How to cut plaster and lath - carefully

[Well Respected Man]

I saw in another thread a recommendation to be careful when cutting the plaster and lath, so that the plaster doesn't crack. After watching a few videos, this is my plan to open up a portion of a wall and/or ceiling.

 - Use blue painter's tape to cover the lines to be cut. This give the plaster a little bit of protection from cracking

 - Use an angle grinder with a diamond blade to cut through the plaster.

 - Use a Multimate or similar tool with a wood cutting blade to cut through the lath.

 - Don't use a Sawzall or similar.

Does this sound good? Any other tips? How should I patch the hole after the plumbing repair underneath it is complete?

[Sibley]

The biggest things in cutting is don't use a hammer. Plaster is basically powdered, reconstituted rock. Hammer = bad.

I think the painter's tape is overkill, but it won't hurt. As long as you're not hammering at it, how the plaster responds is based on the overall condition. If your plaster is old and crumbly, then nothing you do is going to overcome that and you're just going to have to repair the damage when you're done.

I don't know the details of the different tools you mention. I can recommend that you treat it more like masonry or ceramic in terms of cutting blades for the plaster. The plaster is intended to be firmly attached to the lath so separate cutting blades may not really be practical. You can try.

As for patching, it depends. If you're able to remove a nice solid chunk of material then you can use it to help fill the hole. If not, then you can use drywall (which is less ideal), or get blueboard and plaster over it. Similar methodologies can be used as drywall (provide structure if needed, put in your patch, attach the patch to structure, fill and smooth around the patch, paint). But you can get plaster patch rather than drywall mud to do it. If you might need future access, just do an access panel.

[Papa bear]

That will work.  In general, I’ve found that removing the plaster isn’t the issue.  I’ve even used the back of a claw hammer to get the general size of the hole. The plaster is usually held together with horse hair for its reinforcement, so it can hold up to some beating.  It’s the lathe that is the issue.  The plaster is held on by the amount that falls in between and behind the lathe, basically holding itself in place.  If you shake that lathe, it will vibrate off the entire horizontal row of plaster.  So, after you expose the lathe, be very careful to make sure it’s either supported at your cut, or you take your time with something like the oscillating multi tool. If it starts to vibrate, back off and see if you can drive a screw in somewhere to hold it right.

As for finishing, you can keep some of the lathe you cut out, or from a different area of demo, and put it back in, provided that you have a stud nearby.  We usually use durabond compound to fill in the rest, as it’s hard like the plaster.  If you put a big piece of drywall in there as your finish, it will show as it doesn’t have the same texture or feel.


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[Well Respected Man]

Thanks for the replies. I picked up a Multimate clone, and will give it a shot tomorrow. I have a feeling that tool will come in handy over the years, and it came with a free extra battery, which is always nice.

[sonofsven]

The grinder will work. I've used a regular skil saw to cut through everything. The plaster will dull the blade so use an old crappy one.
Either method will create a lot of dust so plan accordingly.

[NaN]

Angle grinder with diamond bit. Just make a line and try to get as deep as you can to get the lath. Plaster will peal off in area cut, but never pull on lath. I just used a pair of wire snips yo cut the lath if the angle grinder did not get it

[JLee]

Angle grinders with diamond blades work great but will make more dust than you would even think is possible.  Wear a good mask, and if you have a shopvac I'd set the shopvac up to catch most of the debris coming off the blade.

[jpdx]

How large of a cut? For smaller plaster cuts you can use a drywall cut-out tool or Rotozip with a tile bit. Have a helper follow you with a shop vac to prevent a dust storm.

[Well Respected Man]

OK, so the ceiling on one side of the pipe turned out to be drywall, that will be easy. On the other side, it is plaster over metal mesh over strapping. The plumbers cut out a rectangle and made a couple of small crunched areas, but no long cracks as there might have been with lath. There are two pieces of strapping on the back of the rectangle they cut out, so now I need to figure out how to reattach that piece. Or @PapaBear are you saying to fill in the whole area with durabond? The hole is about 15" x 20", and the rectangle that they removed is relatively intact.

[Papa bear]

If you still have the piece they cut out, see if you can attach it back to the strapping and then finish as normal.  With lathe, I I like to “replaster” the repair, rather than drywall and tape. With those repairs I use durabond because I feel that it has a more similar feel to it than the different joint compounds. 


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