USING CUT-IN TYPE BOXES IN LATH & PLASTER WALLS
The box shown above came out of a wall in a home built in the 1940's, a wall which consisted of lath and plaster. Lath and plaster in the era that this home was built consisted of pieces of redwood about 1/4" x 1 1/4" mounted horizontally on the face of the studs. These laths were placed with a small gap of about 1/8" between them giving the plaster something to hold on to. On the left side of the box you can see a mounting plate where the box was nailed to a stud. On the right side of the box you can see a channel in which the lath pieces gained support. Without this support the plaster would easily crack if pressure was applied to the wall at that area.
Sometimes a customer will ask that a new receptacle be installed in a lath and plaster wall using what is called a 'cut-in' style box. This is a box which has little flip up ears that will act to hold the box in place once installed in the wall. The problem here is that once you cut the laths- to make the hole where the new box will be installed- they no longer have support and consequently the plaster tends to crack when the new receptacle is used- someone tries to plug something in.