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If you live in a house that was built in the 1940s or before, your walls are probably made of plaster and lath. If your house was built more recently, however, your walls are likely made of drywall. It is important to find out what your wall is made of before you hang your mirror because using a drywall hanger on plaster can damage a plaster and lath wall's surface. Plaster and lath is harder, thicker, and more brittle than drywall. If you're unsure what type of wall you have, try to push a pushpin into the wall. A pushpin will go right into drywall but not plaster. All fasteners have a maximum weight rating. Some fasteners are designed to hold heavy mirrors and picture frames and some are not. You can use a bathroom scale to weigh your mirror. If you're using a bathroom scale, weigh yourself first and then weigh yourself as you're holding the mirror. The difference will be the weight of your mirror. If you're hanging a heavier mirror, you'll want to find a stud in your wall to support it. If you have a wall made of drywall, you can use a stud finder to locate a stud in your wall. A stud finder, however, won't work for a plaster and lath wall. To find a stud behind a plaster and lath wall, tie some string around a strong magnet. Then, holding the top end of the string, slowly move the magnet horizontally across the wall. The magnet should stick to the wall when it moves across a stud. The magnet you use will have to be strong, so simple magnets from your refrigerator probably won't work. Lift your mirror up on your wall where you want it to hang. Then, use a pencil to make a short line where the center of the top edge of the mirror is on the wall. If you're hanging a heavier mirror, have someone help you get the mirror into position. Many different fasteners exist, and they all have different maximum weight ratings, or the maximum weight they can hold up. These include screw-in drywall anchors, expanding plastic sleeves, picture frame hangers and nails, tap-in expanding anchors, toggle bolts, and anchor wire (monkey hooks). Hardware weight limits should be listed on the packages they come in. When hanging a mirror with wire, it is a good idea to use two fasteners instead of just one. Using two will prevent your mirror from moving in place and becoming lopsided. Using two fasteners will also reduce the likelihood that your mirror will fall. Take a small piece of wood approximately 12 inches (30cm) long and mark its center. Center it under the top of the mirror and hold it up tight under the wire until the wire is taut. Use a tape measure to measure the distance from the top of the mirror to the top of the piece of wood. Then measure that same distance down from the mark you made on the wall and make a mark. This second mark indicates the future position of your hanging wire. Now, take the piece of wood and align its center with the second mark you made on the wall. Use a torpedo level to make sure the piece of wood is level. Then, use a pencil and mark on the wall the location of the upper right- and left-hand corners of the piece of wood. These marks will be where you put in your hooks.
Determine what type of material your wall is made of. Weigh your mirror so you know what type of fastener you'll need. Find a stud in your wall if you're hanging a heavy mirror. Mark where you want the top edge of the mirror on the wall. Choose a fastener with a weight rating above your mirror's weight. Prevent the mirror from going askew by using two fasteners. Determine the position of the two fasteners with a block of wood. Make sure the mirror will be perfectly level with torpedo level.