Plasterboard is a relatively soft material that requires special hardware for hanging a heavy mirror. If the mirror hardware cannot be installed into studs, conventional screws will pull out of the plasterboard and require patching and likely damage the mirror when it falls off the wall. Special fasteners, called "hollow wall anchors", are an option when hanging a heavy mirror on plasterboard.
Hold the mirror in position where you want it to hang and lightly mark the location of the top and sides on the wall with a pencil.
Place a sheet of large construction paper on the back of the mirror. Align the top edge of the construction paper with the top edge of the mirror and one side of the construction paper with one side of the mirror.
Rub the back of the construction paper to find the location of the mounting hardware on the back of the mirror. Use the point of a pencil to poke a hole where the screw holes in the mounting hardware are located.
Place one piece of tape on the top of the construction paper and tape to the wall at the location for the top of the mirror. Place the side that was aligned with the side of the mirror located at the mark on the wall for the corresponding side of the mirror.
Position a level on the top of the construction paper and adjust the construction paper until the edge is level. Adjust the position of the construction paper on the top and sides, moving the tape as necessary. If the hanging hardware on the mirror is located at the same point on each side of the mirror, place the level across the opposing mounting hardware holes and check that the holes are level with each other. Adjust the construction paper as needed and tape securely to the wall.
Mark the location for the hollow wall anchors or screws on the wall with a pencil, marking the wall through the holes in the construction paper. Remove the construction paper from the wall.
Position a large finishing nail on each mark and drive it into the wall halfway. If the finishing nail hits a solid surface, use a conventional screw that is at least 3 cm (1 1/2 inches) long for that location since the screw will be going into a stud. If the finishing nail sinks into the wall easily, a hollow wall anchor is required to hold the heavy mirror securely on the plasterboard.
Use a screwdriver, or drill with a screwdriver bit, to install screws into studs. Use a hammer to tap a hollow wall anchor into areas where there is not a stud and use a screwdriver, or drill with screwdriver bit, to turn the screw and tighten the hollow wall anchor. Leave enough of the head of the screw protruding from the wall to hang the heavy mirror onto the screw heads.
Repeat step 8 for the location of each screw for the mounting hardware. Then, hang the mirror onto the screws.
A paper template reduces errors made while transferring measurements to the wall. Always use a flat piece of paper for a template. Creases in paper, such as a flattened paper bag, may distort the location for the screw holes. An electronic device called a "stud finder" can be used to locate studs in the wall. If the hanging hardware on the back of the mirror is 40 cm (16 inches) apart, it is a good idea to hang the heavy mirror on screws secured into framing studs.
Pay attention to the possible location of electrical lines and plumbing when installing screws or hollow wall anchors into the wall. Electrical lines may be located directly above outlets and plumbing lines may be located below plumbing fixtures located on the floor above.