I'm trying to save 100+ year old plaster ceilings. The K&T wiring has to be replaced, but I don't want to damage the plaster at all, it's in really good shape. Is it at all possible or permissible to mount ceiling light boxes NOT flush with the interior plaster walls, but rather attach them to joists and rest them on top of the attic lath? I would like to just get longer screws for attaching any fixture to the new work boxes through the plaster and lath. I'm worried that enlarging the plaster holes for a full 4" ceiling box will damage the ceilings. Currently, the lighting is just screwed into the lath and plaster (with drywall screws) and the hole is only big enough for the old wires to come through. There are no boxes currently.

 

No......that would be completely improper.

The junction box is there to protect the wiring from shorts and physical harm as well.
In the event of a fixture short......the short is contained within the box.

The light should be as close as possible to the front of the box.

Work carefully installing the boxes and patch any oops with plaster.


I've seen that a lot in older houses. I've also seen fixtures fall when the lathe split and the screws came out. Now is a good time to remount them on boxes.

 

Like PJmax mentioned the electrical boxes can be recessed if the work is done carefully. Plaster is very hard on traditional saw blades but my favorite are carbide abrasive blades for a reciprocating saw. Their grit has decent life cutting plaster and since there are no teeth it does not "bite" as bad as a traditional toothed blade. It's more like grinding it's way through than sawing. You can also use a thin carborundum disk in a angle grinder or circular saw. Wear safety glasses and a very good dust mask or respirator because it will produce a very fine dust that hangs in the air like smoke and gets everywhere.

 

I sometimes make an effort to keep a plaster ceiling in place if it has decorative molding. If it's just a plain, flat ceiling, not so much. But, your call.

Are you sure you have K&T wiring? If so, how are you getting the replacement wiring to the ceiling outlets? From above?

You can mount a 4" or a 3" "pancake" box directly to the bottom of the joist, and bring the wiring into it through a standard cable clamp. Be sure to pull a couple of diagonal lines to find the center of the room, if that is where the outlet is. Install a fan-rated pancake or saddle box if you're planning on hanging a ceiling fan there.

Plaster will destroy an ordinary saw blade in nothing flat. I've had the best results when I mounted a diamond or carbide-tipped blade in my jig saw. Using the jig saw rather than my reciprocating saw gives me better control and allows me to keep pressure on the ceiling to stabilize the plaster and lath and reduce shaking.

Goggles, a respirator and a do-rag are the minimum PPE for this job, IMO.

 

A carbide hole saw in at least a half inch drill may do it for round pancake boxes. I recommend 4" boxes. I have run into fixtures that weren't easy to mount on 3" boxes.

 

I use Really Big hole saws all the time for cutting the openings for recessed lights. Thar said, I would not try using one for this job. Why?

I don't know about you, but I can't get a hole saw to do what I want it to unless it has a pilot bit. That works really well when cutting into a finished ceiling in the space between joists, but not so well when the hole is centered on a joist and going through an existing opening. You'd have to either extend the pilot bit out far enough to sink into the joist before the rim of the saw touched the plaster, and then live with the hole in the joist that the bit would make, or try to hit the mark without the bit engaged in anything that would stabilize it. Not my cuppa, thanks.

 

I drill a hole in a scrap of wood the size of the hole saw bit to act as a guide. Either screw the block in place, stick it with Command adhesive strips or have a helper hold it until the hole saw has cut a groove to keep it stable.

 

No. Just to be clear, the face of the mounted box needs to be flush with the surface of the ceiling or recessed by no more than 1/8". It should not stand proud of the ceiling at all.

 

Nice trick, PD. I'll keep that one in mind.

 

RotoZip makes a plaster blade that really works great. It is similar to a rotary drill saw bit only more aggressive. The higher speeds of the rotozip allow it to shave instead of cut the plaster and prevents blow-outs. It does, however, create an unruly amount of dust in the process. It looks similar to their ceramic tile bits but more aggressive.

Once the hole is secured, you can install a ceiling box that is attached to expanding arms. The arms work on a screw/turnbuckle type system that allows you to put it through the hole in the ceiling, corkscrew it out to bite into the joists and then hang the box from that.

If you have a little oops, you can cover the box area with a surface medallion which will probably fit in well with the era and age of the building.

 

A good way to control dust is to have a helper with a steady hand and a good shop vac. The helper keeps the vac a couple of inches from the cutter as it is used moving as necessary to get the most dust.

While more messy a helper with a spray bottle of water misting the area can help to.

 

I thought the light location in this case was directly under a joist. I just read back to the OP and I may have misunderstood that.