Tile to anhydrite screeds incorporating underfloor heating using Norcros tile to gypsum

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 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.

Check the moisture content of the screed once this process has been completed. Once dry, the screed should be primed with Norcros Prime Bond diluted 1: 4 with water which should be allowed to dry before tiling commences.

The general rule is to work from the centre outwards. Measure the width and length of the floor and draw a cross in the centre. From the central cross lay out the dry tiles allowing a minimum 3mm wide gap between the tiles to allow for movement stresses within the floor during service. These gaps will be filled with grout once the tiles have been fixed and the adhesive dried. Make sure there are no awkward cuts, some adjustment may be necessary. Always be aware of how the floor will look when you enter the room. To counteract movement stresses, which can result in tiles cracking and/or debonding a movement joints should be provided.

In heated calcium sulphate floor screeds, movement joints should be detailed:

  • Over structural joints in the underlying construction. These movement joints through the screed and tile bed should coincide with and be designed to accommodate the same movement of the underlying structural joints.
  • At perimeters of floors where the screed abuts walls and upstands and at door thresholds.
  • As a movement joint through both the screed and tile bed dividing the tiling into areas not greater than 40m², or in accordance with the screed suppliers or screed manufacturers instructions. The areas bounded by movement joints should be square to rectangular with the width to length ratio not exceeding 5 to 8.
  • As movement joints at significant changes of width of the screed surface and in doorways (ground plan length over 5m) with several rooms arranged one after another within a floor plan.
  • As a boundary joint isolating areas of screed with separately controlled heating circuits.
  • At doorway thresholds between separate areas of use.
  • At junctions between heated and unheated sections of the screed.

The width and spacing of movement joints should be sufficient to accommodate anticipated thermal movement of the screed between the maximum operating temperature and expected lowest temperature of the screed. The coefficient of thermal expansion of the pumped calcium sulphate screed should be taken to be .012mm/m²/°C. If the detail provided by the screed manufacturer states that there might be changes in length during installation (expansion), these must also be taken into account when calculating the necessary width of movement joints.

Note: Although pumped calcium sulphate based screeds can be laid in large areas without joints, where joints are required in heated screed they will have to be formed during the application of the calcium sulphate screed. This will require the use of formwork or the use of proprietary pre-formed movement joint profiles designed for this purpose.

FIXING Tile to Gypsum Adhesive
Fix tiles in a 2-6mm bed of Norcros Tile to Gypsum Adhesive. Gradually add approximately 4 parts of adhesive to 1 part of water, stirring to a smooth lump-free consistency. The properly mixed adhesive will be thick enough to hold the adhesive ribs without slumping. Mix only enough material that can be used within the pot life of the material which will be around 30 minutes at 20°C — this will be extended in cold conditions and reduced in hot weather/warm temperatures.

Using a suitable notched floor trowel held at a 45o angle, spread the adhesive onto the floor to form parallel ribs into which the tiles should be pushed with a firm twisting action, this will collapse the ribs of adhesive and produce a 3-4mm solid bed of adhesive beneath the tiles. Work in small areas and discard any adhesive that begins to dry or ‘skin over’. Tiles with deeply keyed back profiles may need to back buttered. Every so often lift a tile to check that it is in full contact with the adhesive and that no voids are left in the adhesive bed. Wipe off any adhesive residues from the surface of the tiles using a damp sponge before the adhesive sets. Remove deposits of adhesive which may build up in the gaps between the tiles. DO NOT WALK ON THE TILES FOR AT LEAST 2-3 HOURS, or until the adhesive has set.

Floor tiles, especially natural stone and porcelain can be difficult to cut and good quality tools are required. These can bought or in some instances hired. A slide cutter and nippers are essential, however an electric wet cutter will make the job easier. For intricate cuts a cardboard template will help to achieve more accurate results.

Allow the adhesive to dry. Use either Norcros Floor & Wall Tile Grout, Norcros Flexible Floor & Wall Tile Grout or Norcros 4 into 1 Grout. Clean out any dust, dirt and adhesive from between the tiles and make sure that the joints are dry.

Approximately 4-5 parts grout powder should be added to 1 part water, ensuring that the powder is added to the liquid to prevent the product from flash setting. Manually mix the grout to reduce the amount of air incorporated into the mix.

4 into 1 Wall and Floor Tile GroutFlexible Wide Joint Tile GroutHolding a soft rubber grout float at 45o, sweep the mixed grout diagonally across the tiles, ensuring that the grout is pushed fully into the joints. Clean off as much grout from the surface of the tiles as possible and allow the grout to become touch dry. Clean off the grout residues with a damp sponge wiped diagonally across the tiles. Allow the tiles to dry and buff the surface with a dry cloth. Any stubborn residues may be cleaned off with washing up detergent mixed with warm water within 24 hours of grouting.

N.B. Before grouting commences trial a small inconspicuous area to check that the grout does not stain the tiles. If there is any risk of staining, the tiles should be fully sealed using a proprietary sealer before grouting begins.

The heating may be switched on 14 days after completion of tiling, 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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