The battle of the roof pitches has abated and British architects’ love affair with the vernacular seems to run and run, but performance and procurement are part of the debate. This month we look at sheet roof cladding, tiles and slates and flat and membrane roofs.
The heated debate over flat v pitched roofs that raged after the International Style peaked in Britain burnt itself out long ago. In today’s risk-averse and litigious climate architects proposing flat or near-flat roofs need to be armed with warranties, manufacturer-indemnified details and visual arguments, with acquiescent clients and contractors.
The visual debates are long-standing. Georgian designers’ distaste for gables and visible pitched roofs found an ally in horizontal parapets. Pending the arrival of reliable horizontal or near-horizontal membrane roofs, these normally conceal traditional construction. The focus of a related debate is the notion that a building’s walls and roof should form a visual continuum.
Some designers avoid the joker’s hat effect altogether by crowning facades with horizontal parapets. Others, including ‘blob architects’, use the same cladding for walls and roofs and suppress the visual transition from one to another or dispense with it altogether. But the honest expression of the roof as a building’s hat remains a popular stalwart that also appeases clients, contractors and planners. These debates have a bearing on the three principal types of roof covering:
- sheet roof cladding
- tiles and slates
- flat and membrane roofs
Sheet roof cladding
Available with many finishes and in various materials, including lead, zinc, copper, aluminium and stainless steel, this is suitable for vertical applications as well as roofs.
Products such as VMZinc’s can be installed at three degrees from the horizontal. Farrells specificed VMZinc cladding for Hong Kong’s Kennedy Town Swimming Pool (AJ 01.11.12) because its workability suited the building’s curvilinear form and it was compatible with its marine setting and able to self-heal
through oxidisation. VMZinc manufactures systems for warm roofs, with insulated structure, and cold roofs. VMZinc’s range of finishes has expanding to include six pre-weathered options: Quartz-Zinc, Anthra-Zinc, Pigmento Red, Pigmento Blue, Pigmento Green and Pigmento Brown. Polyester lacquer finishes are also available, but most architects remain hooked on zinc’s classic grey finish.
Copper sheeting also holds its own in marine environments. PBWC Architects specified Aurubis Architectural Nordic Standard copper for the curved skin of
the new RNLI Lifeboat Station at The Lizard. PBWC wisely minimised cladding penetrations and also chose copper because its rich green patina will complement the aqua blue hues of the local coastline. This product is also available with Nordic Green and Nordic Blue factory-applied patinas. Aurubis’s Nordic Brown preoxidised copper produces light or dark brown oxidisation and copper alloys include the Nordic Bronze, Nordic Brass and the Nordic Royal, an alloy with a long-lasting golden colour.
One advantage of certain sheet cladding materials, including copper, is that they can be worked at low temperatures. This enabled work on Jarmund/Vigsnæs Architects’ Svalbard Science Centre, clad in CopperConcept sheet roofing to continue further into Norway’s cold season. Lead sheet is another popular architects’ choice, exemplified in Hopkins Architects’ Haberdashers’ Hall in the City of London. The popularity of the Lead Sheet Association’s series of CPDs, which continues in this issue, testifies to this and demonstrates its suitability for contemporary construction and work to existing buildings.
With increasing demand for solar panels to be securely attached to sheet roof cladding, roof and cladding fastener specialist SFS intec has developed a new curved saddle washer for its SOL-R system which, originally designed for trapezoidal sheet and sandwich roof applications, facilitates fixing to corrugated roofs. This saddle washer enables safer fixing, accelerating installation. The new adapter base snugly follows the curvature of the sheet to protect
against water ingress. Installation can be 66 per cent faster than ‘stair bolt’ alternatives. Post heights and rake conditions are easy to adjust to optimise solar panel positions.
Tiles and slates
‘In previous years there has been a noticeable move towards clay rather than concrete tiles’, says Marley Eternit sales and marketing director Paul Reed. ‘This year there was a shift back to concrete, perhaps because economic trends are increasing pressure on architects. Marley Eternit’s Ashmore range resembles plain tiles and costs £15-16 per square metre. The performance of these concrete products is similar to clay equivalents, although they look
different. Town planners are generally quite accepting. Edgemere is acceptable to many planners, although some may ask for real clay, also available from
Marley Eternit. Availability is a concern to specifiers of natural roofing products, especially in the case of overseas slate products. ‘When teams switch from natural products to concrete or fibre cement during the course of a project, the details aren’t that different’, says Reed, ‘although sizes might vary slightly.’
In environmental matters, Marley Eternit is a defender of the faith. It recently discontinued its resin simulated interlocking slates in order to rationalise its range. This was also the only product in its range without a BREEAM A+ rating. ‘Roofs are increasingly seen as “sustainable platforms”,’ says Reed. ‘These may incorporate photovoltaics and green roof technology.’ There is also continuing discussion about the relative benefits of airtight and ventilated construction. ‘Ventilation avoids condensation, which can be encouraged by vapour-permeable underlays’, says Reed. ‘It’s hard to know where the drive towards air-tightness will take us.’ Last year, Marley Eternit was awarded accreditation for responsible sourcing. ‘BIM files are now available for all Marley Eternit cladding products, although most designers work with our AutoCAD details,’ says Reed. ‘Data is currently more valued than 3D functionality.’ Carbon Trust footprints have now been added to Marley Eternit’s BIM files.
Marley Eternit’s most recent innovation is EcoLogic tiles, which have a coating that helps to remove nitrous oxide from car fumes from the atmosphere, forming a harmless nitrogen substance that runs off roofs and also acts as a fertiliser. So far there has been limited take-up and there are complications with measuring effects.
‘You’d need large quantities to make any impact and no regulatory requirements encourage their use,’ says Reed. Local authorities have been set targets for air quality, but this product will not hit these single-handedly. ‘They may, however, be specified for their feel-good factor.’ It would be good to see more architects thinking beyond CO2 emissions, to take account of other vital aspects of the environment such as nitrous oxide pollution.
A significant proportion of complaints dealt with by the National House-Building Council (NHBC) involve roofing: more specifically, associated workmanship standards. The use of mortar is a particular concern and there is a move towards increased use of mechanical fixings. Mortared ridge tiles, which block
ventilation, are a particular concern.
Discussions are under way to stipulate improved practice in revised standards, without completely banning use of mortar. ‘Between 60 and 80 per cent of our
products are flat and grey and these are very popular in Britain,’ says Reed. ‘Fibre cement slates are often specified for their clean, sleek lines and flat grey concrete tiles are also often chosen for similar architectural reasons. Architects continue to experiment with the performance and appearance of these products and we sometimes see fibre cement slates on curved roofs.’
Flat and membrane roofs
Even the most optimistic architects know membrane roofs must be laid to falls if unprotected, especially as ponding on flat surfaces causes damaging thermal behaviour and failure of waterproofing. But carefully chosen and suitably protected and drained membranes can be dead flat, reducing roof build-ups, and so can the top surfaces of materials that cover them if correctly designed and specified. The stock rationale for dead flat roof surfaces is their capacity to increase usable, albeit unenclosed, areas for people, facilities, equipment and horticulture.
There are various ways to protect membranes from water, thermal action, traffic and negative wind pressure. Ballast, mechanical fixings, chemical bonding or
interlocking tiles can be used to resist uplift on membranes and insulation on inverted roofs. Where ballast is chosen, this can form attractive and durable roof surfaces. Jonathan Tuckey Design’s roof for an extension to a South London Arts & Crafts residence uses brick slips for this purpose.