using cement as a binder, these types of screeds use anhydrous (dry) calcium sulphate and aggregates. In some cases, the binder may be an alpha hemi-hydrate which is a stronger crystalline form. When you add water to these, they turn into calcium sulphate dihydrate which is usually known as gypsum. The vast majority of the binder is used up after somewhere between 3 and 7 days, and the rest of the water will gradually evaporate through the surface.

Before you lay the final floor finish, it is essential to ensure that the moisture content is at the correct level for whatever type of flooring is going to be laid. Values for moisture sensitive floor finishes are usually less than 0.5% water by weight or less than 75% relative humidity (RH) for impermeable floors and less than 1% water by weight for more permeable floors. If it is necessary to add the final flooring more quickly, this can be achieved by using force drying which is fine to use with anhydrite screeds. You could also consider using a damp-proof membrane.

In addition, if underfloor heating is being installed, this can be brought into use when the screed is at least 7 days old.

Removal Of Laitance

However, before doing that it is necessary to remove the layer of laitance which forms on the surface of the screed as it dries. This is a layer of fine particles which appears as the anhydrite cures, and it is too weak to tile on to. Many floor failures are a result of adhesive being contaminated by laitance. It can also have an effect on the drying process by slowing it down.

Laitance must be removed by sanding using a rotary floor sanding machine and this should be carried out between 7 and 10 days after the screed has been poured. Left any longer, the laitance may be more difficult to remove, and it will certainly delay drying. After the surface has been sanded, any dust must be removed using a vacuum cleaner. 

When bringing the underfloor heating into use under a levelling compound in Oxfordshire, the system should be increased by 5°C each day until either the maximum working temperature is reached, or alternatively in accordance with the underfloor heating manufacturer’s recommendations. The temperature should not exceed 55°C and the level should be kept at the correct temperature for a week or more, and then it can be gradually reduced to somewhere around 15 – 20°C. Using dehumidifiers will also help.

It should also be noted that any introduction of water can affect the screed and may also affect any heating pipes.

It is a good idea to add a primer to the surface of the screed before tiling because this also creates a barrier and helps to make the surface more stable. Choosing the right type of adhesive is important and will need to take into consideration the type of tile to be used and also the method of cleaning the floor when in use. If the finished floor becomes wet with cleaning, there may be some risk associated with water penetrating.

When using tiling for the final flooring, it is necessary to leave gaps of a minimum of 3mm between the tiles, which are then filed with a suitable grout.

Of course, as specialists in the installation of underfloor heating and laying screeds, at UK Screeds we can undertake all of this preparation work for you so that ultimately all you have to do is to lay the final flooring, or, of course, have your flooring contractor do that for you.