Natural stone flooring specification: size, thickness, bed and joints

At a glance

Starting points to agree with the partner stone workshop, not design values. No tolerance is quoted from any standard: none could be read.

Size and thickness: where to start

A stone floor tile is thin for its size, so thickness, size and bedding go together. A piece that is fully bedded and supported on every edge carries the load; the same piece laid over hollows does not. Whether a particular stone can take a given size at a given thickness depends on its strength and on the base, and that is the stone workshop's decision, not a table's.

The starting formats below come from one UK stone supplier's technical manual, written for its own limestone: 400 x 400, 600 x 400 and 600 x 600 mm, or gauged widths up to 500 mm in random lengths, all suitable at 20 mm thick, with larger formats needing more thickness and lifting equipment. The same supplier's specification clause gives 20 mm for sizes up to 600 x 600 mm. A softer or more fissured stone may need something else, so treat these as a first draft to agree with the partner stone workshop, not as design values.

Both standards set requirements for the stone product. Neither covers installation (BSI scope), so laying needs the code of practice as well.

Worked example: weight. A stone slab weighs its density times its thickness. For a stone of [2,600] kg/m³ at 20 mm that is 2,600 x 0.020 = 52 kg per square metre, so a 600 x 600 mm piece (0.36 m²) weighs about 19 kg. The density is an example only: ask the stone workshop for the declared apparent density (BS EN 1936) and plan the lifting, the structure and the delivery route from it.

The standards to name, and what changed in 2024

Three things have changed since many older specifications were written. BS 5385-5, the code that stone floor specifications used to cite, was withdrawn on 30 May 2024. Its floor content, with that of an earlier part, now sits in BS 5385-3:2024, published on 31 May 2024. And BS EN 12058 was reissued in 2015, with changed nominal thickness tolerances. A specification that cites BS 5385-5 needs updating.

Stone Federation Great Britain publishes a Code of Practice for the Design and Installation of Internal Flooring, which the supplier's manual cites by clause. Our research found it listed as unavailable and under revision, and we have not read it. Nothing on this sheet is taken from it.

Using them: ‘tolerances as BS EN 12058:2015’ ties the tolerance to an edition; ‘installed to BS 5385-3:2024’ ties the laying to the current code. Ask the stone workshop for a Declaration of Performance with declared values for the stone you have chosen. Stone Federation Great Britain's 2011 buyer's guide likewise tells buyers to look for a product standard on the packaging, such as BS EN 12058 for floors and stairs or BS EN 12057 for modular tiles, and to check that any slip values relate to the finish being bought.

Bedding: the build-up in section

Stone is bedded fully, with every edge supported, on an adhesive or mortar bed that suits the stone. The supplier's manual says the adhesive must meet BS EN 12002 or 12004 and be compatible with its light-coloured stone, and that because water is used in cutting, the bed must suit damp stone or the stone must be thoroughly dry when laid. It also says an uncoupling membrane is normally included to stop stresses in the base reaching the stone, and that the work should go to an experienced stone fixing company, not a tiling company.

Under the stone sits a levelling screed, typically 25 to 75 mm depending on its type, which has to suit the stone and any underfloor heating. The screed needs time to cure and dry, and a heated floor needs its heating tested before the stone goes down.

• Who owns the base? Say in writing who supplies the screed and who signs off that it is flat, dry and cured. The stone fixer should not inherit a base nobody has accepted.
• Which bed? Adhesive or mortar is the stone fixer's choice with the stone workshop, but the specification should name the standard and leave the class in brackets until the fixer confirms it.
• Is there heating? The heating designer, the screed contractor and the stone workshop agree the screed, the commissioning and the first heating cycle. Temperatures and ramp rates are theirs to give; none is quoted here.

Common mistake: specifying the stone carefully and leaving the base and the bed to whoever arrives. The stone does not usually fail by itself; the bed does.

Joints and movement joints

Joint width. The width follows the tolerance of the stone. One UK supplier says stone cut to the tolerances of BS EN 12058 suits joints of 5 mm, give or take 1 mm. The Tile Association, summarising BS 5385-3:2024 for floor tiling, writes that joints between floor tiles should never be under 3 mm and that large-format panels need wider joints. Agree the joint width and the pointing material with the stone workshop before the stone is cut, so the layout can be drawn to it.

Movement joints. The same Tile Association article describes two kinds. Perimeter joints separate the floor from walls, steps, doors and window frames. Intermediate joints divide a continuous area into smaller sections. Both should go through the full depth of the tile, the bed and the levelling screed. For external floor tiling it quotes BS 5385-3:2024 as asking for intermediate joints at intervals of 3 to 5 m. It gives no spacing for interior floors, and neither does anything else we could read, so take the interior spacing from BS 5385-3:2024 and the stone fixer.

Line them up. The usual practice, which we have not found in a standard we could read, is to carry every structural joint and screed movement joint in the base up through the stone in the same place. A joint below that stops at the stone is where a floor tends to crack. Ask the stone fixer to confirm it and draw the joints on the stone layout.

Where the stone meets timber, carpet or another finish, see floor transitions and movement joints. Terrazzo has its own strips and joints: see terrazzo specification.

Before you order: six checks

• Slip. Ask for a wet pendulum test on a sample in its finish. The finish and the test are covered in natural stone finishes.
• Cutting dust. Cutting stone makes dust that HSE treats as a serious health matter. Ask the stone workshop to cut shapes and openings off site where it can, and see engineered stone and silica guidance for the HSE position and the questions to ask.
• Weight and access. Larger formats need lifting equipment, and a floor of stone is heavy. Check the delivery route against the delivery access survey and the structure against the weight in the worked example.
• Protection. BS 5385-3:2024 covers protection from following trades (BSI scope). The supplier warns that wrapped pallets are for transport and limited site storage, not for long outdoor storage.
• Sample. Approve a sample cut from the production slab, finished and sealed as the floor will be, signed, dated and kept on site: see sample hierarchy and approval.
• Sealing and cleaning. As our finishes sheet notes, Stone Federation Great Britain's consumer guidance advises a pH-neutral cleaner, and the supplier advises against bleach, household soaps and detergents, adding that acids can etch and stain. The stone workshop names the sealer and the cleaner.

Natural stone flooring [slabs to BS EN 12058:2015 / modular tiles to BS EN 12057:2015]. Petrographic name to BS EN 12407: [ ]. Denomination to BS EN 12440: [ ]. Finish [honed] to approved sample [ref] cut from the production slab. Face size [600 x 600] mm, thickness [20] mm, to drawing [ref]. Tolerances as BS EN 12058:2015 [confirm current edition]. Joint width [ ] mm. Declaration of Performance with declared flexural strength (BS EN 13161), slip resistance (BS EN 14231) and apparent density (BS EN 1936) before order. Stone workshop to confirm in writing that the thickness suits the stone, format, base and use.

Base: [cementitious levelling screed to BS 8204-1], [ ] mm, accepted in writing by the stone fixer as flat, dry and cured before laying. Underfloor heating, if any, commissioned and tested before laying. [Uncoupling membrane: type ref]. Stone fully bedded in [adhesive to BS EN 12004 / mortar bed], every edge supported, no hollows. Installation to BS 5385-3:2024 [confirm current edition] by an experienced stone fixing company.

Perimeter movement joints at all walls, columns, steps and thresholds. Intermediate movement joints at [ ] m centres [BS 5385-3:2024 clause ref] and over every structural and screed movement joint below, in the same position, through the full depth of stone, bed and screed. Filler and sealant: [ ]. Joints shown on stone layout drawing [ref], approved before fabrication.

Floor protected from following trades with [board / covering] until handover. Sealer [product, as recommended by the stone workshop]; cleaner [pH-neutral product]. Maintenance instructions issued at handover.

Standards and sources

• BS 5385-3:2024: floor tiling code, 31 May 2024. Listing read; text not read.
• BS 5385-5:2009: withdrawn 30 May 2024.
• BS EN 12058:2015, BS EN 12057:2015, BS 8204-1:2003+A1:2009: listings read; texts not read.
• BS EN 12002, 12004, 13161, 14157, 14231, 1936, 12407, 12440: named by a UK supplier; not read.
• UK stone supplier's technical manual and NBS-style clause (Oct 2020): read 5 Nov 2026.
• Tile Association, external floor tiling and movement joints (Jan 2025): read; quotes BS 5385-3:2024.
• Stone Specialist (2013); Stone Federation GB buyer's guide (2011): read. The Federation's Code of Practice was not read.

Questions we are asked

How thick should a stone floor be?

One UK supplier recommends 20 mm for sizes up to 600 x 600 mm, and says larger formats may need more thickness and lifting equipment. There is no single figure: the right thickness depends on the stone, the format, the base and the use, and the stone workshop should confirm it in writing.

What size should stone floor tiles be?

The same supplier recommends 400 x 400, 600 x 400 or 600 x 600 mm, or gauged widths up to 500 mm in random lengths. These are starting points for one limestone. Ask the stone workshop what it offers for your stone.

Is BS 5385-5 still current?

No. BSI lists BS 5385-5:2009 as withdrawn on 30 May 2024. BS 5385-3:2024, published on 31 May 2024, now covers the design, installation and maintenance of floors of ceramic, natural stone and terrazzo. Older specifications that cite BS 5385-5 should be updated.

How wide should the joints between stone floor tiles be?

The joint follows the stone's tolerance. One UK supplier says stone cut to BS EN 12058 tolerances suits joints of 5 mm, give or take 1 mm, and the Tile Association writes that floor tile joints should never be under 3 mm, with larger panels needing wider joints. Agree the width with the stone workshop before cutting.

Where do movement joints go in a stone floor?

At the perimeter, where the floor meets walls, columns, steps and doors, and across larger floors as intermediate joints, both through the full depth of stone, bed and screed. They should also line up with any structural or screed joint below. For external floor tiling the Tile Association quotes BS 5385-3:2024 as asking for 3 to 5 m between intermediate joints; take interior spacing from the code.

Can stone be laid over underfloor heating?

Yes, if the whole build-up suits it. One supplier says the screed must suit the stone and the heating, and the heating must be tested before the stone is laid. The heating designer, screed contractor and stone workshop agree the commissioning and the first heating cycle.

Should a tiler lay a stone floor?

The supplier's manual says to use an experienced stone fixing company rather than a tiling company. Stone is cut, bedded and protected differently, and the stone workshop can usually name fixers it works with.