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09 (III). Metal Roofs

Session 9 (III)

Metal Roofs 9.1. Introduction 9.1.1. Materials 9.1.2. Principles 9.2. Metal sheet roofs 9.3. Corrugated metal panels 9.4. Bibliography

Based on the original presentations by Prof. Julián García julian.garciam@upm.es Translated by Luis M. Martín

A metal roof at Gananoque, Canada

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9.1. Introduction.

In this presentation metal solutions for pitched roofs are studied. These roofs are often built with but large sheets, that enable low slopes, with a minimum of between 5 and 15% in many cases.

These are the minimums established by the spanish regulations:

Slopes and materials according to the spanish CTE DB HS

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9.1.1. Materials Stainless steel. Suitable for roofs and facades of any kind, given its excellent corrosion resistance. Doesn’t need much maintenance, admits welding, a bit malleable, and compatible with metals such as carbon steel, aluminum or zinc.

Different finishes, including tin, satin, glitter and textured.

Aluminum. Suitable for roofs and facades of any kind. Very light, uses oxidation as protection against corrosion, since the oxide film (alumina) formed on its surface prevents oxidation of the core material. Different finishes, ranging from anodizing (artificial formation of a protective oxide film and subsequent sealing of the surface) to lacquering with enamel paints or PVCs.

Stainless Steel and aluminium

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Zinc. Good corrosion resistance, although it may be attacked by condensation on its inner face, -that’s the reason it is usually installed on different types of membranes. Not much maintenance, admits welding, is malleable (although it might not be mechanically settled), and is compatible with standard metal construction. Different finishes, including naturally patinated and anthracite or gray.

Copper. Its use is limited by its high price. Good durability, even in coastal areas (where other metals can cause problems), malleable, admits welding. Can generate galvanic corrosion problems to less noble metals, such as steel, aluminum or zinc, brass but not stainless steel, or lead. Unaffected by condensation corrosion. Different finishes including natural, preoxidated and patinated, etc.

It is also used in alloys with other metals: tin (bronze), reddish, usually in facades; zinc (brass), golden, in gutters and auxiliaries; and aluminum (aluminum bronze), golden, rare.

Natural zinc, natural copper and patinated copper.

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Lead. Extremely malleable material, but very heavy. Low mechanical strength. Suitable for indoor edge solutions, can also be used in full covers. Low maintenance. Can be in contact with steel, zinc or copper. Toxic if inhaled, sensitive to condensation corrosion. Natural finishing.

Titanium. Good material for roofs, but very expensive. Durable, lightweight and sturdy, it is compatible with most common metals. Its expansion coefficient is low compared to other metals studied. It comes in a natural finish, usually very bright, and anodized and textured.

Lead sheets and a Titanium Faรงade at the Guggenheim Museum. Bilbao, Spain.

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9.1.2. Construction principles Expansion of materials on deck. Temperature ranges between + 80 째 and -20 째. An element 10m. long can increase its length up to 2 cm. during the day and decrease other two at night.

Galvanic corrosion. In roofing, galvanic couple problems become compatibility problems. To avoid problems materials with similar galvanic potentials are used in construction.

Expansions and standard potentials of usual roof metals.

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Galvanic corrosion principles Se considera corrosión galvánica la generada por una diferencia de potencial entre dos metales distintos puestos en contacto con un electrolito (agua, o soluciones salinas, por lo general). Esta diferencia, o par galvánico, genera una pila galvánica, en la que el ánodo se corroe, mientras que el cátodo no sufre corrosión, sino reducción; el primero pierde electrones, que el segundo recibe, por vía de los iones del electrolito.

Estas diferencias de potencial son función, obviamente, de los metales puestos en contacto (aunque también del medio; no es idéntico el comportamiento de dos metales puestos en contacto con agua de lluvia y agua marina, por ejemplo). Convencionalmente, las series galvánicas o electropotenciales determinan la sensibilidad de los metales a sufrir corrosión en electrolitos típicos. A mayor diferencia de carga, más rápidas la corrosión y reducción de los materiales. Para los metales que se emplean habitualmente en la construcción de cubiertas, se ha llegado a la siguiente convención: 1.- Aluminio 5.- Níquel

2.- Titanio 6.- Estaño

3.- Zinc 7.- Plomo

4.- Hierro 8.- Cobre

De esta forma, cuando dos metales de la lista anterior entran en contacto, sea por presencia de agua o de aire húmedo, el metal de índice más bajo se corroe. La corrosión es más rápida cuanto más alejados se hallan los metales en la serie, por lo que es necesario evitar siempre contactos directos entre materiales muy separados en la escala: cobre y aluminio, por ejemplo; o cobre y zinc.

Ejemplo de corrosión por par galvánico en una tubería. Microfotografía de corrosión de una placa de latón.

En Callister, 593.

La corrosión galvánica puede prevenirse de diferentes modos. Por un lado, es posible, en determinadas condiciones, aislar eléctricamente los dos metales entre sí, mediante plásticos u otros materiales.

También pueden mantenerse secos los metales, protegiéndolos con plásticos o resinas epoxi; o al menos al material con mayor capacidad de reducción. Otra opción es emplear ánodos de sacrificio, generalmente de aluminio, el metal de más bajo índice en la tabla anterior, para prevenir la corrosión de los elementos que deseamos proteger. Pero, como puede entenderse, casi todas estas estrategias son complicadas de llevar a cabo en un elemento tan expuesto como una cubierta de edificación, por lo que es recomendable emplear, siempre que sea posible, dos metales que tengan potenciales similares, o incluso el mismo metal para toda la construcción.

Para ampliar información puede consultarse Callister, W.D. Introducción a la ciencia e ingeniería de los materiales. Reverté. Barcelona, 1996. Y más concretamente el Cap 18. Corrosión y degradación de materiales, en http://books.google.es/books?id=YiWdEYEHBIAC&pg=PA565#v=onepage&q&f=false

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9.2. Metal sheet roofs A system of overlapped and seamed sheets. All the above materials, with varying thicknesses, are used. Depending on the material and the desired finish, the seaming of the sheets can be different. Sheets can be seamed by hand (traditional system) or mechanically (system of long strips).

Seaming machine and standard spanish materials.

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Metal sheets are now usually connected through mechanized seaming systems. This system allows to construct these roofs at a very high speed, regardless of other aspects of roofs construction.

Seaming machine. Building site with a roof covering.

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9.2.1. International standards

All these details available at http://copperalliance.org.uk/resourcelibrary/pub-156---copper-roofing-in-detail

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9.2.2. Spanish standards

Proceso de engatillado para junta alzada.

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Medidas para engatillado en junta plana y alzada.

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Junta sobre list贸n curvo y junta hueca.

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Diferentes juntas sobre list贸n plano y aleros.

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Junta rehundida y junta alzada con grapa vista.

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Dos ejemplos de soluciones actuales.

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Esquemas de funcionamiento mecánico e higrotérmico de una solución actual.

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Detalle de encuentro ventilado con paramento.

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9.2.3. Case Studies

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Zinc roof construction.

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Lead roof construction.

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Lead roof construction.

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Zinc roof construction.

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Zinc roof construction.

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Zinc and copper roof construction.

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Copper roof construction.

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Lead flashings in a slate roof.

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9.3. Corrugated metal panels Systems based on preformed sheets in waves, nerves or frets. Large sizes that cover the distance forward-ridge. Two basic types: single sheet and sandwich panel.

Single sheet. Preformed in different sections.

Added insulation needed. Overlap parallel to slope: 15 cm. Overlap perpendicular to slope: 1.5 waves or frets.

Different types of metal corrugated sheets.

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International standards

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Sandwich panel. Double metal preformed sheet with internal insulation, usually rockwool, polyurethane or extruded polystyrene in different thicknesses. Overlap parallel to slope: 15 cm. Perpendicular to the slope direction clipped joints are used, with or without flashings.

Possibilities of preforming the top and bottom sheets are the same as those in the considerations above. The inner finishing may be of different materials; microperforated sheets are common.

Different sandwich panel sections.

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Joints. Joints in the direction of the slope are designed in different ways. With single sheet systems, common solutions are simple overlaps; in sandwich panels clipped solutions are used.

Overlaps of different metal roof solutions.

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Relationship with substructures. Metal panels can be screwed directly to the support (where materials allow this: concrete slabs, for instance). If the supporting substructure is a steel truss with rafters, indirect connection systems, like hooks or brackets, are used.

Different relationship with substructures.

Details

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Details

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Details

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Construction of a corrugated metal panel roof.

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9.7. Bibliography Deplazes, A (Ed.) (2009) Constructing Architecture. Materials, processes, structures.

Birkhäuser, Basel Knaack, U.; Klein, T. (2009) The Future Envelope 1: A multidisciplinary approach.

2009. Research in Architectural Engineering Series Knaack, U.; Klein, T. (2009) The Future Envelope 2: Architecture - Climate – Skin.

Research in Architectural Engineering Series Schunk, E. et al. (2003) Roof Construction Manual. Pitched Roofs. Birkhäuser, Basel Schunk, E. et al. (2003) Roof Construction Manual. Flat Roofs. Birkhäuser, Basel VV.AA. La plancha de plomo en la construcción. Asociación nacional del plomo.

Madrid, 1986.

VV.AA. Tejados de cobre. CEDIC. Madrid, 2004.

Online references General: Centro Nacional de Investigaciones metalúrgicas: www.cenim.csic.es Copper: www.infocobre.org.es/publicaciones-cobre-en-arquitectura.html European Copper in Architecture: www.copperconcept.org Plomo: European Lead Sheet Industry Association: www.elsia-web.org Zinc: International Zinc Association: www.zinc.org Acero inoxidable: www.cedinox.es