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Carefully adjust the mantel so that the fireplace is framed evenly, with the mantel extended an equal distance on either side of the hearth. Use the measuring tape to make sure the position is ideal, and follow with the use of a level to make sure the mantelpiece is perfectly horizontal. You want to make sure the mantel is level not only side to side, but also front to back. Use a torpedo level to check the level front to back. Using a piece of chalk or a pencil, make an outline around the edges of the mantel, both along the top and the sides of the fireplace. Once the marking is complete, pull the mantel away from the fireplace and position it face down on a smooth surface. Make a second set of lines, which will serve as the outer edge of the mounting boards, also called cleats.  One way to measure the cleat is to fit it into the back of the mantel like it will be on the wall. Use a measuring tape to measure the length from the top edge of the mantel to the bottom edge of the cleat. Place the measuring tape along the line drawn in step 2, and then use the measurement and draw a new line below the first. For example, if the length from the top of the mantel to the bottom of the cleat is 3", measure 3" down from the line on the wall and draw a second line. Another method to measure the cleat is to measure the size of the inside of the mantel from the top of the shelf to the edge just above where you will place your cleat. Then, measure the length of the side of the cleat you will affix to the wall. Add those totals together. For example, if the measurement from the mantel shelf to the edge is 2" and the length of the cleat is 1-3/8", then your measurement will be 3-3/8". Draw a line under the line on the wall using your measurement. The mounting boards will be affixed to the actual wall surrounding the hearth at strategic points, creating a framework for the mantel. A minimum of 3 mounting boards is required, 1 for the top and one for each side, although additional cleats can be used.  Measure the size of the mounting boards against the new markings on the wall, and use the saw to cut the cleats to size. The top cleat should be one foot shorter than the shelf. Dry-fit the mounting boards in the mantel. The top cleat should be fitted first, then the two legs. They should fit snugly together, but don't have to be fitted perfectly. Make any adjustments to the length of the cleats if you need to. If you are attaching the mantel to dry wall, you will need to attach the cleats to three studs behind the mantel. When you have located the studs, make a mark in the middle of the stud along the cleat line.  Wall studs are meant to support and hold up dry wall on interior walls. When you are hanging heavy objects, such as a mantel, you want to make sure that you hang the object on the studs. The easiest way to find a wall stud is to use a stud finder, which can be purchased from a local home improvement store. Studs are evenly spaced behind walls. In most houses, studs are spaced 16" apart. Studs are usually 1.5" in width. When you attach an object to a stud, you need to attach it in the middle of the stud, which is 3/4" from the edge. Try finding an electrical outlet on the wall. One side of the electrical outlet will be nailed to the stud. To locate which side, you can do the knock test. Using the heel of your hand, gently knock the wall on each side of the outlet. The side without the stud will sound hollow, while the side with the stud won't. After you have determined the side with the stud, measure 3/4" from the side of the electrical outlet. This will be the center of the stud. Using a tape measure, you can mark the studs along the wall every 16". Another way to locate the studs is to look at the baseboards. Baseboards are nailed to studs, so if you find holes or indentations that have been painted over or caulked over, you can measure 16-24" inches from that point to locate additional studs. Hold the cleat against the wall and align the bottom with the second set of drawn lines. The bottom edge of each cleat should line up along the top of the second line. Use the level to make sure the cleats are completely horizontal from left to right across the top and perfectly vertical from top to bottom on the sides.  Drill holes through the cleats and into the wall. Make sure each hole is drilled into the center of the studs you marked earlier. You can also nail the mounting boards to the studs instead of drilling holes. If your wall is brick, make sure that you drill into the brick, not the mortar. Mortar doesn't have structural strength, so you want to avoid attaching the shelf to the mortar. Use a hammer drill, concrete screws, and masonry drill bits. If you are drilling into softer brick, a carbide-tipped bit in a power drill can work. Drilling into brick takes a lot of force and power, so make sure you have the holes exactly where you want them. When you have the holes, complete the task by screwing the cleats into position using the drilled holes. Be sure to turn your hammer drill to regular drill mode before doing this. Push the mantel against the wall, using the drawn lines. The mantel should fit around the cleats, which hold it in position. Then use the drill to insert screws through the mantel and into the cleats. These screws should be roughly 16 inches apart.Fasten the mantel to the cleats along the top shelf and along the two legs. You can also nail the mantel to the mounting boards if you wish. Attach the scribe molding. There will be a gap between the wall and the mantel, so the molding will cover this. You can use nails to attach the molding. Apply wood putty to cover the screw heads, taking care to smooth the putty evenly with the surface of the mantel. Allow the putty to dry, and then paint to hide the holes completely.

Summary:
Position the mantel around the fireplace. Mark the territory. Mark the location of the mounting board. Prepare the mounting boards. Locate and mark the wall studs. Affix the mounting boards to the wall. Mount the fireplace mantel. Put on the finishing touches.