Q&A: What is the Maximum Panel Size for Portland Stone Cladding?

The Age Old Question

After 35 years of quarrying and manufacturing Portland stone, one of the most common questions we are asked is: “What’s the biggest size we can have for Portland stone ashlar panels?” We've heard it from so many architects it’s easy for us to believe that Sir Christopher Wren asked the quarrymen in his day the same question when he was designing St. Paul’s Cathedral and he probably got the same answer as we give today, ‘it depends’.

St Paul's Cathedral

Size Does Matter

When BS 8298, the standard for natural stone cladding, was revised in 2010, it included a formula to calculate the minimum thickness of natural stone cladding taking into account five criterion relating to the design, the building’s location and the technical performance of the stone being used.

By looking at that same formula from a different perspective, we can deduce the maximum panel size for any given thickness of Portland stone. To avoid a plethora of impractical dimensions, we have assumed the thickness will be 40mm or 50mm, the most common thicknesses for stone cladding, both here and in Europe.

Guidelines for Maximum Panel Size for Portland Stone Cladding

Using information from BS 8298, and the Stone Federation’s Guide to BS 8298 we can work out the maximum size for cladding using the two controlling calculations which are: breaking load due to lateral load and the breaking point at the fixing.

The panel sizes below are calculated using the same criteria as set out in the Stone Federation’s Guide example: a four dowel fixing support for the stone, a low wind environment (which is 1500N.m-2), a factory of safety of 6 and the mean flexural strength figures.

Portland Stone - Maximum Panel Sizes

Thickness

Maximum Size For Cladding

Project Examples

Jordans Basebed

Grove Whitbed

40mm

800mm x 625mm

800 x 650 mm

Cliffords Inn

50mm

1000mm x 725mm

1000mm x 825mm

British MuseumFinsbury Circus House

NB – The following table is a guideline only and should not be used as a basis for any specification. We strongly recommend you apply the formula in BS 8298 to calculate the maximum panel size for your project. While Jordans Basebed and Grove Whitbed have been used for this example, we can provide the technical data for all the Portland stone beds in our range to facilitate the calculation.

Big Isn't Always Best

Using large stones for your project may result in more wastage, and therefore increase the environmental impact and probably the price. Please contact us for more detailed information about the effect of ordering large stones from the different Portland stone beds.

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 BS 8298, we can suggest an experienced engineer or stone consultant.