A jack post is a steel post used to support a structure. This post is placed under a beam typically in a basement or ground floor. It is adjusted to fit snugly, thereby preventing the beam in the structure from sagging.
Adjustment occurs through a threaded rod connected to a steel plate until the proper height is accomplished to allow the jack post to fit under the beam tightly. Some versions of this post use a system of pins and holes to adjust the post. It must be placed on a cement or concrete floor in order to support the structure's weight without sinking into the floor.
When a jack post is being placed underneath a structure, it is often a good practice to raise the building slightly prior to placing the post. This is accomplished by placing a wooden beam or a short section of steel jack post on top of a large screw-type jack and turning the jack out until the floor has been raised slightly. The post is then placed under the supporting beam and adjusted to fit snugly. When the screw jack is removed, the jack post assumes the duty of holding the structure and preventing any snagging.
It is also wise to place a steel plate underneath the jack post to allow the pressure from the structure to be distributed out and away from the post's circumference. This will also prevent the post from breaking through the floor that it is resting on. Most top plates on these jacks have a provision for nailing them in place under a wooden beam or for bolting them in place if the supporting beam happens to be a steel I-beam. This prevents the jack from slipping out of position over time.
Since the jack posts are typically placed in an open expanse of basement in a non-supported building, they can be an eyesore if the basement is ever finished or remodeled to be a living space. Most builders will frame the posts with a square wooden structure that is easily blended into the surroundings by painting or wall papering in the same material as the walls. This gives the appearance of the jack being an integral part of the structure. In some new construction, the jack posts are placed at the time of the build due to the unmatched level of support they provide.