Clare Fenton of the Metal Cladding & Roofing Manufacturers Association (MCRMA) explains why durability must be a priority when specifying metal roofing and cladding panels and systems to deliver sustainable buildings
As the physical separator between the interior and the exterior, it is no surprise that steel and aluminium are used by the vast majority of roofing and cladding panel and system manufacturers to ensure specifiers have access to highly durable solutions.
Most products and systems are manufactured from mild steel which is protected by a coating taking the form of a sacrificial metal (i.e., hot-dip galvanising (HDG) with zinc or other metallic coatings which include a range of formulations) and in most cases organic paint coatings for both the external and internal surfaces. These help to prevent corrosion and provide a level of durability dependent on the coating applied and the environmental conditions where the building is located, ranging from 10-15 years up to 40 years.
Aluminium does not corrode in the same way as steel as a result of oxidation. It is a very durable material and grades can be specified for a range of applications. However, generally it does not require any coating for protection purposes, except in some particularly aggressive environments where additional protection may be beneficial or where aesthetics may be a consideration.
Regardless of the material choice, however, the level of durability offered by the finished envelope will depend on a number of factors. Therefore, it is important to understand how these will impact on the design life, and thus overall sustainability goals.
Carbon cost vs value for money
Sustainable buildings have a long design life and utilise materials, products and systems to match the intended lifespan, but the carbon cost also has to be aligned with value for money. Hence why buildings are assessed in terms of their Whole Life Costing (WLC) or Life Cycle Costing (LCC), taking into account the costs associated with construction, operation and occupancy, maintenance, renewal, and the end of its life. However, the impacts of climate change and the increasing urgency to act mean environmental and carbon costs are more important than ever.
While a building’s life expectancy is typically regarded as 60 years, the actual design life can be very different, depending on factors such as the building’s location, the type of building, its use, method of construction and materials used. It is important, therefore, to refer to BS EN 1990 (Eurocode 0) and the UK National Annex to find the indicative design life of various building types and structures in the UK.
This could be as short as 10 years for Category 1 ‘temporary structures’ or 15-25 years for agricultural and similar buildings, up to 120 years for Category 5 ‘monumental building structures, bridges and other civil engineering structures’.
The indicative design life, however, can only be fully realised if the materials, products and systems specified for the external building envelope have the durability in the given use, location and surrounding environment, not forgetting the necessary cleaning, maintenance and repairs.
Maintainable, replaceable or lifelong?
Another useful reference point here is BS 7543. This guide to durability of buildings and building elements, products and components can be used to provide guidance on design service life planning and the means of communication of information on materials durability (including metals) for all members of the construction and facilities management teams. The standard also gives guidance on climatic agents that can affect durability of materials.
Under BS 7543’s categories, metal roofing and cladding systems are generally classed as ‘maintainable’, meaning they offer a very durable building envelope solution, capable of meeting the life of the building with the appropriate material and component specification and maintenance regime.
It is worth noting that the building envelope is increasingly being ‘systemised’, giving specifiers the peace of mind that comes with installing a roofing or cladding system that has been tested as a complete assembly, backed by performance data.
Some components of these systems, such as flashings, rooflights and profiled fillers, may be classified as ‘replaceable’ on buildings with long life expectancies and where the component is easily accessible to undertake the replacement. However, other components, such as fasteners, are not easily accessible for maintenance or replacement and would be classified as being ‘lifelong’ to the design life of the material or system within which they are used.
Design life data and categorisation are important in the main environmental assessment methods – BREEAM and LEED – and metal cladding and roofing systems and products can be easily assessed in line with their criteria. To assist specifiers here, the MCRMA has produced guidance document GD39 which is split into eight sections, some of which are CPD approved, to show how different metal roofing and cladding systems and products are appropriate according to the design goals and enable the accumulation of credits to achieve high ratings.
In an era when new technologies are often heralded as the solution to decarbonisation of the built environment, we must not overlook how important it is to select durable materials for the building envelope, which are properly designed, installed and maintained, to maximise the building lifespan. After all, any carbon savings achieved through the installation of technologies such as ground source heat pumps and MVHR systems would be quickly cancelled out if the metal envelope needs repairing or replacing prematurely given the carbon cost involved in that process.
View the MCRMA’s guidance documents on this topic and find out more at www.mcrma.co.uk.
Clare Fenton is chair of the Metal Cladding & Roofing Manufacturers Association (MCRMA)