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Old plaster tends to break off the wooden lath easily, which makes installation much more difficult. If possible, mount electrical boxes next to a joist for a sturdier support. Be prepared to repair the plaster after installation. When working with lath and plaster, ask someone to help you by standing on the other side of the wall so you can both guide the wires. If there are horizontal fire blocks between studs, you have two options: Use a flex bit to drill through the center of the fire block. Or cut through the drywall at the fire block, and chisel out a notch -inch wide x 1-inch deep (1.9 x 2.5 cm). Cover the notch with a metal nail plate after pulling the cable through. If the wall contains insulation, try to fish the wire between the outer paper or plastic cover and the wall. If there is no cover, press the wire against a stud and use it as a guide. While handling fiberglass insulation, wear gloves, eye protection, and a mask that covers your mouth and nose. If there is no way to avoid fishing your wire horizontally, you will likely need to cut through the drywall. A small cut with a penknife is usually enough to let you guide the wire through. In most cases, you will also need to drill through a wall stud as described below. If there is no open path for the wire, you may need to drill through wall studs or ceiling joists. Follow these guidelines to avoid causing structural damage: Wall stud: drill through the center of the stud, using a maximum diameter of 60% of the stud's width (40% for load-bearing walls). Make sure you're at least 1.25 inches (3.2cm) from the edge of the stud. Ceiling joist: center the hole vertically, not through the top or bottom 3" (5 cm). Avoid the ends of the joist as well as the middle third. The maximum diameter is of the actual depth of the joist (not the depth as labeled). Vital support structures: Never drill through "glue lams" (laminated support beams), or through supports above doors, windows, or arches. Wiring through unprotected holes can allow a fire to spread rapidly between the floors of your house. If you drilled through a fire block in your wall, or through fire-resistant flooring material, seal up the holes with fire-resistant caulk or a similar product.
Approach lath and plaster construction with caution. Drill through fire blocks. Thread wires past insulation. Cut through drywall for horizontal fishing. Drill through studs or joists as a last resort. Seal holes in fire blocks with fire-resistant caulk (recommended).