Here in the 21st century, we look for improvements to our standards of living and try to move with technology in all walks of life. One thing that fascinates me is the relatively ‘new’ acceptance of underfloor heating as a primary source for warming a property.
The Romans cottoned on to this over 2,000 years ago by having a void beneath the floors with warm air circulating. This moved through the floor into the area above, warming the subfloor, which was usually tiled. This is indeed where underfloor heating comes into its own, with tiled floors that absorb heat and warm up themselves, while also allowing the release of heat into the atmosphere. Europe, particularly in northern areas, has embraced this for decades.
The Romans cottoned on to this over 2,000 years ago by having a void beneath the floors with warm air circulating
There are many different systems and methods in the market that can be generally referred to as underfloor heating (UFH). For this article I’m going to discuss underfloor heating only in terms of warm water pipes encased in the subfloor. The recommendations and area sizes are for cementitious screeds and may differ with calcium sulphate-based screeds. It will be assumed that the installation of the screed itself is done correctly and that the UFH has been fully commissioned before applying any tiling.
This is critical, otherwise when the heating is first brought online, any issues with the system will be highlighted – cracks, weaknesses in the subfloor, cold or hot spots, etc. Don’t wait until the beautifully tiled floor is in place before finding these things out: always insist nothing will be installed until the UFH has been properly commissioned.
So what differences need to be highlighted for installing products onto UFH screeds? The easiest way to understand what needs to be considered is to know what temperature variations do to products, including screeds.
Heat causes materials to expand (correspondingly, cold causes shrinkage), so it’s a case of how to accommodate these potential movements.
The first point of note is the subfloor itself should be designed with movement joints between heated zones. These joints must be followed all the way through the flooring and tiling installation. If there aren’t any movement joints apparent, then get the main contractor to confirm that this is designed correctly, and explain that you’re simply applying to the surface of the subfloor so aren’t responsible for any design beneath that paint. Similarly, there should be movement joints at all perimeters and upstands. You may not see many of these as they can be hidden under skirting boards, etc., but do take the time to ask to demonstrate your professionalism and knowledge at all stages.
Your responsibilities don’t stop there. You now have to ensure the products you select to use are correct, will consider movement, and will ensure the longevity of the tiling installation. We have agreed products move under heat variations, so what else is there to consider?
With all the different tile types available to use these days (we’ve come a long way since the Romans), then we also have to understand what may happen to the tiles themselves under thermal movement.
Provided the tiles are manufactured correctly, the likes of ceramics, porcelains, natural stones, etc. are very strong and able to withstand thermal changes. However, when it comes to agglomerates such as quartz, notable temperature changes can cause significant bowing in the tiles and, although they can be used with smaller bay sizes, we advise against using them on heated floors.
Agglomerates such as quartz, notable temperature changes can cause significant bowing in the tiles and […] we advise against using them on heated floors
When it comes to softer products such as travertine, it’s prudent to consider isolating these from the subfloor to minimise any possible cracking under movement. The use of anti-fracture mats here would be a great alternative.
To ensure the bond between tile and subfloor is strong and tolerant of slight thermal movement, it’s advised a minimum C2T or S1 adhesive such as Bostik Vitri-Flex S1 be used. Adhesive should be applied to the subfloor with the appropriate trowel, but also applied thinly to the back of tiles of 600mm x 600mm and above dimensions – in what is termed ‘buttering’ the tile – to give a much more cohesive contact between tile and floor.
100% contact between tile and floor is always recommended, even when there isn’t any UFH, but it is critical here. Any bits of unbanded tile, however small they may be, will be free to move much more than the rest of the floor so all movement will try to occur at these points, resulting in excessive stress and strain on neighbouring adhesive. Tiles will pop long term, so don’t risk it.
There are also guidelines to how large a subfloor area can be covered in a single tiled floor bay. This differs significantly from tiling onto unheated floors and is to reflect the potential movements that may occur. The TTA guidelines are for a maximum bay area of 40sqm. Also, and equally important, no single run should exceed 8 linear metres.
Movement joints should be included to ensure these parameters aren’t exceeded. It’s definitely worth undertaking significant planning with the client and main contractor so they can appraise how the final floor may look.
We know that the grout lines in a tiled floor are the ‘softer’ aspect of the installation and can alleviate some of the stresses of movement. It is essential to utilise a grout joint width of a minimum of 3mm with a flexible grout of class G2, such as Bostik J115 Flexible ‘Smooth Finish Grout. A thinner grout line gives less opportunity for the tiles to expand, therefore any movement will cause stress at the adhesive line, allowing the tiles to peak and lift.
There is a lot of debate in both the tiling and flooring industries about the use of UFH before, during and after installation of products. My understanding is adhesives are designed in laboratories and have optimum performance at between 15-23deg C.
My understanding is adhesives are designed in laboratories and have optimum performance at between 15-23deg C
It makes sense therefore to ensure the installation area is a warm environment and if the only heating available is the UFH, then it’s prudent to utilise this, at cutback temperatures, to give an air temperature between 15-20deg C. Otherwise, you’ll be bringing in external heat sources temporarily, which will achieve the same thing but only while the heating is present.
Once the external heat source is moved, the thermal changes can become significant if the substrate becomes cold, with the risk of thermal shock during the curing of the adhesives and grouts. When it comes to post-installation however, the UFH must not be cranked up suddenly as this will cause thermal shock between materials.
In fact, the UFH should be left for a minimum of 14 days at cutback or switched off completely while the adhesives cure and the tiles settle. Once this has been achieved, the UFH can be brought slowly up to its working temperature.
This should be done at a rise of no greater than 5deg C per day. So, let’s get up to speed with the Romans and ensure that when you install lovely warm tiled floors, they can stay down for as many years as possible – though maybe not quite 2,000.
Written by Martin Cummins
