Ask the Authority: Installing Receptacles Without Damaging Plaster
Q: I need to install an electrical receptacle in a plaster wall. How can I do this without cracking the plaster?
A: When I have to upgrade old-house wiring, my first choice is to mount the electrical receptacles in the baseboards rather than the wall surfaces. This practice greatly reduces the number of boxes I have to cut into plaster, but doesn't eliminate them.
When I have to place an electrical box in a plaster wall (say for a switch), I mark the cut lines on the wall using a plastic, remodeling-type box as a template. Because this type of box has ears on the corners that overlap the plaster, I use a framing square to complete the outline.
Making a clean cut through lath and plaster is a two-step process; first you remove the plaster, then you cut through the lath. If I'm working in an unoccupied house, I'll cut the plaster using a RotoZip tool outfitted with a plaster-cutting bit (www.rotozip.com). This process requires a steady hand, however, and it also kicks up a tremendous cloud of dust.
When I need to work dust-free, I cut the plaster with a simple utility knife. To cut cleanly without wandering, the blade must be razor sharp. But rock-hard plaster will quickly dull a knife blade, so I keep a pocket-sized sharpening stone in my tool belt and hone the blade often.
The hardest part of cutting plaster with a knife is scoring the face. I use a small framing square as a straightedge and apply minimal pressure until I've formed a groove. Once I break through the hard, finish coat of plaster, the softer base coats give way easily. Then it's a simple matter of prying up the rectangle of plaster to expose the strips of lath.
To prevent the lath from shaking and cracking the plaster, I clamp it with adjustable pliers and cut it with a jigsaw that's outfitted with a narrow, fine-tooth, blade.
I use a saw for the vertical cuts only. If I have to rip through a lath strip on the top or bottom of the cutout, I'll make a series of shallow passes with a sharp utility knife.
After the hole is finished, I test fit the box before inserting the wires. Since the ears on a typical remodeling box do not afford much wiggle room, I always cut the holes tight and expect to have to do a bit of fine-tuning. If I have to carve away some plaster, I use a Painter's 5-in-1 tool; if the lath is in the way, I trim it back using a detail sander such as the Fein Multimaster (www.feintools.us).
Although the jigsaw makes quick work of the vertical cuts, a sharp utility knife is more effective (and less likely to cause cracking) for horizontal cuts.
After verifying that the box will fit, the wires are inserted and the box is installed. Because lath and plaster walls are thicker than standard drywall, make sure that you've got about an inch of space between the ears on the front of the box and the swivel supports on the sides before you insert the box.
Tom O'Brien is a veteran restoration carpenter who writes frequently about construction practices and old houses.