Laying the foundation for good design.

April 03, 2012

Assessment of space usage will determine the requirements not only for flooring materials but also for the load-bearing structure beneath. The information can be used to manage client expectations with respect to project cost, materials performance, and choice of finish materials. “The key is to make sure you truly understand how the floor will be used,” says Caroline Robbie, a principal with Quadrangle Architects Ltd. (, Toronto.

Building codes may help determine minimum load-bearing requirements, but the realities of the space, the likely occupants, and the planned usage in specific areas are all predictive of the amount of wear the floor will have to endure, and what kind of maintenance program would be optimal.

As an example, hospital floors suffer from static load and movement of hospital bed and gurney wheels. While resilient flooring is more conducive to rolling patients down corridors, it works best with flat-wheeled equipment, which prevents damage to vinyl, linoleum, and the like. For carpeted areas, narrow wheels work best. In patient rooms and other areas where hospital beds are used, strong adhesives such as epoxy-based products may be recommended by flooring manufacturers to protect the finished surface.


Structural considerations for commercial floors fall mainly into two categories: load capacity and material composition. Load capacity has a number of ramifications for the design of interiors generally and especially for the floor. Building codes will help determine what options are available—and the limits to those possibilities.

“The building codes require substantially more structural capacity for assembly uses than residential or office uses,” notes James Rappoport, AIA, NCARB, vice president of Philadelphia’s Daroff Design ( “Our first review is to determine the use and occupancy of the space and then to confirm the structural capacity of the floor.” According to the American Society of Civil Engineers Structural Engineering Institute, a minimum live-load capacity of 100 lb/sf is recommended for “public assembly occupancies,” such as large meeting spaces, dining areas, and the like. This compares to minimum load capacity of 60 lb/sf for office usage and 40 lb/sf for residential uses.

These minimum capacities may determine the feasibility of a renovation project. “While it’s technically possible to reinforce a floor from a residential or office structural capacity to the assembly capacity, the cost for this reinforcement is very high,” says Rappoport. “We see many assembly projects come to a halt once the client realizes just how expensive it is [going to be].”

The second category—materials composition—further complicates the matter. Let’s look at these factors one by one.

Concrete: Dealing with moisture and leveling. A majority of commercial buildings have concrete subfloors, and many architects favor poured concrete. But that is so in spite of the technical challenges raised by using concrete, notes Gretchen Lotz, RID, IIDA, an architect with IA Interior Architects, Inc. ( “Structural concrete slab substructures are ideal to receive most types of flooring installations,” she says. “But slabs provide myriad conditions—moisture content, levelness, cracking, slab-on-and grade conditions—that affect the installation and performance of flooring.”

Jean Hansen, FIIDA, CID, AAHID, EDAC, LEED AP BD+C, sustainable interiors manager at HDR Architecture (, San Francisco, who works mainly on healthcare and research lab projects, says, “We’re accustomed to working with concrete as the substructure, but we have to make sure that we have the absolute best concrete surface available to put that flooring product on.”

“The main issue is settling, whether it’s an existing floor that has already settled and may be cracked, or a new floor that’s likely to settle and crack over time,” says Mike Krochmaluk, LEED AP, a designer of corporate interiors with STUDIOS Architecture (, New York, N.Y. “Cracks and other imperfections in the concrete will ‘telegraph’ through the floor at the grout joints or by cracking the flooring itself, in the case of stone and terrazzo.” Krochmaluk suggests that an anti-fracture membrane applied as underlayment may solve this problem, by reducing the effects of settling, structural movement, and uneven surfaces. Some membranes also serve as a waterproofing product.

According to the experts, concrete problems almost always are related to settling or moisture. “We are super-sensitive about the moisture content in concrete floors, and we will often take a core sample to test the moisture level, if possible,” says Todd Baisch, AIA, RID, LEED AP, a principal in Gensler’s Chicago office ( Newly installed structural floors are generally too moist to receive floor coverings. Wood installed over new concrete may warp or buckle; adhesives may fail. Carpeting and carpet tiles present a number of potential moisture-related worries, including mold, odor, and deterioration of the material. Manufacturers generally recommend the Carpet & Rug Institute’s installation standard CRI 104, to test for pH levels and moisture before installation.

Building Teams employ a variety of solutions to the concrete moisture issue to help speed projects and ensure a lasting floor assembly. New, rapid-drying concrete mixes are available that have desiccant properties. For wood flooring over concrete, some teams employ a closed-cell foam underlayment between a floating wood floor and the concrete slab, for resiliency as well as moisture control. Consulting the manufacturer’s product information can help find solutions like these.

Concrete may also present concerns related to uniformity and levelness, possibly creating conditions that may even be in violation of codes related to the Americans with Disabilities Act (ADA).

If the floor is not poured to the specifications of the Concrete Institute’s finish and level guidelines, the interiors team may arrive on the job already over budget and behind schedule. “The cost to prepare a poorly installed concrete floor slab could be $3.00/sf or more,” warns Daroff Design’s Rappoport. Leveling compounds may solve the problem, he says, though at an added cost: “Because adding a self-leveling topcoat adds moisture, remediation work can delay the project by weeks.” Still, leveling compounds can be a valuable technology, especially in renovations where the concrete slab has already settled.

Of all the worries about concrete, “concrete moisture continues to be enemy number one in all flooring failures in schools and hospitals as well as commercial offices throughout North America,” says Ray Darrah, a flooring inspector with Cali-Floor Technical Services, Sacramento. When the cool concrete meets warm air, moisture condenses on the exposed surface wherever the concrete is exposed to air for any length of time. “Concrete does not require extended lengths of time to accumulate enough moisture from the air to cause the new flooring materials to fail,” Darrah adds.

Wood subflooring and underlayments. Though concrete dominates the commercial subfloor segment, Building Teams must be prepared to cope with alternatives. “Structural systems encountered vary with each project,” says Quadrangle’s Robbie. “For example, a renovation may have an original heavy timber floor with wide variations in level.”


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