jack post

A jack post is a vertical steel tube assembly that extends and contracts to provide temporary support during construction, renovation, and demolition work. The typical unit consists of an outer sleeve and an inner shaft that can be driven upward against a load, creating a rigid prop between floor and ceiling or between ground and overhead structure. Standard jack posts range from 4 feet to 10 feet in extended length, with capacities typically between 2 and 4 tons per post depending on diameter and wall thickness.

The mechanism works through a threaded adjustment screw at the base of the post. Turning this screw extends the inner tube against whatever load sits above it, creating compression that locks the assembly rigid. Better-grade models use a ratchet or quick-action handle rather than requiring a wrench for every adjustment. The telescoping design lets workers position posts at any height within their range without requiring custom cutting or welding, making them faster to deploy than fixed props.

Common applications and hazards

Jack posts appear everywhere temporary support is needed: shoring trenches, holding up beam soffit during floor removal, bracing walls during interior demolition, and supporting formwork during concrete pours. They are lighter and faster to move than adjustable shores or steel beams, but this speed creates real danger. Posts that are not adequately base-locked, over-extended without support, or loaded unevenly can collapse suddenly, sometimes taking structural elements down with them. The failure mode is typically sudden buckling under load.

The term jack post derives from the screw jack mechanism at its base; early models were openly based on the automobile jack principle. They are also called adjustable posts, acrow props (in some regions), steel props, or telescoping shores. Casual workers sometimes call them lally columns, though that term more properly refers to filled steel tubes used as permanent structural members.

Proper use requires a solid base plate under the foot, adequate bracing to prevent lateral sway, and careful load calculation based on post height and applied load. Many jurisdictions require an engineer to certify temporary support installations before workers can proceed. The posts themselves are consumable items in typical construction economics; they are rented rather than purchased by most builders.