In this article, we’re looking at problems with ceramic and other types of tiles placed on screeds. When looking at tiles on screed floors, the following main points should be considered:
Why failures arise in ceramic tiles on solid floors
Failures of ceramic tiles can often be quite severe in relation to what initially appears to be minor defects to limited areas. For instance, in the case of a small amount of cracked tiles, it can be very difficult to trace the original type and supplier of the tiles. In addition, this is exacerbated due to the variation in colour and texture between batch numbers for exactly the same product. This variation may not be acceptable to meet the high standards of finish of today’s homes. This may result in entire floor areas having to be replaced, despite only a relatively small area of tiles being considered defective. Investigations by Premier Guarantee surveyors and external specialist consultants indicate that there are numerous reasons for these failures. As always, care should be taken when specifying anhydrite floor screeds with tiling finishes. Additionally, floor tiling adhesive must be compatible with the anhydrite screed to prevent de-bonding of the tiles.
Poor bedding of the tiles on the floor with adhesive
With the current popularity of large format tiles, it is important to make sure the background is flat and true before tiling begins. This is particularly important with large tiles when the slightest unevenness of the floor will result in at least one corner of the tile protruding above the expected flat plane of the floor. It is also worth bearing in mind that the tolerances for the tiles are based on a proportion of the size of the tile. When fixing large format tiles it is essential to use the correct adhesive (Class 2 plus F, T, E, S1 or S2 where applicable), also being sure to check that as near to 100% as possible is covered in the adhesive. Buttering the back of each tile as you fix the tiles, in addition to the adhesive on the floor, is recommended with large format tiles. However, large format tiles may not be suitable for fixing in a brick bond pattern. Your Premier Guarantee surveyor may tap a few of the tiles to check for hollow sounds. All tiles, large or small, must be fully bedded. Floors exhibiting hollow sound aren’t fit for purpose, as they will almost inevitably crack. Fixing should be carried out in accordance with BS5385 and BS8000 Code of Practice for the installation of wall and floor tiles.
Movement joints for large areas over 40m2
Movement joints allow for movement and help to prevent tile damage. The recommended minimum width is 6 mm, but it depends on each individual application. Movement joints are either field-applied sealant, or a prefabricated profile with anchoring legs, which are adhered beneath the tile. Movement joints are normally installed where flooring abuts wall, steps, columns etc., on large floor areas over 40m2, or with the longest dimension over 8M, and over structural movement joints. Floors less than 2 metres between walls will not normally require perimeter movement joints, except where there is under floor heating.
A decoupling membrane is designed to isolate the tile top layer from the subfloor. There is likely to be some movement of the subfloor in the following circumstances:
One common problem is the potential for the substrate to contract or expand. In particular, newly laid sand-cement screeds or concrete may contain large amounts of moisture, which, as it evaporates, causes the screed to shrink. However, all substrates will expand and contract naturally due to humidity and/or temperature fluctuation, and this is especially true where underfloor or under-tile heating has been installed. What’s more, certain water-sensitive substrates, such as anhydrite screeds (or calcium sulphate) will lose their cohesive strength if they get wet. Any movement, whether shrinkage or expansion, can cause stress cracks that can transfer through to the tiled surface, causing the tile to either fracture or de-bond from the background. Uncoupling matting helps to prevent these lateral stresses from transferring through to the tiled layer by absorbing these stresses and transferring them evenly over the floor.
Decoupling mats should not however be used as an alternative solution to waiting for the floor screed to dry out properly.
Screed drying out time
BS 5385-3 2015 Code of practice for the design and installation of ceramic floor tiles and mosaics, specifies that a screed shall be left for at least 3 weeks to dry prior to tiling. However, other documents specify longer periods and BS 8204 recommends 1 day per mm, for thicknesses up to 50mm. Normally 3 weeks is the absolute minimum. Many tile manufacturers’ recommendations for thicker substrates suggest 2days/mm. Technically, for a 70mm screed it could be up to 140 days before the screed is ready to accept the tile finishes.
Grouting or walking on the floor tiles should not be for at least 24 hours (unless using a rapid setting adhesive) to avoid disturbing the tiles before they have bonded completely with the adhesive.
Tiling on under screed heated floors
After drying out, the screed should be heated slowly at a maximum rate of 5ºC per day to a temperature of 25ºC and maintained at that level for 3 days before being allowed to cool to room temperature. The UFH must be fully commissioned before tiling is applied and a commissioning certificate supplied.
Additional guidance for Natural Stone, Travertine and Porcelain Tiles
As the name suggests, these are a natural product and may not be as uniform in thickness and general dimensions as other types of hard flooring. Due to this, they may need a slightly thicker bed and larger joints to allow for the variations. Due to their high absorption rate, they are not recommended for kitchens and bathrooms. Natural Stone tiles must be solidly bedded.
Cement-based tile adhesives are the most appropriate for this method. Using a suitable notched trowel, apply the adhesive onto the substrate, spreading only enough adhesive that remains workable. Take the flat side of the trowel and smooth over the adhesive to produce a uniform, ridge-free bed. ‘Back buttering’ is also recommended for each tile so you achieve a strong bond with whole tile. Flexible grout should be used particularly in areas that may be subject to vibration and the possibility of any movement. Stone tiles should be sealed using a proprietary sealer, not only to prevent water absorption but also to protect the tiles from staining and preventing premature wear. The surfaces of natural stone, travertine and limestone tiles are considerably softer and more prone to wear than ceramic, marble or porcelain tiles.
Please note: Chapter 10.2.1.5 of the Technical Manual recommends heavier tiles such as marble, travertine or stone tiles should not to be used on suspended timber floors.
Please note: There are different grades of travertine tile available which are based on the amount of ‘filler’ used in the tile make up (Voids can be present in the travertine). If the tiles are not “double glued” there is a potential for the filler to ‘fall out’ into the voids in the tile adhesive, resulting in holes appearing in the upper tile surface.
A cement-based tile adhesive is recommended with porcelain tiles so you may need to check if your substrate requires sealing prior to fitting. As porcelain tiles have very low absorption rate compared to other tiles, absorption of the adhesive to the tile may take considerably longer than a stone, travertine, or ceramic tile. It’s important that the tile manufacturer’s recommendations for the correct type of adhesive for their product are followed. Additionally, it’s also important that a minimum of 24 hours has passed to let the adhesive fully dry before grouting commences. It is recommended using a rapid setting cement-based flexible grout. Porcelain tiles do not need sealing.
Tiled Floor Wet Rooms
In recent years, wet rooms with tile floors have become popular. Unfortunately, however, there is evidence that many have failed to meet house buyer’s expectations.
Chapter 10.2.1.8 of the Technical Manual clearly states these type of wet room floors must not be constructed over a Timber floor deck substrate.
The main problem appears to be inadequate falls to the outlets, causing the water to reach a level where it comes out of the wet room to a dry area. This is particularly prevalent in upper floor bathrooms and en-suites, or bathrooms in upper floor flats that may overflow through a ceiling, causing considerable cosmetic damage and potential health and safety issues with electrical wiring and lighting. The position with flats may become particularly difficult when the flat below is suffering damage and inconvenience, whereas the flat above, where the issue is, could be completely unaware. It is always going to be difficult for a floor tiler to introduce split falls to direct water to a single outlet that may be placed centrally in a relatively small area, particularly when using large format tiles.