Fixing an interior floor
A separation layer should be placed between the support (concrete slab) and the fixing screed, including around the perimeter. This should be film (polyethylene film 0,15mm thick, bitumen felt type 36S)which in turn should be laid on a 10mm layer of sand. This avoids cracks in the tiles due to relative movements and deformations of the concrete support and the tiles. It also acts as a barrier against water and dissolved salts from the concrete slab finding their way into the tiles causing efflorescence. There are proprietary brands of barrier specifically designed for stone including Schluter-Ditra. Some manufacturers such as Norcross (Permalayer) claim that fixing can proceed 48 hours after the substrate has been laid when using their products. Fixing on top of timber floors requires specialist advice. Sheets of plywood (min. thickness 15mm) or proprietary plastic ply (N&C Nicobond) can be fixed to the timber when using tile adhesive.
A 3 mm space should be left around the perimeter to allow for expansion, 5mm for under-floor heating (this can be filled with expanded polystyrene).
Expansion joints existing in the main building structure must be continued through the screed and the tiling. According to French regulations for surfaces over 40m2 and for every length over 8 m (in long corridors for example) divide the tiled surface with expansion joints, minimum 5mm wide. Use a flexible mechanical joint or a joint compound usually silicon based, (shore hardness A>60), suitable for natural stone (see below under ‘Staining’ and ‘Expansion Joints’) . The expansion joints dividing the tiling area are only inserted in the screed and the tiles.
Define a datum reference line. Spread out the fixing mortar and fix the tiles firmly, making sure the sides are parallel or square to the datum. Tap lightly over all the surface of the tile to compress the fixing mortar and level the tile. Immediately wipe away any surplus mortar that may overflow onto the surface of the tile with a damp clean sponge, alongside the joint only.
Avoid unnecessarily spreading any slurry over the surface of the tiles.
Wash the tiles with clean water as the work advances and protect immediately from subsequent contamination by cement or plaster.
Never use wet slurry to pour into the joints, spreading with a rubber scraper. Although this technique is often used for ceramic or travertine tiles, when used on limestone it will darken the stone and leave it with a dull grey appearance. One will not be able to recover the initial colour and texture of the tiles. The joints between the tiles should be a minimum of 3mm for tiles with sawn edges and 6mm for chiselled edges.
The joints should be pointed with a grouting mortar suitable for stone (dosage 300 kg/m3 - see above under ‘Grouting Mortar’), or with a proprietary ready to use grout. Grouting should proceed one joint at a time with a spatula. Fixing mortar should never be used to grout. Clean the tiles as the work advances with water and a clean sponge. Never butt joint.
Tiles that are honed in our factory on our numerically controlled machines produce a perfectly plane surface. A qualified fixer can execute a finish without lips and there is no justification for grinding a floor after fixing which we strongly discourage.
In full summer and during very hot weather, to ensure a perfect adherence with the mortar screed, wet the bottom face of the tiles which will be in contact with the screed. Some fixers like to improve adherence to the mortar screed, particularly when fixing large size tiles or if the bottom face is smooth from sawing or calibrating, by buttering the bottom face of the stone with white cement slurry or sprinkling a small amount of white cement powder on the screed. With heat the concrete base can be distorted which risks cracking or chipping the stone by compressing two tiles against each other. This is why it is so important to have a separation barrier between the base and the screed.
The base should be dry and executed more than 2 months previously.
Never fix tiles when there is a frost or if the tiles have been exposed to frost within 48 hours.