Understanding Natural Stone – Article written by Alan Sargent
Natural stone, when used for the production of steps, has to pass many different ‘tests’ to ensure that the right material is used for the purpose.
There are three primary aspects the mason demands from stone. It must be predictable, very strong and have a degree of workability that can only be found in high quality materials. Anything less than first class stone may result in a poor quality job, as the stone discolours, delaminates and generally fails after a few years due to a number of factors.
Stone Step Construction
For step construction, three important features are required; risers, treads and the front edges of the step treads.
Risers must be uniform in thickness and height, and fine sawn on all six sides.
Treads must be uniform in thickness, as each slab must marry in visually with its’ neighbour. Any variation in thickness across the step width will show immediately and spoil the overall appearance of the feature.
The front edges of the step treads are usually treated with a fine sawn ‘finished edge’. This may be bull-nosed, rounded, pencil edged or faceted according to the design.
For these reasons alone, the quality and choice of stone is extremely important.
Which natural stone should you use?
Traditionally, steps are created using either York stone (i.e stone from Yorkshire, not simply a generic sandstone which may have been quarried from a wide range of regions, and are erroneously described as ‘York stone’ simply because they are sandstone products.) or Portland Stone, which is a natural limestone that comes from the Portland region. Portland is described as an oolitic Jurassic limestone, and is widely used, both as steps, but also for cladding on many famous Landmark Buildings across Britain.
Both York stone and Portland stone have a similar density. This is proven by the weight of the material. Both weigh around 2,400kgs per cubic metre.
The porosity of both is around 3.5 – 4.0%, with a comparable compression or ‘crushing strength’ of around 38MPa on the Megapascal Pressure scale.
Step quality stone must also be non-slip. This aspect is measured using a pendulum tester, comprising of a bar fitted with a rubber slider, which is passed over the stone and the slip resistance of the device is carefully measured, with different sliders used for smooth or coarser surfaces. The lower the number, the greater the slip resistance – ideally around 36 ptv (Pendulum Test Value) – is considered safe for treads.
Working accurately with natural stone is a skill that takes many years to master, and it is only through experience that a mason will instinctively recognise quality stone, rejecting anything less than ideal for the proposed use.
Through a chain of professionals working together to supply and fix the stone from the quarry to the customer, each stage is carefully scrutinised to ensure the best possible end product.
With thanks to Alan Sargent for his first column on our website. We are honoured to be working with such a legend from the landscape world. Check out his site here: www.alansargent.co.uk | PGCA