Resilient Flooring To Heated 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.
N.B. The maximum operating water temperature of anhydrite screeds is 45oC.
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 dried the heating should be switched on and the temperature raised at a rate of 5oC per day to operating temperature. The operating temperature should then be maintained for 2-3 days before cooling down to room temperature, i.e. above 15oC and below 20oC. This is to ensure any movement in the screed has taken place prior to the levelling compound being applied.
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 of 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 of 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.
The underfloor heating can be switched on a minimum of 14 after completion at its lowest operating temperature, raising by 5 degrees per day until operating temperature is achieved.