Subscribe or Renew Membership Subscribe Renew

Clean cut plaster on expand. metal lath?

Mike_D's picture

What cuts expanded metal lath and plaster without leaving a big broken ragged edge behind?

Our kitchen demo team got a little rambunctious removing one soffit in our kitchen remodel and left a ragged-jagged broken edge behind.

Now I need to straighten that edge so that I can repair the gap with drywall and plaster that disappears when painted.

Is there a way to cut the existing plaster/metal lath combo that won't simply repeat the mess?

Mike D

(post #109022, reply #1 of 24)

A mini grinder would work, but that would be one ugly jobsite afterwards!

I'd screw a 1x to the part you want to save, right at the cut line. If the surface of the plaster is uneven, put a strip of foam sill seal under the 1x.

Then go at it slowly with a fine-toothed metal blade in a Sawzall. Use a long blade so that you can keep your cut angle low, as far off of perpendicular as you can.

Good luck!


(post #109022, reply #3 of 24)

If it's only a soffit, I'd use a small hand held chipping hammer. This is a relatively small hammer with a spring on the handle to help absorb shock. The head has two ends, one is a spike, the other is a small chisel shape. Start chipping with the spike end, chip over a stud so the area where you're chipping has backing and the plaster won't bust apart where you don't want it to. Use small gentle strokes, kind of poking at the plaster to establish your straight line and keep going over it. This all adds up rather quickly and before you know it, you will have your line of demarcation. Use the hammer to gently break away enough plaster on the waste side to expose the mesh. Then cut the mesh with a tin snip. At first you'll have to cut one little rib at a time, but soon you'll probably be able to take bigger bites with the tin snip. If you really, really have to use an electric tool, I'd use a grinder but it will generate a ton of dust. Use a diamond wheel on the plaster. You can even use a metal cutting wheel for the mesh but be aware that will generate sparks.

(post #109022, reply #2 of 24)

You already have a ragged edge, and there is some possibility that you could make it worse rather than better.

If it were me, I'd use a combination of materials to patch new to old. A combination that comes fairly close to the original plaster, built up in layers.

There might hve been a "brown" coat, made from some sort of reinforced rough plaster. That would have been followed by a middle layer (scratch coat) of finer plaster, and a very smooth layer (the skim coat) of finishing lime.

It sounds all difficult and time consuming, but don't forget that you'll probably need to do three coats if all you use is drywall mud.

I won't be laughing at the lies when I'm gone,
And I can't question how or when or why when I'm gone;
I can't live proud enough to die when I'm gone,
So I guess I'll have to do it while I'm here. (Phil Ochs)

. . . I can't live proud enough to die when I'm gone, So I guess I'll have to do it while I'm here. (Phil Ochs)

(post #109022, reply #8 of 24)

Actually the plaster in this house is kind of interesting. It is a mix of "sheet rock" (early gypsum board - looks kind of like hardi board), with an expanded metal lath tacked over it, followed by a plaster overlay/finish. The total is nearly an inch thick in many places. Then, in the kitchen, at some point much later, some enterprising handy man flocked the darn thing. Then it got about 4 coats of paint.

What I plan to do is first to remove the flocking on the rest of the ceiling. I don't look forward to that, but I'm DIY'ing my own kitchen, so I can afford the time.

Then I'll build up the missing (ex-soffit) 18 inches with 5/8" sheet rock, followed by a coat of 20-minute dry mix, followed by a finishing premix skim coat. Since I've zero experience with actual plaster, this seems the closest to my skill set.

I really liked the suggestion to tack a 1x4" straight edge up to where I want to cut a straight line. Then I got to thinking that if I cleaned up the worst of the munched out mess, and then made a pattern from cardboard of it's outline and cut out a matching 5/8" sheetrock replacement baseboard, tacked down the errant metal lath to that, and then "plastered" the result with 20-minute dry mix, I'd be off to a good start of making a "seamless" repair.

I agree that if I were a real plasterer, I'd knock this off in a morning. I wish!

Thanks very much for all the input!

Mike D

(post #109022, reply #9 of 24)

Don't be afraid of plaster. You can find patching plaster in 20-or 25lb bags. I found that it works easier than drywall mud and the resultant surface has the proper hard sound when you knock on it.

When I did a plaster wall repair, I used a scrap of Hardie Backer instead of gyp board as it has a rough texture for better adhesion and is tolerant of spraying with water to prevent its sucking the water out of the plaster before it can be worked properly.

That rough and jagged margin of the tear-out will actually work in your favor whether you patch with plaster or drywall because it will make a sort of keyed joint. A clean-cut joint is more likely to crack and show up.

Your original plasterers may have used the expanded metal lath for the ceilings and soffets in order to get better hold-up while they worked and during cure.


(post #109022, reply #21 of 24)

Humm, patching plaster - I hadn't even considered that. I'll look into that. I don't suppose Lowes or HD carries it do they? Actually, there's a drywall jobber outlet not too far from here, so if they don't I can probably get it there.

How fast does patching plaster set up?

Speaking of stuff I hadn't considered, hardi backer would certainly hold it better - the rock lath had holes in it for that purpose, and of course, drywall does not.

I hadn't considered spraying the drywall to keep it from sucking the moisture out of the plaster, either. Sort of like bricks when you are doing brick and mortar?

Mike D

(post #109022, reply #22 of 24)

I think I got my patching plaster at HD. Ace hardware has it too.

As I remember, it dries in 90 minutes; working time was 60 minutes or more.

"I hadn't considered spraying the drywall to keep it from sucking the moisture out of the plaster, either. Sort of like bricks when you are doing brick and mortar?"
Exactly. Also good to do it for stucco repair.

Edited 10/17/2009 9:43 pm by brucet9


(post #109022, reply #23 of 24)

I think the suggestions that you try to plaster rather than drywall are good. I'm not sure what 'patching plaster' is... Beware of something like 'Fix-It-All'. It sets up rock hard and fast... it's really meant for small holes. There is a product here called 'One Kote'. It can be troweled to a smooth surface but unlike regular finishing plaster, it can be thick, too. So it can all be done in one fell swoop. The drawback is that it, too, may set up on you before you're ready. It all kind of depends on how much area you need to plaster. And it depends on the nature of the original plaster. If, as mentioned earlier, it is gritty then it's probably a sanded base plaster. If you have a lot of area and you've got gritty original base plaster, it would be good to match it with a sanded base plaster. It's easy enough to mix... 2 or 3 sands to 1 plaster... add water slowly so it doesn't become soup. I use a brand of base plaster called 'Red Top'. It will give you a long working time, at least a couple of hours if the weather isn't brutally hot. The beauty of trying to match materials is that they will expand and contract at approximately the same rates, thus reducing the chances of cracking.

A couple of tips: Make sure that you overlap the old lath with new lath by a couple of inches. Make sure the lath isn't flapping around... it has to be nailed stud to stud.

Don't go nuts if you can't find rock lath. Lots of plasterers just put a 'scratch coat' on the mesh itself without any backing behind it. If you really want backing, use some 3/8 drywall, but paint it first with Plaster-Weld. It will keep the drywall from sucking in too much water and it will help adhesion.

Mesh has an up and a down. If you look carefully, each cell is designed to hold the plaster in. Look at the bottom rib. It should bevel in a way that cradles the plaster, keeping the plaster from spilling out. Another way to tell is to run your hand along the mesh from bottom to top. If you feel resistance, then the mesh is oriented correctly. The top rib of each cell is beveled in a way to 'cut' the plaster from the trowel, cleanly depositing the plaster in the cell.

You'll need to pay a little attention to the edge of the old plaster. If you have a compressor, blow off the loose dust. If there's no compressor just use a dry paint brush to clean the dust off. I like to prime the edge with primer. That keeps the edge from sucking water from the new plaster. If the edge sucks too much water, you'll get a crack. I find that just wetting the edge doesn't really solve that. After the primer is dry, then paint the edge with Plaster-Weld and you're good to go.

If you use One Kote or 'patching plaster' you can bring the new surface up flush with the old surface. If you use a sanded base coat, the old surface should be proud of the new by about 1/16 of an inch to allow for the finish plaster which will then make everything flush. In plastering, there's really no such thing as feathering. It's called 'joining', which means the new edge meets with but does not overlap the old edge.

A great reference that's really worth having around is a book called, Plastering Skills by Van Den Branden/Hartsell. Plaster is a wonderfully intricate trade that is sadly becoming extinct, its virtues eclipsed by concerns of time and money. When I can, I practice it. I believe it looks better, creates a better wall, and affirms that time and money isn't always everything.

(post #109022, reply #24 of 24)

Great input! Thanks. I found the book on Amazon and ordered it. Looks like a little more reading before I start.

P.S. Someone asked "how much do you have to do".
I have two 18" wide strips - one 12ft long and the other 10ft long, so about 33 square feet.

Edited 10/18/2009 4:22 pm ET by Mike_D

(post #109022, reply #4 of 24)

If you are going to patch with true plaster the ragged edge means nothing. A good plasterer won’t even question this situation. I would advise using plaster, but if you really want to cut the plaster.  A Roto Zip with a plaster bit and a shop vac held right next to the bit is the only way to go. The bits are available at all the home improvement stores. They are carbide and require a 1/4” ferrule. They run between $15 -$20 and last a very long time.

(post #109022, reply #5 of 24)

Sorry, I forgot to mention this will only cut the plaster. I would leave the metal lathe and go over it.

(post #109022, reply #6 of 24)

angle grinder with diamond wheel, dusty as haddies however gets the job done

(post #109022, reply #7 of 24)


Used originally, a plaster cutting sawzall blade (Milwaukee has (had?) a short, not much set in the teeth.  Whether you could make it work now is possible.  Easy to control the depth of cut by angling the saw head.  Find some and give it a try. 

Lennox made some longer plaster cutting blades, much harder to operate those.

These are not carbide grit blades.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


(post #109022, reply #10 of 24)

If it is cementitious plaster (gritty sand in it) you'll need the angle grinder/diamond wheel which will cut everything (shop vac going full force).   Cementitious plaster will eat pretty much anything else including Sawzall blades.

Don't ask me how I know this ;o)


(post #109022, reply #11 of 24)

I had a short stubby "plaster" sawzall blade that was actually very effective. I don't remember if it left a clean cut on plaster but it was the cat's meow on drywall.

Is anybody out there? 

(post #109022, reply #12 of 24)

Actually, I have practically every "modern" (1950 era) plaster system in that kitchen. Part of the walls from the ceiling down to the tops of doors and windows has a cementitious (grey sandy) base directly over expanded metal lath with a plaster skim coat. Below that is about 3/4" of plaster over the older smaller sheet rock.

The stuff is hard and heavy. Doesn't dent, deadens sound. I like it. I hope to save much of it.


(post #109022, reply #13 of 24)

"older smaller sheet rock" sounds like rock lath.


(post #109022, reply #14 of 24)

"Rock Lath".
Yep, per Google, that is what I have.
Mike D

(post #109022, reply #15 of 24)

Except it's a little odd to have that AND the expanded metal lath.

As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #109022, reply #16 of 24)

Yes it is.

(post #109022, reply #18 of 24)

It's all an education to me, but, yes, it does seem odd to have three different approaches in the same room, all seemingly original. Now when I'm done it'll have 4.

(post #109022, reply #17 of 24)

Angle grinder with continuous rim diamond wheel.  It will cut the lath too.

Tape the round nozzle of your shop vac right next to the guard and have at it.

If you have a variable speed angle grinder it will throw a lot less dust on 3-4,000 rpm vs the 12,000rpm that the regular units do.


(post #109022, reply #19 of 24)

Based on other input, I'm now just trimming off the worst shreds of metal lath with shears and incorporating the rest back into the repair. I'll report on my progress in a day or two when I can use my hand again. I managed to unlatch one of the support legs on my dry-wall bench when I moved it, with predictable results when I got back up on it and started back to work. Rodeo! :)

Not the bench's fault - I hooked it with my foot to move it and didn't notice I'd unlatched it. The now modified latch now requires a determined effort to unlatch it.
Mile D

(post #109022, reply #20 of 24)

I use a hammer and a crummy semi-sharp chisel and go along cutting the mesh back to the edge of the plaster in as straight a line as can be expected.
Butt the new drywall up to the ragged edge, fill in the gap with setting type taping compound slightly below the surface of the drywall, then put on fiberglass tape and setting compound
I've done this a bunch of times over the years, and it works for me.

"The best lack all conviction while the worst are full of passionate intensity"