By Colin Stanyard, Technical Manager at Mapei UK
Raised access flooring systems are often installed as a means of concealed delivery of electrical power, telecommunications cabling and data transfer as well as being used for the distribution of air flow beneath the flooring. Raised flooring systems are regularly situated in commercial offices, schools, colleges and universities, retail parks and leisure facilities.
Access flooring systems consist of pedestals, also referred to as ‘legs’ or ‘upstands’, which are adjustable and support the load bearing panels and substrate. Stringers are also available to either clip or bolt-on to pedestal heads providing enhanced lateral strength and stability, creating a matrix across the floor. This is particularly necessary in heavier duty applications and where high level finished floor heights are required.
Pedestals should be constructed from corrosion resistant steel and incorporate a locking nut to fix them into position and prevent vertical movement. They will contain holes for fixing the panels and slots for the incorporation of stringers. Angled heads are also available to allow for the inclusion of ramps.
Types of raised access flooring
Although their aim is to facilitate the provision of services under the flooring, several types of access flooring are available, the selection and design of which will depend on the needs and requirements of the building and its users.
In light to medium duty areas, gravity loose lay raised access floor systems may be appropriate. This type of system allows for frequent access to the void beneath the panels and is a good option for commercial offices. However, as the panels in these systems are not fixed, they would not be suitable for a tile or stone finish.
Systems where tile or stone finishes are to be installed would often be described as a partial-access floor system whereby calcium sulphate based tongue and groove panels are installed on a conventional 600mm grid access floor under structure. The boards are glued together along the tongue and groove joint and laid in a staggered pattern on the pedestals. Once installed, the floor is flat and smooth and joints will not track through the applied finish. Access can be incorporated with conventional access floor panels or access frames.
Other panel systems comprise a galvanised steel encapsulated particle board core. In general, these types of access flooring systems would require further treatment prior to the installation of tile or stone flooring e.g. over-boarding with a suitably strong tile backerboard or plywood. It may also be possible to use a de-coupling membrane.
As with any other installation, it is essential to prepare the substrate to which the access panel system will be installed. Therefore, sub-floors must be dry and free of any dust, laitance, debris, oil, grease, paint or other contaminants which may be detrimental to the pedestal adhesive bond. The sub-floor surface should be reasonably flat and smooth to allow for satisfactory seating of the pedestal base plates. The overall level of the sub-floor should be checked to ensure that variations are within the adjustment range of the access floor support pedestals. The application of Mapei’s Latexplan Trade self-levelling compound, or a moisture barrier such as Mapeproof One Coat, may be necessary at this stage. Check with the manufacturer regarding the necessary temperature of the sub-floor to allow full curing of the pedestal adhesive.
The strength, composition and condition of the sub-floor must be appropriate for the envisaged application. One of the most common problems experienced is failure of the fixing of the pedestal base to the concrete sub-floor. This detachment is usually caused by failure of the concrete and not the adhesive bond however the dynamic impact test shall be carried out on site by the fixing contractor.
Sealing of the sub-floor within the under floor void is normally only required when the void is to be used as a HVAC plenum. When sealing of the sub-floor is specifically requested it is essential to ensure that the sealant is compatible with the proposed pedestal adhesive.
It is possible to install over sub-floors that are not concrete such as existing timber or asphalt. These should be evaluated for suitability and compatibility with the access flooring with consideration also being given to the pedestal adhesive. Mechanical fixings may be required for certain sub-floors. Sub-floors that deflect will cause the same effect in the access floor.
Standards and Associations
The Access Flooring Association (AFA) promotes the use of access flooring systems to specifiers, users and installers and provides guidance as to the appropriate standards of specification, manufacture, installation and maintenance of the products.
BS EN 12825 is a recognised British Standard approved by CEN as a voluntary specification for private projects and mandatory for public projects. The standard classifies Raised Access Floors based on the load at which failure occurs, referred to as the ultimate load. This classification is also based on a chosen safety factor and deflection under working load.
A class 3A3 panel reflects the safety of the flooring, the ultimate load (the point at which the system would collapse), deflection class, and the safety factor. BS EN classification is based upon the panel’s actual structural performance.
The PSA specification is a long established performance standard used by the Property Services Agency and now referred to as MOB PF2 PS/SPU outlining grades of raised access floor, light, medium and heavy. The PSA specification outlines a method of test for structural performance in point load and uniformly distributed load. This ensures compliance and provides guidance as to the working areas where each floor grade may be used.
Installation of tile and stone finishes
As with any tiled installation to floors, reference should be made to BS 5385 part 3 “Design and installation of internal and external ceramic and mosaic floor tiling in normal conditions – Code of practice” and part 5 “Design and installation of terrazzo, natural stone and agglomerated stone tile and slab flooring – Code of practice”.
Solid and suspended floors are affected by several factors which are assessed at design stage, including the load the flooring has to support, the type of tile used, and the resistance to water or water vapour.
When possible, selection of the tiles or mosaics and bedding method should be made at the structural design stage so that the appropriate depth can be allowed between the base and the finished floor surface. The chosen flooring should be suitable for the conditions, size of units, and loading likely to occur.
When a sub-floor is designed, due allowance should be made for the ultimate weight it might have to support, including the flooring installation. Mapei’s Ultralite S2 Quick is a lightweight, fast setting adhesive for the installation of tile and stone.
With any of the systems available, it is essential that the substrate is flat, that there is no excessive ‘lipping’ in the flooring and that there is no movement. All panels should be firmly fixed to the pedestals, which in turn must be firmly locked into position to prevent future movement. It may be necessary to further strengthen the accessible substrate with 18mm exterior grade plywood or an appropriate tile backer board or calcium sulphate board.
Where plywood is used, it is recommended that the boards are screwed to the panels with perimeter gaps being filled with a flexible joint sealant such as Mapesil AC silicone sealant.
In the case of a ground floor area there may be a risk of some moisture ingress. In this situation the reverse and edges of the plywood sheets should be primed with Mapei’s Eco Prim T prior to fixing the boards in position. Alternatively and dependent on requirements, the application of a suitable moisture barrier e.g. Mapei’s Triblock P may be necessary.
De-coupling/ anti-fracture membranes may offer the additional performance necessary to prevent the transmission of stresses to the tiled finish resultant from thermal or moisture movement. It should be noted that these systems are only designed to cope with ‘in plane’ or lateral movements, and vertical deflection cannot be accommodated.
Underfloor heating systems
Low profile piped underfloor heating systems can be installed over Raised Access Flooring systems and guidance should be sought from the product manufacturer with regards to their design, installation and where some access may still be required.
Where electric cable systems are to be used, it is recommended that an insulating tilebacker board is first installed over the panels to improve thermal efficiency. The cables can then be encapsulated in the self-levelling compound Mapei’s Ultraplan Renovation Screed 3240, to provide a flat surface upon which to tile.
Wet service areas
The application of a suitable waterproofing (tanking) system to the Raised Access Floor System together with the incorporation of appropriate drainage will permit the use of such accessible systems in wet areas including wet rooms. The Mapeguard WP 200 system offers waterproofing and de-coupling/anti-fracture performance in one product.
All raised access tile and stone installations should follow the guidelines given in the relevant codes of practice (BS 5385 parts 3, 4 and 5) and consideration at design stage should make provisions for movement joints within the tiled finish.
Contact Mapei’s technical department for more information regarding the correct products to specify for your raised access tiling project.
Mapei UK Ltd
Head office, Mapei House Steel Park Road, Halesowen, West Midlands B62 8HD
0121 508 6970
Specification Centre, 6 Great Sutton Street, Clerkenwell, London EC1V 0BX
020 302 9610