Watch your clients' eyes light up when you talk to them about specifying flooring: glass floors are the current trophy detail. No longer exclusively the domain of high-powered offices or retail spaces, they are being increasingly requested on domestic projects.

Engineer David Crookes has no doubt that glass floors are the 'must have' detail of the moment. Despite costing around four times more than a timber floor, a growing number of private clients are demanding glass. 'It's an iconic ideal – a structure that supports you invisibly,’ explains Crookes. 'Glass defies gravity and is a non-structure structure. Louis Kahn would have used a lot more of it if he could have done.’

Glass weighs as much as concrete, yet its attraction lies in looking effortlessly light. Inevitably this introduces specific issues to be addressed: the transparency of the material means that mistakes and services can't be hidden; the floor has to be thick enough to avoid deflection; and, crucially, the structural loading has to be correctly calculated for the span required.

Although using glass at floor level is not new – pavement lights were designed over a 100 years ago – developments in glazing technology have allowed the boundaries to be pushed further. By the 1980s Pilkingtons had produced a standard glass floor plate of 750mm2, and 10 years later engineer Dewhurst MacFarlane designed the first laminated glass floor, with Rick Mather Architects, spanning a massive 3.9m in the 'Now and Zen' restaurant in Central London.

‘It’s important to balance what’s practical with what's possible,’ says Mick Clews, a glazing consultant. ‘Very large panes may look beautiful, but how do you get them into the building?’ The larger the span, the thicker the glass needs to be, and the heavier the floor, so for best results both an engineer and a glazing consultant should be involved. They can advise how the composite floor should be designed and which type of glass would best suited to the application. ‘Glass doesn’t fail in a safe way – it’s a brittle material,’ warns engineer Jonathan Sakula of Dewhurst MacFarlane. ‘It has to be laminated, so that if one leaf breaks, all the load can be carried by the other’

Although toughened glass can take higher stress loads than float glass, and can sometimes span further, once it breaks it shatters into tiny pebbles. Float glass is seen to be more suitable for flooring: it cracks before giving way, giving at least some warning underfoot.

Many engineers believe that the way forward is to specify heat-strengthened glass: it has some compressive stresses built in to give it more strength than float glass, but with the advantage that, should it fail, there is not enough energy locked in to make it shatter like toughened glass. ‘The optimum solution is a laminate of both heat-strengthened glass,’ says Sakula, ‘but in reality this is still rare. For ordinary everyday uses, float glass is used.’

Fire-rated glass can also be used to achieve more than one hour's integrity, although costs can be prohibitively high, and because building control officers are still nervous of the material, they can be difficult to convince. ‘Glass is non-combustible, it will withstand heat for longer than timber,’ argues Malcolm Armfield, of glazing consultant Armfield Glass. ‘Most fire officers perceive glass as scary – they tend to go over the top.’

Loates-Taylor Shannon Architects teamed up with Armfield and David Crookes to design a fire-rated glass entrance hall in London. With a skylight above, and a basement dining room below, the glass floor punches daylight through the house. The hall leads to the only staircase, and because there is no secondary means of escape the glass had to be fire-rated to one hour. This was achieved by using two layers of laminated glass (one of which is fire-rated) separated by a cavity that acts as thermal insulation to keep the temperature down so that heat doesn't transfer to the top layer. The floor was split into three panels, partly because of the limited size of fire-proofed glass, but also because of the size of the front door. Despite costing £8,000 for three one metre panels, the client pushed for it. ‘It was the most economical solution we could find, given the scenario,’ says Mike Loates Taylor.

Preserving the illusion of transparency presents another challenge: that of framing the glass to maximise the glass area and minimise the sub-frame without compromising structural integrity. The standard framing system is a steel tray grid that supports the glass panel on all four sides, but this is not the only solution. ‘Inevitably, people are using more and more unusual fixings and ambitious structures,’ says Sakula. ‘Using a central prop, for example, will provide bigger spans and control deflection by providing an exoskeleton for the glass, resulting in a stiffer plate.’

At Thames Court office development in the City of London, architect Kohn Pederson Fox installed four storeys of glass floor to encourage daylight to penetrate the heart of the building. A dramatic framework of mild steel cruciform members forms a grid system which is then filled with structural glass panels and supported by cantilevered steel members, a macho design that is in itself a feature.

In some cases the entire structure itself can be made from glass, using load-bearing glass beams (generally triple-laminated) supporting the glass panels. Although this gives maximum transparency, it is most commonly used on roofing rather than flooring because the depth of the glass sections, greater than that of steel, may compromise storey height.

Dirk Jan Postel used glass beams on the glass bridge he designed to link two buildings for his Rotterdam practice, Kraaijvanger Urbis. ‘I couldn't think of anything to span between the two buildings, which are wonderfully expressive concrete structures: I decided that nothingness would be the best solution.’ The resulting invisible walkway is the most expensive building by area in Holland: its cost per square metre of £6,500 is the same as as Foster's Hong Kong Shanghai Bank.

Postel insists his bridge has weathered the last five years well, as all shoes are cleaned by 30m of carpeted office floor before they reach the glass, but some scratching is likely to mark most glass surfaces with time. One way to detract attention from this problem is to obscure the surface by acid etching, ceramic screen printing or sand blasting. As well as offering the advantage of producing a non-slip floor, it also provides a ‘modesty’ screen over which skirt-wearers can walk with confidence.

KPF enlisted the help of artists Langlands and Bell to make a feature of this requirement. The resulting public art takes the form of international airport destinations. Unfortunately, this solution means that the sensation of walking on a clear surface is lost, removing the ‘game’ of total transparency, and giving a sense of solidity that detracts from the thrill.

Despite knowing that a glass floor, when correctly designed, is as strong as any other, psychologically, the material is still perceived as dangerous 'People's reaction is very noticeable,' asserts Armfield 'They tend to slow down as they approach the glazed section and tread more carefully over it.'

Glass floors: what you need to know

  • Get the right people on board: a good engineer, preferably one with some experience in glass floors; a specialist contractor; and a glazing consultant. This is a highly specialised area and should not be tackled by a general contractor, glazer or metalworker.
  • Loading requirements will depend on the floor’s setting and how much traffic will pass over it – domestic loads are much lighter than commercial. BS6399: Part1, 1999 outlines minimum imposed floor loads, but it may be acceptable to design a floor to a sensible and safe lower load. Your engineer will advise you on your limits.
  • If the glass floor extends outside there will be a risk of condensation, so a thermal break will be required.
  • Consider where to put the services, as fitting a glass floor will inevitably mean loss of a ceiling void.
  • Glass floors cannot withstand lateral compression, so allow adequate jointing for thermal expansion, especially in lime and mortar buildings where some movement is likely.
  • Each panel needs even support on all four sides, as glass cannot withstand unpredicted loading on one point.
  • The edge support should be 3mm thicker than the glass laminate.
  • When considering the overall thickness of the floor, don’t forget to add the resin interlayer into the calculations. This will add around roughly 1.6mm to the total thickness.
  • There should be no direct metal-to-glass contact. Use nylon washers or neoprene gaskets to cushion the panel
  • Deflection must be limited to 0.5-1mm, depending on glass size.
  • Because glass is so heavy – 2.5kg/m2 per mm of thickness – try to limit panel sizes to 1m2 or less. This will make it more affordable and also cut down on weight. Thicker panels are also harder to laminate.
  • Panel size is sometimes limited by practical considerations such as ease of installation. Very large panels may need to be lifted into the property with a crane.
  • Acid-etching, sand-blasting and ceramic-baked screen printing can all be used to obscure the glass surface and provide privacy.
  • Consider maintenance carefully, as glass will tend to scratch and mark. If your client voices concerns, suggest obscuring the surface to minimise the effect.
  • Detail an access point in the frame so that if the glass is damaged, the panel can be easily removed and replaced.
  • Glass floors are most suitable for dry areas. If it is to be near an entrance, where it is liable to get wet, obscure the surface to reduce slip. BS 5395: Part 11984 specifies the minimum coefficient of friction.
  • If installing a fire-rated floor, remember to leave the fire-rated emblem exposed for the building control officer’s inspection.
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