Electric undertile heating is an increasingly popular method of heating, accounting for around 40% of the underfloor heating market.
Tiling onto a timber substrate can be problematic at the best of times. So how should you tackle it when there’s the added complication of undertile heating? PCI’s technical team at Degussa offers some advice on getting the job right.
Wood is a long way from being the best substrate for ceramic tiling – a wooden floor will move as it is walked on or loaded with furniture and it will shrink or expand depending on temperature and moisture levels. How much the wood moves, bends, bows, stretches and shrinks depends on the weather, the heating system, foot traffic and so on. In contrast, tiles are rigid and don’t move, flex or bend. So if you apply enough loading to a tile that isn’t fully supported, it will crack. Using undertile heating may seem to be an added complication, but it can be overcome by following the correct method of installation.
Before you begin, remember that u nderfloor heating is generally considered to be ‘comfort heating’, with a temperature of 25-27ºC. It is not normally the sole source of heating, nor should it be set to a temperature higher than 27ºC over a wooden substrate. Electrical systems suitable for undertile heating use over timber include wires, mesh, foil and sheet systems. Whichever system you are installing, you will need to take the same basic steps to ensure that you limit the movement of the wooden substrate as much as possible, as temperature differentials will be greater than with other heating systems.
Subfloor Preparation
The deflection common in domestic timber floors makes them unsuitable for tiling, so before doing anything else, new floors will need to be strengthened as per the recommended standards. They will need to be able to withstand an additional dead load of up to 0.8kN/m², and the probable dynamic loading, without excessive deflection.
Options available for preparing the substrate:
Or, the BS5385 standards suggest to overboard again first. The ‘belt and braces’ approach of double boarding will strengthen the floor so that there is less deflection, avoiding cracked tiles. When overboarding, the top layer of boards should be laid at right angles to the bottom layer.
Tile Backer/ Construction Boards
A third option is to use a tile backer/construction board. On top of the original boarding, apply a 10mm minimum PCI Pecidur ® tile backer board, which will limit the movement of floors and the expansion/contraction and moisture absorption. The surface then receives mats or wires and single-part Rapidflex over the top.
A built-in advantage of using PCI Pecidur ® is its thermal insulation properties – at 10mm it has a U Value of about 3.4 against 12.8W/m²K of 12.5mm plasterboard. It reflects heat upwards into the floor tile, acting as an efficient thermal barrier that will allow floor tiles to heat up in as little as an eighth of the time it would normally take. At a seventh the weight of plasterboard, PCI Pecidur ® is also waterproof. A distinct advantage of using a construction board instead of plywood is it is easy to handle and more lightweight to carry and work with.
Using PCI Pecidur ®
Insulated, waterproof construction boarding such as PCI Pecidur ® can be used for overboarding at a minimum thickness of 10mm. To fix PCI Pecidur ® in place, first screw down wooden floor boards and prime with PCI Primer W. Allow the primer to dry. If the flooring is uneven, level with a levelling compound such as PCI Periplan ® Extra. Then fix the PCI Pecidur ® boards in place with a PCI flexible tile adhesive. Boards can be joined by butting them up in a staggered fashion, in the same way that a brick wall is bonded. Unlike other construction boards PCI Pecidur ® boards do not need to be screwed down saving time. They are fixed in place using a PCI flexible tile adhesive; Tilefast 6 Rapidflex.
If there are any gaps between the boards when they are laid side by side on the floor, lay PVC or electrical type adhesive tape over the gap to stop tile adhesive filling the gap which would create a ‘cold bridge’.
Tips for Overboarding
Key points from BS 5385: PT 3-14:4 are as follows:
Construct new timber bases with noggings between joists.
Screw WBP exterior grade plywood to both joists an d noggings at 300mm maximum intervals.
Seal lower faces and edges o plywood against ingress of moisture using a non-aqueous sealer.
All junctions between boards should be supported by noggings or joists.
To further reduce/eliminate movement, an additional layer of sheets of minimum 10mm thickness, resistant to moisture and thermal movement, should be screwed over the plywood at 300mm centres, ensuring joints in the two layers don’t coincide.
In general, the top sheet should be laid broken bonded in both directions.
Where the board has a rough and smooth side, the latter should be used for tiling.
Installing the Heating
Most electric undertile heating systems on the market are relatively easy to design and install, following the manufacturer’s guidelines. Measure the area of floor to be heated in square metres, prepare the surface to ensure it’s clean and follow the instructions to install the cable evenly over the area. Test with an appropriate meter before you start and again when the installation is complete.
TIP: Often, cables & matting can be nipped or dragged when using an adhesive directly onto them. By using a floor leveller to fix these in place can often save time in the long run.
Leveller – PCI Periplan for 2-30mm thickness. Allow to dry for at least 6 hours.
Tiling over the Heating
Timber substrate
Apply flexible cementitious Tile Adhesive
Install PCI Pecidur ®
Install Underfloor heating system
Apply floor leveller / screed – optional
PCI Adhesive
Tile
Grout
Adhesives
Once the heating cables or mat are fixed in place apply suitable PCI Tile Adhesive for selected tile or stone. Consideration should also be given to the type of boarding or underlayment used.
Direct Timber fixing – (Overboarding/ Ply method): PCI Timberflex
Direct Fixing to Pecidur Boards: PCI recommends the use of Tilefast 6 Rapidflex, Floorfast 20 Rapid or PCI Nanolight.
Direct Fixing to Underlayments: PCI recommends the use of Tilefast 6 Rapidflex, or PCI Nanolight
The PCI Carra Adhesive should be used if using natural stones
Grouting
When grouting, you should always use a polymer-modified product, such as PCI’s Fastrack grey grouts or Groutfast 20 Rapidflex. Special additives in these two products help them to bond to the edge of tiles so that they resist movement.
Perimeter Joints
When tiling over heating and onto timber floors, there should be a minimum 6mm wide perimeter joint filled with silicone so that a little bit of movement in the tiling can be accommodated. This will eliminate unsightly cracked joints around the edge of the room caused by shrinkage when the heat is switched on.
For more information, contact PCI’s Techincal Services Department on 0161 794 7411