Flooring

Portland Stone flooring can be used in a wide variety of locations from domestic  houses to busy offices and public buildings. There are a number of factors that need to be considered by the designer including the paver size and thickness, but most importantly the substrate that will provide the strength.

Flooring Substrate

See typical flooring substrate

Portland Stone tiles fixed with adhesive on a levelling bed

Levelling Screeds

Levelling screeds are normally used as an intermediate layer between the structural concrete base and the stone flooring. 

They are normally either washed sand and white cement or a proprietary levelling screed.

When using a sand and cement levelling screed a number of factors need to be taken into consideration, including the bond to the concrete base, the use of insulation with the base, whether reinforcement within the screed is  required and the desired surface regularity. The screed is normally designed to be between 40 and 70mm depending on the construction techniques, but the minimum thickness is 25mm for bonded sand and cement screeds. Where the required screed thickness is greater than 50mm the use of unbounded sand and cement is recommended.

When insulation is required under the screed, e.g. with underfloor-heating, a floating screed is often installed. This type of cement and sand screed normally has a minimum thickness of 75mm. However, in lightly loaded domestic locations a minimum thickness of 65mm is acceptable. With some proprietary systems the minimum thickness requirements may be less. BS 8204-1 contains the required guidance on the installation of screeds to receive stone floorings.

Probably the most common problem with stone floors is caused by shrinkage cracks in the screed being reflected in the stones. It is vitally important that an appropriate curing and drying period for the base and the screed is built into the project programme. Under normal conditions a seven-day curing period when the screed is covered followed by a drying period of 1 day per 1mm of screed.

Proprietary screeds can be used as a levelling screed and have the advantage of a more rapid drying time and can be used less than 25mm thick. It is important for the designer to establish that the screed is suitable for use with the Portland Stone and the proposed location. The manufacturer's instructions must be followed as this will have a bearing on the required drying times and how soon the floor can be trafficked. It should be noted that Calcium Sulfate screeds require the same drying time as sand and cement screeds.


Under-Floor Heating

There are two systems currently available; hot water pipes and electric matting.   

The electric systems are now more commonly used with stone flooring applications.  It is important that the designer ensures that the system selected is suitable for Portland Stone flooring and follows the manufacturer's installation instructions. In all cases the heating system should be tested before the stones are laid but after the levelling screed has cured and dried. This testing procedure may take several days and will involve slowly heating and cooling the floor. It is important that the fixing contractor is suitably experienced in all aspects of the installation process.


Movement Joints

The stone will reflect the movement in the supporting substrates. 

This movement will arise from continuing drying shrinkage of the slab as well as seasonal thermal variations and under floor heating systems.  The inclusion of metal mesh reinforcement in the heated screed, as described in BS 8204-1, can help minimise the likelihood of cracking.  

It is the designer's responsibility to assess the magnitude of the stress in the flooring and the bed and decide where the movement joints should be located.  

There are three types of movement joints:

  • Structural Movement Joints
  • Perimeter Movement Joints
  • Intermediate Movement Joints

Structural Movement Joints in the flooring should be sited directly over and be continuous with the structural movement joints in the base.

Perimeter Movement Joints should be incorporated into all flooring designs around all structures: wall, columns, steps etc. Most Perimeter Movement Joints will be located under the skirting.

For larger areas of flooring or where more movement is expected, Intermediate Movement Joints may need to be incorporated into the design. These movement joints normally divide the stone flooring into panels not exceeding 40 square metres in area and should extend through the heated screed and any reinforcement.

iii. Stone sizes  

Albion Stone'€™s Portland Flooring is produced in the following range of standard sizes at 20mm thickness:

  • 400x400mm
  • 600x400mm
  • 600x600mm

As the stones have a thickness greater than 12mm they are not subject to BS EN 12057-Modular Tiles, but BS EN 12058-Slabs for Floors and Stairs, but they are still often referred to as flooring tiles in the industry.

Non-standard sizes and longer lengths can be produced to order. The 400x400 tile is approximately 7.5kg and a 600x600 tile is approximately 17kg. Increases in the module sizes or thickness will increase the weight of the units and may require specialist lifting equipment.

Our Portland Stone tiles are manufactured on a dedicated tile plant to a tolerance of +-“ 1mm on the width and length and +-“ 1mm on the thickness  (although the BS 12058 states that +2mm is acceptable).


Laying the Stone

It is strongly recommended that a Fixing Company experienced in Limestone is selected to lay the Portland Stone Flooring rather than a tiling company. 

The process of fixing, although similar, requires different skills.

Most Portland Tiles are now laid onto adhesive, although Washed Sand and White Cement bedding with a white cement slurry on the back of the tile is still used in some applications.

The designer must ensure that the proposed adhesive and grouting material is compatible with Portland Stone and suitable for the project location, including any under-floor heating systems. Water is used as a coolant in the cutting process for the tiles; therefore it is important that the installation process is suitable for damp stones and allows time for the floor to dry. All adhesives and grouts should be mixed and applied strictly in accordance with the manufacturer's instructions. We recommend that a small sample area is completed and approved before the main fixing operation commences.

An uncoupling membrane (such as 'Schlüter ®-DITRA') can be installed over the  substrate to neutralise very small movements and cracks in the screed, thus preventing any stresses being transferred from the screed to the stone.


Joints

The tolerance for the standard tiles allows the designer to have 4mm joints +-1mm.

Larger stones or stones thicker than 20mm will need to be designed around joints from 5mm +-1.5mm. Some very large stones may require a greater tolerance.

The maximum deviation between floor unit surfaces either side of the joint should be 1mm for commercial application. Where the stones are used in a domestic situation and liable to be trafficked with bare feet, lipping is not normally acceptable.

The grouting operation should not start until the bedding has thoroughly cured, which should be at least 12 hours. It is advisable not to unduly delay the grouting as the open joints can collect dust and other debris.  The joints should be filled to a full depth with either a white cement or Portland Stone dust/white cement mix or a cement-modified grout, which is a similar colour and compatible to the stone. The grout selected should not be as strong as the stone; therefore epoxy-based grout should be avoided.

There is a risk that the grouting material can migrate into the Portland Stone.  Different grout manufacturers recommend differing methods for ensuring that the 'picture framing' effect (grout smudged into the pores on the stone around the joints) does not occur, including the sealing of the stone before the grouting procedure and the use of grouts with water retaining agents or rapid setting grouts. It may also be preferable to flood grout the whole face, as this will eliminate the risk of 'picture framing', but it is important to ensure that the grouting product is suitable for this application.

We suggest that a sample area is completed and approved before grouting the entire floor.

Tiling on Different Structures

Stone Flooring on an Existing Timber Structure

Portland Stone flooring can be laid onto an existing timber structure providing a number of procedures are carefully followed. 

However, we recommend that a Structural Engineer checks the floor for stiffness and loading.

The procedures include layers of plywood and or wedi board with an uncoupling membrane (such as 'Schlüter ®-DITRA') to stiffen the floor and prevent the movement from the flexing of the timber joists. Existing floorboards can either be removed or incorporated into the structure. An uncoupling membrane  (such as Schlüter ®-DITRA) can be incorporated in the design and will help prevent seasonal movement cracking the stone tiles. Seasonal movements occur as the moisture levels change and affect the timber.

It is recommended that the maximum size of the stone to be laid on an existing timber structure is 400x400mmx20mm thick.


Stone Flooring onto Pedestals

Portland Stone flooring can be laid onto a pedestal system, but it is important that the applied loadings (uniformly distributed loads and point loads) are established.

In most cases the system will suit the small tile sizes to reduce the span. Larger stones may need an increased thickness. To ensure that there is no lipping between the stones, the thickness will need to be manufactured to +-0.5mm, which will have some cost implications.

Treads are Risers

Portland Stone treads can be laid on a concrete base using a similar method to the standard flooring. 

The risers are normally set on top of the treads and secured into place with a stainless steel fixing.

To comply with building regulations, it may be necessary to incorporate a contrasting material with an increased slip resistance to the nosing

Wet Areas 

Portland Stone has been successfully used in bath and shower surrounds and steam rooms; however, we would suggest that the designer seeks specialist advice on the sealing and on-going maintenance.

The stone has also been used for swimming pool surrounds, but advice should be sought depending on how often the stone will be wetted with the pool water and the chemicals used in the pool.

We do not recommend Portland Stone for the following applications:

  • Swimming pool internal linings
  • Public toilet floors
  • Kitchen worktops
  • Vanity units
  • Shower trays

Construction Maintenance

Delivery and Packaging

All products leaving our works are packed in a manner to ensure safe delivery to site.

This entails protection by CFC free polystyrene strips, shrink-wrapping and delivery on suitably sized pallets, frames or crates.  These normally contain a maximum of 1 tonne for safe handling on site. It should be noted that it is the customers' responsibility to ensure safe unloading of delivery vehicles.


Storage and Handling

A suitable storage area will need to be set aside for the stone, which should be stored on firm, level surfaces and, to avoid contamination should be sited away from wet or muddy areas. 

Storage should be as near as practical to the areas of working in order to minimise handling, damage and waste. Pallets should not be stacked on top of each other. Should it be necessary to store the material on site for any length of time, it should be protected from the elements and the environment.

The designer will need to be aware of the manual handling regulations and ensure appropriate lifting equipment is available to move and fix the heavy stones.

Sealant and Impregnators 

Over time, the action of foot traffic on a Portland Stone floor will hone the surface and form a natural patina on the surface of the tiles.

This action will allow the untreated tiles to develop their own rich tones and character. Despite the benefits of this natural process, it is random and will be concentrated on the areas with the highest foot traffic. For a more consistent finish, a sealant or impregnator will be required.

If a sealant is applied to the surface of the tiles, a barrier is formed and moisture can become trapped just beneath the tile. Over time, this build-up of moisture can cause discolouration and possibly even structural problems in the substrate. Conversely, impregnators allow the floor to 'breathe', so moisture can pass harmlessly through the floor tile and prevent the potentially damaging moisture build up. This is true of both water-based and solvent-based impregnators. For this reason, Albion Stone recommends the use of impregnators rather than sealants for Portland Stone flooring in most applications.

To some extent, all Limestone tiles will allow remaining moisture in the concrete floor slab (which requires at least six weeks to fully dry out) or the screed (which requires at least three weeks to fully dry out) to pass through them; this is one of the many advantages of Limestone floor tiles.

When compared with solvent-based impregnators, water-based impregnators have superior stain resistance but limited water repellence. They are also less durable and may therefore could require more frequent re-application; however, they are non-hazardous, odourless and have a shorter drying time than solvent-based impregnators. Water-based impregnators are typically used where stain resistance is most important, e.g. kitchen floors, halls/lobbies, receptions and internal wall linings. We recommend you contact Tensid, whose addresses and contact details are listed in the Reference Section.

When compared with water-based impregnators, solvent-based impregnators have improved water repellence but inferior stain resistance. They are also more durable and may therefore require less maintenance, but they are flammable and emit an odour; they also require a longer drying time than water-based impregnators. When used internally, consideration must be given to adequate ventilation both during application and drying. Solvent-based impregnators are typically used where water repellence is most important e.g. bathroom floors and pool surrounds. We recommend you contact Tensid ,whose addresses and contact details are listed in the Reference  Section. Impregnators should be applied strictly in accordance with the manufacturer's instructions.

Regular Maintenance

Cleaning and Stain Removal

The floor should be swept frequently with a soft brush or vacuumed to remove any dust and loose dirt.  

Highly trafficked floors can be kept clean with the occasional use of a cleaning machine with a soft-padded scrubber/drier. For lower trafficked floors, use a mop with clean water, approximately once per week.

All spillages should be treated promptly with a damp cloth and with continuously replaced clean water and not left to soak into the tile. In most cases, this initial treatment will minimise any potential staining. However, it may be impossible to remove some old dried-in stains, particularly if they were caused by acidic liquids.

Bleach, household soaps and detergents should all be avoided. Tensid products can be used to remove most common household stains in Portland Stone floor tiles and should be applied strictly in accordance with the manufacturer's instructions. Acidic products such as cola and red wine may attack as well as stain the limestone and should always be cleaned immediately.

Specification Clause