Raised access floors

For most fully accessible floor systems, the easy removal of individual tiles is critical. The tiles should be capable of lifting, rotation through 90° and reinsertion without difficulty. However, for this to be possible, the tiles need to be manufactured to within fine tolerances. Although it would be reasonable to expect that reputable manufacturers are able to achieve this, it is not always the case, and examples of poorly fitting floors are not uncommon. However, assuming that the dimensions of the panels are within tolerance, the following factors might affect the fit of the tiles.

Temperature and humidity during installation

Installation of the tiles during hot weather could cause problems if the floor plates then contract during colder weather, leading to tightening of the joints between panels. Similarly, if the panels are not fully encased, small dimensional changes arising from changes in humidity could give rise to either loosening or tightening of joints. Generally, for installation to proceed, the tiles should be stored and laid in dry and watertight areas at a temperature above 5°C and humidity below 75% RH. It is usual to provide a perimeter expansion gap of 10mm - failure to do this could lead to difficulties in the ability to lift tiles and replace them.

Movement or sway in the pedestals

The installation of the floor should not rely upon the perimeter walls or partitions to provide lateral stability; the floor should be capable of supporting itself without additional lateral restraint (other than within the floor system). Possible causes of sway could be due to the design of the pedestal, for example being extended close to its limit, failing to tighten off the lock nut, detachment of the pedestal from the floor or lack of stability (in the case of a deep void additional stringers are usually needed to control lateral movement). Alternatively the movement could be due to too much play in the pedestal head (as a result of missing fixing screws or poor design). Most MOB standard floors have positive fixing lugs on the tile and or pedestal and many medium and higher grade floors have additional corner fixing screws.

According to the MOB specification, all pedestals should be mechanically fixed as well as adhesively bonded unless it can be shown by testing that additional mechanical fixings are not needed. The usual method of fixing is to bond the pedestal plate to the floor with a resin adhesive: quite often an installer will omit mechanical fixings altogether. While the bond so formed can be very strong, the surface of the structural floor may have been contaminated with dust prior to fixing or, and particularly with lightweight concrete floors (lytag), the surface of the concrete is weaker than the adhesive and permits the pedestal and the adhesive to shear away.

These tiles show signs of poor alignment; the tile in the top right has been forced in against the restraint lug, distorting the galvanised steel sheet cladding. (Photograph courtesy of Jane Dalgliesh.)

Incorrect location of a fixing nipple in relation to the panel edge has resulted in damage.

Similarly, a very smooth power floated floor could create adhesion problems and manufacturers will often call for the surface to be roughened prior to the application of the adhesive.

A field test developed for pedestal fixity is included in the MOB but not the BS EN document. Essentially a small rig is set up above a pedestal supporting a 3kg bag of sand. The bag is supported by a 1m length wire and swung through a 90° arc to impact the side of the pedestal, resulting in a simple pass or fail result. The standard suggests a 0.5% test frequency. The test is onerous and unpopular with installers, but if there is doubt over the adequacy of fixing, the swing bag test should be deployed. For pedestals in excess of 300mm additional mechanical fixing will almost certainly be required. Loads on pedestals are not always in the vertical dimension - rolling loads can induce sideways movement and the sudden overturning of a pedestal could prove very serious for users or for fixed equipment supported by the floor.

Deflection of the structural floor could, if severe or coupled with some of the other factors described above, serve to lock the panels in place.

If floor plates could be constructed to perfectly modular sizes it would not be necessary to cut tiles at thresholds or perimeters. Such accuracy is seldom possible, so cut tiles are often required at perimeters and thresholds. Cut tiles need proper support and therefore perimeter details need additional scrutiny to ensure that proper support (additional pedestals) has been fitted and that the tiles are tight and secure. Cut tiles should not be less than 150mm wide to avoid fixing difficulties and should generally be mechanically fixed rather than simply retained under gravity. Loose tiles at door threshold positions could be particularly dangerous and so perimeter stringers should also be placed in these positions.

Additionally, if the tile has been cut the core will have been exposed, so additional site sealing is required to protect the core from moisture and or abrasion damage.

Site work

While the installation instructions for raised floors would caution against the creation of long rows of opened tiles or the formation of islands during fitting out and cabling operations, such activity is seen frequently. Foot traffic or rolling loads from pallet trucks, bins, etc. can in these cases allow slight movement to occur in the pedestals, with resultant difficulty in the reinstallation of tiles.

Tiles should normally be lifted out and replaced vertically and not hinged back into position - such action can distort the edge of the tile and lead to a raised lip.

If a row of tiles is removed, the sudden release of compression in the entire floor plate could lead to permanent but slight movement, making it impossible to reinsert the row of tiles without physical reduction in size. For this reason alone, continuous rows of tiles must not be removed.

Severe lipping and edge damage to galvanised steel-encased chipboard floor tiles in a new building. Attempts to prize up the floor panel have damaged one edge, while excessive gaps between the panels are visible. Slight in-plane movement has occurred during fitting out, possibly as a result of a continuous row of tiles having been removed. Alternatively, the lipping may have been the result of incorrectly positioned tiles placed on 'nipple' based caps during the natural raising and replacing of floors during the fit-out phase.

Dragging electrical and data cables around pedestals should be avoided to prevent pedestal damage or displacement. Raised access floors usually need to be earthed in order to comply with statutory requirements (in the UK, the latest edition of the IEE Wiring Regulations). Pedestals will often contain a copper or other conductive contact system beneath the tiles to avoid the need for an electrical earth contact to each and every pedestal. However, contacts can become damaged in older installations or during refurbishment, so in some circumstances earth continuity testing will be advisable.

Carpets, both broadloom and tiles, are common floor finishes in general office areas. They are usually attached to the floor with tackifier adhesive in dabs or continuous rows rather than over the surface of the tile as a whole. Accumulation of adhesive or overzealous application can lead to contamination of the joint and the heads of corner fixing screws, making the removal of tiles very difficult. Care should be taken to remove old adhesive properly and lay new carpet in accordance with the manufacturer's requirements. Contact adhesives can be very difficult to remove and may result in panel delamination during attempts to remove carpets or finishes bonded with these adhesives.