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Fixing External Stone

Before fixing the stone should be “shuffled”. The stone will arrive in much the same order as it comes off the saw, which means that shades of colour and particular markings in the stone are liable to be next to each other. Mixing the stone up will spread these variations naturally. Stone tiles, especially in larger sizes, can vary in thickness, so it may be necessary to build up the adhesive to minimise “lipping”. Always start with one of the thicker tiles.

Exterior stone should be a minimum of 30mm thick. Exterior paving (including Treads) should be fixed to a “fall” to allow water to run off. Cross falls should be 1:40/60 & Longitudinal falls 1:80/100.

Expansion joints should be considered for the sub-structure and accordingly the stone.

The unfinished edges of the stonework around a patio or terrace should be solidly infilled and finished and there should be a “Mowing Strip” or an edging detail (www.everedge.co.uk) between stone and soil or turf – particularly with white stone. Contact between the stone and moisture retaining elements (soil, turf, etc.) should be avoided.

It is important to minimise moisture retention in the stone. Falls will take water from the surface but voids between adhesive or exposed edges can all allow water to be sucked into the unseen faces of the stone and if retained these could result in frost-damage in hard winters. Good stone will pass tests for frost resistance; it may not be frost proof.

Using a screed:

The foundation (eg. MOT “Type 1”) should be firm (perhaps “whacked” seven times!), clean and dry. The recommended depth of screed is at least 50mm. The mix should be 4 parts washed sharp sand to 1 part white cement. Mix flexible adhesive or a plasticiser into the mix which will act as an anti-frost element. It should be dragged across the area to create a solid bed with no voids and made 5-8mm higher than the finished level desired, to allow a compact bed when tapping the stone down. It can be made wetter than usual to ensure a solid bed.

Steintec make a good range of mortars, including a bonding agent for the back of the stone.

Make sure the back of the stone is clean. A PVA bonding agent may be applied to the back of the stone. Butter the back of the stone with an adhesive slurry (cement may dry too quickly) before fixing the stone in the screed. Tap the stone down firmly to ensure solid bedding.

Using an adhesive to a new Concrete screed:

Before fixing the tiles ensure that the base is true and level, free from contamination and loose areas and properly prepared to give a good bond. Make sure the background to be adhered to is thoroughly dry, solidly bedded, and free from dust and friable matter before starting to tile.

Fix tiles with adhesive (with flexible additive*) in accordance with the recommendations of the adhesive manufacturer. Apply a PVA bonding agent to the back of the stone. Butter the back of the stone with adhesive slurry. Press tiles onto the adhesive and tap firmly into position. It is essential that each tile is laid on a solid bed of adhesive with no voids. If adhesive gets on the face of the tiles clean it off immediately. *works as frost resistant additive for exterior use.

Loose laying or placing on Harmer (or similar) adjustable supports:

The stone should be supported appropriate to the thickness of the stone, taking the advice of the support manufacturer. Thinner stone will need closer support – centres of 10-20cms, for example.
Thicknesses can vary by +/-2mm so provision should be made to accommodate the tolerance with adjustable supports. Rubber softening strips may be needed between stone and support to allow some “give” and to help prevent damage resulting from a piece of grit acting as a fulcrum between the stone and the hard plastic of the support.


The gap between tiles may be between 3 to 10mm depending on personal preference. The tiles should not usually be butted up*, as the size of the tiles will inevitably vary slightly and the stone will not have cleaned or calibrated edges. Sawn edges will sometimes be slightly ragged which is disguised by the grouting/pointing.
*except Khan Roman Pattern

If you are fixing the stone to look like an internal floor (perhaps continuing the indoor floor out through a glass wall), internal floor fixing principals might need to apply in regards to the gap between tiles and the method of filling that gap.


If using a “slurry grout” system, apply one coat of IMPREGNATOR to the stone before grouting, to help in washing the grout off textured surfaces. (Make sure the stone is clean and dry (use a WATER-based Impregnator if the weather prevents the stone drying) before the application of the protection and allow it time to cure.)
The grout should be mixed to suit the stone for colour and flexible additive should be used for frost resistance. It is advisable to test a small area to make sure the grout (when dry) is suitable.
Grout the joints so that they are completely filled. The grout should be very liquid and pushed in firmly so it goes right into the gaps. If it does not, it will crumble out. Most stone can have the grout pulled across the face to fill any voids and to avoid “picture framing”. As the work proceeds, thoroughly clean off the surface using a damp cloth. Tool the joints smooth.


Cement, sand and silver sand can also be used instead of grout, mixed in proportions to suit the stone colour. Using dry will permit moss growth between the tiles.


These are recommendations, which should not be thought of as definitive. The above mentioned products will not make stone totally impervious to all stains.