Today’s ceramic tile designs are so colourful, varied, and beautiful that it is all too easy to overlook the basics of trouble-free installation. These include selecting appropriate adhesives, grouts, trims and even movement joints: all topics we will cover separately.
In this How To article, the technical team at Porcelain Superstore just wants to underline the vital importance of using the correct tiling background. It doesn’t matter what tile you choose - ceramic, porcelain, natural stone, glass, etc - if you don’t use the right substrate, or prepare it properly, you run the risk of compromising your beautiful new tiling scheme.
But don’t worry. Creating a suitable base for tiling is quite simple. There are many high performance, low cost products out there to help for the DIY tiler. And, if you are having your tiles installed, most professional tilers are familiar with a comprehensive range of proven and effective options.
BS 5385 states that plywood and other wood-based sheet or boards should not be used for direct tiling to walls; while gypsum and gypsum plasterboard should not be used for direct fixing in wet areas. To fill the gap, there is an increasing range of proprietary tile backer boards on the market today, in a variety of materials, thicknesses, and weights, that provide a suitably rigid substrate for tiling.
Weight-bearing capability is particularly critical when it comes to wall tiling. For instance, a standard gypsum plaster wall, or 12.5mm plasterboard wall skimmed with gypsum plaster, can take a tiling weight of no more than 20kg per sq. metre.
Remember that this is an aggregate weight that includes the adhesive and grout as well as the tile itself.
For an unskimmed 12.5mm plasterboard wall, the maximum tiling weight is higher at 32kg per sq. metre. For walls featuring a cement:sand render finish, there is normally no tile weight restriction.
Tile Backer Boards
Recently, following changes to the Building Regulations, more and more wall and floor tiling installations feature purpose-manufactured tile backer boards. These include lightweight foam-cored boards that have a maximum tile carrying capacity of 60kg per sq. metre. Glass reinforced cement-based boards can achieve 50kg per sq. metre, with Gypsum fibreboards weighing in at 40kg per sq. metre.
These are guide figures only, as certain boards may be capable of supporting higher weights. It is always a good idea to check out the technical advice from the board manufacturer to confirm its load-bearing capability, and other performance criteria.
Today’s tile backer board options include foam-cored boards with a glass fibre-reinforced polymer-modified cement coating; glass reinforced cement boards; magnesium oxide boards; and extruded polystyrene (XPS) boards. These can all provide a suitably rigid substrate for tiling.
It is important to bear in mind, while evaluating tiling backgrounds and their maximum recommended weights, that when multiple substrates are installed using adhesives alone, such as when bonding a tile backer board to plaster, the recommended maximum weight of tiling will be determined by the substrate with the lowest value. And, to reiterate, the maximum weight of tiling is defined as a tile plus any adhesive and grout.
Points to Remember
It is also, of course, vital that the tiling substrate, both walls and floors, are clean, dry, and free from debris or any contamination, such as spilt oil, etc. All substrates need to be dimensionally stable and able to support the tiling for the intended duty and traffic. The use of sheets or boards that are subject to movement due to changes in moisture content should be avoided. Plywood or other wood-based sheets or boards – once commonplace in floor tiling installations, showers, etc - should not be used for direct tiling.
It is worth noting that priming may be required to improve the bond to some surfaces. This should be made clear in the manufacturer’s datasheets. In wet areas, such as showers and wet rooms, a waterproofing system may be required as part of the substrate preparation. We will cover wetroom installation tips in a separate technical guidance note.
The growing popularity of large format tiles, means that it is now more important than ever that the substrate is flat, plumb, and true before tiling begins. With these XXL format tiles, it is also important to ensure that the back of the tiles are free of dust or contamination prior to adhesive application.
Surface preparation is particularly important when it comes to calcium sulfate screeds. These self-levelling screed products are increasingly common but can be difficult to identify. Once dry, they look very similar to a sand:cement screed.
Porcelain Superstore’s advice is to always err on the side of caution. If you are not sure if your floor has a calcium sulfate screed, assume it is and seal the floor prior to tiling as a sensible precaution. This is because calcium sulfate screeds can suffer from a number of potential problems such as the formation of ettringite, resulting from the cement and water in the tile adhesive reacting with the gypsum in the screed, which could cause tile debonding. Additionally, as calcium sulfate cures, a weak layer of laitance is formed on the surface. This layer is too weak to tile onto and must always be removed prior to tiling.
In every case – and this is of vital importance - any screed must be allowed to fully dry out and then be prepared as per the screed manufacturer’s recommendations. Generally anhydrite screeds should be allowed to dry at a rate of 1 day per mm of screed thickness for screeds up to 40mm thickness, and 2 days per mm for any additional thickness over 40mm to achieve a moisture content of no greater than 1% w/w or 75% Relative Humidity (RH). One fully dried, the surface should be mechanically sanded and vacuumed to remove all contamination, dust, and laitance. The floor should then be sealed, and allowed to dry, before tiling starts.
To cope with the potential of structural or thermal movement - particularly with underfloor heating - many floor tiling projects feature an uncoupling system. These are designed to accommodate moderate lateral stresses between the substrate and the tiles, protecting the floor from damage. Some may also be used for waterproofing purposes, and may be suitable for use with underfloor heating systems.
In all cases, the uncoupling system is placed beneath the tiling layer. They can be used over a range of structural substrates, including timber, screeds, and concrete.
To find out more, visit pcsboard.com/deltaboard-brochure/