For residential applications, steel flooring systems require new tools, attention to fastening, and a detailed plan for mechanicals. By Matthew Power

Illustrations by Harry Whitver

Look around virtually any new subdivision, and you're likely to spy the glint of galvanized metal. Steel floors have been gaining market share in the last couple years, with good reason. They address many of the seismic, termite, and quality control problems that have complicated the lives of builders in recent years. And they allow homeowners to create the big, open floor plans they want.

Nader R. Elhajj, a project manager at the NAHB Research Center, says that adding steel flooring capability to your next project will require about 20 percent more labor time on the first job as workers learn the ropes. And you'll need to buy some new, relatively inexpensive tools. But after the first time, the steel floor process takes no longer than a solid-sawn wood or engineered I-joist floor.

Steel joists weigh less than solid-sawn wood. Typically, they are sized and cut in the factory, so you should end up with minimal waste. But there's a catch: The steel industry's distribution network is far from complete.

"You still have to contact a drywall supplier, a commercial building material supplier, or go direct to the steel supplier," notes Elhajj.

Steel floors are covered in the new International Residential Code (IRC) and currently cost about 3 percent or 4 percent less than dimensional wood floors. That could change, Elhajj adds, as new tariffs take effect. "In a lot of jurisdictions, like Hawaii and parts of the West, codes won't let you build everything with wood any more. Something like this is a good solution."

Gear Up

To work with steel, you need to assemble a collection of basic metalworking tools, including hand clamps, a chop saw with carbide blade, an electric "power nibbler" (along with hand-powered aviation snips), a screw gun with adjustable clutch and 6.5 amp motor, and a heavy duty 1 1/4-inch hole punch. Finally, don't forget gloves. Metal not only responds sharply to cold or heat, but it also has unforgiving edges.

Fasten to Last

Get an organizer to store the lengths and types of fasteners needed to assemble a steel floor platform. Learn the difference between self-drilling and self-piercing screws. Use appropriate screws for each task, as directed in the "Builders' Guide" (see "Helping Hand," below). Screws should penetrate through the steel a minimum of three exposed threads. Screws that connect pieces of steel should go through the thinner piece first.

Share the load

Whenever possible, steel joists should break at load-bearing walls. For interior walls, they must have a minimum 1 1/2 inches of bearing support, and 3 1/2 inches where they rest on exterior walls. Overlap joists back to back by at least 6 inches. Fasteners must clamp the joists together, plus attach them to the track or top plate. Where continuous joists end, they should be anchored to load-bearing walls with clip angle or bent stiffener.

Block or rock

Like engineered wood I-joists, steel joists require lateral bracing. This can be accomplished by combining a 1 1/2-inch-by-33-MIL steel strap screwed to the bottom of the joists with a section of solid blocking on either side of the strap at the end of the joists. This arrangement can be repeated at a maximum 12-foot interval. Another alternative that greatly simplifies blocking: Attach gypsum board directly to the underside of joists. This method also meets typical fire protection requirements in conditioned living spaces.

Lock down

When installing metal joists, try proprietary systems that offer easy solutions, such as the TradeReady Joist Rim Track. Joists simply slot into pre-machined fittings and screw into place. When installing joists at the first-floor level over a wood sill, remember that the key to a sturdy structure is linkage. Each component attaches to the one below it. The sill is anchor bolted to the foundation. The foundation is screwed to the rim track by way of flat metal plates, and joists attach to the rim plate. Be sure to insulate or caulk gaps between sill and concrete, and rim track and sill.

Plan penetrations

Most steel joists include standard punched web holes at 24-inch intervals. Make a detailed layout of plumbing, HVAC, and wiring in advance. Be certain that holes line up precisely. If you must cut holes larger for plumbing or ducts, they may need a reinforcement patch. Holes should not be located within 10 inches of the edge of a load-bearing wall. If they are, they will need to be patched for strength. Flanges on joists should never be cut, but screw-sized holes are allowed anywhere in the web.

Sound Control Tips

Steel floors can get noisy. Keep unwanted noise down by:

  • Insulating between joists. (Use wire hangers as shown.)
  • Providing air chambers in water plumbing to prevent pressure "hammering."
  • Specifying oversized water pipes to reduce pressure.
  • Using duct liners to quiet air noise.
  • Boxing all recessed ceiling fixtures.
  • Helping Hand

    To get the PDF format of the complete "Builders' Guide to Residential Steel Floors," from which this story is derived, point your Web browser to:

    Note: If you are having trouble with the linked .pdf file, check that you have the latest version of Adobe Acrobat .