Fixing Large Format Tiles with Underfloor Heating

Martin Pouncey
TTA Technical & Training Manager

The Tile Association (TTA) is the UK trade association for the whole of the tiling industry, with a core purpose to build excellence in tiling and the tiled environment. In this article, Martin Pouncey, TTA Technical & Training Manager, discusses some of the key considerations for installing large format tiles and panels over underfloor heating systems, in line with the British Standard Codes of Practice.

Large format tiles and panels are a popular choice over underfloor heating systems, increasing the surface area of tiles that keep feet warm. A key requirement when cutting and installing large format tiles and panels is to manage stresses. When used with underfloor heating systems, there will be greater adverse tension affecting the tile due to thermal expansion and movement. It is therefore important to minimise tension and maximise strong bonding.

Pre-drilling pilot holes at each intersection is recommended when cutting “L” shaped lines around objects such as windows, doors and electric sockets. Cutting shapes using only dry scribe cutting tools and angle grinders should be avoided as this creates potential stress and cracking at the intersection of cut lines.

Here’s a step-by-step guide:
1. Adapt the flat workbench to create a void for drilling holes
2. Fix the tile or panel with clamps, securing it in place
3. Start the hole by tilting the drill bit
4. Continue drilling using oscillating movement without pushing excessively, cooling the drill bit with water if necessary
5. Complete four holes (min 8mm) in each of the corners
6. Use a continuous crown disc to cut out the aperture

Adhesives and Grouts
Solid bed fixing (including back combing of the tile) is required when fixing large format tiles and panels. Polymer-modified cement-based adhesives and grouts should be used to withstand thermal effects:
– Adhesives must have a minimum C1S1 classification, conforming to BS EN 12004
– Grouts will be either 1) a normal cementitious grout classified as CG1 with a suitable dispersion admix or 2) an improved cementitious grout classified as CG2, conforming to BS EN 13888
These should be mixed and applied strictly in accordance with the adhesive manufacturer’s instructions.

Movement Joints
Stresses in large format tiles should be localised with movement joints, which require a minimum joint width of 3mm. The maximum area bounded by intermediate joints within the heated screed should not exceed panels of 40sqm in area for ceramic and natural stone tiles and 25sqm in area for resin agglomerate tiles. The panels formed should be square to rectangular with a length to width ratio of no more than 8×5 metres.

Intermediate joints through the screed and tile bed should be used to accommodate differential movement occurring either between zones of heated screed or concrete operated by separate control systems or between zones of heated and unheated screed or concrete.
Further information is available for purchase in TTA’s Technical Publications, free to download for TTA members.

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