Shoring and back propping for openings, slabs and loaded floors

Dead shoring carries vertical load. Needles — beams passed through holes cut in the wall — pick up the masonry above the intended opening and transfer it into props or shore legs on both sides, which in turn land on spread bases.

This is the arrangement behind almost every load-bearing wall removal and every large structural opening. The detail that matters is what the props land on: a sole plate or spreader on ground that can take the load, and back propping below if the props are standing on a suspended floor.

Raking shores act at an angle against an external wall, resisting lateral movement where a wall is bulging, has lost restraint, or has had a floor or return wall removed from it.

The angle, the wall plate and the foot bearing all matter — a raking shore that is too shallow pushes rather than supports, and a foot without a proper spread simply sinks into the ground and lets the wall move anyway.

A flying shore spans horizontally between two structures, holding a wall laterally without any legs down to the ground. It is used where a building in a terrace is demolished or gutted and the party walls either side need restraint, and where the ground below cannot be blocked.

It is more involved to design and set than a rake, but it leaves the ground clear, which on tight London and terraced sites is often the deciding factor.

Back propping distributes load down through one or more floors below the level being loaded, so no single slab takes more than it was designed for. It is standard practice on concrete frame construction while slabs are curing.

The arrangement runs vertically in line, floor on floor, with head and base spreaders. Props out of line, or a level missed, means the load path stops and concentrates exactly where it should not.

Where the front of a building is being kept and everything behind it removed, the facade needs a frame to hold it in both directions for the duration of the works.

That is a designed structure with calculations, tied and braced, and it is priced and planned as its own package. Talk to us early on these — the retention scheme usually drives the sequence of the whole job.

Plant on a floor, a scaffold tower standing on a suspended slab, a stack of materials in a fit-out — all of these apply loads the floor was never designed for and all of them need propping below.

It is the most commonly skipped of all the categories, because the load looks modest. Concentrated point loads on a thin slab are exactly the ones that cause cracking.