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COVERING EVERY ASPECT

FOCUS ON: CLADDING

Until a few years ago, the majority of buildings were designed with perpendicular facades and there was a clear distinction between different building elements such as glazing and metal-panel systems. This allowed the architect to work with a specialist from each discipline and develop the facade by integrating different elements.

The current trend for buildings of less rigid, more organic forms has increased demand for cladding products.

In addition, the trend to incorporate an ever-expanding palette of new materials makes the interfacing details critical.

No one could have failed to notice a recent renewed interest in traditional building products such as terracotta tiles, timber and render, especially for residential projects. Most of these have been updated in their design, performance and quality, and often offer a sophisticated method of fixing, such as a pre-engineered metal carrier system.

A good example is the way Allies and Morrison used terracotta tiles for an office development in Farnborough.

Mark Taylor, of Allies and Morrison, says: 'We took a traditional terracotta rainscreen system and modified the aspect ratio of the sections to produce an additional layer of detail, which, when drawn into the window bays, gave added depth and shadowing to the facade.'

Metal panels have always been popular. Although there is still a high demand for prefinished and post-coated metal paint systems, there is a renewed interest in anodisedaluminium panels, with some attractive new finishes, such as Keronite, due to be available soon. Keronite is an electrolytic conversion process for aluminium, originally developed for the aerospace industry. When treated, the aluminium achieves a very hard wearing ceramic surface.

Metal sheets on their own are not flat, but flatness and rigidity can be achieved by forming the edges and providing a core and balancer to the sheet, making it a composite structure. For a metal rainscreen, one way to manufacture a very flat panel is by structurally bonding an aluminium honeycomb core between two panels.

This is not necessarily an expensive way to produce a flat panel. The composite structure of the assembly means lightgauge metal skins can be used instead of heavier-gauge metal.

One cost advantage of this panel is its inherent spanning capability compared with a formed single-skin panel.

The greater spans reduce the need for secondary support or carrier extrusions.

There is a popular belief that coated aluminium will have a longer life expectancy than coated steel; this is often not the case. The meaning of terms such as 'life expectancy', 'warranty' and 'guarantee' can often be confused. The Prisma range of coatings, available in prefinished steel sheet from Corus, offers one of the most robust guarantees available.

For many buildings there is no longer a distinction between facade and roof.

An example of this is the Gateshead Music Centre by Foster and Partners - compare it to one of Foster's earlier projects, the Renault Centre in Swindon.

On Renault, the glazing element and the insulated cladding system were both standalone systems coming together at only two main perpendicular interfaces.

Even though Gateshead is constructed using (mainly) the same materials as Renault (metal and glass), interfaces between the elements occur all over the envelope in different geometric conditions. This is a much more demanding challenge for both architect and manufacturer.

MAKE is a practice which understands that for high-performance buildings, fully glazed facades can be a very expensive construction option. By introducing more solid elements within the facade, construction and building running costs are reduced. MAKE has used this mix of glass and solid elements to develop facades. On its design for a building at Hampstead Road in London, it has developed a cladding system of floor-to-ceiling circular windows. Jason Parker of MAKE says: 'In order to achieve the optimum thermal performance in our buildings, we avoid using triple-glazed facades, which are very costly, or complex double-skin constructions, which use valuable site area, and are difficult to clean and maintain.

'By contrast, a facade with approximately 50 per cent solid elements and 50 per cent glazing enjoys a greatly improved thermal performance compared to the typically wasteful figlass boxfl.'

SELECTING A SYSTEM Cladding systems that are appropriate for buildings of a traditional 'rectangular' form do not always lend themselves to more random facades.

For example, most insulated cladding systems are based on either eliminating water ingress or on drainage management within the joints.

This works well on a facade that has a regular layout, especially for the vertical (the main water-drain) joints, but for a facade needing a more random modular layout, a more exible construction method should be considered.

One way to achieve this is to use a rainscreen form of construction.

By using an inner (warm) wall, a vented cavity and an outer skin facade (a cold wall), the fully sealed inner wall provides an air barrier while the outer 'screen' acts as an effective initial weather barrier.

Insulation can then be incorporated either within the inner wall or in the vented cavity, which also provides pressure moderation and a drainage zone. As the joints between the facade modules are normally open, this method of construction has the exibility to provide a random facade layout unrelated to the structural grid.

When developing an elevation, one of the most fundamental decisions for the architect is to establish whether the selected products are front sealed or back ventilated. This often misunderstood aspect of how a product performs can lead to very difficult, if not insurmountable, detailing issues.

While some facade products perform as ventilated, rear-drained facades, others, such as curtain walls and insulated panel systems, have drainage in the joints. Other products, such as bolted or structural glazing, are fully front sealed, normally through site-applied silicones.

The positioning of the windows can often determine the choice of cladding system.

As a general guide, facades designed with recessed windows tend to suit a rainscreen type of construction, while flush facades work better with frontsealed or joint-drained systems.

BEST PRACTICE Some projects successfully achieve an organic profile by use of innovative detailing and the selection of appropriate products. A good example is the multi-sports Hub in London's Regent's Park, designed by David Morley Architects.

In this case, instead of using a proprietary roofing system, the architect, in conjunction with the manufacturer PSP Metal Facade Solutions, developed a metal panel system, normally used for vertical facades, for use as a roof.

Each modular panel was manufactured to provide a fully integrated flush roofi ng element which achieves a three-dimensional form.

Unfortunately, in other cases, some buildings which try to achieve a more organic form end up as poorly designed structures with crude and ill-thought-out detailing.

This is often the result of an unrealistic budget, and of using unsuitable products in an inappropriate manner.

INSTALLATION

At one time installation contractors tended to specialise in a single system or product, but now most have expanded their operation to offer a complete range of facade systems.

One of the main benefits of this approach is the resolution of interface details between the different products. If the contract is let to a reputable 'envelope' contractor, it should be easier to achieve workable interface solutions, with a single point of responsibility.

For certain projects a unitised facade may be the answer. This minimises valuable site time and eliminates the requirement for scaffolding or expensive access equipment.

Red Architectural, for example, used a unitised system on a building at Farnborough Business Park for Slough Estates, designed by Allies and Morrison. This contrasts with a more highly glazed building that the practice completed about five years ago, on the same estate, using a system from Sch³co. Another good example is the CIPD building in Wimbledon by GMW Architects. The envelope included bolted glazing, curtain walling, metal cladding, windows and roofl ights. Sean Keenan, managing director of the contractor Red Architectural, said: 'As a specialist envelope contractor, we were able to develop all the interfaces within our own design office ensuring the success of the project.'

Ron Fitch is a design consultant working with PSP Metal Facade Solutions

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