Lightweight Natural Stone and Glass Ventilated Rainscreen Facades and the Requirements of Modern Architecture - alsecco (UK) Ltd


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Architectural Facades

Hello and welcome to Lithodecor part of Alsecco UK limited and the DAW group of companies. I am going to present Alsecco RIBA accredited CPD entitled lightweight natural stone and glass ventilated rainscreen facades and the requirements of modern architecture.


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Learning Aims

The objectives of the CPD are to provide an understanding of the comparison between handset stone and lightweight stone ventilated rainscreen panels. To indicate the method of achieving robust lightweight rainscreen stone facades, to address specification considerations and to provide an understanding of the Airtec system.


The agenda for the CPD firstly introduces Alsecco UK and Lithodecor. Then we will look at the historic use of stone as this forms the basis of our considerations in the CPD.


We will then move on to consider traditional construction methods using natural stone. And then as architecture has changed, look at the requirements of modern architecture, and how ventilated rainscreen cladding aligns with that. We will then present Airtec stone and Airtec glass in more detail, including reviewing a number of projects.


Alsecco UK and DAW

Alsecco UK is a member of the DAW group of companies whilst not well known in the UK is Europe’s second largest manufacturer of architectural paints and coatings, producing some 500 million kilos of paints and coatings per annum worldwide. In 1958 the DAW group pioneered the development of External Thermal Insulated Composite Systems or ETICS as it's known in Europe, referred to as external wall insulation or EWI in the UK. The DAW group is the 2nd largest supplier of ETICS systems in Europe as well as supplying EWI systems in the UK, Alsecco has supplied ventilated rainscreen cladding systems for over 30 years.


In 1988 the DAW group acquired Lithodecor manufacturers of the Airtec system at that time, Lithodecor were developing the two systems and continued with the help of DAW group to do so over the following four years, over 200,000 square metres of Airtec panels have now been produced and installed with the Airtec systems, now branded as Lithodecor.

Historic use of Natural Stone

It may be useful to put ventilated rainscreen natural stone cladding, into context by going back many years to look at the historic use of natural stone. This is a picture of the Hales quarry back in 1913, which had a huge influence on the use of stone in the city of Edinburgh. There were only two main stone quarries in Edinburgh and yet the use of stone in the city is vast, and we still appreciate the iconic buildings today.


The late 18th and early 1900s was characterised by a large skilled workforce, but labour was relatively cheap, so as a result many of our large cities utilised natural stone, influencing cities like Bath and Oxford too.


Shown here is a red sandstone quarry in the West of Scotland still being quarried today. Note the use of the old derek's used to winch the blocks to the service. We only have to consider cities such as Leeds, not necessarily thought of as a stone city where, we have historic buildings using portland stone on the left for the Civic Hall and sandstone used on the Town Hall on the right, to see how influential the use of stone was in many of our cities and towns. Town halls and public buildings across the UK were constructed often in monolithic proportions using sand stones, limestone and in some instances granite from the local quarries.


We only have to consider Edinburgh, Bath and of course London, to realise the huge legacy of great buildings left from the previous generations.


Stone Industry - Modern Day

We just looked at historic quarrying and, in some respects, very little has changed when we look at the present day the only difference being the use of hydraulic machinery. Although still quarried in the UK, the stone industry is much smaller now, with a limited number of quarries in the dimension stone sector still producing building stone. In this instance sandstone, a sedimentary stone formed in layered deposits, is being pulled from its natural horizontal bed. In some granite and slate quarries wire saws are used to cut blocks from the quarry face. In the Portland quarries in Dorset the latest methods include cutting channels into the quarry floor and inserting huge airbags which when inflated simply push the large blocks from their original position, ensuring no damage is caused by blasting. Once extracted from the ground these huge chunks of stone require cutting to shape. This stone is being cut into slices with rapid cutting diamond wire saws and then the finished sizes will be achieved using secondary saws.


Traditional Ashlar Construction

Moving onto traditional ashlar construction, which is where we have a cavity wall with an outer leaf of natural stone. You can see an example here where a red sandstone has been used. Traditionally the ashlar cladding would be between 75 and 100mm thick and as you can see illustrated here, would have relatively tight joints of say 5mm or sometimes a 3mm joint. The two basic principles of handset stone are support usually at each floor level and restraint. Each stone would have a number of dowels fixed as restraint fixings perk to the inner course, usually block work or concrete depending on the framework at the building. Course Heights would be 2 specification although, they would often be 300 or 400 millimetres and covered by British standard 8298 2010.


This is a detailed drawing taken from that standard and the immediate thing we notice is the size of the cell and above the handset ashlar considerable in weight, meaning every floor level would need a robust support structure. The ashlar taken above there is not only supported but restrained by the dowelled fixings. Here you can see the dowells restraining each course of stone.


So, we begin to conclude the natural stonework is extremely heavy therefore, scaffolding is a necessity it is also a wet trade, so you are limited to the number of courses you can fix in a day, usually around 4 as the jointing needs time to dry.


So, in summary there are weight constraints with this type of construction which requires the need for scaffolding; and the nature of the wet installation requires time considerations, not just for the lead time to obtain the stone but also for drying on site.


Stone on pre-cast concrete units

In an attempt to reduce complexity and time, a number of buildings moved to using stone on precast units. The main reason for this was tight city centre sites especially, in London where roads weren't able to be closed and also there was no space on site for stone storage; and in many cases no room scaffolding. So, the answer was to put natural stone onto huge pieces of precast concrete.


This building, clad in Portland stone in the centre of London, has a repetitions design, which lends itself for using stone on precast concrete.


The first thing you would have to consider with this construction method is how you were going to split up the stonework into those precast elements. So, if we look at the T sections, we can see they are repeated which, you will be able to see later in its completed form. So, in the early project stages the architects and constractors would need to work together to set out the building, ensuring it is suitable for this type of construction.


On the left while the stone is being prepared in the quarry, the wooden formers would be produced to lay down the stone. The stone is laid face down in the former's a set number of dowels are then put into the stone at specific angles and the joints masked. The back of the stone is sealed to prevent leachates from the concrete mix into the stone causing discoloration. Once that is completed the concrete is poured onto the back. Remember we looked at the tea section drawings earlier this is it in its finished panel form on the left. The issue here is factoring in the panel curing time to the project time scale, which is typically between 22 and 25 weeks, as well as factoring obtaining the stone itself. The main advantage of this type of construction was, that once produced the panels could be transported to site and craned into position on the framework of the building.


So, to recap this method of construction: a robust frame is required due to the weight constraints of the completed panel, plus cranage to move the panels into position and with the main issue, being the long lead time.


Ventilated Rainscreen Cladding

As modern methods of design progressed this therefore posed the question as to how to align natural stone, a very heavy product, to rainscreen cladding. Considering the requirements of modern architecture with the introduction of products such as terracotta cladding and wooden cladding, many architects wished to use a natural stone finish with rainscreen cladding. The problem therefore for the stoneworld was how to make an inherently heavy product light and not only that, but to create very large panels, as you can see in these photos.


When using natural stone in its handsets or precast format, it was relatively simple to specify as BS8298 covered those construction methods we have mentioned. However, where the rainscreen cladding there is a lack of British standards, testing therefore is very important.


As far as the principals here with the British standards that are relevant none of them actually refers specifically to a product, but just to the design considerations such as the wind loading and structural aluminium elements and, also impact testing. Perhaps as a result the CWCT who set the standards for systemised building envelope systems, play a huge part in the UK with regards to the testing.


Our recommendation is that if you are specifying any product on rainscreen cladding not just natural stone, that you ensure it's not only tested by such as CWCT, but you also go back to European standards body that can offer approvals in the UK. We have the BBA and in Germany we have the DIBT the Deutscher Institute of Building Technology.


It is important for British standards and building regulations, that we have an approvals body to a European standard. An obvious advantage of ventilated rainscreen cladding is that the building can be made watertight fairly quickly, and often contractors are able to work inside while the exterior of the building is being completed. Here you can see that the rail system has already been completed, before the panels are brought to site.


Since the panels are light in weight they can be fixed back to systems, such as SFS and can be fixed from scissor lifts, mast climbers or hydraulic platforms. So basically, the large former panels are constructed of site and then a rapid fix can be achieved on this dry site.


So, if we just recap on the innovation of rainscreen cladding and the main advantages. The panels are light in weight and can be of large format, it's a fast track build with panels produced off site and simply transported in and attached to the building framework.


Site access ability is generally not an issue as minimal equipment is required, and project time scales can be dramatically reduced.


Airtec Stone

So, if we move on to look specifically at the Airtec ventilated rainscreen cladding system. Within reason any worldwide stone can be used ensuring huge choice for the building designer, from grenades to basalt to sand stones, limestones and slate. As long as the stone is accessible from a quarry and is readily available it can be used with the system.


Unlike the precast concrete panels which is a wet pour system, for this system blocks of lightweight aggregates are cured which will be the backing panel. In this instance a blown clay aggregate is being cured, each block in the images numbered having already been cured and each block is tested by the DIBT to ensure it is the correct constituents in that block. The stone and backing material other than cut to size. Once cut the stone slabs and the Airtec backing are adhered along with a lightweight mesh. These sandwich panels are then put through the vacuum bags shown here, which not only assist the adhesion process but also the compaction of the two layers.


Once the panels are completed and cut to size, a number of holes are drilled with CNC core drilling technology, in designated positions typically 150mm in from each corner. Ceramic plugs are then inserted into these holes. The purpose of the ceramic plugs is to carry a screw thread sleeve which clips will be attached to.


The principles of rainscreen cladding systems are a helping hand bracket at the back onto a T section. The Airtec system then begins with the horizontal rail onto which, the clips which are attached to the panels, are fixed. This shows a section of a panel with the clip already attached, on the outer edge on the right, we have the natural stone then, the lightweight aggregate, and then adhered to that, the ceramic plug with a threaded sleeve through the centre, onto which the aluminium clip is bolted.


The clips are set to the correct torque on site and in this instance lined up as there is more than one row of clips. The panels are then simply clips onto the horizontal rail. One of the main advantages of their system is its weight, for example a 40mm thick piece of stone would weigh approximately 100 kilos per square metre. Whereas the Airtec panels are about 1/3 of that weight. It does depend on the type of stone for example, with granite the panels will be about 43 to 44 kilos per square metre, whereas a lightweight limestone would be around 40 kilos per square metre.


As outlined large format panels can be produced limited only by the size of the stone that can be quarried. The fixing system is invisible giving a smooth seamless effect with mitred corners, also available as part of the system, supported off the face panel up to a 250mm return. So, if you have window reveals or window soffits as well as the corners of buildings then the return gives the appearance of solid stonework. The system offers very high impact resistance, tests have been carried out by the CWCT, both hard and soft body impacts on the natural stone with no effect. With the hard body impact on the glass panels although, the glass shattered none of it came away from the backing panel, which clearly from a safety perspective is extremely important.


The Airtec system can easily be combined with other facade systems, as the rainscreen panels are fastened to a fairly standard helping hand brackets and T section, then it can be combined with other materials, such as terracotta cladding which we will look at later.


It is important to use a system that is fully tested and approved, and we have mentioned a number of times the importance of this.


If you look at the building regulations 2000 materials workmanship approved document to support regulation 7, it states that, it is a requirement that the system is fully tested and approved by a European approvals body.


As part of the DAW group, Alsecco offers all the technical and design back up you might require. Advice and support is offered from design stage to architects through to main contractors and subcontractors on all aspects. All requirements are checked against approvals before production will be considered, including not only panel sizes, but fixing positioning, wind load etcetera. We also will regularly visit site to ensure that the system is being installed correctly and be on hand to discuss any queries the installer might have.


Airtec Stone Projects

We would like now to introduce a number of projects which we hope you will find interesting. Southampton regional business centre situated in the Guildhall area of Southampton, used Portland stone to maintain its setting. This project was one of the first to use Airtec stone in the UK, and the building is home to the Capital Group of companies and Southampton City Council. In this project the system was used to encase columns and creative soffit with a drip groove. This building received a BREEAM excellent rating and just to reiterate, Portland stone was used to mirror the historical character of the setting, while still being relevant for such a modern building.


This building was part of the redevelopment of the Portsmouth ferry terminal, where the designers wanted to create an ammonite and shell like appearance to the façade, to fit in with its nautical and coastal surroundings. Airtec stone was used for both the exterior and interior of the building, with a Jura limestone finish.


In this instance the lightweight cladding carried through into the inside of the terminal building. You will note the large panels with 90 degree returns creating the effect of monolithic pieces. This building was awarded a BREEAM very good rating. The Airtec system was specified here, at the time, for the biggest shopping centre in Europe in Stratford east London, the Westfield Stratford city shopping centre directly opposite the Olympic park. A Portuguese white limestone in large panel sizes was used for a number of shop facia. Again, achieving a BREEAM rating of excellent.


The same stone was used for the redevelopment of a shopping centre in Wandsworth south London, the development is part of a retail led regeneration scheme of the area. The panels were used to create the dramatic and angular columns and in total around 1200 square meters was installed to this modern shopping centre. This was an interesting project at Ruskin college on the outskirts of Oxford, where a Portuguese stone with a heavy shell content was used to create a modern contrast with the existing listed building.


This new extension to the college's Rookery building has created a dramatic impact with the overhang shown here. Moving back to east London and the Olympic village, this project used two types of limestone. Firstly, a white limestone and then an unusual bluestone called Marshall kelk which has a high content of fossilization for the inset panels. The building was originally built for athlete accommodation for the London Olympics, but is now being transformed into luxury accommodation, in the area now known as East Village.


Moving onto Farringdon station, a very large project using Jura limestone plus also a few 100 square metres of Airtec glass in white. Both systems were used on the interior as well as the exterior and here you can see both Airtec stone and glass used in the ticket halls, with Airtec stone extending down to the platform. This was part of a much larger scheme to develop Farringdon station, which is still under construction. As a result of the helping hand brackets and T section, standard products for most rainscreen cladding, it is possible to incorporate different materials on the same façade.


On this project in the North of England on the base of the building there is extensive use of black granite, as part of the Airtec system and, then above installed on the same backing structure is the terracotta rainscreen cladding. This mixed use centre houses both a library and swimming pool, whilst also creating a public service hub for the community, an integrated plinth was also used at the base, providing increased impact resistance and durability for this high traffic area.


A stone from Gloucestershire was used on this relatively small scheme in Corby Northamptonshire. Quite a soft limestone was chosen so the panels had to be produced slightly thicker than usual. The main finish of the building was render but this elevation was finished in the natural stone system for aesthetic reasons, since it was opposite residential housing.


Airtec stone has been used here to dramatic effect in the centre of Dresden’s main square, where the new natural stone facade needed to match the surrounding building in its structure and shape. On this redevelopment project the Airtec system was used as an over cladding to the existing building. It literally acts as an overcoat to the existing façade, standing off around 300mm from the original façade. The tiered effect was incorporated to match the adjacent buildings and make sure it was in keeping with the local architecture in this historic city. You can see the stonework on this side elevation on the right the Marshall kalk limestone was used again for this project. On the left you can see a sanction that was required to complete the tiered effect on this project.


Airtec Glass

We would now like to introduce Airtec glass, the exact same system only with a glass finish. Airtec stone as explained was developed first and then a glass finish was then adhered to the same bagging to create Airtec glass, which has now been installed on projects for over 10 years. Very large panels can be achieved with this system, the largest we have produced is 4.7 square meters, but the DIBT approval covers larger sizes. Safety is an important aspect when dealing with glass and should there be a breakage, the glass is retained on the panel. All RAL colours are available and we would provide a sample to ensure the colour is as expected. It is also possible to incorporate designs on the glass which will be shown later. Airtec glass was specified for a prestigious project at Oxford University, the new earth Sciences Department, creating a very dramatic and sophisticated finish with a blue green glass. Reveals into the windows were not required on this project but these can easily be produced if required. The building was awarded a BREEAM rating of very good. It was a very significant project and created a lot of interest.


This is a project in West London again using a combination of materials. The brightly coloured panels that you can see are the Airtec glass system. The coloured panels provide a striking effect for this new build apartment block.


One of the first projects to use Airtec glass in the UK was for this M&S store at Gemini park in Warrington, different materials were used with a timber rainscreen cladding surrounding the black Airtec glass panels. The system was also used along the soffits, where cut outs were used for the lighting system, and the building was awarded a BREEAM excellent rating.


Both Airtec stone and Airtec glass were used here to create this amazing patchwork finish for the University of Potsdam in Germany. As the light changes different effects are created on the façades, the stone and glass systems were easily combined to offer a seamless transition of glass and stone.


An interesting point is that one type of black granite was used but with three different finishes, a polished finish, a leather optical finish and a brushed finish. The compatibility of the two systems has ensured a smooth sleek finish. The method of baking the colour onto the back of the glass, makes it possible to print on the reverse of the glass. With this project it was printing advertising wording on an elevation of the hotel.


Whereas on this project in Saint printed montage on a memorial in Erfurt in Germany and we are just working on a montage using the system at Berlin airport. This gives a clearer view of the intricacy of print, which is possible which, is really effective.


That brings you to the end of the CPD, so I will just recap on the content as follows: firstly we introduced Alsecco UK and Lithodecor; then we looked at the historic use of stone; and the more traditional construction methods through the years; we then looked at how the requirements of modern architecture have influenced these methods; and how then ventilated rainscreen cladding was developed; the latter part of the CPD looked in more detail at the Airtec system and then ran through some interesting projects, illustrating the features described as part of the system.


Many thanks for watching and listening to our CPD. We hope you found it educational and interesting.

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