Raised access floors

Originally introduced in the 1960s, since the late 1970s and early 1980s raised access floors have been commonplace in office buildings and are now virtually the norm for new commercial developments. Their use has been extended to laboratories, cleanrooms, studios, data centres and many similar environments where there is a desire to allow cabling, pipes and services to be run beneath a floor and where a degree of flexibility is required to accommodate future changes.

There is a wide variety of different types and specifications of raised flooring systems ranging from the fully accessible to the semi-accessible (or in some cases limited accessible) but in generic terms most current systems comprise a walking surface of 600 x 600mm tiles supported on pedestals of some kind. Access tiles may be of timber products (such as plywood or chipboard), sometimes encased with metal or cementitious or calcium sulphate cored metal-faced panels with a variety of finishes including carpet, vinyl or plain steel. See below for common types of raised access floor panels.

Pedestal systems vary from the sophisticated adjustable metal jack types to systems as simple as an upended clay brick held in place with a plastic tray. Less accessible types of floor may involve the use of larger sheets of flooring material supported on foam faced timber battens - although these systems are mainly used in residential situations to provide sound insulation and to create a void for services. The key distinguishing system between these floors and products for commercial use is that in the latter system, modular tiles can be lifted easily to enable voice, data and small power services to be reconfigured with minimum effort and disruption. In some cases, the use of a raised floor enables the provision of floor mounted cooling or air-conditioning systems, with treated air being discharged at floor level rather than overhead.

Common types of raised access floor panel

• Steel encased, chipboard core: very popular flooring system for general office-based (medium) applications. The chipboard core is bonded with epoxy or polyurethane adhesives to a galvanised steel outer casing. Different thicknesses of steel and core can provide a variety of different loadbearing characteristics. Panels are usually held in place by gravity or locked down with corner screws or toggles if additional fixity is required. Panel casing must be sealed with tape when the panel is cut; panel lips can be damaged easily by misuse or careless fitting. Gravity fixed tiles can be pre-finished with carpet or vinyl floor finishes.
• Chipboard: less common use in speculative offices or where limited accessibility is likely to be required. Although flooring grade chipboard is used, the panels are more susceptible to moisture movements than encased versions.
• Steel faced cementitious flooring: very popular in Grade A developments during the 1980s and 1990s. Constructed from a steel flat top plate, with a profiled bottom sheet (often in the form of a series of hemispherical depressions) and filled with foamed concrete. Robust floor type, with good fire resistance and acoustic properties. Tiles are often locked down - screw heads can become filled with tackifier adhesive and difficult to turn. Edges of the tiles prone to lipping and distortion if misused.
• Steel faced anhydrate core: similar to cementitious types but with a calcium silicate or calcium sulphate core. Good fire and acoustic properties.

In some installations stringers are needed to add additional stability to a floor (particularly where there is a deep void of say 600mm and above) or where the underfloor area is used as a plenum. In this case an air seal is needed to prevent air escaping between the tile joints and the stringers may not be required to provide a structural function. Stringers are usually of galvanised steel, clipped or screwed to pedestal heads. Cut perimeter tiles will not fit onto a standard pedestal and so it is usual to provide alternative pedestal heads with perimeter stringers to provide adequate support to the cut tile.

While simple in concept, the installation of raised access floors is not always straightforward and can lead to difficulties in subsequent serviceability. In some circumstances the risk of collapse can pose unacceptable risks to health and safety, or risk damage to sensitive electronic equipment. Therefore, care is essential when selecting and installing a suitable system.

Proprietary raised floor system using aerated blocks held in a plastic tray. (Photograph courtesy of Allen Gilham.)