Questions on Access Flooring

I enjoyed your article on access floors in the January issue of

EBN [Vol. 7, No. 1]. You demonstrate the wisdom of using access flooring in new office buildings, but I wonder what their applicability might be for other types of construction, including schools, libraries, and office renovations. What governs the selection?

Although you wrote of some of the inherent disadvantages with raised flooring systems, I think there are some negative considerations that you failed to mention. Isn’t it necessary to at least compartmentalize, somewhat, the floor of an office building so as to limit the rapid spread of fire between the access floor and the structural slab? Another problem that I have noticed in the past—and perhaps this is no longer so noticeable with current models of access flooring—is unevenness, the feeling of walking on a temporary floor with variable deflection and irregularity where sections are poorly fastened. Of course, using carpet as the wearing surface for access flooring softens any irregularities, but for those of us trying to get away from using carpet, the difficulty of making a smooth, uniform floor becomes more of an issue. You mention the use of concrete and other floor panels—are these subsequently covered with carpet to produce a monolithic surface, or is there a successful way of dealing with the joints and fastenings?

Your suggestion that access flooring permits reduced floor-to-floor heights seems too good to be true. You state that by eliminating supply ducting in dropped ceilings, it is possible to reduce the ceiling plenum from 18-24” to 12” in thickness. But you also say, at the beginning of your article, that “when used for air distribution the floor is raised at least 14” above the slab.” By these figures, it would seem to me that the floor-to-floor heights would increase with use use of access flooring by 2” to 8” per floor. Am I missing something?

G. Mackenzie Gordon, AIA

Lakeville, Connecticut

Editors’ response:

To address these questions in order, access flooring should be applicable to schools, libraries, and other commercial buildings, though adding them to existing buildings may prove very difficult unless a complete gut-rehab is being done. From inquiries we have made, it appears that compartmentalization for fire protection might be required beneath fire separation walls (to carry a fire wall down to the floor slab) but otherwise would not be required. Unacceptable flex has indeed been a problem in some installations, such as Union Station in Washington, D.C., but this should not be a problem if suitable panels are specified. Use of concrete access floor panels should eliminate the flex you refer to. InterfaceAR and several other manufacturers are producing such panels that can be left unfinished, finished with a high-pressure laminate, or covered with carpet tile. They are corner-bolted to the support structure.

Finally, as for a reduction in the floor-to-floor height, the example dimensions provided failed to provide such a reduction. Dan Nall of Flack & Kurtz Consulting Engineers in New York tells us that a more typical ceiling plenum used with an access floor system would be only 6-7” (150-180 mm) deep—enough room for lighting and sprinkler pipes. Combined with a 14” (350 mm) access floor, the total plenum depth would be 21” (530 mm) vs. 24” (610 mm) for a conventional ceiling plenum. That’s only a net savings of 3” (80 mm) per floor. Nall says the best you can usually hope for is 4-6” (100-150 mm) reduction in floor-to-floor height, and you may not get any. We regret overstating this potential advantage in the article.