Props are adjustable for height by a large diameter screw thread, formed on the outside of the tube itself. The screw thread provides a fine adjustment over a short range. A loose pin through a series of holes in the inner tube gives a wider range of coarse adjustment. Use of a screw thread also allows the prop to be tightened when already in place, to adjust the load that each one bears.
Props are made in a range of five standard sizes:
- size 0 from 1.1 m to 1.8 m,
- size 1 from 1.75 m to 3.12 m,
- size 2 from 2.0 m to 3.4 m,
- size 3 from 2.4 m to 4.0 m,
- size 4 from 3.2 m to 4.9 m.
The safe load for a given prop varies according to the distance between its ends and the eccentricity of the load relative to the longitudinal axis of the prop.
The safe load for a standard prop is a maximum of 35 kilonewtons at a length of 2 meters (without using any attachment and when not supported by horizontal lacing). This allows 1.5° out of plumb (out of a vertical position) when loaded concentrically. Safe load is reduced when props are further extended, or by up to half when the load is not concentrical.
Props are mostly used for temporary supports during building repair or alteration work, known as shoring, rather than scaffolding. A typical use is to support an existing horizontal beam whilst its original masonry supports are removed or repaired. When masonry itself is to be supported, holes are first knocked through the brickwork and a ‘strongboy’ is placed through the hole. A pair of props are then used, one under each end. Existing windows or doorways may also be supported directly, or via needles.
The base and top plate of props are quite small, so they are suitable for supporting a vertical load but are at risk of toppling if there is any sideways force. In such cases, props should be strutted or ‘laced’ with scaffolding poles. A recent improvement to props was to shape this baseplate with notches, allowing pallet loads of horizontal props to be stacked neatly, rather than randomly piled.