Tile to anhydrite screeds using cement-based adhesives
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. 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.
Alternatively, allow the screed to dry to a Relative Humidity of not greater than 85%, ensuring that the floor is prepared as above prior to laying Norcros Permalayer anti fracture membrane in a bed of Norcros Rapid Porcelain Grey adhesive.
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 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 joint should be provided around the perimeter of the room. A gap no less than 6mm wide and 6mm deep or the depth of the tile should be left at the junction between the walls and the floor. This should be filled with silicone sealant at the end of the job, or if being covered with skirting boards or kitchen units etc. may be left open. In large floor areas, intermediate movement joints should be provided at 8 to 10 metre intervals in both directions. Refer to the How to Sheet ‘Movement Joints in Tiled Installations for recommendations regarding movement joint requirements in heated anhydrite screeds.
Fix tiles in either Norcros Rapid Porcelain Grey or White , Norcros One Part Flexible Grey or White or Norcros S1 adhesives. For large format tiles use Norcros Thick Bed Stone & Porcelain or Norcros S1 adhesives. Mix the adhesive in accordance with the recommendations on the bags/ product data sheets, gradually 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 20oC—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, (16-24 hours if One Part Flexible adhesive is used) 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 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.
Holding 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.