Thanks to new prescriptive tables, light-gauge steel floor
framing is as simple to use as I-joists or solid lumber
Steel framing has so many advantages over wood -- including
dimensional stability, consistent pricing, and resistance to
rot and termites -- that I'm convinced it will claim more and
more of wood's market share in the years ahead. But for
builders who have never used anything but wood, the prospect of
diving into steel framing all at once can present a big mental
barrier (see "Making the Switch to Steel
One alternative is to start at the bottom, and begin the
transition to steel with the floor framing system. As the
former project manager of a family-owned company that has been
framing entirely with steel for years, I found that the
material's benefits were especially attractive for floor
Allowable Spans for
Cold-Formed Steel Floor Joists (1,2,3,4)
33 ksi Steel
30 psf Live Load
40 psf Live Load
For SI: 1 inch = 25.4 mm, 1 psf = 0.0479 kN/m2, 1 foot
= 0.3 m
1. Table provides the maximum clear span in feet and
2. Bearing stiffeners shall be installed at all
support points and concentrated loads.
3. Deflection criteria: L/480 for live loads; L/240
for total loads.
4. Floor dead load = 10 psf (0.479 kN/m2)
Courtesy of the NAHB Research Center
1. Steel joists permit unsupported spans of well over 20
feet. By varying the gauge of the material, it's possible to
increase the allowable span without increasing the depth of the
Advantages of Steel Floor
As you can see from the table at right, at 24 inches
on-center, a 10-inch-deep 54-mil steel C-joist can span 15 feet
11 inches at 40 psf live load (see Figure 1). To get the same
span from wood joists of the same depth, you'd have to go to a
12-inch spacing. If you go to a 12-inch spacing with the same
steel joist, you can span up to 20 feet. That's a nice option
if you want to build a bonus room over a garage without having
to add a beam. You can span even farther with heavier-gauge
joists, but I've found that if you go beyond 16 gauge, it gets
difficult to drive screws into the material.
Although steel is usually referred to by gauge on the job
site, current tables reference it by mils of thickness.
Sixteen-gauge steel is 54 mils thick, 18 gauge is 43 mils, and
20 gauge is 33 mils. For easy reference, steel framing is also
color coded, with 16 gauge being green, 18 yellow, and 20
Unlike wood I-joists, steel joists also allow you to vary
the gauge of the material to suit the span, without increasing
the depth of the joist. That's useful when you have two spans
of different lengths that lap over a beam. With I-joists you'd
have to frame the shorter span with an unnecessarily deep
joist, or go to a shallower joist and face the problem of
getting the floor surfaces to line up at the beam. With steel,
you can use a 16-gauge joist on the longer span and go to 18
gauge for the shorter one -- maintaining a constant depth
without throwing away material.
2. A conventional wormdrive saw with an abrasive
blade (left) is the preferred tool for most cuts made
on site. The 14-inch chop saw (right) can cut several
stacked pieces of material at once, speeding repetitive
Working With Steel
The steel framer's tool kit is pretty simple. For cutting
joists and tracks, we use a standard 7 1/4-inch wormdrive saw
equipped with cheap abrasive blades that we buy from a local
supplier for about $3 apiece, or a 14-inch chop saw with the
same type of blade (Figure 2). We tried a new carbide-toothed
blade from Tenryu that lasted longer and provided a nicer cut,
but the results didn't seem to justify the $40-per-blade price
tag. Also, we found that any vibration in the blade had a
tendency to cause the teeth to chip.
All steel-to-steel connections are screwed together with
self-tapping screws. Our crews like the Quik-Drive QD2000MT
housing for strip screws, mounted on a DeWalt screwgun.
Whatever gun you prefer, stick with a 2,500 rpm model; screws
driven at 4,000 rpm tend to burn at the tip and are therefore
more difficult to set.
In addition to the power tools, each framer also needs
self-locking clamps (usually one large and one small will do
the trick) to temporarily hold joists and tracks in place until
they can be screwed together. A pair of aviation snips can also
come in handy at times.
Planning and Layout
When you're framing with steel, you need to spend extra time
previewing plans and doing takeoffs, because you can't just
make a run to the local lumberyard for material you forget.
While many contractors and subs view this as an extra burden,
our company found it to be a blessing in disguise.
By spending a few extra hours in the office detailing and
laying out the plans, we save twice that much time in the
field. Our crews aren't left standing around waiting for
missing material to be delivered, and the detailed plans show
them exactly where each piece of steel is to be used. Finally,
most of the material is ordered precut, which eliminates the
need for cutting and saves time and material.
Dealing with foundation irregularities. It can
be risky to order precut full-length joists, because you don't
really know the exact dimensions of the foundation until the
forms are stripped. Unless the foundation is dead on, the
joists won't fit. Ordering them a little long and cutting them
down is a waste of time and material, but if you wait for the
actual foundation measurement to place your order, you may not
get the material by the time you need it.
Figure 3.Lapping joists at a
central beam eliminates the need to cut joists
precisely to length, making it possible to order
material before the exact dimensions of the foundation
are known. Web stiffeners are not required at the
To avoid these kinds of problems, we plan the floor layout
with a central I-beam where we can lap the joists (Figure 3).
That lets us adjust the joist length as needed without any
cutting. The joists are still ordered to length, with an
additional 6 inches to allow for the lap. This also saves time,
because screwing the joists together back-to-back at the lap
eliminates the need for web stiffeners over the beam.
Span Tables and Approved
Until just a few years ago, all steel framing had to be
approved by an engineer. But now, steel framers can draw on
span tables and approved details from the Prescriptive
Method for Residential Cold-Formed Steel Framing
by HUD, the American Iron and Steel Institute, and the NAHB
Research Center (available from the NAHB Research Center, 400
Prince George's Blvd., Upper Marlboro, MD 20774-8731). The
prescriptive method is recognized by the 2000 International
Building Code and 2000 Residential Building Code, and we've had
no problems with the building inspectors here in Indiana. But
as we all know, each municipality and local building official
is different, so I would suggest opening a line of
communication early on. The North American Steel Framing
Alliance (NASFA, 1726 M St. NW, Suite 601, Washington DC
20036-4523; 202/785-2022; www.steelframingalliance.com) is an
excellent source for both information and assistance in
educating your local officials.