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My walls are very old prefab 3 inch thick concrete. I want to make a floating bathroom countertop. My guy helping me remodel, is not an expert (I’m in Costa Rica) He says I can’t build this because the walls won’t hold up my sink. I think there must be a way?! Maybe to build a backing to attach the countertop to?

Jet
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  • So you, as the expert, think you can get a weak wall support a “floating” top? Add feet, braces or an extra framework. – Solar Mike May 20 '23 at 06:48
  • I definitely don’t think I’m an expert? That’s why I’m asking. My guy here in CR is skilled in McGuyver builds – Jet May 20 '23 at 06:55
  • Well you diss his advice… again frame, feet or braces… – Solar Mike May 20 '23 at 06:55
  • He only knows what he knows. I feel like there is a solution perhaps, that he has never seen or learned to do – Jet May 20 '23 at 06:58
  • Yes, the solution is rebuild the wall. You *might* be able to get away with making the wall thicker somehow. How, is another question. – Huesmann May 20 '23 at 11:44
  • 3" concrete brick, or 3" hollow-core concrete block, or 3" hollow-core with cores reinforced and grouted, or 3" solid poured reinforced concrete? All would be 3" concrete wall, but they will perform very differently. – Ecnerwal May 20 '23 at 13:20

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3" thick concrete wall is not able to hold up a counter???.. sounds strange to me. You need to get creative like I did when I did my bathroom. The wall I attached my counter to is a 2x4 wall, I would say that's not stronger than a 3" thick concrete wall.

This is what I did.

I used a 4" thick wood slab for the counter top, live edge is what it is called. I made mine from a Sequoia Redwood burl. Any naturally rot resistant wood will work fine. Make sure to oil the top periodically to keep water from getting into the wood, optionally you can seal it with urethane. I am not one for the glossy type finishes (except on my cars) so I went with oiling it.

This is the trick I did. I used 2 round hardened steel bars that measured 5/8" (~16mm) diameter and 18" long to hold the counter top. I had a framed wall so I opened up the wall and put 2 2x4's with a piece of 1/2" OSB sandwiched between them into the wall where each of the steel bars was going to be installed. I closed up the opening and measured up the standard 36" and marked the wall with a horizontal line. That line is where the top of the counter will be. I then went into the room behind it and measure up the same amount on the wall where the counter was going to be mounted and drew another line. I measured down 2" from that line and marked the wall in the 2 locations where the 2x4's I added were located.

I then got the counter top set into position against the wall bracing it with 2x4's and I also placed 2x4's from the edge of the counter to the wall opposite the counter. You will understand why I did this in a minute.

I went back into the other room and using a 5/8" wire pulling drill bit (really long drill bit) I drilled 2 holes through the wall and into the back edge of the counter top. I marked the drill bit at 19" before drilling the holes so I knew how deep I was going in. Then using a 3lb hand sledge hammer I hammered the steel rods into the wall and into the back of the counter top. It was hard hammering the thing in, I should have rubbed some Ivory bar soap on the rod so it would have gone in easier.

Concrete I would drill the holes a little bit smaller to start off with and see if the bar goes through the holes in the concrete. You want a snug fit so the bar doesn't wiggle at all. You will want to use Ivory soap to get the bar into the wood. You will have to drill with a smaller bit first then take the counter off and go back down the holes in the counter with a 5/8" bit and put the counter back into position.

Using any kind of a screw is not going to work, it will simply tear out of the concrete if any weight gets placed onto the counter. Using steel that is hardened means you would have to put a monster amount of weight on the counter before they would bend. It would be very hard to pull the bars out of the wall because when they are being hammered in the end of the bar is going to mushroom making the end larger than the hole you drilled. You might have to cut the end of the bar off to make it flush with the wall then grind it down a little to be able to patch over the ends.

test fitting the counter

checking steel bar length in wall

all finished

  • IDK how strong 3" concrete is. Probably plenty strong in shear, but not very strong in bending. IDK if there's even any reinforcement in 3" concrete. – Huesmann May 20 '23 at 11:45
  • the forces being applied are not sheering or bending. I am not sure what exactly you would call the forces being applied using the steel rods. The wall on the counter side becomes a pivot. in order for the counter to move downward on it's outside edge the back end of the rod would have to move up. This is causing counteracting loads on the wall (no pun intended). I would call the load more of a compressive load in which the concrete is probably rated to some 3000 or 3500 PSI. I don't see that happening from a bathroom counter. –  May 20 '23 at 12:42
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    The top of the wall is being bent into the room, the bottom is being bent out of the room. Concrete and wood are very different materials, particularly if the concrete is unreinforced. – Ecnerwal May 20 '23 at 13:58
  • With a concrete wall it is not going to be sitting on the floor, it is going to be pinned or have rebar. I cannot imagine that the wall was simply poured free standing especially being 3" thick and at least 8' tall. the wall would easily be knocked over. There must be rebar in the wall otherwise it could fall over. There has to be attachment at both the top and the bottom to keep it from doing that. –  May 26 '23 at 17:15