Propping is the temporary support of a building element with adjustable props, so that the element does not fail or deflect before it can carry its own load. A prop is a height-adjustable steel tube with a base plate and a head plate; in the UK it is often called an acrow prop or a floor prop. The Dutch term is onderstempelen.
When propping is needed
Three classic situations:
- Freshly cast concrete. Concrete only reaches its strength over days or weeks. After striking the formwork, some props stay in place until the concrete is strong enough — known as back-propping
- Composite and precast floor slabs. A lattice-girder or composite slab acts as permanent formwork during construction and is not yet structural. It is propped at the spacings specified by the supplier until the structural topping has cured
- Alterations. Before a load-bearing wall is removed in whole or in part, the floor or wall above is propped. The props only come out once the new beam is in place and fully bearing
Rules of thumb
- Follow the supplier’s propping schedule. For precast floor units the manufacturer sets the spacing of the prop lines and the permitted load per prop — that is a condition, not a suggestion
- Always prop down to a sound bearing. On an upper floor you must also prop the floor below, or you simply move the problem downwards
- Use a spreader beam (a timber or steel member under the prop heads) so the load is distributed rather than applied at points
- Set props plumb and restrain them. A prop out of plumb can kick out; at greater heights props should be braced together
- Do not over-tighten. The prop should take the load, not jack the structure up — over-jacking can cause damage
- Only strike when released. For concrete, once the striking strength is reached; for alterations, on the engineer’s approval
> Removing a load-bearing wall without propping can bring a floor down without warning. Temporary works are part of the structural design, not an optional extra.
Related terms
- Acrow prop
- Formwork
- Temporary works
- Striking
- Girder
- Needle and prop
- Base plate grouting
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.