Raised Access Flooring

The question needs rephrasing, because as put it nearly always leads to the wrong number. The figure that appears first in the data sheets is the distributed load, and it is also the one that in a real office never defeats the system: the weight of people and desks is modest and well spread. The critical case is always the concentrated load, the one bearing on a few square centimetres: the foot of a full filing cabinet, the castor of a document trolley, the leg of a safe, the upright of a shelving unit. These loads act at the worst possible point, the centre of the panel or an edge, and they are why panels deform or break. The practical consequence is twofold. In design you choose the load class by reasoning about the actual furniture rather than the generic occupancy, and you flag the zones intended for heavy loads, where a higher class is often worthwhile. In testing you test where that furniture will go, because testing in the middle of the room tells you nothing useful.

Because something is moving, and in a system of resting elements the possible movements are few and all identifiable. The first is uneven panel bearing: if it does not rest uniformly on all four sides, it rocks imperceptibly underfoot and produces a small knock, clearly audible in a quiet space. The second is a pedestal out of adjustment, which leaves one side slightly low and creates the same effect on every panel resting on it. The third is sliding between panel and pedestal head, typical where the gasket is missing or the floor has been loaded sideways, for instance by a trolley. The fourth is edge deterioration, which appears after years and mainly affects the most heavily used panels. The good news is that all these causes are correctable without demolishing anything, which is exactly what the system was conceived for: lift the panel, adjust or replace what is needed and close it again. The bad news is that almost nobody does it, and the noise gets accepted as a characteristic of the floor.

It can be done, but it is rarely worthwhile, and the reason involves two distinct problems that must be considered together. The first is load: a partition delivers a continuous line load along a line that almost never coincides with the pedestal grid, so it ends up bearing on panels at points they were not selected for, typically mid-span. The second is reversibility: the partition restrains every panel it stands on, and those can no longer be lifted, which means a continuous band of plenum becomes inaccessible precisely where the services feeding the two rooms run. There is also an acoustic consequence that surprises people: if the partition stops at the floor, the plenum remains continuous beneath it and becomes a direct path for sound between rooms that were meant to be separated. The correct solution is to take the partition down to the slab, passing through the floor, and interrupt the plenum there; alternatively, where the partition must remain removable, the grid is designed so that it bears on a dedicated line of pedestals.

No, and it is one of the system's most concrete advantages: the pedestals are adjustable, so a raised floor absorbs irregularities that a traditional screed would require correcting with wet trades, drying time and extra weight. The limit does exist, though, and lies not in adjustability itself but in its direction: pedestals extend, they do not shorten below their minimum height. This means the highest point of the slab determines the lowest possible finished floor level, and that a survey made from scattered samples is risky, because a single unrecorded hump forces the whole floor up. There are then two substrate conditions that matter more than flatness and are checked less: it must be dry, because the pedestals are bonded and residual moisture compromises adhesion with effects that appear months later; and it must be clean and free of friable material, because the adhesive works on the substrate, not on dust. In practice, an uneven but dry and sound slab is a far better condition than a perfectly levelled but still damp one.