Problem
Underfloor heating can cause an array of issues if not tiled onto correctly, from cracking due to heat expansion to damaging wires with a trowel. There are two solutions for this problem, one for each electric underfloor heating and piped underfloor heating.
1. Movement due to thermal expansion and contraction.
The screed and the adhesive, being of a similar material, expand at a similar rate to each other. However, the tiles usually have a slightly lower coefficient of thermal expansion - so for a given temperature rise they will grow proportionately less. The result on the tile-fixing products is twofold: Stresses build at the interface between the tile and the adhesive. At some point this will be too great and the weakest part of the system will yield - usually the bond between the tile and the adhesive. As the base expands in relation to the tiles they try to move slightly further apart stretching the grout joints. Cementitious products are inherently weak in tension and the bond onto the tile edge can fail.
2. Damage to wires from the trowel.
One of the most common reasons for problems with the installation of an undertile warming system is damage made during fitting. When covering the wires with adhesive it is quite easy to accidentally cut a wire with the trowel.
3. Switching on too soon weakens the adhesive & grout.
Cement-based adhesives and grouts set hydraulically as water is involved in the hardening reaction. If the adhesive is allowed to dry out before it has properly hardened it will tend to be weak and crumbly. It is important that the heating system is off when tiles are fixed and remains off until the adhesive and grout have fully cured.
Solution 1
Tiling onto electrical under-tile warming mats (solid substrates)
Weber’s highly polymer-modified adhesives and grouts have enough flexibility when set to accommodate thermally induced movement. Weberfloor Flex can be used to cover/protect the wires that otherwise could get damaged during application of the adhesive.
Stage 1. Preparation.
Ensure that the floor is rigid, sound, clean, dry and free from any contaminating barrier. Prime the substrate and allow to dry.
Stage 2. Under-tile warming.
Install the under-tile warming system in accordance with the manufacturer’s instructions and test that it works. Turn off and allow to cool. Apply Weberfloor Flex self-levelling compound until the warming elements are covered by at least 3mm. Allow 1-2 hours before foot traffic.
Stage 3. Fix the tiles.
Fix the tiles into a bed of Weberset Pro Lite Rapid, Weberset Rapid SPF or Weberset SPF at least 3mm thick. Leave joints at least 3mm wide for grouting and make provisions for movement. Grout the joints and use silicone sealant to fill perimeter movement joints.
Stage 4. Allow to cure.
Keep the heating/warming system turned off for at least 5 days to allow the cement to cure. Bring the system up to its operating temperature gradually in stages over a few days, do not exceed 27°C and no more than 5°C per day.
Solution 2
Tiling onto electrical under-tile warming mats (wooden substrates)
Weber’s highly polymer-modified adhesives and grouts have enough flexibility when set to accommodate the thermally-induced movement associated with under-tile warming on timber substrates. Weberfloor Flex can be used to protect the warming elements prior to tiling and also helps stabilise movement in the floor.
Stage 1. Preparation.
Ensure that the floor is rigid, sound, clean, dry and free from any contaminating barrier. The wooden floor must be capable of supporting the expected dead load and probable dynamic load, without excessive deflection. Additional strength can be provided, where necessary, by taking up the existing boards and stiffening with noggings. Alternatively, the required rigidity can be achieved through overlaying the existing timber boards with either WBP plywood or tile backer board. Plywood should be at least 18mm thick, primed on the reverse face and edges with Weber PR360 and screwed every 300mm. Tongued and groove chipboard or floorboards must be screwed to joists using two screws per board at every joist. All boards should be primed. All joints between boards should be filled with the silicone sealant to prevent leakage during application.
Stage 2. Under-tile warming.
Install the under-tile warming system in accordance with the manufacturer’s instructions and test that it works. Turn off and allow to cool. Apply
Stage 3. Fix the tiles.
Fix the tiles into a bed of Weberset Pro Lite Rapid, Weberset Rapid SPF or Weberset SPF at least 3mm thick. Leave joints at least 3mm wide for grouting and make provisions for movement. Grout the joints and use silicone sealant to fill perimeter movement joints.
Solution 3
Tiling onto under-floor heating pipes (solid substrates).
Piped heating systems are buried in reinforced cement/sand floating screeds of not less than 65mm thick. If a polymer-modified levelling screed is used it must cover the pipes by a minimum of 15mm. Weber’s highly polymer-modified adhesives and grouts have enough flexibility to accommodate thermally-induced movement.
Stage 1. Preparation.
The heating pipes should be installed according to manufacturer’s instructions, fixed down and tested prior to being encapsulated in a screed or levelling compound. If the pipes have been laid in a reinforced cement/sand screed this must be allowed to dry fully prior to tiling. A sand/cement screed should be left for 3 weeks with the under-floor heating off to dry. After this period the heating system should be turned on and raised by a maximum of 5°C/day until the maximum recommended operating temperature is achieved. This temperature should be maintained for 3 days and then the system turned off and the screed allowed to cool to 15°C before tiling commences. If Weberfloor Flex levelling compound is used to cover the pipes instead of a cement/sand screed, the drying time will be considerably shorter. Ensure that the cured surface of the floor is rigid, sound, clean, dry and free from any contaminating barrier. Prime and allow to dry.
Stage 2. Fix the tiles.
Fix the tiles into a bed of Weberset Pro Lite Rapid, Weberset Rapid SPF or Weberset SPF at least 3mm thick. Leave joints at least 3mm wide for grouting and make provisions for movement. Grout the joints and use silicone sealant to fill perimeter movement joints.
Stage 3. Allow to cure.
Keep the warming system turned off for at least 5 days to allow the cement to cure. Bring the system up to its operating temperature gradually in stages over a few days.
