Knowledge base

Heated floors cause fluctuations in temperature which makes the screed, adhesive and tiles expand and contract. While the screed and adhesive expand at a similar rate, tiles have a lower coefficient of thermal expansion meaning they ‘grow’ at a different rate. This can cause the tile to crack or de-bond due to the stresses placed at the interface between tile and adhesive.

To counteract this issue, adequate preparation is crucial, as is the correct selection of adhesive and grout.

In this guide, we will be taking you through the process of tiling on underfloor heating to ensure that the risk of any issues are mitigated.

TILING OVER HEATED SCREEDS (HOT WATER PIPE SYSTEM)

Heated pipe based heating consists of a run of pipes embedded within a pre-laid screed, and is often calcium sulphate based due to its flowing characteristics. The surface preparation and priming required before laying tiles is the same as for screeds without underfloor heating. Before any tiling is carried out, the following criteria must be met:

PREPARATION

The screed must have been left for the minimum cure time before the heating is switched on (this is generally 21 days for sand/ cement screeds).

For calcium sulphate screeds please see our guide on tiling onto Anhydrite.

The underfloor heating must have been fully commissioned and tested. This is not a simple pressure test but must be a full heat up and cool down of the system. This is normally a 7-day cycle. Only by doing this can any screed weaknesses be identified.

All movement joints must be identified and followed through in the tiling. Screed movement under temperature change is a major cause of tiled floor failure so the need for movement joints is extremely important. Movement joints should be present between all different heating zones, door thresholds, upstands and perimeters.

Traditional cement based subfloors and calcium sulphate screeds must be tested for moisture levels and confirmed to be 75% RH or less using a hair hygrometer. Proprietary cement-based screeds may cure and dry at different rates so always check with the manufacturer.

FIXING TILES

Do not run the heating at high temperatures as this will force dry the adhesive and grout, causing cracking and lifting. Set the heating on at a ‘cutback’ temperature whilst tiling – this means a floor temperature of a maximum 15˚C.

Make sure you plan ahead so you are able to exit the room without stepping on any tiles as the adhesive won’t have set. If the room is essential, you can tile half of the room at a time, so you are still able to access the room.

Always ensure the correct flexible adhesives and grouts are selected for the installation, we recommend you use BAL Rapid Flex Fibre or BAL PTB Fibre Plus for large format tiles. Mix the recommended tile adhesive in a mixing bucket as per the instructions on the bag, to lay tiles, mix and apply a flexible adhesive using a thick bed solid bed trowel and a minimum of 3mm spacers.

For a sustainable and more eco-friendly option, Regener8 is a great adhesive available in S1 rapid and standard setting bags of 20KG (white and grey).

GROUTING

Always mix to the instructions on the bag, different grouts require differing quantities of water.

Make sure to add the flexible BAL Micromax3 grout to the water in your mixing bucket and not the other way around, this will ensure a lump free consistency once mixed. Allow mixed grout to stand for 2-3 minutes before re-mixing for 30 seconds.

Avoid drawing in too much air to the mixture by mixing at too high a speed or by over mixing. Bear in mind not to mix too much at one time as it will harden quickly creating wastage.

Once the tiling and grouting has been carried out, the temperature must remain the same for a minimum of 7 days. After this time, the underfloor heating can be brought up to full working temperature slowly. A maximum water temperature increase of 5˚C per day is advised.

TILING OVER ELECTRIC UNDERFLOOR HEATING

There are numerous manufacturers of ‘radiant mat’ electric underfloor heating so always contact them directly for specific advice. The systems consist of electric cables or mats which need to be fixed to the substrate in a prescribed pattern to ensure even heat distribution throughout the floor.

When installing electric underfloor heating, the subfloor must be sound, strong, stable and suitable to receive a cementitious self-levelling compound. Any expansion joints in the floor design must be followed through into the finished tile installation and must not be covered over. This is most often between different heated areas and/or substrates.

Once the heating has been laid onto the insulation boards, care should be taken to avoid snagging (therefore damaging the cables when applying adhesive). There are two options which can be used when installing tiles onto these systems:

1. Applying a self-levelling compound to embed the matting or loose wire (always use 2-part latex self-leveller on matting)

1. Tile directly onto the matting or loose wire

Before any preparation or tiling work is carried out, the system must be tested by a fully qualified electrician to confirm it is fitted correctly and functioning. This should also be done during and after installation to ensure no damage has occurred.

Select the most appropriate self-levelling compound for the substrate. Ultra-Level it 2 is a two-part self-levelling compound suitable for difficult to bond to, dense surfaces such as ceramic tiles or where the substrate is mechanically fixed only e.g. backer boards. It’s also perfect for laying over the Promat heating system, the high polymer content bonds to the matting, preventing potential movement within the substrate which could cause the tiling to crack or de-bond from the surface.

Apply the selected compound by pouring onto the floor and gently smooth with the straight edge of a metal trowel. Either cover the highest point of the cables by a minimum 3mm or fill in between the cables without applying compound over the surface (this will enable the tile adhesive to be applied without snagging the cables). Allow the smoothing compound to cure. Time for this will depend on the thickness, substrate and atmospheric conditions.

You can then proceed to lay your tiles in the same way as a heated screed (hot water pipe system) however it’s recommended that both products are left to cure for a minimum of 14 days before gradually bringing the temperature up by 5˚C per day