20th March 2017
New Era Brings New Questions
Over the years, architects have often asked us, “What’s the biggest size we can have for Portland stone ashlar panels?” But recently, the focus has changed. The new era of sustainability and conservation has brought new challenges for building designers. These days we’re just as likely to be asked “What is the Minimum Thickness for Portland Stone Cladding?”
Reducing the thickness makes a lot of sense. It reduces the amount or raw material used and therefore reduces the environmental impact, it also costs less and reduces the weight of the structure. So what’s the catch? It will also affect the maximum panel size you can use.
40mm, 50mm or 75mm?
While BS8298, the British Standard for stone cladding, has a formula to calculate the exact minimum thickness it’s impractical for a factory to cut stone at every conceivable thickness. Imagine a 62mm for one project and 47mm for another and so on. It would require more raw material in the long run because there would be very little opportunity to re-use the off-cuts for a 62mm project and which would increase the waste in the factory. And when the waste goes up, the price follows.
Some projects still use the old 3”, which was always the default position, but more commonly 50mm and even 40mm as the reduced thickness. That way, any off-cuts can be stored and re-used for another project, reducing waste, saving raw material and minimising costs.
For limestone, 40, 50 or 75mm are are the most common thicknesses for stone cladding, both here and in Europe. The BS 8298 Code of Practice for the design and installation of natural stone cladding is currently being re-written and we are helping with the drafting. We hope the new code will give clearer information relating to the use of the calculation and the impact of the differing technical data used.
We have produced the following projects in these thicknesses. We strongly recommend you apply the formula in BS8298 to calculate the minimum thickness size for your project. We can provide the technical data for all the Portland stone beds in our range to facilitate the calculation.
|40mm||Clifford's Inn||Jordans Basebed & Grove Whitbed|
|50mm||CitzenM Hotel||Jordans Whitbed|
|Canada House, Ireland||Portland Roach|
|St James's Market||Jordans Whitbed|
Contact us if you require any further detail on specifying Portland stone for cladding or any application. If you need help with the formula in BS8298, we can suggest an experienced engineer or stone consultant.
Please see the cladding annex which forms part of our technical manual, this annex sets out the two separate calculations that need to be completed relating to breaking due to lateral load and breaking at fixing point.