Plunge cut in place cement board?

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Old 11-13-13, 01:43 PM
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Plunge cut in place cement board?

I have to access the wall cavity behind a tub/shower to repair a valve leak.

The bottom side is of course the tub, the top portion is all 12"x12" porcelain tiled. No access from the other side of the wall as it's my neighbor's space.

I do not have any spare tiles, previous owner did not leave any spare nor remember where her tile man bought them.

So, my plan is to use an oscillating tool with a grout removal blade, to carefully cut all the way around a piece of the 12"x12" tile, then cut the supporting wall board behind it the same way, and remove the whole thing in one piece without damaging the tile. Make my repair, then I will have to glue some backing strips around the perimeter of the square hole, then glue the piece of tiled wall back on and regrout.

The only issue I see is, I am not quite sure when cutting the piece out, once I make it through the grout around the tiles, how do I cut the cement board behind it? I don't think the grout blade would work, even if it could, it's not deep enough to cut the total thickness of tile PLUS cement board..

Is there a way to plunge cut the cement board with a special blade? Somehow I have a feeling even if one is available, it will take forever.

I have a 4.5" angle grinder, may be a diamond blade will cut most of it but when it gets to the corner I don't want to overcut. How about a sawzall with a carbide blade will that work if I start a slot with the ginder?

Trying to figure out the best way to cut the cement board with minimum dust and quickest. If that is possible.
 
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Old 11-13-13, 02:02 PM
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Cement board can be cut with about anything at the expense of the blade. A carbide fine toothed blade in a cordless circular saw will do it but the blade will be toast when you're done but it will cut it. You can also use a abrasive blade in a circular saw. You can also use a reciprocating saw. You might need a couple standard steel toothed blades but they also have carborundum abrasive blades that will last longer. Whatever you pick wear a dust mask because there will be a lot of dust.

I am more concerned with how you will waterproof the wall after removing the piece. Grout is not waterproof so removing that section will open the wall up to water intrusion.



 
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Old 11-13-13, 03:06 PM
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Don't make your cut too deep, as any plunge cut will put you precariously close to the horizontal plumbing pipes, so be careful. Switch to a regular wood cutting blade to penetrate the cbu. Kiss it first as it will be toast when you are through.
 
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Old 11-13-13, 05:34 PM
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Pilot Dane, if I follow you correctly, the current tile installation done by the previous owner, is this:

Cement board over wood studs.
Tiles set over cement board looks to me using thinset.

There isn't a waterproofing membrane like a Schluter membrane or a painted on Aquashield, I guess the thinset IS the waterproofing in this case as the grout nor the cement board are both porous. Now this is a tub so these walls are rarely wet unless someone is splashing water.

Not sure what I can do to waterproof it better.
 
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Old 11-13-13, 06:16 PM
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Plunge cut in place cement board?

I have to access the wall cavity behind a tub/shower to repair a valve leak.
Where exactly is the leak, a valve leak is different from a plumbing leak. The valve should be centrally located and visible through the opening after removing the shower trim kit. They make over sized replacement covers to conceal damage to tile on a retro fix.
 
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Old 11-13-13, 10:33 PM
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The leak is "near" the valve, one of the inlets from the side. The inlet is FIP and has threaded into it a male adapter which is sweated to a short piece of copper, it is the male adapter to copper connection that seem to have a pin hole in it.

The wall tile opening only exposes the valve "cover" plate foot print, there is no way to get to the fittings leading to it, let alone trying to solder.
 
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