Laying the foundation for good design.

April 03, 2012


Regardless of underlayment or finish, in some cases raised floors offer advantages for the occupants. A novel types of system known as low-profile access floors has gained attention recently for accommodating quick changes of electrical systems and data cabling. In other circumstances, the classic “post-and-plank” raised floors—sometimes called pedestal or simply grid systems—can be used for the same reasons. These systems come in two-foot-square panels, with a higher profile that allows for HVAC air delivery through flexible ducts or an underfloor plenum.

Designed properly, raised floors should not sound hollow or reverberant when walked on, and should not have any unwanted movement. “For a large law firm’s new, 45,000-sf building designed for their use, the client had already determined they would use raised-floor construction, mainly to allow all telecommunications and data cabling to be routed below the floor,” says WXY’s Pew, who specified concrete-filled, 2x2-foot tiles that were then overlaid with carpet tile. “It was almost impossible to tell that you were walking on a raised floor,” he says.

Whether low-profile or standard heights of 12-18 inches or more, raised floors are basically elevated structural platform systems sitting directly on the slab or other substructure, typically on some sort of damping padding. Created for data centers and computer rooms to route mechanical services and cables, wiring, and electrical supply, the taller versions were originally invented to cool the undersides of mainframe and midrange computing systems.

Today there are new concerns centered around information technology, says Franz. “The building IT infrastructure creates challenges for where wires can be run and how to cool the ever-increasing heat load of more equipment,” he says. “Raised floors have become a great way to have easily accessible wiring raceways that also allow for modular flexibility. They also allow ductwork to be run in a more concealed manner or for mechanical systems to focus on managing temperatures closer to areas of human occupancy, not 10 or12 feet or more in the air.”

Another reason to use raised floors is that they can ease wire management and restacking of office layouts, which is ideal for high-churn workplace environments. The low-profile type systems are not used for underfloor air, and only allow 2-3 inches of height for cable runs. A new CSI MasterFormat specification section (09.69.33) was recently added to cover the product, as it is unlike other raised-floor and underlayment products. The steel interlocking sections are typically installed by a flooring contractor and can be easily rearranged by facility staff, usually without the use of tools.

Some Building Teams prefer using raised floors because of their benefits in the occupancy phase, but others see life cycle arguments against them. “While we have some raised-floor projects in our portfolio, we still believe that a blended deck, steel underfloor duct, or other wire-distribution systems within the structural floor slab are preferred for first cost and life cycle cost when compared to an access floor installed above structural concrete,” says Daroff Design’s Rappoport.

But Quadrangle Architects’ Robbie points to another benefit of raised floors: clearing the view corridors on the floor plate of electrical feeds from the ceiling and duct chases on core or exterior walls. “The systems are always modular in nature, so flooring choices are either integrated into the system or must be modular, such as laminate tile in clean rooms or carpet tile in open office areas,” she says.


Building Teams can also provide for raised or recessed features that protect the appearance of flooring systems in special-use areas or high-traffic zones. Entrance areas, lobbies, and vestibules have to deal with foot traffic carrying moisture and particulates from the outdoors. When entry-area flooring transitions are well designed, they can absorb or remove significant amounts of dirt and moisture from visitor and occupant foot traffic as they enter, reducing O&M demands for the building.

Clean mats are a common solution, though some Building Teams consider them the least aesthetically pleasing. They cost practically nothing to install; when they need maintenance, they can easily be removed and replaced with other mats, to be cleaned elsewhere. Moreover, they can be used with practically any floor type: hardwood, resilient, carpet, even stone. Many owners and facilities managers prefer them for their ease of operation and low cost.

“Some clients prefer to have a maintenance program in place using clean mats,” particularly in healthcare and research laboratory facilities, where cleanliness is extremely important, says HDR Architecture’s Hansen, “There are some great carpet tile alternatives. The entire vestibule can be finished with tiles specifically designed to work as walk-off mats, and the tiles can be replaced easily when necessary.”

Hansen, like many designers, prefers to specify recessed grille systems. Recessed systems, normally used in vestibules but occasionally installed in lobby areas, are long-term solutions that reduce the frequency of required maintenance. Many designers feel they do a better job of keeping the building interior clean while being more aesthetically pleasing.

The system works by covering a floor area that has been installed recessed, usually between a half-inch and one inch below grade or below the finished floor, with a walk-off covering: grilles and grille mats are two common types. Both coverings use the grille texture to remove particulates from shoes as traffic passes over, but grille mats boost the moisture absorption of a clean mat as well. Cocoa mats, rugged fibrous mats made from coconut husk and typically backed with vinyl, represent a lower-cost alternative with a long track record.  Many proponents find them versatile and cost-effective, says WXY’s Pew: “They are the most straightforward and economical choice when you need a custom size or shape.” Adds Gensler’s Baisch, “We still use cocoa mats. They’re durable, not super-expensive, and only need to be replaced every three years or so.” He notes, however, that “there are better walk-off mats now.”

Today’s improved walk-off mats use special materials and often combine grille, mat, and recessed sections to protect the floor. Maintenance practices typically require the removal of the covering so that the recess well can be cleaned; some recesses may include plumbing for drainage, allowing the well to be cleaned with a hose. Recessed flooring specification may result in receiving LEED-CI credits under IEQ 5, for pollutant control, if the vestibule dimensions meet the criteria. Quadrangle Architects’ Robbie, based in Toronto, says, “We use a standard minimum depth of three meters for vestibules, which is in line with accessibility guidelines and good practices.” In the United States, 10 feet is the required depth for the LEED credit. This treatment also satisfies most ADA requirements.

“Recessed systems are preferred over mat systems because of the raised profile and potential tripping hazard with mats,” Robbie explains. Even the best recessed grills and mats may lack a clean, pleasing aesthetic an owner is looking for, so Robbie’s team at Quadrangle Architects has been recommending a novel modular recessed-flooring system combining stainless steel grilles and custom cell mats that exhibit good durability and wear resistance. The products install flush with the surrounding floor finish and, in some cases, mimic the adjacent floor finishes.

Of course, it may be ineffective to install a recessed flooring system in an existing slab due to cost or other constraints. In these cases, says Interior Architects’ Lotz, “Low-maintenance flooring installations for lobbies can be designed successfully with thin-set tile or pavers, composite flooring, or high-impact applied vinyls, without cutting into the existing structure at a premium cost.”

Rappoport offers another possible way to create a low-maintenance lobby without a recessed system—raising the floor: “You can start with the zero elevation and build up from there, but you need to add drainage to avoid mold and mildew below the finished floor surface.”



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