Considerations when designing with dimensional stone

‘Dimensional stone’ simply is a term that refers to any stone which is to be processed to specific dimensions. This is an entirely separate process to that of aggregate quarrying. Dimensional stone needs to be treated gently, with blasting kept to a minimum to ensure the integrity of the quarry block.

Quarrying

The process begins when the ‘overburden’ is removed and the fractured rock is quarried to access the consolidated quarry block that’s needed for dimensioned stone. Since the block should not be fractured in any way, there is no blasting and instead, the block is dragged out mechanically using a ‘claw’ tool. Some of the largest blocks need splitting into manageable sizes and this is achieved by drilling a series of holes and using a small quantity of black powder to ‘pop’ the blocks in two. They are then removed to the production unit.

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Processing

After selection, the block is ‘primary sawn’. This transforms an uncut block into slices and is done either with large circular saws that have diamond-tipped blades – cutting one slice at a time – or with ‘frame saws’ which have a series of blades that cut each block into slices in a single operation.

The stone slices or ‘scants’ are then sawn-to-size – for splitting and pitching in the case of walling stone – or cut-to-size and profile for finished masonry. For some projects, hand masonry is required and this is done to specification.

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Lead-Times/availability

Stancliffe have long recognised the importance of reliable supply to construction projects throughout the British Isles.

The production planning and actual manufacture of dimensional stone makes up only one part of the total lead-time applicable to any project. Research and selection, designing the stone, designing the fixings, let alone the contractual obligations of the tendering process, can all add to a lengthening timescale.

Tight construction programmes are nothing new to Stancliffe and we are available to advise the design team at the earliest stage of current lead-times for specific products. It is not unusual for these to change over a period of time and, providing we are actively involved with the project team, we will keep the relevant project professionals fully briefed on changes and the consequent implications for the scheme.

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Selection criteria

The key issues to consider when specifying natural stone are as follows:

Colour

Colour varies considerably in natural stone and this is part of its attraction. Red or buff sandstones and some of the limestones, for example, may vary even within one quarry – and so it becomes important to discuss the range of colour with the quarry at an early stage of the project.

Finishes

A number of traditional finishes are available:

– smooth-faced Ashlar – typically fixed with tight joints.
– pitched faced, split faced and tumbled – all types of walling stones usually laid with a 10 mm joint.

However, numerous other surface finishes are available – some produced by machine, others hand-tooled. In some instances, finishes are reproduced to match existing stonework or perhaps to accentuate a band course. Specifying the texture is critical and you should always ensure that the precise description is agreed with the quarry at the time of bill preparation.

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Physical suitability

Up-to-date descriptions and information on the physical suitability of particular stones, including test procedures and results, should always be available from the quarry – although specific projects may require special testing.

There are also additional sources of guidance on this subject – the British Geological Survey ( www.thebgs.co.uk ), for instance, can provide a list of operating quarries throughout the UK, together with petrographic analysis of samples and matching suitable available stones.

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Design guidance

Relevant British Standards include:

BS 5628 Pt3: 2001 which addresses walling stone with a 10 mm joint using wall ties and covers dimensional tolerances, movement accommodation, mortar recommendations and fixing information.

BS 8298: 1994 relates to Ashlar Cladding and covers dimensional tolerances, movement accommodation, fixing information and testing methods. Outline fixing details include handset masonry, and illustrate the basics of restraint and support fixings. Details are also provided of performance testing to ensure suitability for rainscreen cladding.

BS 5642: covers all issues relating to cills and copings.

Unit sizes and bed heights

When looking at design, detailed consideration should be given to stone unit sizes, since these will vary depending on the stone chosen. The natural bed heights of our Salterwath Limestone, for example, are limited to a maximum of 400 mm – whereas in our Stanton Moor Gritstone quarry, bed heights of 1500 mm can be achieved. Once again, early discussion with the quarry will be advantageous in ensuring that design requirements are realisable.

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CDM

When discussing unit sizes CDM considerations come into play. Designers should be aware of the mass of the units and then appropriate measures can be adopted. Specialist fixing companies will ensure that adequate or special scaffolding and lifting equipment is included in the fixing rates.

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Cost management

Consultation with the quarry can often help in cost engineering a project to fit your budget.

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Standard products

Standard stone products are available, generally consisting of units designed for use with a 10 mm joint and a 2-3 mm unit size tolerance. These can often be fixed with non-specialist masonry fixings and non-specialist labour – although knowledge of stone and its fixing is still advisable.

We have also developed a set of standard masonry units which may be used as an off the peg solution.

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Bespoke products

When a standard solution is not appropriate then a bespoke solution should be developed. Achieving designs where the stone seemingly hangs in the air through to arrangements with complimentary materials, project-specific designs allow the designer to show what stone is capable of.

The involvement of the quarrying company, alongside fixing companies and fixing manufacturers will ensure that the design team is given the most appropriate advice. Stancliffe is delighted to instigate this partnering process.

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British Standards in Natural Stone

BS 5628 code of practice for use of masonry.
BS 5642 cills and copings.
BS 8298 code of practice for design & installation of natural stone cladding and lining.
BSEN 12440 natural stone denomination criteria.
BSEN 12670 natural stone terminology.

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Natural stone test methods for the determination of:

BSEN 1925 water absorption by capillarity.
BSEN 1926 compressive strength.
BSEN 1936 real and apparent density; total and open porosity.
BSEN 12370 resistance to salt crystallisation.
BSEN 12371 frost resistance.
BSEN 12372 flexural strength under concentrated load.
BSEN 12407 petrographic examination.
BSEN 13364 the breaking load at dowel hole.
BSEN 13373 geometric characteristics of units.
BSEN 13755 water absorption at atmospheric pressure.
BSEN 13919 resistance to ageing by SO2 action in the presence of humidity.

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Further sources of information