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

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


Samples

The initial selection for a short list of stones to be used on the project should be made from Indicative Samples. 

These are small samples that show the typical colour and texture, but cannot show the natural variations that are present in all stones. The stone selection should be confirmed with an inspection of buildings  (case studies) built using the same stone and a visit to the mine to view the natural faces and the Range Panels.


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 '€˜Mine 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.


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

Stone Design

It is important to remember when designing stone cladding that the finished stone will be cut from a raw block. 

This may seem like an obvious statement but the principle is important for cost-effective and environmentally sound use of this valuable product.


Unit Sizes

The '€Mine 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.

 

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Fig. Stone Detailing

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Fig. Optimum Detail Design

Weathering

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 building to suit the 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  Stone strength

2  Panel size

3  Wind loadings

4  Fixing location in the stone

5  Type of fixing

It is also necessary to consider the strength of the stone around the fixings. The breaking load at the fixing will be dependent on the area of the panel, the estimated wind loading and the stones’ strength.  Please see Cladding Annex 1.

To complete the calculations you will need the Technical Data for our stones. 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

 


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 1: 2000 section 7.2 and conforms to the European Standard BSEN 1469. 

Many stones will fall into the category of >50mm thick and length or width <600mm and will therefore be governed by a tolerance +– 2mm.

Albion Stone will typically manufacture within this tolerance for the majority of production and allows for an installation tolerance of +– 1.5mm on a joint size of 5mm in most cases. Even the larger stones (>600mm) will be within the +– 3mm tolerance and in the majority of cases will allow for +– 1.5mm on the joints for site installation.

On several projects the design team have required exceptional tolerances (eg +– 1mm). The only reliable way of delivering these tolerances is through checking a very large sample of the finished stones using electronic callipers


Quality

Albion'€s Portland Stone bears the BSI ISO 9001 certification kitemark.

This is your assurance that samples are regularly subjected to rigorous, independent testing to ensure that they comply with stringent standards for safety, product performance, or reliability. In addition, this also means that the quality systems of the factory where this Portland Stone is manufactured are systematically assessed.

The certification is therefore Albion Stone'€s commitment towards maintaining the highest possible standards.

Technical Data

The European Standard BSEN 1469 requires that all stones comply to a regime of initial testing and Factory Production Control testing. 

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.  The results from these tests have been incorporated into the Technical Data Sheets for the individual stones. The figures on these sheets have been prepared using the methods set out in the harmonised European Testing Standards into the highest expected values (HEV), the lowest expected value (LEV) and the average. Due to the large number of tests, these figures provide the design team with a high level of confidence.


CE Certificates and Declarations of Performance

The figures from the Technical Data Sheets have been used to CE mark Albion Stone'€s products. 

The CE marking of all natural stone became a legal requirement in July 2013, but Albion Stone have been CE marking all their stones since 2008.   CE marking provides a legal confirmation to the client that the product is suitable to be placed on the market.

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


Stone Testing

Despite this wealth of data, some clients will still desire additional testing. 

Often this will be required to confirm a particular aspect of the project; 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. In some instances the design team may wish to conduct their own tests.  BS 8298 Part 1 (Table 3) provides guidance on the frequency and extent of the testing required. The use of Portland Stone in all locations of the UK can demonstrate a '€good previous history'€.  Current test data is also available, therefore only limited testing is required to verify that current production is consistent with past results.


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 

Impact Damage

All exposed surfaces are liable to impact damage. This  damage can be described as either soft body or hard body.  A soft body impact is generally associated with a person or another heavy cushioned item falling against the stone.

Hard body impacts are associated with smaller lighter objects such as tools being dropped onto the cladding. Both types of impacts have the potential to cause damage and although it is very difficult to protect against hard body impacts, there are a number of ways to increase the strength of stone cladding to protect against soft body impacts; including stone thickness, spacing between the fixings, and fixings  design. An assessment of the potential risks needs to be completed by the designer or engineer, with testing in line with BS 8298 Part 1: Section 6.14.

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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: 2010  Code of Practice for the Design and Installation of Natural Stone Cladding and Lining(replacing BS 8297 & 8 - 1994) BS 6399-2: 1997 Loadings for Buildings, Part 2: code of practice for wind loads.

    • Part 1 - General

    • 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

Specification Clause