Holes in the wall, concrete behind drywall, shelving disaster.. help!
November 17, 2013 7:19 PM   Subscribe

I'm trying to put up shelves on a wall that, I believe, is drywall over concrete. I made holes for plastic anchors, but the drywall part isn't deep enough for them to fit. Help, I'm about as handy as the two I have at the of my arms!

I want to hang a set of Ikea Ribba shelf on the wall of my condo. I made the holes hoping to use plastic anchors but then realized that there is a much more solid surface behind the drywall. Its the wall that borders the stairwell in the building so it must be concrete, right?. It rings negative with the stud finder, and bends any nail I try to hammer into it.

So my question is:
I) Can I use a tapcon screw? I have a drill, although its not a hammer drill.
II) What do I do with the holes in the wall? Can I fill them in with something, and then drill through that with the tapconscrew? There is about an inch of drywall. Or should I just choose a different drill site?
III) Is there any chance this could be anything else besides concrete?

posted by cacofonie to Home & Garden (7 answers total)
Best answer: Are you sure on the 1"? That would be strange, I'd expect 5/8" rock max. It might be installed over metal furring, if so, there should be a small gap behind it.

Could be concrete, could be concrete block. No hard and fast requirement for either - could have been stud/multiple layers of gyp board and gotten the required fire rating.

Tapcons are probably the easiest solution. You do not need a hammer drill. Don't expect to take them out and re-install them with any reliable amount of holding power. And they tend to be blue, hex-headed critters, might not be the look you are after.

Next is a masonry bit and putting in an expanding anchor. If it's concrete block, there's a chance you could hit a cell that's NOT full of concrete, in which case a molly bolt or toggle bolt would work.

For patching, if the hole is over about 1/2" diameter, get some setting type drywall compound - comes as a powder, you have to mix it, it sets up chemically, not by drying, so much less shrinkage. Put it in the hole just a little proud of the surface. If you work it with a damp sponge as it gets stiff, you might not even need to sand. Regular premix mud or spackle is going to shrink and probably crack on drying that thick

But wouldn't whatever you are going to screw to the wall cover the hole?
posted by rudd135 at 8:33 PM on November 17, 2013 [1 favorite]

Best answer: OK, the ADD is kicking, and Mefi won't let me revise the comment.
If it's a 1/2" gap, it's gyp on resilient furring channels - which would be there to acoustically isolate your gypsum board from the stairwell structural wall. You'd be creating a sound path if you hard anchor into the masonry and fasten something to your gypsum board.
7/8" or larger gap indicates furring channels, which are not designed at acoustical insulation.
I'd really expect some sort of furring between masonry and drywall that's supposed to look good.

Also consider that what ever fastener you put into the masonry is really sticking out the 1" or whatever it is - and will be subject to bending if you put a good deal of weight on it. Tapcons are pretty stiff, regular big box store finished looking screws not so much.
posted by rudd135 at 8:47 PM on November 17, 2013 [1 favorite]

Response by poster: Ok! I kind of feel like you're speaking another language, but I think I follow.

So I ran over and remeasured the thickness of the drywall, or "gyp". Its about 3/4". Whats weird is that the middle hole that I made is about 1 inch deep, but the two lateral holes are both exactly 3/4".

I've attached an artist's rendition of how this looks to me.

I think, unless I figure out another option, I will go ahead with the tapcon solution and just drill slightly above the current holes so that the shelf covers it. Is the acoustic thing a big worry with three small screws?
posted by cacofonie at 9:08 PM on November 17, 2013

Best answer: Well if the wall construction was indeed built with acoustic isolation in mind, then the idea is to mechanically decouple the drywall from the concrete or whatever is behind it so that sound travelling through one component (drywall or concrete) does not have a way to transmit into the other component. By connecting the two decoupled entities with a few screws, the isolation is essentially nullified. How much of a worry this is depends on a few things, but the nice thing is that if you attach the screws and find that the noise of people in the stairwell is a bother, you can at least take the screws out and recover most of the isolation you had before (be sure to plug the screw holes with something substantial).

If there is no air gap behind your drywall (i don't see one on your diagram), then screw all you want.
posted by maximum sensing at 11:07 AM on November 18, 2013

"Whats weird is that the middle hole that I made is about 1 inch deep, but the two lateral holes are both exactly 3/4"."
Almost sounds like hard gypsum plaster work - trowel applied rather than boards. If it's old, (say up to 1930) it will have some horse hair in it. No kidding.

Your tapcons sound fine. Use the correct drill bit (size will be on the package of screws), drill straight and don't wobble around, put the tapcons in one time, straight, and push firmly or the threads will just chew up the sides of the hole. If that happens, you can always pick up some lead expanding anchors.
posted by rudd135 at 7:17 PM on November 18, 2013

Response by poster: Ugh. I made a big mess.

So I went out and bought the Tapscon screw. I used the included drill bit. I opted for the 2 inch 3/4 and 3/16.

I started screwing in, making a second hole about 1 inch above the first hole and at first the drill just hit the solid under substance and didnt move, and I worried that it wasn't concrete.

Man, drilling was HARD without a hammer drill (my drill here), but slowly, with time (i.e. 30-60 seconds x multiple times), it started going in. The problem was the silly drill started coasting in to the left, as if the substance there was softer.

The result, is a big mess.

i'm not sure what to do now. Theres a good hole now in the concrete at the top left corner, but I'm worried that the screw is basically floating in air until it gets to the concrete.

Blagh. What should I do now? Make ANOTHER hole and try again and keep it steady?
posted by cacofonie at 6:54 PM on November 19, 2013

Response by poster: Ok, just to document this for other folks, my week-long adventure has ended in success!

I ended up making another hole two inches to the right of the whole that I messed up.

The trick to using the tap con bit was that my drill was "high torque" mode. When I switched it to "high speed" mode, it went much easier, but was still difficult, needing 120-200 seconds of drilling for a 1 3/4 hole. You really need to hold it steady, both for the bit and the screw part.

Thanks again for your help!

I think next I'll try and rebuild my car engine.
posted by cacofonie at 1:37 PM on November 22, 2013

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