Tiling to concrete / screed floors over underfloor heating systems
A reinforced cement: sand screed generally 65mm to 75mm thick incorporating hot water pipes.
How to tile over underfloor heating
Minimum drying times for new concrete/ sand: cement screeds before installation of heating/ tiles are:
Concrete = minimum 6 weeks
Sand: cement screed = minimum 3 weeks
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 the tiles being fixed. On completion of tiling, the heating should be allowed to cool completely and should not be switched on for a minimum of 14 days to allow the adhesive & grout to fully cure. After this time, the heating should be switched on at a rate of 5oC per day to operating temperature.
N.B. It is essential to ensure that the tiles selected are suitable for use on heated floors.
Remove all traces of dust/ dirt/ laitance and prime with Norcros Prime Bond diluted 1: 4 with water. To protect the tiles from thermal movement stresses, Norcros Permalayer anti-fracture membrane may be laid in a 2mm bed of Norcros Rapid Porcelain Grey adhesive. Allow to dry.
Guidance on the design of underfloor heating is set out in BS EN 1264 Water based surface embedded heating and cooling systems. The maximum surfaces temperatures for ceramic/porcelain tiles laid onto heated concrete/screeds are:
|AREA||MAXIMUM FLOOR SURFACE TEMPERATURE oC||DESIGN ROOM TEMPERATURE |
|HEAT EMISSION W/m2|
|Bathroom or similar||33||24||100|
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, in accordance with British Standard BS5385-3:2014, movement joints should be provided:
a. Over existing and/or structural movement joints
b. Perimeter movement joints should be inserted at door thresholds and where the tiling abuts restraining surfaces such as perimeter walls, columns, curbs, steps and plant fixed to the base
c. Internal floors, which might be subjected to significant thermal changes, i.e. direct sunlight in atria, or underfloor heating etc., the floor area should be divided up by intermediate movement joints provided at 5m intervals in both horizontal directions to provide 25m2 bays. N.B. These bay sizes may be increased to 40m2 with a maximum bay length of 8m when Norcros Permalayer anti-fracture matting is used.
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.
Fix tiles in a highly polymer modified adhesive such as Norcros Rapid Porcelain Tile Adhesive Grey, Norcros Rapid Porcelain & Stone Adhesive White, Norcros Rapid Porcelain S1 adhesives, or Norcros Thick Bed Stone & Porcelain Grey/White or Norcros Rapid Porcelain S1 adhesive for large format tiles.
N.B. Before grouting commences trial a small inconspicuous area to check that the grout does not stain the tiles. If there is a risk of any staining, the tiles should be fully sealed using a proprietary sealer before grouting begins.
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.
Allow the adhesive to dry.
Use either Norcros Flexible Wide Joint 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.
Commonly asked questions
You should allow around 3-6 weeks before turning on underfloor heating after tiling. This is because turning it on too early may result in an unstable subfloor, as the flexible tile adhesive needs time to dry.
If the underfloor heating system and tiles have been fitted correctly, there should be no cracking. Having said that, poorly fitted underfloor heating or a poorly-designed floor screed could result in cracking. Turning on the underfloor heating after tiling too soon may also result in cracking, always allow appropriate setting time and follow operating temperatures.
A flexible tile adhesive is the recommended type of adhesive for underfloor heating. This allows room for movement due to thermal temperature changes.