Resilient Flooring to Anhydrite Screeds Using Anhydrite Levelling Compound
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 tiled 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 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.
The moisture content of the screed should be ascertained by the use of a Hygrometer in accordance with BS 8201:1987. The flooring should not be installed until a reading of not more than 75% RH is achieved. Prime using Norcros Prime Bond diluted 1:4 with water and allow to dry. Apply 2 coats o f the Norcros Pro Moisture Suppressant using a roller. Allow to dry between coats (approximately 1 hour). Apply the second coat at a 90 degree angle to the first coat to ensure 100% coverage is achieved. Allow to dry (approximately 1 hour).
Wash the roller in clean water after each application. Apply one coat of Norcros Key Bond onto the dry moisture suppressant using the
roller and allow to dry.
Apply a layer o f Norcros Anhydrite Levelling Compound.
Into a clean pail add 4.2 litres of clean fresh water and slowly add the 20kgs of Norcros Levelling Compound. Mix using a rotary paddle drill to form a creamy lump free consistency.
The material should be mixed for a minimum of 3 minutes. NB: Using excessive water may weaken the leveler 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 the trowel / flow lines to give a more uniform surface appearance. The mixed material should be applied at a thickness between 0mm to 50mm in a single application. the best results, an overall thickness of at least 2mm 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 foot light traffic typically 3 hours after application. A 20kg unit will cover approximately 5 meters square at 3mm of thickness.
Flooring can then be installed as manufacturers recommendations.