A basement post is usually installed to counteract a sagging frame in a wood-frame house. Typically the posts are eight-foot adjustable devices that can be placed under a sagging beam or floor joist. They are made from metal and have an adjustable screw top that can be raised to fit the space between the floor and ceiling of the basement. These adjustable posts can be used to shore up a sagging floor or even raise the floor if need be.
Measure the height of your opening to make sure an eight-foot adjustable metal post is just what you need. If so, then go ahead and purchase as many posts as you need. They should be locally available in most hardware stores.
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Set the post on a wooden block in the exact location where you want the post to be. For example, let's say you have a wooden beam that runs the length of your basement. It is cracked, but not sagging, so you want to place the post under the crack. Make a square shaped wooden block from a 2 X 12 inch or 2 X 10 inch piece of lumber and set it underneath the crack. Then, place the post on top of the block and turn the adjustable top until it starts to move upwards.
Cut a wooden block to go between the top of the post and the beam or floor joists, for which it is supporting. You can take a 2 X 6 or a 2 X 8 and use that piece of wood for the block after you make a square-shaped cut. You might want to double up on the wood, which means you will have to cut two square blocks and then place one on top of the other.
Raise the level of the floor. If you turn the lever that attaches to the post in a clockwise manner, you will slowly raise the level of the floor above you. Never raise the floor more than ¼-inch per day and you may have to build an additional makeshift beam to spread the force of several jacks across the span of many floor joists. A 4 X 4 or a 6 X 6 makes a good temporary beam, or you can nail together several 2 X 6's to get the same effect. Overtime floor joists may sag because of wear and tear or poor design, so you could insert several metal posts underneath an eight-foot temporary beam that spans seven floor joists placed 16" apart on center. To accomplish this you would probably need three posts placed equal distances apart underneath the beam. Raise each one a little bit at a time till you get your desired lift. Don't forget to go only a ¼ inch at a time.
Build a cement pier underneath the place that you will place your jack. This is only necessary, if your cellar has a dirt floor or the concrete slab is in disrepair. Most modern homes have poured slab floors that are 4 to 6 inches thick and can support some lifting. If your structure requires anything more than this, then it is best to consult a professional.