Clean cut hole in plaster

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Clean cut hole in plaster

Postby Bryce on Thu Jul 18, 2002 1:56 pm

I'm installing a whole house fan in my old house and I need to cut a big hole in my plaster ceiling. What's the best way to cut a clean hole and keep from damaging the surrounding plaster?

I've used a Rotozip tool with a masonry bit for small holes but I don't think that will cut it for a hole this big. I've also thought about a circular saw with a masonry wheel, but I'm not sure how easy that will be working above my head! Any ideas?


Re: Clean cut hole in plaster

Postby Chris on Thu Jul 18, 2002 3:53 pm

Trace the hole out on the ceiling with a pencil. Score the line with one of those knives used to score cement backerboard (you can also use a utility knife for this, but keep the blade short so it doesn't snap off). Then, go around the inside of the scored line with a nail set and a hammer, gently creating a "perforation". The plaster should pop out (or in your case, fall out) once you do this with a minimum of damage to the surrounding area.

Attic Hatchway Story

Postby Allen on Thu Jul 18, 2002 4:04 pm

I had to go through the same experience in the hallway of the upstairs of our 1924 house when we installed an attic hatch staircase a couple of years ago. I employ the same method when I have cut the rest of the holes in my plaster walls from outlet holes to big square holes for new ventilation and a medicine cabinet.

I don't know of an absolute PERFECT way of cutting into old plaster without breaking away surrounding plaster. I had a heck of a time with the medicine cabinet and broke away so much surround plaster that I had to install pieces of drywall to fill in the gaps.

However, that was just that one case. Everybody's house is different and all old plaster is not going to behave the same way.

So the way I cut into plaster is to place masking tape over the area I wish to cut my hole (assuming it is going to be a big hole) that will have to be rectangular to admit a box. I draw lines on the masking tape where I wish to make the cut. I cut into the lines with a mat knife as deep as I can and remove the top portion of plaster in a hopefully neat square. I then chip out the rest of the hole with the chipping hammer and cut the lath out with the Rotozip, using the wood blade, not the plaster/lath blade.

When we installed the attic hatchway, I went into the attic and found the old wiring which was in the way of the cut. We turned off the power and cut the wire and spliced in new wire and routed it away from the cut. We first attempted making the cut-out in the ceiling using the Rotozip with the plaster/lath blade, but that the blade wasn't deep enough to cut the lath, and it started burning anyway. We resorted to chipping out all the plaster along the cut line and then, while standing on a ladder, cut through the lath using a circular saw. Actually, my wife's uncle did that part. I just held the end of the piece that was being cut out.

The big piece came off in one big dusty crash, and there was a huge dust storm upstairs, which was like fog. Before we could continue, we had to wait for the dust to settle. Then we had to remove part of a floor joist. Then we installed the hatchway.

There wasn't severe damage to the surrounding plaster, and putting door trim around the hatchway hid the plaster cut. All in all, it was a difficult and messy job, but it turned out fine.

If you apply similar principles to how you install the whole-house fan, the surround plaster should be OK. I have thought about installing a whole-house fan in our house, but decide not to after recalling the attic hatchway.

Re: Clean cut hole in plaster

Postby Doug Seibert on Thu Jul 18, 2002 5:45 pm

Bryce.................I disagree with the previous posts............I use a circular saw and a carbide tipped blade.

I would frame out the opening from above and drill holes thru the plaster at the inside corners of the framing. I connect those marks with a solid line I can see even if its dusty.

Using the circular on the ceiling takes some practice but produces a clean cut. Wear eye goggles and dust mask, wrap a towel around your head. Cut the ends parallel to the lath FIRST.....that way the plaster won't break out farther than you want......HERE's the trick....Screw a scrap of wood across the 'hole' crossing those saw cuts to support the cutout.....Next cut the the cross lath edges to free the opening.

It's dusty and hard on the saw......I use a special 'beat-up' saw just for these cuts.......but it cuts a quick, clean hole.

I've had whole house fans in every house I've owned........the best performance came when the fan was mounted vertically (upright) and a plenum used to direct the air to and from the fan.
Doug Seibert

Re: Clean cut hole in plaster

Postby Bryce on Thu Jul 18, 2002 5:57 pm

All these responses sound great. Thanks for all your help. I'm not looking forward to the messy job, but it will be worth it to cool down my house at night.

Just one question about the last response... Why would you choose to use a carbide tipped blade instead of a masonry blade? Is it just so you can go through the lath and plaster at the same time? I was just thinking of cutting through the half inch of plaster with the masonry blade and then taking my jigsaw to the lath.


Re: Clean cut hole in plaster

Postby Doug Seibert on Thu Jul 18, 2002 10:27 pm

The jigsaw with its's in and out stroke seems to shake the lath so much it loses it's 'key' to the plaster..........the carbide has no problem with the plaster and lath..........doug
Doug Seibert

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