Portland Stone Internal Cladding and Linings can be used in a variety of locations ranging from Commercial and Public Buildings to domestic properties.
The architect and structural engineer should agree, with those selecting and designing the cladding stones, their fixings and the supporting structure, the requirements of the design and the maximum movement of the cladding, together with the movement of the building frame and how this is to be accommodated in the cladding.
The cladding’s fixing system and the stone thickness should be designed to resist loadings that are anticipated during its life:
i. Wind Loading / Air Pressure changes, there are two calculations that need to be completed
ii. Point Loading
iii. Barrier Loading
The strength of the individual beds of stones are in the Portland Stone section of the website.
Albion’s Portland Stones have been regularly tested for the last 30 years so there is a wealth of reliable test data to allow for the loading calculation to be completed so the fixing systems can be designed, and the stone thickness calculated.
The factor of safety to be used to complete the stone thickness calculation, is now a calculation based on the reliability and robustness of the stone’s technical data. The factor of safety calculation has partial load factor, γf, that accounts for the uncertainty in the loading and a partial material factor, γm, that accounts for the natural variations in the physical properties of the stone. Section 5.6.3 in BS 8298 part 2 explains the importance of the technical data is used in the partial material factor part of the calculation. The key aspect of the calculation when using one of our stone’s, is our wealth of historical data that is incorporated into the figures which allows a very low factor safety to be used.
The stone panel thickness should be calculated at an early stage of design, to influence key decisions relating to the stone design and supporting construction. There are two calculations that need to be considered:
There are example calculations in Annex G in BS 8298 Part 5.
The worked example is based on a typical Jordans Whitbed Portland stone panel of 900mm in length × 600mm in width, supported by dowels in a stack-bonded layout giving a fixing distance along the length of 540mm. Therefore, the maximum distance between the fixings is the height at 600mm. It is assumed that the stone is used in an atrium with an opening ratio of less than 5%.
For this example the calculations based on the prevailing Technical Data for the Jordans Whitbed show that the stone can be used at 30mm thick.
Stone Cladding is liable to accidental impacts such as people stumbling or cleaning equipment striking the stone which can be divided into soft and hard body.
The likelihood of these impacts and the risk should be assessed for each part of the cladding. The stone cladding should be designed to withstand reasonable levels of impact, but additional measures can be applied to specific areas if required such as making the stones thicker, supporting the stones or altering the fixings.
The actual stones should be designed to ensure cost-effective and environmentally sound use of this valuable product.
We would suggest 600mmx400mmm as a typical ashlar and 900mmx600mm as a maximum. Even at these sizes, assuming 30mm thick, the weight respectively at approximately 2,300kg per cubic metre would be 16.6kg and 37.3kg. If the stones are designed at a size larger than the average block size (see latest mine report), the production will have a higher carbon footprint through additional wastage and may increase the cost and need a longer lead in period.
The fixing designer needs to have experience, knowledge and/or qualifications necessary to design the cladding and fixings. There a number of differing fixing systems for differing applications that should be considered including.
i. Load bearing and restraint fixings
ii. Combined load bearing and restraint fixings
iii. Undercut back anchors
iv. Kerf and mortices
v. Soffit fixings
vi. Adhesive bed, with supporting and restraining fixings.
The BS 8298 Part 5 has detailed drawings of the fixings and their suggested layout relating to the stones and the joints, together with information relating to the anchoring to the structure.
The cavity between the stone cladding and the backing structure will allow space for the fixings and bolt heads, stone, and structural tolerances but the control of fire will also need to be assessed.
Portland Stone at 20mm thick can be supported with fixings on a vertical adhesive bed without a cavity, but the supporting structure’s tolerances will need to be considered carefully. Stone solidly supported have greater resistance to impact loadings.
The joints will normally be 5mm to allow for manufacturing tolerance, and if narrower joints are specified, the impact on jointing material should also be assessed. The mix suggested in BS 8298 Part 5 is, 1: 2: 8 or 9, white cement, lime, Portland Stone dust.
Compression and movement joints and should be set out as stated in BS 8298 Part 5. Any joints that accommodate movement should be filled with a suitable sealant.
Manufacturing of Stone Cladding
The stone is manufactured in accordance with the tolerance set out in BS EN 1469 which are incorporated into all the BS 8298 standards. Typically, this standard sets out that there are greater manufacturing tolerances for larger and thicker stones.
Tolerances for the dowel hole and mortice are also set out in BS EN 1469 and wherever possible should be completed as a factory, not site operation.
All Albion Stone’s production is certificated to the ISO 9001 kitemark, meaning the quality system is independently audited by the BSI representatives. As part of this system, we have a programme of Factory Production Control (FPC) where each stone is inspected, and independent stone testing is regularly completed in accredited laboratories in accordance with the schedule in BS EN 1467. These results are recorded on the Technical Data sheets (link), UKCA certificates and Declaration of Performance, which can be provided once an order has been placed.
The design/manufacturing information should be provided to Albion Stone in line with any lead-in periods and to give the opportunity to instigate economic production runs for similar stones. The production will be scheduled to ensure that the stones are ready for delivery in line with an agreed predetermined call‑off that is compatible with the site fixing sequence.
All stones will be clearly marked with the agreed identification number and palleted and wrapped awaiting despatch.
The pallets of finished stone are wrapped in recyclable plastic to protect the stone during the transportation, unloading and limited site storage, not for prolonged external storage.
All stone need to be carefully handled on site by experienced fixing operatives. The designer should also ensure that the relevant lifting and manual handling regulations are considered at the design stages.
The best method of achieving satisfactory finished work, free from unsightly staining, mortar accretions and smearing, is prevention and good workmanship from an experienced Portland Stone Fixing Contractor. Care should be taken to avoid damage and suitable protection may be required, but some minor site damage can be repaired using a suitable filler. However, in most cases a replacement stone will be required, and Albion Stone will attempt to complete the work as expediently as possible.
On completion of installation of stone, the face of the unit should be cleaned of all dust, rust and other stains. All cleaning, protection and repairs should be carried out in accordance with BS 8221‑1 and BS 8221‑2.
Sealing or impregnating the walling is an option if the stones are in an area that is possibly subject to staining.
However, in most situations we would recommend leaving the stone to develop a natural patina on the surface.
Cleaning and surface repair should be carried out in accordance with BS 8221‑1 and BS 8221‑2 and the Stone Federation guide to Best Practice on the Cleaning of Internal and External Masonry Surfaces guidance by suitably qualified operatives but for small and day to day cleaning in most minor internal situations, cleaning with a damp cloth and diluted Lithofin Easy Care will remove any surface grime. Any spillages that could potentially stain should be treated promptly with clean water. Bleach, household soaps and detergents should not be used on Portland Stone. For any further contamination to these areas, contact the Lithofin Technical Line for guidance on cleaning these areas
Albion Stone, a fourth generation family business pride ourselves on having a helpful and skilled workforce as well as modern and traditional manufacturing process to enable us to produce the highest quality Portland Stone with minimal environmental impact.