Think zinc

Jonathan Lowy of VMZinc looks at the precise requirements of a complex metal roof and addresses the aesthetics and challenges involved

The new library at Magdalene College in Cambridge – designed by Niall McLaughlin Architects – sits next to part of the old Jacobean manor making the use of red brick and gabled roofs no surprise. However the building was somewhat designed from the inside out with single, double and even triple height reading areas being resolved before the exterior was designed in detail. Both the red brick and timber feature heavily on the exterior and interior but a material that is only used on the exterior is the standing seam zinc roofing. The 45 degree pitched zinc roof combines with glazed gables that give the library a wonderful luminosity with baffles reducing glare from direct light in the reading areas.

Much of the wood used on the interior is Cross Laminated Timber and this was also used as the structure for the roof. Light grey pre-weathered 0.7 mm thick zinc in 25 mm high standing seam panels, with 530 mm centre to centres, were installed over a non-vented, warm roof. A Brooft4 test demonstrating no fire spread or penetration was also carried out.

Non-vented roofing

One of the common misnomers of non-vented roofing – which is also known as warm roofing – is that the build-up is thinner than more traditional vented roofs. In reality, as all of the insulation is above the roof structure, warm roofs are nearly always ‘thicker’ than their cold roof counterparts. This does however increase the thermal performance of the roof as well as reducing thermal bridging.

The 90 mm CLT structure is entirely covered with an aluminium foil bitumen backed vapour barrier (polyethylene film VCLs are not recommended as they do not self-seal when perforated by fixings.) This type of vapour barrier also acts as a high performance air barrier. 90 mm of rigid insulation with a high U value was then fitted with a specific breather membrane covering the insulation. As with all zinc standing seam roofs the panels are held in place with stainless steel clips, with five, one piece fixed clips pinning the panel towards the ridge and then two piece sliding clips, allowing thermal movement, installed at 330 mm intervals towards the eave. Clip spacings are reduced at verges, eaves and ridges. As the roof is not vented, it is vital to use a back side coated zinc – in this case Quartz-Zinc Plus – which protects the zinc from temporary humidity on the underside of the panels. The zinc roof was installed by a specialist hard metal roofing company, with a 50 year material warranty.

Zinc as a building material

Since zinc was first used as a roofing material at the beginning of the 19th century, its malleability and flexibility has meant that it is used as a material to produce gutters, downpipes and all of the flashings for not only zinc roofs but also tile and slate roofs alike. At Magdalene College the low profile ridge caps and valleys are all made from the same light grey pre-weathered zinc giving this spectacular roof a harmonious feel. A further addition to the harmony are the brick chimneys, which are a fundamental part of the natural ventilation strategy keeping the interior at an impressive 21oC during the recent record breaking heatwave.

While zinc is best known in the UK in its standing seam roofing and cladding form, the same material can also be used to create rainscreen facades – as can be seen at Lampwick Quay in Manchester. Here, the architects specified several finishes of pre-weathered zinc in various forms of rainscreen cassettes to form an impressive facade.

The large choice of pre-weathered zincs now available – ranging from engraved zinc, to zinc with discreet colour and a dark slate colour zinc which celebrates its 44th anniversary this year – offer designers an extended palette of aspects. The finish choices then combine with the wide array of systems now available on the market. By using heavier gauge zinc, typically 1 mm or 1.5 mm, back vented rainscreen zinc panels offer yet another aesthetic to fully supported panels such as roll cap, standing seam and flat lock systems.

Jonathan Lowy is operational marketing manager of VMZinc