Resilient Flooring To Anhydrite Screeds
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 75% (0.5% w/w). 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. Once dry, the screed should be primed with progressively stronger coats of Norcros Prime Bond as follows:
- Dilute 1 part Prime Bond to 4 parts water, apply with brush . Allow to dry.
- 1:3 dilution applied at 90 degree angle 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.
Next, apply a layer of Norcros Pro 50 or Pro 30 Levelling Compound.
In to a clean pail add (4 to 4.4 litres of water for Pro 50) or (5 litres of 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.
Flooring can then be installed as manufacturer’s recommendations.