light- and medium-duty applications, friction anchors tend to
be the least expensive.
Concrete screws. Concrete screws are typically
available in 3/16- and 1/4-inch-diameters for light-duty
applications or 3/8-inch-diameters for heavier-duty
applications. Screw lengths range from 1-1/4 to 6 inches, but
usually with only enough thread for embedments up to 2-1/2
inches. Most manufacturers recommend drilling a pilot hole 1/4
inch longer than the screw to allow room for displaced concrete
or mortar to collect.
Concrete screws have hardened-steel threads, which cut a
thread in the sides of a predrilled hole. If you’ve used
concrete screws, you’ve probably stripped out a fair
number of holes. That's because the tolerance between the screw
shank and the thread diameter is very tight, so the pilot hole
must be of an exact diameter: Too big, and the threads
won’t bite deep enough; too small, and the shank either
pulverizes the sides of the hole or jams. Depending on how far
you were able to drive a concrete screw before it jams, you'll
either bend it or strip the head.
To ensure a perfectly matched pilot hole, several concrete
screw manufacturers package "tolerance-matched" bits with their
concrete screws. You’ll pay more for Tapcons (ITW
Buildex), Tappers (Powers-Rawl), or Scru-n-Taps
(Star Fasteners) compared with generic brands sold in home
centers, but we’ve noticed fewer stripped holes using
tolerance-matched bits. All three manufacturers also offer
drivers that slip over the drill bit, so you can drill the
pilot and immediately drive the screw without changing
Concrete screws.The tolerance between the
screw shank and the pilot hole is critical when using concrete
screws. Several manufacturers provide "tolerance matched"
carbide drill bits for their screws, but even with these,
it’s easy to strip the holes if the base material
isn’t up to specs. In poor base materials, try drilling
the smallest hole possible.
Concrete screws work well for temporary installations, such
as attaching formwork to existing foundations. But they are not
the best choice for applications that must be removed, then
reinstalled. In theory, a concrete screw can be unscrewed and
reseated. But you lose considerable holding power, even if
you’re lucky enough to reseat the threads. Mostly, you
wind up stripping the concrete.
Because of the narrow tolerance between the hole and the
threads, concrete screws require a solid base material, such as
a sound poured concrete wall or slab. In brick or block,
manufacturers recommend installing the screw in the mortar
joint for optimum holding power. This will work best in a new
brick or block wall with consistent mortar joints, but on older
structures that have been repointed, the joints won’t
always be stable. If you're having trouble with poor base
materials, it may help to drill a smaller pilot hole or use the
next larger diameter screw.
PAFs. Powder-actuated fasteners (PAFs) are one
of the most commonly used concrete anchors in the trade, but
not the strongest. They are certainly the fastest way to fasten
to concrete, provided you drive the pin into a sound base
material. In weak base materials, like soft concrete or old
mortar, they are likely to pull out, especially if you’re
trying to take a bow out of a board.
As common as they are, and as careful as manufacturers have
been about training and licensing, PAFs are often misused.
Operators tend to use the fastening tool like a nail gun, yet
concrete cannot sustain a close network of stresses. Keep pins
at least 3 inches from each other and from the free edge of
Each shot exerts a tremendous shock load, so unless
you’re firing into good, dense concrete or mortar, the
impact of a PAF often causes the base material to crumble or
spall. To avoid overstressing the base material, manufacturers
recommend using the lowest load possible. This means you have
to work up from the lowest power load until you find the one
that just seats the pin with the proper amount of embedment.
This rule may be one of the most overlooked in the field; most
operators tend to go with whatever loads they have on hand.
Hammer-in. There are several distinct kinds of
hammer-ins, although they are typically lumped into one group.
Many light-duty (non-structural) hammer-ins look like rivets.
After predrilling, you insert the plug and hammer home the
pre-assembled pin, which spreads the sides of the plug. These
fasteners are available in zinc, nylon, and Zamac (an
Other light-duty hammer-ins, such as Star Fastener’s
Strikr, look just like a PAF that you pound in with a small
sledgehammer. In masonry and green or lightweight concrete, you
don’t need a pilot hole if you use the
manufacturer’s installation tool. This tool resembles a
"pea shooter" — a pounding rod that slides inside a metal
tube. The tube holds the pin straight, and it gives you more to
hold onto as you work. In dense concrete, however, it’s
hard work, and even with the driving tool you don’t
always get a good seat for the pin. A slightly undersized pilot
hole often works better.
One of the strongest hammer-ins is the crook-style pin,
typified by the Rawl Spike. This pin has a curved shaft
which compresses as it is pounded into a pilot hole, exerting
force at three points on the sides of the hole.
Ramset/Redhead recently introduced two hammer-ins —
the King-Con and Redi-Drive — that have
holding values comparable to the Spike. The King-Con is made
chiefly for attaching 1-by or 2-by furring strips or sleepers
to concrete. It has a wide spiral flange that’s slightly
larger than the pilot hole, while the shaft is slightly
smaller. This makes the King-Con very quick to pound in, but
requires a tolerance-matched bit.
Hammer-ins.Two of the
latest light- and medium-duty concrete anchors include
Redhead’s King-Con (top) and Redi-Drive (bottom).
King-Cons have been developed for attaching strapping
and 2x-stock. Redi-Drives, which have the highest
holding values among hammer-in fasteners, have been
targeted to the electrical trades. Both require precise
pilot holes, but can be installed with a 3-pound
The Redi-Drive has a straight shank that’s slightly
larger than the hole, so it provides continuous holding along
the entire length of the hole. As a result, Redi-Drives have
one of the highest pullout values among hammer-in fasteners.
But this rigorous holding power also makes Redi-Drives more
difficult to pound in, requiring about twice as many hammer
blows to seat as the King-Cons. Redi-Drives have been targeted
for commercial electrical and hvac installations, and are sized
to fit through predrilled holes in standard fixtures (the
King-Con’s spiral is too wide for this application).