Considerations in selecting stone

Selecting stone for external paving

As many of you know‚ we have a passion for the use of natural stone. There is no doubt that in the external environment‚ stone has qualities unmatched by other materials. There are many examples of very old paving which remain a pleasure to behold while standing up to all the vicissitudes of wear and tear. Manufactured products never seem to have this wonderful ability to improve with age.

In many urban situations‚ the paving is 50% of the total visual impact. Investment in this sphere has massive effects on the whole streetscape. Without it looking good‚ no town‚ city or office can have quality‚ can seem as though we care for it and will thus be respected by its citizens. It matters in the drive and garden‚ too.

Of course‚ it is the good examples of paving which survive‚ there have been failures in the use of stone as in everything. A good supplier’s wish is to assist with the choice of materials to give the best possible chance that they will be appropriate and practical for their purpose. His future depends not only on the quality of his material but how it is used. He should offer criticism‚ if appropriate‚ with reasons and ensure the designer has the best possible information on which to make a decision.

Paving suffers‚ not just from the weather‚ but abrasions and impact‚ frost‚ salt‚ skateboarders‚ market traders‚ pub deliverymen… no building façade has to do so. Quality is necessary‚ but the skill is in ascertaining which stones have the particular characteristics that are needed.

Through experience‚ we have found that we can be of most use if we are given the essential requirements for the particular application. We have evolved a checklist‚ which is given below‚ of the main matters needing consideration. In public spaces there are now (early 2010) some very useful British Standards (and some further revisions nearly ready to be published) that make a major contribution to successful design‚ especially is this the case in vehicular situations using either setts or flags. In most private and small commercial applications‚ an accurate description of the vehicular usage will enable us to assist. Guidance is given in BS EN 1341 which leads to a determination of the breaking load to be applied in the calculations for the laying of flags. It is surprising how often flags can be used now that there are high quality mortars readily available and flags are much more economical to lay than setts. For street design‚ further information of great use is contained in BS 7533‚ especially part 10 for setts and part 12 for flags. The revision of part 10 about to be published provides for the first time a standard method for the laying of natural stone setts where traffic levels exceed 200 standard axles per day and‚ the most onerous situations‚ where they exceed 1000 standard axles per day. For full information‚ you need to refer to BS to calculate the number of standard axles per day precisely‚ but a guide can be gained from the fact that a bus counts 2.9 standard axles. Buses of any size are particularly harsh on a road surface because they tend to follow the exact same line each time‚ thus increasing the likelihood of fatigue failure.
Then begin to select colours‚ design and sizes which suit the chosen materials.

Checklist

  • Purely pedestrian or vehicular (or shared surface)
  • Potential build-up for supporting the paving
  • Ethical issues
  • Environmental issues
  • Thickness constraints
  • Planned life
  • Fast food outlets
  • Oil drips from vehicles
  • Staining from other sources
  • Potential damage from: Market traders‚ pub deliveries‚ skateboarders‚ lorries on pavement and vandalism
  • Maintenance - chewing gum - street cleaners : Vacuum and jet washing
  • Frost requirements‚ salt regime
  • Slip/skid resistance
  • Disabled access
  • Building context and adjoining materials
  • Relationship to internal paving
  • Colour consistency
  • Compatibility of street furniture and kerbs
  • Can the quarry produce the size/shape desired
  • Start date/procurement timetable
  • Tolerances required
  • Details to be incorporated - gulleys‚ drains‚ inserts…
  • Supervision of the work
  • Contrasts in one material
  • Combinations of materials

Use stone paving for giving a sense of place‚ showing history or geography‚ providing information and entertainment‚ art and having fun -aesthetically‚ economically and with sustainability.

Michael Heap
CED Ltd 13.02.10

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