What did I do wrong? (Tapcon screws)

The building where I live has continuous steel guard rails running next to the front stairs and then around a wheelchair ramp. They are bolted into the concrete at their bases. On one side, the bolts rusted out, so there was nothing holding that rail down at the base of the stairs. It didn’t move, though, because the rest of the rail was still tied down at about six places. I told the HOA I would fix it with those blue concrete screws (Tapcons). A Tapcon rep told me they would work fine, as long as I didn’t reuse the old anchor holes, but drilled new ones.

So I sanded off most of the rust on that mounting plate, and started drilling holes. I drilled 3/16" holes in the steel plate and concrete, using first an ordinary twist bit for the steel (maybe 1/8" thick) and then a masonry bit for the concrete. I used the prescribed Tapcon masonry bit of the right size (they say use 3/16" bit for 1/4" screws) and drilled to the prescribed depth (about 1 1/2"). Then I started driving the 1 1/4" screws.

And I couldn’t get a single one to seat fully! I stalled out a 1/2 corded drill, even when driving a screw, backing it out, then redriving. I tried a socket wrench and managed to twist the heads off three of the screws. They are all now standing proud and awkward. The plate is fasted to the concrete though, because the screws self-tapped into both the concrete and the plate.

What did I do wrong? Should I have made larger holes in the steel plate? Should I have blown out the dust in the holes?

Yours truly,


Tapcons are not my first choice for anchoring but they do have some fans.
I recall that may of my crews had comments similar to yours about their failings
We also tried a similar product from Hilti - perhaps not as readily available - and maybe in larger diameters - that we had better luck with.

Back to your question:
They do recommend that the hole be 1/4 inch deeper than the fastener - so you seem OK there
They also recommend that you blow the dust out of the hole - so maybe that’s one issue.
As you also recognize it is often useful that the diameter clearance hole in the material to be fastened be a bit larger than the diameter of the fastener.
You may also know that not all concrete is created equal. Old well-cured concrete may be much harder than you might expect causing problems. Concrete with lots of hard embedded aggregate may also prove tricky.
Drilling a slightly oversized hole might help - but possibly at the expense of anchor strength.
Some folks also like to lubricate anchors with oil or candle wax (much as you would do with wood screws)

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I agree with @fred s Answer, however I would add( having ran over 5,000 Tapcons) fill your holes!, Frankly at best you get about a real 5# torque, if your lucky, Slam some treated lumber in the holes! Pulling on concret just wont work expanding works every time.

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Good thought - I’ve also heard that they work better with a oversized hole and a sliver of wood driven in.

Before there was any of this new-fangled stuff (e.g. Tapcons, PATS, Epoxy anchors, Red-Head expansion anchors, power spikes etc.) there were Rawl Plugs (made of fiber - then plastic later on) and lead shields.

The appeal of powder-actuated tools (PATS) and fasteners when the hit the market - was that no drilling was required - but you really should have training an a license to use the tools (some of the early ones had the force of the charge - up to 38’s - acting directly on the pin). And - while there are many choices in anchor pins and charges to drive them - they are not amenable to all masonry or all applications.

While taking a few more steps than using a PAT, the appeal of using Tapcons or the similar sorts of fasteners - especially in the Situation that “Koko…” describes - is that there are still less steps, not as many parts and a bit less tooling involved than using some expansive anchors (maybe requiring 2 drill bits, hammer drill, drive pin-punch and hammer). This is especially true since the railing was already in place. Too bad that he encountered problems.

Last year I screwed down 3/4" plywood sheets on my garage floor as I mainly do wood work and wanted something that was kind to dropped tools but tough enough for wheeling around heavy carts. I used the tapcon screws and had a variety of problems. First the screws are very poor at cutting through wood, so I found that drilling an oversized hole in the wood was best. Second the concrete I was drilling in to was really tough. I went through numerous masonry bits with cheaper ones not even drilling two holes. This was with normal hammer drills, both cordless and corded, I then switched to an SDS drill and the same bits cut multiple holes without problems. I used an impact driver to put the screws in, but as soon as they started to slow down I would back them out again and redrill the hole to make it a little deeper and try to get any dust out. Eventually i got every screw but one seated correctly and there is no movement in the floor at all, only 6 screws per sheet. However it was certainly not as straightforward a process as I first thought and I’m not sure I would recommend tapcons to anyone else.

I guess flathead Tapcon or other brand concrete screws have an advantage over other things you might have used for your application. In theory - nothing to stand proud of the floor surface - to trip you up or catch on wheels etc. - and if the plywood wears screws make it relatively easy to pull it up compared to using PAT-Driven pins, a T-Nailer or T&G plywood wiith construction adhesive.

For your type of application we did use a T-Nailer - and had a pair of this one:


Thanks, Fred, Cr8ondt, and Graham! I talked it over with a Tapcon consultant, and I think you were right Fred, the issue was the tight hole in the steel plate, not so much the concrete. The threads in Tapcons are meant to drive into concrete, not steel, so they don’t create clearance at the start of the threads, the way some self-tapping wood screws. So the screws would cut the steel plate, get hot, soften the coating and expand, and wedge tight. I should have drilled larger holes for the plate, then smaller holes in the concrete. Live and learn.

Yep, the reason I decided on Tapcons was that the anchor holes would be smaller. Every hole I drill has to be through the steel plate first, because the rest of the rail is fixed in place. The old holes are filled with rusted out bolts, so not usable. Drilling the anchor holes was actually fairly easy with a hammer drill. Interestingly, they say you don’t have to drill the holes too deep. Tapcons are supposed to drive only 1 1/4" or so into the concrete. Any deeper and the threads grind themselves to bits.

I feel your pain. I’ve had a only about a 50 percent success rate to screw them all the way down. I think I will try the wood trick next time

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I would not use Tapcon concrete anchors I would use GRK Caliburn XL screws they are superior either the 1/4" or 19/64"

Thanks! I’ll keep them in mind.


Hi - can you tell me more about this process - of putting lumber into the holes? I am having issues of my tapcons pulling out.