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I'm trying to drive a 2 1/2" long 3/16" diameter tapcon into a 50 year-old concrete slab/subfloor to attach a new toilet flange.

The instructions on the case of Tapcons said to drill a hole in the concrete with a 5/36" bit. Did that with a Bosch Bulldog SDS; that part was easy.

Next the instructions say to drive the screw in with a #2 Phillips head bit. So I put a new #2 bit in my Bosch 1191VSR drill and switch it into hammer drill mode. But I can only get the screw about 1/2" in and then it just stops turning, and the bit comes out of the screw head and begins to strip the screw.

I feel like there's some technique I'm missing, or else my bits are defective. But I tried two different ones, and the same thing happened each time. Or maybe it's my drill? From stationary, I can not get the screw turning. I need to back it out a bit and start again. It feels like the torque is too low at slow speeds.

What am I doing wrong?

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    Just a guess here but I've never used hammer mode on a tapcon. For the pre-drill yes, but not for the tapcon. Does the tapcon have a combination hex socket head? Use that instead of Phillips. – Tyson Dec 9 '16 at 22:28
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    I'd have to go along with the idea that the problem is hammer-drill mode. Drive the screw in with rotation only. Have you noted that the threads on the screw are smashed off by the hammer portion of the drive? – fred_dot_u Dec 9 '16 at 22:31
  • It's just a Phillips. Yes, the threads on the screw appear worn down. – iLikeDirt Dec 9 '16 at 23:24
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    @Ecnerwal i.imgur.com/NU5JYgS.jpg – iLikeDirt Dec 9 '16 at 23:43
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    I just put several 3/16x3" (about 20) tapcon screws in my basement (20 years old): don't use Phillips tapcon screws and hammer drill mode; try to get a box that includes the drill bit; if you have to use Phillips, pulse it (1-2 seconds of power, stop 1 second, repeat). Always try to buy the hex-head tapcon screws especially for long runs. Phillips are useless in tapcon screws. – Der Kommissar Dec 10 '16 at 6:03
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Aside from not using hammer mode, as suggested in the comments, I will report that I have had HIGHLY variable results with Tapcon's in old poured concrete - I don't recall what they claim to work in, but I begin to suspect that concrete block or green (not yet fully cured) poured concrete is more their cup of tea, or perhaps "concrete made only with "soft" rocks for aggregate." Some work as they should, some stick partway in, some go all the way in and don't hold; I think it's luck of the draw on what the concrete right at that particular hole consists of. Sometimes the threads shear right off (bit of hard aggregate they can't cut into?)

You can switch to driving them with a ratchet, but you may find that the threads shear off, or the entire shaft shears.

Since this is a vertical-down hole, you might try vacuuming it out before driving the Tapcon.

I've gone back to traditional anchors. Our local aggregate tends towards hard rocks, not crushed limestone.

They work OK as big blue overpriced wood screws ;^) which will let me use up the boxes.

  • You absolutely must blow out or vacuum out the dust from the pilot hole you drill. – cfx Jan 7 at 18:45
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All the Tapcons, Conserts, or other brand of concrete screws I have driven over the years, I have never, while following the directions driven a concrete screw no deeper into any concrete that has been well aged over an 1 1/4" in.

The specs that I seen for the most part say the screw is rated to go in between 3/4" to 1" max for the rated hold. To go in deeper since i had the wrong length screw to make do, I drill the shank size to a depth, then the thread size to get the recommended hold into the concrete.

As a rule, I do not use an impact or hammer drill to run the screw in. I have a 1/2" variable speed drill that will power them in all the way or break them off. Which has happened when driving them in too far. My logic is concrete does not like vibration, and any vibration will damage what threads are cut into the concrete and not give the screws the ultimate holding power. It has worked like a charm while setting wood plates using 2 1/2" screws, it draws the flathead screw below the surface of the plate, showing me it is holding well.

  • Plus one for 1" maximum drive depth, but once I started using an impact gun to drive them I never looked back. You might snap a bunch before you get the hang of it but IMO there's no other way (and I now, having driven my ~1000th Tapcon, can regularly exceed that 1" by up to another 1/2" with confidence - in fact, I'll take it out and use a longer one if it didn't feel right). – Mazura Dec 10 '16 at 3:15
  • I also use an impact if not paying attention old concrete can break up but I have used them for years where acceptable with few failures, even read head anchors don't always hold and they require a larger hole. – Ed Beal Jun 8 '18 at 21:05
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Use 1/4" diameter Tapcons (the 3/16" screws are worthless). Drill the hole with a 3/16" masonry drill bit with a hammer drill, one inch farther than the fastener is long, pulling the bit out of the hole several times as you go to clear the spoils, being careful not to ream the sides as you do.

Drive the screw with an impact gun, and stop half a second after you hear a change in pitch, which is when it's bottomed out.

If it doesn't feel right or is still not snug, finish tightening it with a socket wrench. After you drive your ~100th Tapcon, you won't need to do this step, and will seldom snap the heads off the fastener anymore because you were overzealous with the impact gun.

Use hex-head whenever possible, as the Philips type require a hardened #2 bit or you'll break bits left and right.

Assuming you lack an impact driver, and the screw does get stuck half way out, remove the screw (and throw it away if it's a damaged Philips head) and then do ream the sides of the hole a few times and try again. Repeat as necessary, and avoid over reaming before you've tried it again.

  • If at any time it feels like or if you think for one moment that you won't be able to fully sink the screw, stop and take it out before you strip the head, because once you do you're screwed.

I used to use compressed air to blow out the hole before I had an impact driver and sometimes used lube, but since I acquired one I've never had to.

  • I haven't snapped the head off of a 1/4" screw in a long time. What will usually happen with those, if it goes wrong, is that you'll strip out the hole.This can be remedied by drilling even farther and then switching to a longer screw. If you snap the shaft, you'll have to relocate. You should make yourself up a kit containing different lengths and just use a 5/16th nut driver - not that sheath thing. – Mazura Dec 10 '16 at 5:37
  • Late to the party, but for 1/4" tapcon, you want a 3/16" bit. confast.com/articles-drill-bits-for-tapcon – Aloysius Defenestrate Feb 10 '18 at 1:02
  • @AloysiusDefenestrate - Whoops. Good catch. – Mazura Feb 10 '18 at 1:19
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Aaaargh, NEVER use hammer on a hammer drill with a Phillips driver bit. Those heads cam out with the slightest provocation, bouncing them up and down will guarantee they part company and trash the head and the bit.

An 'impact driver' is a totally different beast. It hits down when it drives right, synchronised the way a hammer drill isn't.

If the concrete is too old and hard to use, then drill an oversize hole, and use an epoxy-in fastening, expansion bolt, or the good old-fashion expansion plug.

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When drilling down, as for a toilet flange, the cement dust will settle in the drilled hole. When you try to drive the tapcon in, the dust would prevent it from going the depth it needs to go. Try getting as much dust out as possible or drill the hole deeper so the dust is below the depth of the length of the tapcon. Drilling deeper for tapcons is not a problem, drilling not deep enough will make the tapcon spin out therefore negating the fastening ability.

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I just installed a new toilet flange over 20+ year old concrete. Used the appropriate concrete bit with a hammer drill. Cleaned out the holes. I too couldn't get the screws to drive in hammer mode or drill mode. I pulled, hammered, cussed... To no avail. I tried backing one out in hammer mode and it drove in!! So guess what? I drove all 4 in that way... Screw in until snug then reverse with down pressure on hammer mode and it's solid as a rock.

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I ran into this problem driving into concrete with an impact driver. The screw would just bind up and twist my arm.

Try putting s little bit of water into the freshly drilled hole. After doing that, my screws went in very easily.

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