Portland Stone has been used for centuries as a building material and these buildings stand as testaments to the stones strength and durability. Originally the stone was used on its own or as a facing to brick masonry. With the advent of framed structures, where the load is carried by the frame and not by the external walling, the stone used to cover the framework is referred to as cladding.
Portland Stone cladding can be sub-divided into three categories based on the fixing method and function:
The decision on the type of cladding needs to be made early in the design process and will be determined by the function of the cladding, the size, height and location of the project, the type of structure and the programme.
It is vital to understand the geological variations that make up the typical character of any natural stone that is proposed for a project. At the pre-tender stage of the project a shortlist of stones should be researched to ensure the geological features are clearly understood, the are adequate quantities of appropriate form available for the project and the technical properties have been researched.
The initial selection for a short list of stones to be used on the project should be made from Indicative Samples. The range panels (reference samples) held at Portland show the designer the typical geological characteristics in each bed of Portland Stone.
The Quarry Range Panels all have a rubbed finish using a 50 grit (defined as rough ground in 'BS EN 1469'). Other finishes are available including; a honed, much smoother finish (120 grit) and various tooled finishes
Inspection of the Buildings
Albion Stone will be able to provide a list of buildings constructed using the same stone.
Inspection of the Mine
Most project teams will arrange at least one visit to the Quarry/Mine at Portland, Dorset.
The designer should be aware of the recommended lead-in period for the supply of stonework cladding.
A typical programme for a medium sized office development in Central London has been set out below:
This may seem like an obvious statement but all stone is cut from a raw block. This principle is important for cost-effective and environmentally sound use of this valuable product which should be considered at the design stage.
The 'Block Stock Report' will provide the current information about the typical and maximum sizes of each of the stones.
As all stones are cut from the solid raw blocks, anything cut away, such as notched quoins of 'U' shaped stones, is waste and therefore additional cost.
The design of the façade will affect the weathering rates of the stonework.
In order to derive the benefit from a ‘designed’ thickness of the stone, it is vital that the designer makes an early selection of the stone and then designs the cladding to suit the selected stone
All exposed surfaces are liable to impact and each building should be assessment to ascertain the likelihood of impact damage and point loads.
For Portland Stone cladding systems, U-values may be determined by a simple thermal resistance calculation. Please see Cladding Annex 2
The manufacturing tolerance on the thickness and face dimensions of stones is set out in BS 8298 Part 2 section 7 and Part 4 section 6 and conforms to the European Standard BSEN 1469.
Albion's Portland Stone bears the BSI ISO 9001 certification kitemark. This is your assurance that the stones are regularly subjected to rigorous, independent testing to ensure that they comply with stringent standards for safety, product performance, and reliability
Albion Stone's Portland Stone is one of the most tested stones in the world due to the high number of prestigious buildings that have been built using the stones. A summary of all of our test results is declared on our Technical Data Sheets, CE Certificates and Declaration of Performance (DoP)
UKCA Certificates and Declarations of Performance
The figures from the Technical Data Sheets have been used to UKCA mark (CE mark for EU exports) Albion Stone's products.
No further testing is normally required for our Portland Stone as it invariably falls into Testing Regime 1 as defined in the BS 8298 due to the extent of its use and the wealth of technical data available. However, in exceptional circumstances it might be required.
If it is decided that a testing regime is appropriate, a Sampling Plan needs to be prepared and the acceptance criteria should be established based on the actual physical requirements of the stone and a valid factor of safety.
We recommend that the following standards are read in conjunction with this Technical Manual: