Fixing Natural Limestone Tiles

 

RECOMMENDATIONS

The correct fixing of natural limestone tiles is as important as the choice of the stone. It is essential to choose a qualified artisan who takes responsibility for the whole operation. Incorrect fixing and maintenance may result in stains, cracks and disappointment. The following recommendations are intended as a guide based on our experience. They are mainly based on general practice for fixing and maintaining Burgundy limestone in France. They will need to be adapted to local building regulations, the availability of local building materials and to local climatic conditions. Please check compliance with local codes of practice and standards and manufacturer’s instructions for any products used. ** indicates a reference to building standards listed on the last page. We cannot accept liability for any failures which may arise when following these guidelines, nor for the performance of any of the manufacturer’s products mentioned.

PREPARING NATURAL LIMESTONE TILES FOR FIXING

Being natural stone, some variation of colour, sizes, thickness (if not calibrated), pattern etc. is to be expected (refer to acceptable tolerances)**. To obtain the most natural effect and to display the full beauty of our limestone it is important to mix the entire quantity of tiling for each area. If there are several pallets of stone it is preferable to open them all and take tiles randomly from all the pallets. This will result in a harmonious blend of colours and will avoid concentrations of lighter, more patterned or more fossilized materials. Tiles that have been recently processed or stored outside will still be damp. Opening the pallets and allowing the tiles to dry will allow the final colours to appear. Any tiles that have suffered small chips or minor damage during transport and unpacking can be set aside for perimeter cuts.

SOME BASIC RULES

  • CONCRETE SLAB: If fixing on top of a concrete slab, it is essential to check that the concrete is completely dry, sound and free from significant cracks and that the surface is free from contamination. Poor quality substrates may need to be sealed. Dry concrete is a good foundation. The recommended minimum curing time is 6 weeks.**

  • MATERIALS USED FOR PREPARING SCREEDS AND MORTARS: When preparing screeds, fixing mortars and grouts for jointing, great care should be taken to ensure that the materials are suitable for limestone. If using premixed proprietary brands, check with the manufacturer that the product is suitable for limestone (see below under ‘Staining’). SAND: Sand can be used for the 10mm thick separation layer, screeds, the fixing mortar and the grouting compound. It must be clean (see below under ‘Staining’). The recommended size graduations should be respected.** BINDING MATERIAL (MATRIX): For a base screed, use artificial cement (preferably high silica cement, relatively free from alkali) or hydraulic lime (see below under ‘Staining’). For fixing mortars and grouts for joints only white cement or hydraulic lime should be used. Please note that hydraulic lime mortar takes longer to reach maximum strength.

  • BASE (LEVELLING) SCREED: It may be advisable to lay an initial levelling screed. This can compensate for uneven concrete, incorporate under floor heating systems, plumbing and electrical conduits, cover flexible wooden floors or create falls. Tiles can later be fixed on a good levelling screed using tile cement or adhesive. The screed is composed of: 1 part cement for 3 to 4 parts white or washed river sand. During cold or humid weather or if the sand is already damp the amount of water should be decreased. It should be laid on top of the separation barrier previously placed on the concrete slab. Before fixing the tiles the screed should be allowed to dry out. Allow a minimum of 3 weeks or 1 week for every 25mm of screed depth (longer if hydraulic lime is used). ** It is usually a minimum of 30mm thick, and 65mm if a piped water under floor heating system is installed. For large areas, or when laying on top of building expansion joints, it will be necessary to incorporate expansion joints (see below under ‘General Fixing Advice’). There are proprietary premixed screeds available. For urgent work it is possible to reduce the curing time by replacing the regular cement with a quick setting cement such as BAL Quickset.

  • TRADITIONAL FIXING MORTAR SCREED: Natural stone tiles may be fixed using a traditional mortar screed, made from sand and white cement or hydraulic lime. The thickness of the mortar varies between 20 and 50mm according to the type and dimensions of the tiles to be fixed. Fixing natural stone tiles or slabs with a liquid mortar is not recommended. The mortar should be mixed to obtain a homogeneous consistency according to building standards.** It is recommended that the mortar is prepared with a mechanical mixer. Hydraulic lime based mortar or mixed hydraulic lime and white cement mortar is preferable. Lime makes the mortar more pliable and elastic and diminishes the risk of cracks and subsidence of the tiles. The presence of lime also creates a ‘self defence’ which increases the impermeability of the tiles and joints. Hydraulic lime is said to be more eco friendly, re-absorbing some of the carbon dioxide released during manufacture and facilitating stone reclamation. Alternatively flexible additives can be used, especially when fixing on top of flexible wooden floors or incorporating under floor heating. Typical composition of a lime/cement mortar: 175kg of white cement, 175 kg of hydraulic lime to 1 m3 of fine clean sand The fixing mortar should be prepared with as little water as possible. 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.

  • PROPRIETARY PREMIXED FIXING COMPOUNDS: Building regulations and new standards are starting to restrict on site mixing of screeds and mortars. There is a large variety of premixed tile fixing mortars available. It is essential to check with the manufacturer that the mortar is suitable for fixing limestone. Different formulations are available, for example flexible mortars to avoid cracking of tiles due to movement in the substrate and thick bed for when thicker mortars are required. Amongst the principal manufacturers are Mapei, Laticrete and Easipoint.

  • TILE ADHESIVES AND CEMENTS (THIN BED): Calibrated tiles may also be fixed using tile adhesives and cements. These must be suitable for limestone to avoid staining (see below under ‘Staining’). This is useful when the floor height is limited. The floor should be level (there should not be more than a 5mm gap under a 2m straight edge or a little more if a thick bed adhesive is used). Proprietary manufacturers of tile adhesives for natural stone include Mapei, Sika, Laticrete, BAL and Norcros. They supply a range of adhesives including products for thicknesses up to 35mm, and products with increased flexibility.

  • GROUTING MORTAR: One can prepare one’s own mixture with fine sand and a binding material (white lime or white cement). Do not use grey cement. Too strong a cement mix will block evaporation through the joints. Ready mixed grouting products are available from builder’s merchants including Sika, Lafarge, and Weber & Broutin. They have many qualities: they do not shrink or crack when drying and they often contain additives that delay setting time and improve waterproofing. If adding colouring agents, do a trial on a dry piece of stone to check the result.

  • EXPANSION JOINTS: When possible chose mechanical expansion joints. When using flexible compounds, make sure that the product does not contain solvents that may cause discolouration (see below under ‘Staining’). There is a wide range of products, mainly silicon based, formulated specifically for stone including products by OttoSeal (S70) and General Chemical Engineering (Uniseal Silicon). See below for rules under ‘General Fixing Advice’.

FIXING INTERNAL FLOORING

  • BARRIER ON CONCRETE SLAB: The support structure should be fully cured and preferably have been poured at least 2 months previously. A separation layer should be placed between the support (concrete slab) and the fixing screed, including around the perimeter. This can be geo-textile sheeting or a bed of sand 10mm thick. One can also create a barrier against both water migration and movement by laying a waterproof film (polyethylene film 0,15mm thick, bitumen felt type 36S or a geo-membrane). . 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.

  • GENERAL FIXING ADVICE: Do not soak tiles before fixing (in dry, hot conditions the back face should be dampened to improve adhesion). Excess water is often the cause of mortar collapsing as well as a reduction in its waterproofing properties. A space should be left around the perimeter to allow for expansion (this can be filled with expanded polystyrene). Expansion joints existing in the main building structure must be continued through the screed and the tiling. 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, available at hardware stores and suitable for natural stone (see below under ‘Staining’ and above ‘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. The joints between the tiles should be a minimum of 3mm for tiles with sawn edges and 6mm for chiselled edges. After fixing do not hose down the stone with water, nor cover with a waterproof film.

  • FIXING WITH TILE ADHESIVE OR TILING CEMENT: To use tile adhesive or tiling cement, the support should be completely dry, clean and level. Tiles should not be fixed directly to a concrete base or screed unless it has cured for a minimum of 30 days. A rule of thumb guide is that the concrete support should cure for as many weeks as its thickness in cm. Check that the product is suitable for natural limestone (see below under ‘Staining’ and ‘Tile Adhesives’) and that the technical specification covers the size and thickness of the tiles to be fixed, particularly for vertical applications. The ambient temperature during fixing should be between 5 and 30°c. Divide the surface with expansion joints as detailed above (‘General Fixing Advice’).

  • GROUTING THE JOINTS: Some manufacturers of impregnators recommend applying a first coat of their product on the face of the tiles before grouting. This reduces the risk of the grout adhering to the face of the stone and makes it easier to clean. It does have a disadvantage: it is very difficult to avoid the impregnator running down the edges of the stone. This will stop the grout adhering to the edges, forming a small crack which can open up and allow water ingress. The joints should be pointed with a grouting mortar suitable for stone (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. 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.

  • FIXING TILES ON A FLOOR INCORPORATING UNDERFLOOR HEATING: Before starting, check with the heating supplier for their recommendations. As a general rule the layer of floor in which the heating is installed should cure for a minimum of 30 days (or as many weeks as there are cm of floor thickness). When the heating is activated it should be increased to its maximum by 5°c at a time per 24 hour period. The floor should then be left with the heating on for three days, after which the temperature should be decreased in the same steps until it is off. Then the heating should be left off for 48 hours before starting to fix. You should wait 7 days after completing fixing before reactivating the heating if the tiles were fixed with tile adhesive, and 28 days if a traditional mortar screed has been used. Again this should always be done by steps of 5 °c per 24 hours.

  • FIXING TILES IN WET ROOMS: This should only be undertaken by experts. All natural stone is to some extent porous, and should not be used in wet rooms unless tanking (use of a waterproof membrane) has first been properly installed. After fixing, the tiles should be treated with a reliable impregnator for stone (see below under Maintenance).

FIXING AN EXTERIOR FLOOR

The ground should be stable and well compacted. The support should be absolutely flat and clean. The various layers of the foundation, the fixing screed and the tiling depend both on the nature of the material and the usage of the area: light traffic for pedestrians only, motor cars or occasional heavy duty lorries.** For tiles 30mm thick or less, fixing should be done with a full mortar bed on well compacted sand or preferably on a concrete slab. Install a damp-proof separation barrier with a non-woven film (geo-textile) between the support and the fixing screed, including a return around the perimeter. Install drainage around the perimeter. For the mortar see details of the ‘Traditional Fixing Mortar Screed’ above. The area already laid must be protected from foot traffic for 24 hours and from any load for at least 7 days after fixing.
For thick paving or cobbles, it is possible only to fix the perimeter stones with mortar, the interior stone being bedded down in sand or a very weak sand/cement screed. This will allow the stone to be re-used at a later date and meet stringent recycling requirements.
One should wait 24 hours after fixing the stone before grouting the joints.
For expansion joints see above under ‘General Fixing Advice’. The joints around the perimeter should be filled with flexible mastic. The joints between the tiles should be a minimum of 10mm.
To ensure water run-off, the slope of the support and the tiling should be a minimum of 10mm per metre. Fixing directly onto waterproofing membrane is not permitted.
Fixing on raised flooring pedestals can be done complying with the recommendations regarding size and thickness of the stone.** Check also with the pedestal manufacturer.
Do not fix during freezing weather nor lay tiles that have been exposed to frost within the previous 48 hours.

PROTECTION OF FLOORS AFTER TREATMENT

Whenever the construction programme allows, one should insist that the paving is laid last, after all the other trades, and just before the painting. If that is not possible and other trades have to intervene after fixing, it is essential to protect the floor effectively, and from experience we recommend laying sheets of 30mm thick expanded polystyrene. These should be attached to each other, and to the skirting around the perimeter, by large strips of duct tape which will also ensure the effective sealing of the protection. For stairs it is advisable to protect the front edge of the steps with right angle sections of pine or light wood. This protection always pays as it avoids cleaning operations and replacing damaged stone.

FURTHER TREATMENT

The honed finish or surface treatment, obtained in our factory with our numerically controlled machines, ensures that the tiles will be perfectly flat. A qualified stone fixer will be able to lay a floor without any lips. Grinding or polishing the floor after fixing is not required and we strongly advise against this procedure.

AVOIDING PROBLEMS

STAINING: Staining that could be difficult or impossible to remove may be caused by the incorrect use of materials for fixing or by allowing water to penetrate underneath the stone:

  • White cement, hydraulic lime or a combination of both should be used in any screed, mortar or grout in contact with the stone. Grey cement should be avoided as it generally contains higher quantities of alkali than white cement, including sodium and potassium oxides. If grey cement is used these alkali will hydrate and migrate slowly through the stone, reacting with minerals in the stone and accumulating on the stone surface. This shows up as efflorescence and will often cause staining. Hydraulic lime is almost free from alkali and is widely used for stone restoration projects.
  • The quality of any sand used in screeds and mortars is important.** Use only white or washed river sand. Any impurities such as soil or clay particles and any dissolved salts will find their way to the surface of the stone and cause staining.
  • When preparing mortar, cleanliness is important. Impurities such as cigarette butts and bits of wood can cause surface staining, particularly on lighter coloured limestone and marble. The mortar should be prepared with as little water as possible. Excessive water will evaporate through the stone increasing the risk of staining.
  • A waterproof barrier laid over the concrete slab will prevent the transfer of salts from the slab which could later cause efflorescence on the tiles.
  • For expansion joints, when possible prefer mechanical joints. When using compounds, make sure that the product does not contain solvents. These may leach into the stone and cause discolouration. There are specially formulated products safe to use on limestone and marble.
  • When grouting, avoid too strong a cement mix. This will block evaporation through the joints and could encourage efflorescence around the edges of the tiles

CRACKS: Cracks can occur, running through adjacent tiles, if basic rules are not followed:

  • A separation layer (barrier) on top of a concrete slab, timber flooring or whatever substrate is used for outside paving will prevent differential movements and stresses in the substrate being transmitted to the stone. These movements may cause cracks to develop in the stone and to run through several adjacent tiles. If the joints are too narrow spalling can occur through the compression of two adjacent tiles against each other.
  • The use of a flexible mortar or adhesive, either a proprietary product or a mortar containing hydraulic lime, will decrease the risk of cracks developing.
  • The rules concerning the positioning of expansion joints must be closely followed (see above under ‘General Fixing Advice’). **
  • For outside paving, the dimensions and thickness of the paving and the quality of the substrate must correspond to the rules for each type of traffic.** Cracks often occur when trucks are driven over areas designed for pedestrian traffic only.

FROST ACTION: Frost can cause damage to limestone used for outside paving. Check that the type of limestone chosen is suitable for the local climate where it is to be installed.

  • If you e-mail us your postal code we can advise on the suitability of different stone for your area.

 

For more information and quotations contact: robert@burgundylimestone.com

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