Depending on the area you live in, it may or may not be legal to side over asbestos. The main reason why most individuals usually choose to side over asbestos is financial. Removing asbestos is often cost-prohibitive for most people. Alternatively, if you leave the asbestos on your home, you can safely seal the old siding underneath the new siding, and, because asbestos is an excellent insulator, you will actually be adding to the energy efficiency of your home by leaving it on. As long as the asbestos does not break apart and you are not breathing in the fibers, you should have no health or safety problems with your old siding.
Inspect the outside of the home for any damages to the asbestos siding. Make sure that there are no loose or broken pieces.
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Measure the house for siding and the backer board. The simplest way to do this is to use a tape measure to measure one of the clapboards of the original siding on the house in inches. Then, count the number of total clapboards all the way to the peak of the house (you will be counting for "extra" siding, but that's OK, because you usually end up needing it). Multiply the length of the clapboard by the number of clapboards on the home and then divide that number by 100, and you will know roughly how much siding you need, measured in squares. So, for example, if one of the clapboards measures 10 inches and there are 20 of those clapboards going up the side of the home, 10 x 20 = 200. Then, divide that number by 100. You get two. That represents the amount of square feet, or "squares" of siding you need. This process also tells you how much backer board and vapor barrier you'll need. Repeat this process for the entire house.
Install the vapor barrier. This is essentially a thin plastic sheet that helps to keep moisture from penetrating the walls of your home and causing mold and mildew. In this case, it will also help to contain the asbestos siding. Vapor barrier is typically nailed on, so you will have to be careful not to damage the existing siding that is on the home when putting your vapor barrier on.
Cut the backer board to fit the dimensions of your home. Now that you have all the measurements you need, all you have to do is cut the backer board to fit the dimensions of your home (i.e. cut to fit around the windows, door jambs, etc.). Select one area of the house (at the bottom of the house) to work on and start there and work your way up. Hold your backer board firmly in place and use corrosion-resistant screws to install your backer board to the home. Since the screws will be going through the asbestos siding, this step is critical. If the board is not held firm, there is a chance that you will end up breaking up the underlying siding and having it fall to the bottom behind the backer board.
Install the starter strip at the bottom of the house. It is important that your starter strip remain straight around the entire bottom of the house; otherwise, your siding will not go on straight.
"Hang" the siding. Now that you have your backer board up and set firmly in place, you can put the siding up safely without worrying about the asbestos falling down behind the siding. Start by hooking the siding on the bottom of the starter strip with the nail holes facing upward. When you nail siding to the house, remember to nail the siding "loose." Do not hammer the nail all the way into the home. The siding has elongated nail holes to allow the siding to expand and contract, and if you nail the siding tight to the house, your siding will buckle. Continue the process of hooking the siding pieces together and nailing them to the house--working your way up to the top.