I'm planning on removing about half of the ceiling in the living room in my 1930's home to work on some electrical and install recessed lighting. The existing ceiling is plaster coated over gypsum lath board. A test cut shows that the gypsum base is about 5/16" thick and the plaster topcoat is about 5/16" thick.

Since I don't want to demo the whole ceiling, I'm planning on cutting a straight line into the ceiling, and then I'll install new 5/8" dryawall butting up to the part I cut out. I have to make a 20' cut in a straight line to accomplish this.

Are there any recommendations on how to make this cut? I thought a circular saw with a diamond masonry blade might work at cutting through the plaster, and I can finish the cut through the gypsum with a drywall hand knife. However, not sure how safe it would be to use an inverted circular saw. Any thoughts?

 

Welcome to the forums.

Wouldn't be perfectly straight in all likelihood but my first thought would be my sawzall.

 

I'm cutting the line perpendicular to the rafters so my concern would be hitting those with the sawzall.

With the circular saw I could control depth of cut and could minimize vibrations and hopefully keep the plaster intact.

 

Angle grinder with a diamond blade. Tack on a straight edge, (1/2" plywood perhaps) which can help guide your cut. Have someone hold the vac. hose right behind the blade as you go and you will get most of the dust. This will cut the plaster and gyp. lath in one operation and will be easier to handle than a circ. saw and give better cut than sawzall.

 

If you care about dust, use a sawzall, along with someone holding the shop vac.. It makes way less dust than either a skilsaw or grinder.

Anyone who knows how to hold a sawzall can follow a chalk line and can hold the saw at the right angle to only cut as deep as the plaster is.

 

No way would I be using a sawsall on plaster.
#1, Plan on a ton of blades, a few inches and the teeth will be shot.
#2, Almost no control on depth or width of cut.
#3, The vibration is going to knock the plaster loose.
Why are you doing this work from below the ceiling and not from above?

 

Keep in mind the op said it is 5/16" plaster.... and gypsum lathe, not wood lathe. Nothing is going to "shake loose" when there is gypsum lathe under the plaster.

I would do this with one blade. Maybe two if I felt generous that day.

 

Keep in mind the op said it is 5/16" plaster.... and gypsum lathe, not wood lathe. Nothing is going to "shake loose" when there is gypsum lathe under the plaster.

I would do this with one blade. Maybe two if I felt generous that day.




You might cut gypsolite basecoat but sand aggregate basecoat is not nearly as forgiving on blades.

 

The op did not say what type of plaster it is. Either way, it sounds thin. Grinder will certainly work... its just going to make a lot of dust. Vac will get most but not all. Milwaukee Axe sawzall blades last longer than most..I'd suggest 6 tpi.

 

Remember the safety glasses, dust mask, and gloves.

 

Any advice for cutting joint where the ceiling meets the wall? I’ve read that some type of metal corner lathe is common in this age of house.

 

That is a "corner-rite", a light gauge expanded metal lath. Even though diamond blades are not used to cut metal, the grinder and diamond blade would handle it with no problem.

 

Next question, are there any issues with having drywall butted up next to plaster in terms of forming a good joint that won't crack?

 

You'll need to tape the joint and make sure the edges of both materials are well fastened.

Paper tape is stronger than mesh tape, BTW.

 

In many cases its best to cut 2 or 3" away from a corner. Unless you really need to tear it off all the way to the corner, staying away allows you to flat tape the joint... rather than have to coat BOTH sides of a corner unnecessarily.

Again, sawzalls tear through plaster with metal mesh easily... grinders work too but are way dustier. You will want to prefill the joint between plaster and drywall with setting compound... and let that dry before you tape.