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In our 100-year-old co-op building in NYC, I'm replacing a failed door stop with a heavier one to prevent our (very) hefty entrance door from smashing the doorway's marble.

The door stop is intended to be bolted into the floor with a sleeve anchor very similar to this one and a 5/8" pre-drilled hole.

Unfortunately, as I drilled through the tile/concrete with a hammer drill, I discovered that the floor only extends maybe 2" down, after which it seems to be loose fill/sand. If I try to use the sleeve anchor in this hole, it looks like the "expansion" part of the sleeve will be expanding into thin air, or at best only partially in the concrete floor.

What should I do? I'm considering somehow inserting as much concrete as possible into the hole, letting it cure, then drilling and continuing as before. Unfortunately I only have this 5/8" hole for access and I'm not experienced with concrete—I'm not sure if it will be thin enough for me to get enough down there before it sets.

Clarification: this is the ground floor entry way from the street, elevated maybe 3-4’ from ground level and accessed by four steps from the sidewalk.

I guess this isn’t “the” slab of the building, but just something odd used for the entry way floor. There is no access underneath the floor.

And as to the oddness of the whole thing, hey, 1913 construction in NYC? Not sure.

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    Is this on the lowest floor of the building (you're drilling into a floor slab), or is there a story/level/crawl below where you're working (you're drilling into a floor-ceiling assembly)? – ThreePhaseEel Feb 13 '20 at 1:47
  • Get a shorter anchor. – Greg Nickoloff Feb 13 '20 at 1:54
  • You can’t just pour a bunch of concrete into the hole and expect it to “hold” when you install the expansion bolt. I like Greg’s idea to get a shorter bolt. – Lee Sam Feb 13 '20 at 2:04
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    Sleeves will probably just spin. Use (lag) shield anchors; the old school ones made from lead. – Mazura Feb 13 '20 at 2:12
  • If you are not on the ground floor chances are under the sand/loose fill is the wood subfloor. If that is the case just get a 3" wood screw and see if you can get it to grab the subfloor. If you can grab the subfloor with the wood screw then you can find a lag screw with the correct diameter for your door stop. – Fresh Codemonger Feb 13 '20 at 2:30
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First - what kind of construction uses a 2" slab for a floor? Trying to get more concrete down there is unlikely without breaking out part of the floor because it needs something to displace in order to do what you want. Even if you got some down there it's very unlikely to hold. Why not go to a 3/8 x 2 1/4 expansion bolt. I know you're rightfully concerned with the 2" slab holding but it should be okay unless that door is a real behemoth.

Another option is to improvise an extra large (i.e. 6"x 6") block that you can molly-bolt onto the 2" slab. What this does is take advantage of the shear strength of the molly by exerting force laterally when the door strikes the block or stop as opposed to a narrow stop that will want to tilt when it's struck exerting diagonal forces (up and down) on the 2" slab. If you go with an over-sized stop you could also strengthen it by using more than one anchor spreading out the load.

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You can find drop in anchors made for shallow materials

drop in anchor

such as this one at McMaster Carr. It accepts a 5/8" thread and requires 2" embedment.

You'd just have to cut a short piece of 5/8" threaded rod, thread it into the anchor, then spin on the stop.

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This is kind of dodgy, but so is a 2" thick concrete floor.

Take a 3 foot threaded rod. stick two nuts onto the top end (the top nut should not have any rod protruding). put the tip of the rod into the hole and drive it down into the sand using a heavy hammer, when the nuts reach the floor remove the nuts and squirt a a few cheap tubes of cyanoacrylate glue into the crack. allow 15 minutes for the glue to sent and fit the door stop. and tighten the nuts down, but not too tight.

A less dodgy solution could be to pour a mixed two-part casting resin down the hole then insert a 6" threaded rod , the resin will bond the sand together into something concrete-like

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  • If you're going to do that, you might as well get proper concrete epoxy (chemset). – SomeoneSomewhereSupportsMonica Feb 13 '20 at 5:44
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    I'm not sure I'd want to blindly drive a 3 ft rod into the ground in a random location under a 100 year old building in NYC - it's hard to say what other wires/pipes are there. Though I guess the good thing is if you puncture a water pipe, after you dig up the floor to fix it you can replace that 2" slab with something more substantial. – Johnny Feb 13 '20 at 5:47
  • @SomeoneSomewhereSupportsMonica the problem with chemset is that it's loaded with filler so it wont soak into the sand. – Jasen Feb 13 '20 at 6:09
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    Ooh. I wouldn't do three feet. I was thinking three inches into the slab – SomeoneSomewhereSupportsMonica Feb 13 '20 at 6:50

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