Resilient Flooring to Anhydrite Screeds Incorporating Underfloor Heating


Sometimes referred to as a calcium sulphate, gypsum-based or flow screed, anhydrite screeds are a mix of fine and coarse aggregates, with a calcium sulphate (gypsum) or calcium sulphate dehydrate binder which are designed to provide a level surface ready to receive a resilient or engineered wood finish. They have a fluid consistency and can easily be pumped on to sites, meaning that large areas can be laid in a short time and the encapsulation of underfloor heating pipes is improved. Anhydrite screeds experience minimal shrinkage, with a low risk of cracking or curling and can generally be laid thinner than a conventional cement-based screed. They are not suitable for external or wet areas unless a suitable tanking membrane is applied.


The screed should be allowed to dry for a minimum of 1 day per millimetre up to 40mm thick, with an additional 2 days per millimetre above 40mm to a Relative Humidity (RH) of not greater than 85%. Surface laitance should be removed by sanding and vacuuming, this is normally carried out within 4-5 days after laying and will also aid the drying process of the screed. The drying rate can also be improved by using heaters and dehumidifiers. The underfloor heating must be commissioned in accordance with the manufacturers recommendations, however it may be used to aid the drying process by using the following procedure:

1. Allow the screed to dry for 7 days

2. Increase the heating system temperature by 5oC per day to operating temperature and maintain at this temperature for a minimum of 7 days

3. Reduce the screed temperature by 10oC per day until the screed surface reaches room temperature or not less than 15oC.

Prime Bond

Check the moisture content of the screed once this process has been completed. Once dry, the screed should be primed with progressively stronger coats of Norcros Prime Bond as follows:

¨ Diluted 1 part Prime Bond to 4 parts water . Allow to dry.

¨ 1:3 dilution applied at 90o to the first coat. Allow to dry.

¨ If the screed is still after the first 2 coats, apply a third coat, diluted 1:2 with water and allow to dry.

Any excess primer which pools on the screed surface should be removed

Apply a layer of Norcros Pro 50 Levelling Compound or Pro 30 Levelling Compound


In to a clean pail add (4 to 4.4 litres of water for Pro 50) or (5 litres o f water for Pro 30) and slowly add the 20kg of Norcros Levelling Compound. Mix with a rotary drill paddle to form a creamy lump free consistency.

The material should be mixed for a minimum of 3 minutes.

N.B. Using excess water may weaken the leveller causing it to separate, crack and debond


Pour the mixed material onto the prepared subfloor and allow to flow to give a smooth finish. Minimal work with a smoothing trowel is required. The use of a spiked roller will help eliminate trapped air and smooth out trowel/flow lines to give a more uniform surface appearance. The mixed material should be applied at a thickness between 3mm to 50mm (Norcros Pro 50) or 0mm to 30mm(Norcros Pro 30). For best results, an overall thickness of at least 4mm should be maintained.

Norcros Levelling Compounds are self-smoothing, but should any imperfections remain they can be removed by rubbing with a carborundum stone when dry. The underlayment will accept light foot traffic typically 4 hours after application. A 20Kg un it will cover approximately 5 meters square at 3mm of thickness.

Flooring can then be installed as manufacturer’s recommendations. N.B. The heating must not be switched on for at least 14 day s after completion, raising the temperature gradually at a rate of 5oC per day.

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Technical Support

For technical information about Norcros Adhesive products please call:

01782 524140

Technical Support

Technical Support

For technical information about Norcros Adhesive products please call: 01782 524140


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