An affordable premium
Rainscreen façades incorporating a natural finish such as stone or terracotta, have historically been the preserve of high-end private developments due to premium system cost. In response to specification demand for these materials, façade innovators have pioneered the development of a new generation of system, making it affordable for all types of public sector project. William Harwood, T.I. Dynamic Facades, lifts the lid on the engineering behind the technological innovations.
The primary function of a rainscreen is to protect the inner leaf of a building from wind driven rain and ultra violet light. However, the availability of sophisticated finishes, such as those made from natural stone and terracotta materials, has made the rainscreen as popular for its aesthetics as for its practicality.
Moreover, advances in natural rainscreen technology have made these stylish designs achievable within a budget usually comparable with using a lower-end metallic cladding system. Through continual innovation, façade manufacturers are also creating greater market choice, most notably with the development of higher performing and cost effective terracotta rainscreen systems.
When the terracotta rainscreen was first developed, the only way to achieve the required strength, durability and wind resistance was to use a twin skin design.
While early systems offered many of the benefits expected of a façade system, such as increased build efficiency, there were some challenges experienced on-site, most notably with tile cutting. This is because each tile essentially consists of two tiles, with a hollow section in between. So for each tile that has to be cut, there is twice the thickness, which increases the risk of breakages.
Advances in twin skin terracotta rainscreen system design have helped overcome some of these early challenges, engineering the material to be lighter weight and easier to cut.
Manufacturers have also responded to feedback from the industry, by making a wider choice of tile heights and lengths available. This reduces the amount of cutting required on-site as most floor slab and punched out window dimensions can be accommodated by using one common tile from the standard range. Installation is further simplified by the availability of special tile sections for corners, gutters and windows.
Regarding aesthetics, modern twin skin systems are available in a wide range of colours and final finishes, such as natural, enamelled, slate or metallic. Some of the more popular systems are manufactured with an anti-graffiti treatment as standard. By protecting the tiled exterior of the building from vandalism, this ensures a high quality finish is maintained, even over time.
At present, twin skin is still the most commonly specified type of terracotta tile, because the majority of architects have practical experience of it. However, the development of a new generation of single skin terracotta rainscreen, which offers performance equal to that of the thicker twin tile, is already starting to reverse this trend.
The single selection
The twin skin tile was used for the original terracotta rainscreen and, over time, has built up market confidence in its reliability. Launched to market more recently, the single skin terracotta rainscreen is much thinner and lighter, leading to the misconception amongst some specifiers that performance levels in terms of strength and durability will be compromised. The consequence of this is that twin skin remains the first specification choice at architect level, although it is often not the most practical solution on-site.
The most advanced single skin systems weigh an average of 10kg per square metre less than traditional offerings. As well as ensuring health and safety legislative requirements are met, this also helps to significantly increase speed of installation. Moreover, because the tiles are a lot thinner, cutting is a lot easier, which reduces the amount of wastage through breakages.
As well as simplifying cutting and installation, single skin terracotta rainscreen systems are available in a wide choice of colours and finishes. Some products are available in over 50 different shades, for example.
The objection relating to compromised performance is totally unfounded, as the only complete terracotta rainscreen to be fully BBA (British Board of Agrement) certified, is a single skin system. This means it is proven to require little or no maintenance and its overall performance has been tested up to a height of 100 metres. Most twin skin terracotta rainscreens have only achieved elemental accreditation from the Centre for Window and Cladding Technology (CWCT), with the exception of just one, which is certified by the BRE (British Research Establishment).
Key to the performance of the single skin system is its innovative manufacturing process. A porcelain base baked at a high temperature significantly reduces the water absorbency of the system, making it more resistant to the freeze/thaw phenomenon that causes some silicon sprayed alternatives to fail.
A lighter way
In addition to more affordable terracotta rainscreen systems, façade innovators have also developed a new breed of stone façade that makes it more cost effective to purchase and install.
Although stone is naturally a durable material, to withstand aggressive environmental conditions and provide a viable exterior finish, stone façades have traditionally had to be approximately 40mm thick, in order to deliver the required performance. The consequence of this is that the system generally takes longer to install, resulting in increased labour costs.
To help make stone more competitive, façade innovators have developed lighter weight systems that are durable and easier to install than conventional constructions. Where stone is concerned, this has been achieved by creating a facing that is reinforced with a high strength fibre, aluminium or aluminium honeycomb backing. Stone facings are manufactured 6mm to 15mm thick and bonded to one of the backing systems, with the type of backing determined by the specification requirements of a project.
Sandstone, limestone, granite, basalt and marble can all be manufactured in this way to achieve a durable, high performance finish, without the need for a stonemason. While this offers an on-site cost saving because it substantially reduces site programmes, it also brings the overall system cost down because less of the raw material is required per square metre.
Behind the rainscreen
The full potential of a natural rainscreen is realised when used with precision calculated carrier systems, which are available in a range of grid formats to allow complete design versatility and fast installation.
To accommodate the full range of design requirements, they are available in module sizes ranging from small, typically 150mm x 400mm, to very large at 300mm x 1,200mm
One of the main shifts in carrier system technology is the development of vertical hangings. Compared with standard horizontal subframes, vertical systems significantly reduce the number of fixings required, and the fixings used are designed to be virtually invisible. As well as reducing the complexity of the installation, this preserves the aesthetics of the exterior envelope.
Large format panels deliver the clean finish demanded by many premium developments, and remain easy to install owing to the sophisticated carrier system and minimal weight of the tile. At the same time, smaller tiles provide a cost effective solution for breaking up the monotony of a lower end finish, such as render. This is because the vertical carrier system will easily integrate with the primary frame of most alternative exterior finishes.
Engineered with ease of use in mind, some vertical carriers make it impossible for a tile to be fitted incorrectly. This contributes to increasing the speed of installation, because it is much simpler to find the correct place for the tile.
Once installed, a vertical carrier gives greater flexibility in terms of tile replacement and removal. Individual tile access makes the installation of service lines at the end of a site programme straightforward, as the tiles are easily removed and then replaced, until the most appropriate point of entry is identified.
The major advancements achieved in vertical carrier and natural rainscreen technology have significantly brought the overall cost of these façade systems down, making a high quality and durable finish more affordable than ever before.