Recommendations for the design, manufacture, installation, and maintenance of Portland Stone Facades
We encourage the Architect and Structural Engineer to consider the stone selection together with the designing of the stone facades and the fixing system at an early stage of the project.
The façade’s fixing system and the stone thickness should be designed to resist loadings that are anticipated during its life:
i. Wind Loading, there are two calculations that need to be completed
ii. Snow Loading
iii. Point Loading
iv. 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 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 facade design and supporting construction. There are two calculations that need to be considered:
- Flexural failure of the stone panel
- Breakout failure at the stone’s fixing point
There are example calculations in Annex E in BS 8298 Part 2 and Annex C in BS 8298 Part 4.
The worked example is based on a typical Jordans Whitbed Portland stone panel of 900 mm in length × 600 mm in width, supported by dowels in a stack-bonded layout giving a fixing distance along the length of 540 mm. Therefore, the maximum distance between the fixings is the height at 600 mm. It is assumed that the stone is for a low rise project in central London, which is a non-freeze location, but the freeze-thaw results are used as a precaution and the low wind classification is as in Table B.1. in the document.
These examples based on the prevailing Technical Data for the Jordans Whitbed, show that the stone can be used at 50mm thick.
Stone Cladding is liable to accidental impacts (people stumbling into the stone) and deliberate impacts (vandalism) 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 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 600mmx400mm as a typical ashlar and 900mmx600mm 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. If the stones are designed at a size larger than the average block size (see latest mine report), then the production will have a higher carbon footprint through additional wastage which may increase cost and need a longer lead in period.
The cladding and stones should be designed to avoid uneven washing of the façade by rainwater, traditionally this was achieved using projecting stones and drips. Biological soiling is becoming more prevalent as air quality improves in inner cities but is a visual issue and unlikely to cause any damage to the stone, see section 4.3 Soiling Patterns, Natural stone masonry in modern Scottish construction. Staining of stonework can also be caused by moisture from the backing structure and rising damp so the location of cavity trays and the dpc needs to be carefully considered.
The cavity between the stone cladding and the backing structure will allow space for the fixings and bolt heads, stone and structural tolerances, insulation, an air gap to minimise transmission of rainwater and allow for free draining of any trapped moisture.
The cavity design needs to incorporate cavity barriers to prevent the spread of fire both vertically and horizontally and reduce the effect of fire on the fixings.
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
The relevant BS 8298 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.
Pre-stressed columns of individual stones can have a high strength bar or cable through the column with loading plates at each end to disturb the anchor force.
The type of pointing in a joint will often depend on whether they are intended either to be loadbearing or accommodate movement. The pointing should be weathertight and are normally either cement lime mortar, sealant or for some applications such as prestressed columns, hydraulic lime mortar. Mortar joints are typically 5mm wide to allow manufacturing tolerances, and if narrower joints are specified, the impact on jointing material should also be assessed. The mix suggested in BS 8298 section 5.13.3.2 is 1 white cement:2 lime:8 or 9 Portland Stone dust.
Open joints in rainscreen cladding should designed to permit air to circulate behind the stone and water and moisture to drain from the cavity.
Compression and movement joints should be set out as stated in the relevant BS 8298. Any joints that accommodate movement should be filled with a suitable sealant.
Birdsmouth and mitred joints are not good designs for Portland Stone and should be avoided. In certain circumstances a glued and pinned return may be preferred to a stone cut out from a solid quoin, but the length of the return needs to relate to the thickness of the stone, See table 6 in BS 8298 part 2.
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 1469. These results are recorded on the Technical Data sheets, 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.
After initial installation, the stone may dry inconsistently but slight discolouration will normally settle down very quickly and will not require any additional cleaning. We do not recommend sealing the stone.
Maintenance work is not required in all but exceptional circumstances, to protect the stone during it design life. In the low pollution levels in most UK cities, the design life will be extremely long and almost certainly in excess of the other products used on the facade.
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 by suitably qualified operatives on a cycle depending on the building design, the local climate and orientation of the facades and the levels of atmospheric deposits deemed acceptable by the client.
Where nameplates or other fitments that require regular cleaning are fixed to Portland cladding, measures should be taken to avoid staining the surrounding stone during the cleaning operation. A masking plate that fits tightly around the nameplate or fitment should be provided.
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.
