Calcium Sulphate Subfloor Preparation

Calcium Sulphate, Anhydrite or Gypsum subfloors are becoming increasingly popular with building contractors and are presenting more challenges for flooring installers to achieve superior installations.

Often referred to as flowing, poured or liquid screeds, these are non-structural surfaces that provide lightweight, faster and more cost effective alternative to the traditional sand and cement or concrete subfloor slabs.  For example, a 37mm gypsum screed weighs approximately 64kg/m2 whereas a concrete slab weighs 86kg/m2 at the same thickness. So the benefits in designing these surfaces into the construction of a new multi floor building are clear, and it will still have the same compressive strength as the concrete screed.

Drying times for Calcium Sulphate based screeds vary depending on several factors, such as thickness, ventilation, moisture content in the screed, relative humidity in the building environment in general and temperature. Before, during and after installation, the area should have adequate ventilation, a minimum constant temperature of above 10°c. During this drying process laitance will form on the surface, these rise to the surface after the screed has been poured.

Current British Standards and CFA recommendations, state that the subfloor should be dry and, in a condition, to receive following trades, this is true of any subfloor. It is the responsibility of the Building Contractor to hand over a floor in this condition, however, in the real world, the area is rarely kept at the optimum temperature and there is not adequate ventilation or airflow. It is common to see in some circumstances that the contractor has not yet fitted the windows or even made the building water tight. Whilst flooring installers are responsible for ensuring that their work is carried out to the relevant standards and within the manufacturers’ current recommendations, they you are not a construction or screeding expert.  The onus is on the Building or Main Contractor to sign off the floor as fit for following trades, and that they are dry and in the case of Calcium Sulphates that the proper preparation has been carried out.  Removal of the laitents should take place within 7-14 days of the installation. The longer these are left, the more compacted and harder they become making removal a more difficult task and also impairs the drying time. This is where the difference of opinions on site generally start as the builder will insist the floor is dry as ‘it has been down for ages’ and doesn’t require any preparation.

So in most instances flooring contractors will arrive on site, the surface is covered in a hard, patchy white/cream sometimes flaky residue or laitance. This is the early warning sign of a poorly prepared floor which is not suitable to receive a levelling compound or flooring adhesives. If correctly prepared the fine aggregates used within the gypsum should be visible. The surface should be smooth, clean and free of any laitance, which will impair the bond of any products installed over the top. The laitance must be removed by mechanical means, such as a WOLFF Neo 230 grinding machine with diamond segments. A good vacuum extraction system will also be required as the Calcium Sulphate dust particles are very fine. The system picture shown is the WOLFF Neo 230 grinding machine with a pre-separator and Vacuclean 2 industrial vacuum.

As with all floors, surface preparation is key. The floor must be dry (below 75%RH, clean and dust/contaminant free). Once the floor has been ground sufficiently and the fine aggregates in the screed exposed, the floor should be thoroughly vacuumed before applying a coat of primer, UZIN PE 360 is one of the products recommended by many  Calcium Sulphate screed manufacturers. When this has dried, a coat of Calcium Sulphate smoothing underlayment can be applied such as UZIN NC 110. 

More information on these products can be found at or by contacting UZIN Technical representatives. UZIN also have a range of surface treatments for Calcium Sulphate floors reading above 75% RH and depending on the levels of moisture and the installation type, these are a quick and easy way of saving time on site.

Recommended Machines

NEO 230

Ideal floor grinding machine for small to medium areas as well as for edge grinding

Pre-separator 30L

Powerful pre-separator that separates large quantities of coarse dirt and fine dust before vacuuming

Vacuclean 2

High-performance industrial vacuum cleaner for coarse dirt and dust with large filter surface

Installation systems for screeds, floors and wood flooring

Machinery and special tools for substrate preparation and installation of floor coverings

Complete product range for installation, renovation and maintenance of wood flooring

Resin and cementitious floor finishes combining performance and design

Installation systems for tiling and natural stone

High quality painting, plastering and drywalling tools