Backpropping and reshoring both use temporary supports during concrete construction, but the terms should not be treated as interchangeable. In UK practice, backpropping describes propping below the slab supporting the falsework so that construction loads can be distributed into lower slabs or foundations. North American reshoring normally describes a different sequence in which the original shores are removed, the slab is allowed to carry its own self-weight, and supports are then reinstalled for subsequent construction loads.
The distinction matters because the temporary support sequence changes the load path through the permanent structure. A design based on one sequence should not be changed on site to another simply because both arrangements appear to consist of rows of props beneath a concrete slab.
Backpropping and reshoring temporary support arrangements in concrete construction. Editorial illustration: London Construction Magazine.
What is backpropping?
The CITB temporary works glossary, drawing on BS 5975 terminology, defines backpropping as propping installed at levels below the slab that supports the falsework in order to distribute the load on the uppermost slab to suitable lower slabs or foundations. The current UK technical standard is BS 5975-2:2024, Temporary works – Falsework: Design and implementation. It replaced the falsework-design content of BS 5975:2019 and includes revised guidance for calculating load transfer through flat slabs during backpropping.
In practical terms, backprops allow the load from a new concrete pour, falsework and construction activity to be shared with lower parts of the permanent structure rather than relying on the immediate supporting slab alone. The lower slabs themselves therefore become part of the temporary construction load path and must be checked for the loads they receive at their actual age and stage of construction.
What is reshoring?
“Reshoring” is particularly common in North American concrete construction terminology. WorkSafeBC guidance comparing backshoring and reshoring explains the structural distinction clearly: in a reshoring sequence, the slab is allowed to deflect under its own weight after the original formwork support is released, transferring that self-weight into the permanent columns and walls before the reshores support later construction loading. In the backshoring arrangement described by WorkSafeBC, the support is not lowered when the formwork is removed. The slab is therefore prevented from taking the same deflected shape, and its self-weight continues to be transmitted through the temporary supports and floors below. WorkSafeBC warns that loads in backshoring systems can be materially greater than those in systems designed as reshoring.
LCM clarification: UK and North American terminology does not map perfectly. The UK term “backpropping” has a broader BS 5975 meaning based on distributing falsework loads through lower levels. CITB also separately defines “re-propping”. It is therefore safer for drawings, calculations and method statements to define the actual striking, unloading and reinstallation sequence rather than relying on the words backpropping, reshoring or repropping alone.
Backpropping vs reshoring: the structural difference
| Point | Backpropping / backshoring arrangement | Reshoring arrangement |
|---|---|---|
| Support sequence | Temporary support remains engaged as loads are transferred through lower levels | Original support is released before reshores are installed |
| Slab self-weight | May continue to be carried partly through the temporary support system, depending on the designed sequence | Slab first takes its own self-weight through the permanent structure |
| Initial slab deflection | Restricted by the support arrangement | Permitted before reshores are engaged |
| Later construction loads | Distributed through the designed propping system and supporting floors | Reshores provide a path for subsequent loads while the slab already carries its own self-weight |
| Critical design issue | Load accumulation and distribution through several floors | Whether the young slab can safely carry itself when initially struck |
Why the load path changes when concrete is struck
Fresh concrete initially loads the formwork and falsework rather than behaving as a finished structural floor. On a multi-storey frame, those loads may then pass into one or several previously cast slabs through props and backprops. When the primary support is released, the new slab starts to behave as part of the permanent structure. Its self-weight creates bending, shear and deflection and transfers load into its permanent supports. If props are subsequently installed or retained, later loads from construction above can again be shared between connected floors.
This is why the age of the concrete matters. The temporary works assessment needs to consider the actual strength and stiffness available when the floor is loaded, not simply the specified 28-day concrete grade. Pour sequence, cycle time, early-age concrete properties, slab geometry, reinforcement, construction live loads, stored materials and the stiffness of the supporting floors can all affect the resulting load distribution. LCM has examined this issue separately in its analysis of back-propping load transfer and structural risk, including the effect of partially cured slabs and multi-level load redistribution.
When is backpropping used?
Backpropping is commonly considered where the slab immediately beneath a concrete pour cannot safely carry the full construction load on its own. Lower floors can then be brought into the load path so that the load is distributed through a larger part of the structure. Typical applications include multi-storey reinforced concrete frames, thick slabs, podiums and transfer structures, as well as situations where heavy construction plant or concentrated temporary loads need to be supported by several structural levels. The correct arrangement depends on the permanent structure and construction sequence; the presence of a heavy slab does not by itself prescribe a particular number or pattern of props.
When is reshoring used?
A reshoring sequence may be used where the new slab has developed enough verified capacity to carry its own self-weight but still needs temporary support against later construction loads from floors being built above. Allowing the slab to take its own weight before the reshores are engaged changes the subsequent load-sharing behaviour and can also allow primary formwork equipment to be removed and reused sooner. That programme benefit does not remove the structural requirement: the young slab must be checked for the load condition created at the moment it is struck.
How many floors should be backpropped?
There is no universal rule that a concrete frame should always be backpropped for two, three or four floors. The number of supporting levels must follow the project-specific temporary works assessment. The calculation can depend on slab thickness and span, reinforcement, concrete age and strength, permanent floor capacity, wet-concrete and formwork loads, construction live loads, prop layout and stiffness, pour sequence and the condition of every lower slab brought into the load path. Transition floors, transfer structures and changes in slab geometry can make a rule that worked on one part of the building unsuitable elsewhere.
The current BS 5975-2:2024 is particularly relevant here because BSI says its revision includes an updated method for calculating the percentage of load transferred through flat slabs during backpropping. A statement such as “we always backprop three floors” is therefore not a substitute for an engineered load-transfer assessment.
Backpropping is temporary works
HSE's temporary works guidance includes propping for walls and floors and falsework for concrete pours within temporary works. HSE states that temporary structures must be designed, installed and maintained to withstand foreseeable loads and that, in all but the simplest situations, a design including calculations will be required. The temporary works design therefore needs to look beyond the catalogue capacity of an individual prop. It must establish the loads actually reaching the prop, the effect of its extension and restraint, the capacity of heads and bases, and whether the slab or foundation receiving the reaction can safely carry it.
Management is equally important. HSE describes appointment of a Temporary Works Coordinator as an effective way of controlling temporary works rather than a separately named statutory appointment. The current UK procedural framework is BS 5975-1:2024. LCM's BS 5975 temporary works compliance guide explains the wider system of design briefs, checks, inspections and permits. Where a TWC is appointed, the role is procedural rather than a substitute for engineering design. LCM's guide to Temporary Works Coordinator duties under BS 5975 explains how design, inspection and site control are coordinated through the temporary works process.
Why site changes to propping can be dangerous
A prop is only safe as part of the load path for which it was designed. Removing a prop for an MEP route, changing its position, altering the striking sequence, introducing additional stored materials or placing heavy plant on a young floor can all change the forces acting through the temporary and permanent works. A published CROSS temporary works propping report illustrates the wider point. During an office refurbishment, a multi-part propping arrangement introduced an effectively unbraced hinge into a heavily loaded support system. CROSS noted that temporary works design needs to consider stability, not simply member stress, and stressed the need for temporary works to be designed, checked and installed correctly.
LCM clarification: the most useful instruction on a propping drawing is not simply “backprop” or “reshore”. It is the defined physical sequence: when falsework can be released, what concrete strength or other acceptance criterion must first be achieved, which props remain loaded, which are removed or reset, what construction loads are permitted and which lower floors form part of the temporary load path. Before the next pour or striking operation, the project team should therefore confirm the approved temporary works sequence, verify the required concrete condition, check the supporting floors and prop arrangement against the design, and ensure any site change is reviewed before it is implemented. The system should be loaded, altered and dismantled only in accordance with the checked temporary works information and project control procedure.
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Expert Verification & Authorship: Mihai Chelmus Founder & Editor, London Construction Magazine | Construction Testing & Investigation Specialist |