Old House Wiring: Installing An Old Work Box In An Extra Thick Wall... A "Behind The Scenes" Look

By Bruce W. Maki, Editor

When adding or changing wiring in an existing house, there are two fundamental tasks: running cable and installing junction boxes. If a wall surface is being removed, then a junction box (or j-box) can be of the new construction type, which are typically nailed onto the side of a wood stud. But when a new j-box is placed in an existing wall, it's normally best to use a "remodel" box or "old-work" box.

There are several designs of old-work boxes available. One of the more popular boxes uses two plastic tabs to grab the wallboard. These work great... as long as the wall material isn't too thick.

But people who have worked on old houses know that wall surfaces can sometimes be much thicker than the standard ½" drywall commonly found in newer houses.

In the 1907 house that I live in, we have recently been tearing out some old plaster and replacing it with drywall. In the process we have made some minor changes to the wiring, such as adding a few closet lights.

When we cut a door opening for a new closet I realized that I had an opportunity to illustrate what happens behind the wall when an old work junction box is installed.

The newly cut doorway. Behind the wall there is no plaster, lath or drywall.

To lay out the hole for the junction box, I marked the wall at 42 inches above the floor. This represents the bottom of the j-box.

This 42" height above the floor is not a rule, just a commonly used practice.

I made marks at 4" and 6¼". This 2"x4" junction box is actually 2¼" wide. Since the new door was going to have 3½" wide casing, I wanted to keep the switch box from interfering with the trim.

This is the back side of the wall being cut. When we remodeled that room 5 years ago we left the lath on and screwed "blueboard" over it. Then we had a plasterer apply a 2-coat veneer plaster.

There are pros and cons to leaving the lath on the wall, but I no longer leave it on unless there is a good reason.

I placed my custom old-work box template on the marks. Actually I screwed up this template and made it too wide, but it will suffice for now.

I drilled holes at each corner of the cutout. With simpler rectangular holes I can usually get away with drilling only two starter holes (at diagonally opposite corners) but this cut-out requires that I drill four holes.

To prevent the metal base from marking the painted wall I applied some duct tape to the bottom of my jig saw.

Using a carbide plaster-cutting blade I slowly made the first cut.

The view from behind. This is what can happen to lath when plaster is cut... the boards will flop around if there is only one nail holding the short end. When the blade is nearly done cutting through a piece of lath it often starts to shake violently.

Since I knew I could never make a second cut on such a loose piece of wood, I yanked it out. That one little nail didn't put up much resistance.

At this point I realized I had made a mistake... the first cuts should have been the horizontal cuts, because they don't cut across the lath.

Next I made the horizontal cuts.

The piece of lath that I'm holding is quite badly split, not all of the splitting was caused by my first cut. This is a common problem when cutting into plaster in old houses... you cannot tell the condition of the wood lath behind the plaster, and when you try cutting the plaster some of the boards will vibrate and move, damaging the already fragile plaster.

This is the type of problem that has driven my attitude towards old plaster. It's so brittle and weak that I no longer favor any attempt to save old plaster. I simply advocate replacing plaster with something more durable, like veneer plaster on blueboard.

After making the horizontal cuts I made the final vertical cut.

This is an old work box by Carlon. It costs about $1.85 at Home Depot.

There are two little blue wings or tabs that flip up when the screw is turned, and then the screw tightens the tab against the back of the wallboard to clamp the j-box to the wall. It all works great... unless the wall is abnormally thick. Which happens all the time in old houses.

Plaster on wood lath is supposed to be 3/4" thick (3/8" plaster on 3/8" lath), but I have seen some big variations in thickness. I've seen plaster and lath that was well over an inch thick.

I pushed the old work junction box into the hole to test the fit.

The box after being pushed into the hole. Note how two of the plastic corner flanges have plenty of overlap on the wall surface.

One problem with cutting a hole for a remodel box is that if the hole is made too big there is not enough surface for these flanges to hold onto.

Yet, it can be a real chore to get one of these boxes to fit in the hole, so it's tempting to cut the hole kinda oversize.

The Problem: With the wing-tab in the position as-purchased, the wing can't turn because the wall is too deep This is still a problem with a conventional old house plaster wall, which is normally about 3/4" thick.

I unscrewed the clamp screw about 3 turns.

This can be quirky... the screw may back out and the wing stay in place. You can see the screw head closer to the surface of the j-box. When this happens I have to push the screw back in its hole, or else the wing won't turn when I try to tighten it.

Beware: if you back off the screw too much, the little plastic wing can fall off the screw and fall down inside the wall. If the j-box is way up high on the wall (like a switch box would be) then there is almost no chance that you can rescue the little plastic wing. It's a good idea to buy extra old-work boxes in case you goof one up.

Now the wing can rotate.

But... there is also a chance that the wing-tab rotates beyond its little stop bumper. You may or may not be able to tighten the wing when this happens. What I have noticed is that it just doesn't feel right when I'm turning the screwdriver... because the wing tries to rotate against the corner of the j-box, and the screw tilts away from the box.

These are all things that can happen behind the wall while you are trying to install an old work box. If the hole was cut for a tight fit you can't see much happening. If the hole is oversize you might be able to see the wing turning. Shining a small flashlight in there certainly helps.

My advice is just to be patient and take your time. I recall taking well over an hour to install an old work box in one cantankerous old house, a process that can be done in two minutes with ordinary drywall and no other obstructions.

Of course, I've been just faking this installation the whole time, to illustrate the problems that can arise when doing electrical work on old houses. When installing a remodel box you would first be threading the cable into the box and then inserting the box into the hole. But my example describes what would happen if a person was just test-fitting the box, something I often do.

With the wire in place, the back view of an old work box would look like this.

With remodel electrical work the cable cannot be fastened to the studs, so it just dangles inside the wall cavity. That is not a problem in itself... the problem arises when some turkey starts pulling on that wire from up in the attic. Consequently remodel boxes have spring-clips that prevent the cable from being pulled back out of the box.

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Written February 12, 2005