Frequently Asked Questions (FAQ)

Q: Should I be selecting stone from small samples or looking at a range of samples or even the slabs?

A:  Always, always look at a range of samples. Never select from one small sample. The material will vary over a slab and from block to block. Ask the supplier for pictures of the slab and a range of samples that are typical for this material. See my article in the Natural Stone Specialist magazine for a fuller explanation of the current British European Standards regarding selection of material.

Q: The samples have a mesh and resin on the back. Why is that used? Is it because the stone is too weak?

A: In brief, most suppliers of thin materials 10, 20 and 30mm thick for interior use – kitchens, bathrooms and internal floors, coat the back of the stone in resin and mesh. This is not always an indication of weakness, but must be mostly removed before installation to provide a proper key between stone and adhesive. See my article published in Architect's Journal for more detail.

Q: Who’s responsibility is it to provide fully prepared surfaces for the stone to be fitted to – is it the builder or should the stone installer prepare the surfaces?

A: Depending on the application there are various standards of preparation required to suit the thickness of the bed and therefore the choice of installation method.  For instance, a floor receiving a 20mm tile to be fixed on a thin bed adhesive (up to 7mm thick), would have to be prepared to leave a maximum of 2mm deviation over a 2mtre straight edge, known as SR1.

A large format paving stone, on a sand and cement bed up to 100mm thick, would only need a standard surface preparation to SR3, which is 10mm deviation under a 2m straight edge.

See my article in the Natural Stone Specialist Magazine for a more in depth look at this issue.

Q: The contract says I have to provide a Collateral Warranty and a performance bond. What’s the risk to my company if I sign?

A: A performance bond is designed to give the principal client some surety in the event that the contractor or the subcontractor becomes insolvent. The bond is usually supplied by a financial institution, such as a bank or as an insurance policy via an insurance company. They will have a time limit, based on the contract. They are expensive and the terms should be carefully scrutinised by the providers of the Bond, who will probably want guarantees from the company Directors for the duration of the Bond. The main risk is if you default on the contract and the Bond becomes payable. If you have provided a guarantee as a director you could be liable for the value of the bond.

Collateral warranties are designed to provide a direct contractual link between subcontractors and the principal client or funder of a project, usually where the subcontractor has a design responsibility.  They may be stepped down to individual owners – for instance if the development contains flats that are sold on, the warranty may pass to the new owners. They are usually limited by time. Typically 12 years, but can be longer.

The obligations under a collateral warranty will vary but in principal they refer to the sub-contractors exercising due skill, care and diligence in any designed works and may extend into any selected materials.

In all contracting situations we strongly advise that as a sub-contractor you ask a construction law firm to advise you on all contractual matters before you sign. Forewarned is forearmed, as they say.

Q: The joints in the finished stone masonry wall are different sizes and thickness. Is this acceptable?

A: It will depend on the type of stone finish and the size of the jointing specified.  For instance, a split face finish will have a variable edge and therefore joints won’t be even.

But a piece of ashlar sawn six sides should be more accurately cut. The supplier conforming their product to BS EN 771-6: 2011 Specification for masonry units, will declare the Dimensional Tolerance of the stone. The higher the degree of accuracy declared and produced when the stone is cut, the greater uniformity of the jointing that can be achieved.

See my article in the Natural Stone Specialist magazine which elaborates.