Portland Stone Cladding

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.

New London Stock Exchange

Portland Stone cladding can be sub-divided into three categories based on the  fixing method and function:

Traditional Handset Cladding
Stone-Faced Pre-Cast Concrete Cladding
Rainscreen Stone on Metal Frame Cladding

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.

Selecting the Stone

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.

Extract from BS 8298 Table 2.  Albion Stone's Portland Stone has a good history of use in the UK and several other countries and conforms to BS EN 1469 and the DoP has declared values for all essential characteristics


Test regime 1 means that no production control testing is normally required.  Test regime 3 means that extensive initial testing and production control testing should be factored into the project costings and programme.  

The Stone Federation has produced a document explaining the processes that should be considered when selecting a stone, 'Selecting the Correct Stone'

If you'd like to request a copy please email the stone federation at enquiries@stonefed.org.uk or email us directly at enquiries@albionstone.com


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.

Indicative Stone Samples

These are small samples that show the typical colour and texture, but cannot show the natural variations that are present in all stones. 

Range Panels

The range panels are designed to show a good representation of what the different beds of Portland Stone will look like on a building facade and give the project team the opportunity to select their own choice of Portland Stone with confidence.

Surface Finish

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.

The design team will be able to view the stonework on these buildings either accompanied or unaccompanied by Albion Stone.

When viewing the stonework, examine the stone at close proximity and note the variations; shells, banding of shells, pore structure etc that make up the stone'€™s individual geological characteristics. It is useful to remember that the stones '€˜case harden'€™ (free calcite from within the stone migrates to the surface and forms a protective coat) over a period of time and this process tends to dull the natural variations of Portland Stone.  

After examining the façade in detail, we suggest you view the building as a whole from a reasonable distance, say from across the street. Examine the stone as part of the building structure and note the characteristics that are still visible.

Inspection of the Mine

Most project teams will arrange at least one visit to the Quarry/Mine at Portland, Dorset.

The visit provides an opportunity to examine the Mine and the stock to familiarize yourself with the geological variations from bed to bed. It is important to provide information on the quantity of stone required and the programme for the project so that the availability of particular stones can be established. The potential and merits of a block pre-purchase can also be examined. Albion Stone produces a quarterly '˜Block Stock Report', which provides information on the availability, geology and sizes of the blocks in stock.  Although long stones are available, it is advisable to check that the size of the typical stones on the project are readily available from the chosen blocks.


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:



Stone Design

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.

Unit Sizes

The 'Block Stock Report' will provide the current information about the typical and maximum sizes of each of the stones.

A building design that requires a large number of stones larger than the average block size is likely to involve significant waste and a longer lead-in period, although may still be possible. As a guide, we would suggest 600x400 as a typical ashlar and 900x600 as a maximum. Even at these sizes, assuming 50mm thick, the weight respectively at approximately 2,300kg per cubic metre would be 27.6kg and 62.1kg.

Stone Detailing

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.

In an attempt to replicate a solid stone such as a quoin, some designers consider using glued and pinned returns. The length of the return is limited to 4x thickness for 40 to 50mm thick and 3x thickness for 50mm and over.  The designer should be aware that the glued and pinned joint will contrast the other cement lime mortar joints and should only be used in exceptional circumstances.



Fig. Stone Detailing



Fig. Optimum Detail Design


The design of the façade will affect the weathering rates of the stonework. 

Traditional designs have large projections with robust drip details to throw the water off. The designer needs to be aware that if these projections are omitted from the design, then water run-off may wash the façade unevenly causing surface staining. A more regular stone cleaning programme will be required to keep these projects in pristine condition.

Stone Thickness

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

The overall cost savings for reducing the thickness could be significant, not only for the stone but also the structure itself. The factors that affect the calculation for the thickness are:

1  The Lateral load on the building

2  The fixing system

3  The panel size

4  The stone's technical properties including the flexural strength and the breaking load at dowel hole*.  

5  The partial load factor and partial material factor

 Please see Cladding Annex 1a.

* Albion Stone's Technical data is expressed in LEV and mean figures calculated using over 5,000 test results over 30 years so allows the designer to use a lower partial material factor.  Please see the Technical Data Sheet for each stone listed below

Bowers Roach

Bowers Basebed

Grove Whitbed

Fancy Beach Whitbed

Jordans Roach

Jordans Basebed

Jordans Whitbed

 Impact and Point load calculations may have a bearing on the thickness calculation.  

Impact Assessment and Point Loading

All exposed surfaces are liable to impact and each building should be assessment to ascertain the likelihood of impact damage and point loads. 

A soft body impact is generally associated with a person or a heavy cushioned item falling against the stone. The stone is more vulnerable at the ground level below 1.5m from pedestrians and vandalism where there is no incentive to exercise care.   Further up the facade is only accessible to operators with a high incentive to exercise care although there is a chance of an accident or misuse.  

Hard body impacts are associated with smaller lighter objects such as tools being dropped or thrown onto the cladding.

Generally, the best impact resistance is achieved by using a thicker panel (lower bending stresses, greater thickness of stone over fixings), with shorter spans (lower bending stresses); more flexible fixings (energy absorbed by deformation of fixing or material around fixing); or a more flexible support frame or backing wall (energy absorbed by deformation of supporting structure).

Point Loading should be considered for the project; for example where access equipment is used at low levels.  Design loadings should be provided by the project maintenance consultant.  

Energy Conservation

For Portland Stone cladding systems, U-values may be determined by a simple thermal resistance calculation.  Please see Cladding Annex 2

Manufacturing Tolerances

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. 

The manufacturing tolerances set out in BSEN 1469 and therefore transposed into BS 8298 vary depending on the size of the stone.  Typically, the larger the stone, the greater the tolerances.  Tighter tolerances can be agreed, but cost implication for rejections and more importantly the additional inspection process that will probably involve electronic calipers, need to be considered.   Most mortar mixes work better with traditional 5mm joints, which will typically accommodate the normal manufacturing tolerances. 

With the increasing use of thinner cladding, the potential impact of the strength around the fixing calculation needs to be considered when stating the tolerances for the dowel holes.  These tolerances are more easily achieved in a factory environment. 


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

As part of the quality system we have a programme of Factory Production Control (FPC) the stones are regularly tested in line with the international standards.

  • Geometrical characteristics BS EN 13373 Every production lot
  • Visual appearance BS EN 1469 Every production lot 
  • Flexural strength BS EN 13161  At least every 2 years
  • Water absorption BS EN 13755 At least every 2 years
  • Apparent density and open porosity BS EN 1936 At least every 2 years
  • Petrographic examination BS EN 12407 At least every 10 years
  • Breaking load at dowel hole BS EN 13364 At least every 10 years
  • Frost resistance BS EN 12371 At least every 10 years

The FPC as part of the 9001 and CE certification demonstrates Albion Stone's commitment towards maintaining the highest possible standards.

Technical Properties

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)  

As part of the DoP we maintain a factory production control system which is incorporated into our 9001 quality assurance scheme. This ensures that all our test results are current in accordance with the British standard and EN regulations.

Our Portland Stone conforms with BS EN 1469 and with a DoP and a good history of use in all locations in the UK our stone complies with test regime 1 in table 2 of the BS 8298.  Therefore, no further testing is normally required but limited testing for exceptional circumstances may be necessary.

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. 

The CE marking of all natural stone became a legal requirement in July 2013, but has now been replaced in the UK with the UKCA.   The UKCA, CE and DoP certificates are issued when the order is placed.  

Please request UKCA, CE certificates and Declarations of performance by emailing us at enquiries@albionstone.com

Stone Testing

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.

For example, it may be necessary to demonstrate a particular strength perhaps a load bearing section of stonework that needs to reach a figure that is close to the LEV of the chosen stone. A limited testing regime may be instigated to check the strength of a particular number of blocks. 

Sampling Plan

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.

The Sampling Plan will include the aim of the sampling, the method of obtaining the samples and the traceability of the actual samples back to locations in the quarry. The Sample Report will give details of the actual samples to be sent for testing.

Click here for the Sampling Plan
Click here for the Sampling Report 

British/European Standards

We recommend that the following standards are read in conjunction with this Technical Manual:

  • BS EN 1469: 2015 Natural stone products-Slabs for Cladding: Requirements

  • BS EN 12059: 2011 Natural stone products-Dimensional Stone Work: Requirements

  • BS 8298: 2019  Draft Code of Practice for the Design and Installation of Natural Stone Cladding and Lining, replacing parts 1 to 4 of BS 8298 2010.

    • Part 2 - Traditional Handset External Cladding,

    • Part 3 - Stone-Faced Pre-Cast Concrete Cladding System,

    • Part 4 - Rainscreen and Stone on Metal Frame Cladding Systems

    • Part 5 - Internal cladding