How to specify natural stone: a checklist for architects

"White marble, Calacatta type" is not a specification: it leaves the stone, the quality and the performance wide open. A good specification describes the stone with data that can be checked, and sets out how the material is approved before it is cut. Here is what to include, with a model clause.

1. Identify the stone: a trade name is not enough

The same trade name can be used for different stones, and many imitation materials borrow stone names (see natural stone or imitation). Alongside the name, state:

• Stone type: marble, quartzite, granite, limestone, onyx… They do not respond the same way to acids, wear or frost.
• Origin, if known: country and, where possible, quarry.
• Technical data sheet with the EN test results for that stone.
• CE marking and Declaration of Performance (DoP) under the relevant product standard.

2. The product standard for each use

The DoP sets out the performance the manufacturer declares for the product. Ask for it together with the test reports. Outside Europe CE marking may not apply, but EN test reports remain a solid basis for comparing stones.

3. Which tests to ask for

• Any use: density and porosity (EN 1936) and water absorption (EN 13755).
• Strength: flexural strength (EN 12372 or EN 13161) and compressive strength (EN 1926).
• Floors: abrasion (EN 14157) and slip resistance (pendulum, EN 14231), plus whatever the local code requires. See slip resistance.
• Exterior use with frost: frost resistance (EN 12371).
• Ventilated façades: breaking load at the dowel hole (EN 13364) and, for marble, EN 16306. See ventilated stone façades.

Do not write minimum values from memory: they depend on the use and are set by the engineer. What the specification must require is test reports for the actual stone.

4. Format, thickness, finish and edges

• Format: whole slabs, or pieces cut to size from drawings.
• Thickness: slabs usually come in 2 cm (¾ in) and 3 cm (1¼ in). Anything else as the calculation requires.
• Finish of the exposed face, area by area: polished, honed, leathered, bush-hammered… (see finishes).
• Exposed edges: profile (square, eased, bevelled, mitred…) and whether they are polished or match the face finish.
• Tolerances on dimensions, thickness, squareness and flatness: state them or refer to those of the relevant product standard.
• Reinforcement: say whether mesh or resin on the back is acceptable. Onyx usually has both, and semi-precious slabs are made of pieces of stone bonded with resin onto a backing.

5. Reference sample and natural variation

Natural stone varies from block to block and within a single slab. The specification should accept this and set limits:

• A reference sample approved by the architect.
• The acceptable range of colour, veining and markings. Range samples or photos showing the extremes work better than adjectives.
• What counts as a defect and what is the natural character of the stone (fossils, crystal areas, veins).

6. Slab approval and block reservation

When appearance matters, a sample is not enough: the actual slabs, numbered, are approved before cutting. We send photos and videos of each slab so they can be approved remotely. On large projects a whole block or a run of consecutive slabs can be reserved so all the material matches, and a slab layout plan is drawn, sometimes with a dry lay. See large-format slabs and slab layout.

7. Quantities

Stone is bought in whole slabs, so the material to supply is not the net area. The fabricator or engineer works it out from the layout; with bookmatching, waste is higher. Keep the measured installed area separate from the material to be supplied.

8. Installation and site protection

• Installation: the system or adhesive is set by the engineer or its manufacturer to suit the stone. For example, serpentine-type green marbles such as Verde Alpi are sensitive to moisture during installation, so reactive adhesives (epoxy or polyurethane) are usually recommended over water-based or cementitious ones, which can make the pieces curve.
• Protection: floors and countertops covered until the end of the works, with no acids or harsh products used to clean off residue.
• Inspection on delivery: check the material against the approved sample and the numbered slabs.

Model specification clause

Adapt it to each project. Square brackets mark the gaps to fill in.

Want to specify a particular stone? Ask us for a quote and photos of the actual slabs: we will tell you what is in stock and, if they suit, you can reserve them. We ship worldwide.