I'm installing a grab bar (I didn't buy one yet) next to a toilet on top of tile on a plaster and steel lath wall under a light switch. The globs of masonry under each tile are about 1" to 1-1/4" thick and are embedded in steel lath. I plan on removing the masonry under four or five tiles. This will allow me to make sure there's no squeeze-out behind the lath that could interfere with a hollow wall anchor (if I choose to use one) and it will allow me to use something less brittle than the 50 year old masonry under the tile - but what material should I use? I'd prefer wood or something strong and trowelable. I'd rather not buy a 4 x 8 foot sheet of something.

Maybe I'll also cut the lath and put wood or a steel plate behind it.

ADA standard say "Allowable stresses shall not be exceeded for materials used when a vertical or horizontal force of 250 pounds (1112 N) is applied at any point on the grab bar, fastener, mounting device, or supporting structure." I'm not trying to be ADA compliant (I don't think it's even possible given the dimensions of the bathroom) but I'm using the standard as a guide, so I want it to be really strong.

 

The right way would be a solid wood backer behind that wall.

 

Yeah, I still may cut the lath to get wood behind the lath, but I'm also wondering about what to use directly under the tile (between the lath and tile). I'm thinking either hot mud or concrete patch. I already have hot mud.

 

I think I'll put wood over the existing lath and attach it with some kind of anchor, then put more metal lath over the wood and attach the new lath to the old. Then hot mud over the new lath, then the tile.

 

Hot mud will not suffice and the only that gives the masonary wall strength is the lathe and a THICK wall. The thingeasiest way to get a strong support is to open up the wall from the back side and install wood blocking. You can also rebuild the entire wall and insert you blocking needed. Keep in mind that someday someone may rely on the as the sole means on shifting thier body weight. What you propose will pull straight out of the wall and is not ADA compliant.

 

Yeah, but I'm working on how to do that. Here are some more photos to show wall construction:



I may cut the lath on the hallway side of the wall, insert blocking, and screw the blocking to the lath on both sides of the wall. But I'm still considering attaching the blocking to the "truss studs" somehow.

 

Leave the wire lath in place, it adds strength as a whole structure. Screw the wood block to the lath and into the main metal support if possible. Back fill your tile side with a wall or bed mud to capture the lath and in turn the wood blocking. Finish your tile wall and make sure your grab bars are installed with screws long enough to penetrate all the cement and get to the wood blocking.

Apologies for the typos on the previous post, was out of town and working on a tablet with a small virtual keyboard. That plus big fingers don't mix too well.

 

AS others have said, you must use wood to secure the grab bar. Nothing else will hold. If not, over time any other fastening method will work it self loose and catastrophic injury may occur.

 

Maybe I'll try to get one of the wings of an anchor to grab a flange of the stud. What I'd really like to do is squeeze something through the lath that will encase the studs and be a good substrate for screws (either for screwing on a wood block or screwing in the grab bar directly). There must be some kind of concrete that's strong enough.

 

I think the strongest methods of attaching wood blocking all require cutting the lath. I'd be able to sandwich the lath between two 1" thick blocks of wood and screw the blocks to each other with the screws going through the lath. The wood would securely grip the lath along the entire surface of the wood. Using hollow wall anchors to attach a block of wood to only the outside of the lath wouldn't be nearly as strong or slip resistant and still may require some cutting of the lath for the anchors to fit. To attach the wood to the studs would require small screws that fit in holes drilled in the 1/2" flange of the studs. The strength of the studs would be compromised from holes, so some existing parts of the wall will be destroyed either way, and I wouldn't trust the strength of such small screws that press against the edge of a hole in the relatively thin metal stud.

 

First let me make sure I understand you.
Are the pictures you are showing actual pictures from inside the wall?
Are what you are calling studs 3/4 cold rolled steel channels?
If so I think your idea of a sandwich is pretty good. I would use 2 X 6's maybe even 3/4" real plywood about a foot wide and two feet long
Place one board on the inside of the wall spanning as many channels as you can. If there is room use 3' lumber.

Then on the outside of the wall screw another piece as large as practical to the board on the inside. Use bolts with washers on both boards and draw it up as tight as you can.
The keys in the plaster are not straight and the board will not lay flat against the whole length of itself. When you bolt it if you crunch a little of the plaster not to worry.
Then fit new lath if necessary or use the existing lath to cover the opening you make to do the work. Plaster as thick as you show in the first pictures will give you plenty of room to get a good plaster surface.
Use real plaster if you can get it. If not settle for StuctoLite or Gypsolite.

 

Yes, pretty much. The "top of studs" photo is from a different wall, but I know two walls in the apartment (which I own) look like that and I'm assuming the one I'm currently working on looks like that too. "3/4 cold rolled steel channels" sounds right. They're definitely channels of about that size.

My plan was to break off the scratch coat and expose the lath that will go against the wood. I can use thicker wood that way and it will lay pretty flat. I'm buying a set of hooks today and maybe a narrower chisel than what I've been using so I don't warp the lath too much. I try to chisel at an angle, not perpendicular to the lath. It's labor intensive but so far I've had the patience.

 

Now I'm leaning toward a non-destructive method so I don't have to cut the existing lath.

 

If you go this route use multiple ties learn to make a saddle tie and use that. How are you going to get the wire around the channels which are embedded in plaster?
I thnk if I were doing this I would try to use number twelve wire but might settle for number fourteen in order to get it twisted tightly. You want ths to be strong.
There is nothing inherently wrong with using wire to hold thInge, after all it holds up walls and ceilings, just usa plenty and draw it up tight. You might break some ties. That would be a sign you are getting it tight enough. Use two when you think one is enough.

 

Yes, this was a situation. I am breaking away the plaster from the lath where the wood will go but I can't squeeze wire behind the channel for the bathroom wall because there's another channel directly behind it for the hallway wall, and that channel is partially embedded in plaster. My options are to drill a hole through the channel or to break away some plaster from the hallway wall and pull part of the lath out of the way. And I'll probably go with 14 gauge galvanized wire if I can find it.

 

If you can find it galvanized annealed wire is better than hardened or non annealed. It can take a little more twist.

BTW, for everything but tying lath to the channels 18 ga wire is doubled. That is for tying to hanger channels and other parts.

 

I'll definitely get annealed (I have "soft annealed" in my notes) and I'll use 12 or 14 gauge wire around the studs and 18 for lath-to-lath. I'd like to use 3.4 lbs/sq yd lath, which is another challenge to find. I'd order the 3.4 lath sight unseen but if I get 2.5 I have to see it in person to make sure it's a match.

 

I couldn't make 12 gauge hanger wire tight enough without breaking it. I ordered 16 gauge wire for my next attempt (I thought is was thicker but I was reading an electrical wire gauge chart by accident). A band clamp may work too. They seem strong:

 

Use 14 or 16 and use more of them. The trick to drawing wire tight without breaking it is to pre bend it so it lays flat where ever it should then when you twist it get a hold with the nippers and rock them one way and twist. Little then the others way and twist a little. Each time you do th should Re prying with the nippers and stretching the wire so it keeps twisting on new wire rather than twisting about the same section. Hanger wire might not be soft enough. Black wire is softer than galvaniiZed and will work for what you want to do. It should stay dry and not rust.
I once drove about 800 miles on the Alaska highs way with some no. 10 black form wire holding the axle of a trailer together with a splint. The adage of a farmer fixing things with bailing wire is notar fetched. 18 ga soft galvanized tie wire comes in 25# hanks in 28" or 36" lengths. I use a lot more for things other than lathing than for lathing itself.