BAL Adhesives has emphasised the importance of preparation, adhesive and grout considerations when fixing natural stone.

‘Natural stone has become a favourable choice for commercial and domestic installations over the years owing to its durability and timeless beauty,’ says the company. ‘Because it comes in a wide range of colours, sizes and natural variances ie stability, porosity, moisture sensitivity etc, installation is a matter of knowledge and balance.

‘One must have the knowledge to identify the variables that can affect for example adhesion to a suitable substrate, and ensuring the right balance is met between stone, substrate and the environmental conditions it will be subjected to onsite and in-service.’

Whether you’re fixing or specifying marble, limestone, granite, sandstone, quartz, agglomerate or slate, they require different ways of fixing – including the correct selection of suitable tile adhesive – depending on where the stone is being fixed. The first consideration is ensuring your background is prepared correctly in accordance with the recommendations given in British Standards BS 5385 Part 5.

Substrate preparation
New concrete floors must be air dried for at least six weeks before tiling, while new sand:cement screeds (unheated) require a minimum of three weeks.

Anhydrite or calcium sulfate screeds take about one day per mm up to 40mm to dry, and two days per mm for screeds greater than 40mm. It’s important all laitance is removed after between two-to-six days, while moisture levels should be less than 75% relative humidity (0.5% CM). This can take anywhere from 40-80 days depending on thickness.

Priming is also key before tiling commences on calcium sulfate screeds as this creates an effective barrier between the gypsum and cement-based products – protecting against an adverse reaction eg ettringite.

Says BAL: ‘There are fast-track solutions on the market including BAL Quickset Cement, or using BAL Flexbone 2Easy uncoupling solution, which can be laid direct on new walkable screeds, or anhydrite at 1.5% CM. If you’re tiling onto a screed that incorporates underfloor heating, there are more considerations before fixing can commence. This includes the proper commissioning of the heated screed before tiling. For heated floor screeds use of suitable uncoupling matting, such as BAL Rapid-Mat or BAL Flexbone, is recommended for installation of stone tiles.’

The company continues: ‘For new walls, expect to fix over new Portland cement:sand render after at least two weeks’ drying – unless you’re using BAL Quickset Render or similar, which sets in two hours at 20deg C. No such waiting, though, if boards – such as new BAL Board – are the background substrate. With BAL Board, the weight limit is 100kg/sq m, making it perfect for use with natural stone tiles up to single storey.’

When installing natural stone with tile adhesives, special attention should be given to your choice of adhesive. Some carbonate-based stones, such as marble, travertine, and limestone, may be susceptible to drawing in residual moisture from the tile adhesive, which has the potential to cause problems. ‘Because of this we recommend fast setting, cement-based white adhesives such as BAL Rapid-Flex One on walls and floors, or BAL Pourable One for floors with requirement for greater bed depths and when installing uncalibrated materials,’ says BAL.

‘BAL Pourable One can offer bed thickness up to 25mm and can be grouted after three hours. The cement chemistry in this product uses up more of the water during hydration, thereby reducing the risk of water staining. The use of BAL Pourable One can reduces the need to back butter – where surfaces are flat and even – providing sufficient coverage when the tiles or slabs are bedded down.’
Grouting

The risk of spoiling the effect of natural stone is greatest when the grouting stage is reached. It’s generally wise to choose a grout colour that’s similar to the stone tile colour particularly with porous stone. On very porous stone, the result can be a rough ‘picture frame’ effect around stone edges, unless tiles are sealed with a suitable sealer first. The risk can be reduced by using a rapid setting grout such as BAL Micromax2, says the company.

‘When grouting natural stone, it’s advisable to carry out a trial on a small area to determine the potential for staining first. When in doubt, use a suitable protective sealer such as BAL Protective Sealer and repeat the trial to ensure a satisfactory result is achieved.

‘Finally, to ensure installation success, it’s important that movement joints are incorporated in the floor tiling. For perimeter movement joints, a suitable neutral curing silicone or similar is generally advised.

That’s because acid curing sealants (eg acetoxy silicones) are bad news for some types of natural stone, especially marble and limestone. They chemically react with carbonates in the stone causing de-bonding at the edges and in some cases breakdown of the stone.

‘In high traffic areas, sealant movement joints should be avoided altogether because they will not protect the stone edge from damage. In these cases, proprietary pre-formed movement joints are recommended. Overall, there’s much for fixers to consider, but by taking the appropriate steps at each stage, there’s no reason the beauty of a natural stone finish can’t be successfully installed and maintained for years to come.

‘For technical advice and training that can be trusted, contact BAL technical advisory service on 03330 030160 or our innovation and technology centre on 01782 591120.