What is back-propping and when do you need it?
Back-propping re-supports a slab after the deck formwork is stripped, so the new slab shares its load with the levels below while it is still gaining strength. It is required whenever a slab is loaded — by the deck above, by stacked material, or by its own weight at low strength — before it reaches the strength the permanent design assumes.
On a fast cycle the top slab is almost never strong enough to carry the deck above on its own. Back-propping is how that load is shared downward until it is.
How the load actually shares
Load distributes between the propped levels roughly in proportion to their stiffness, which changes daily as each slab gains strength. That is why the number of back-propped levels comes from a calculation against measured strength gain rather than a standing rule.
Back-props are not formwork props left in place. They are re-set to a designed layout with the deck removed, and they must land on the same line at every level.
The three common mistakes
Props that do not line up between levels, so load is dumped mid-span instead of at a support. Props over-tightened, which jacks load into a slab that was not designed for it. And a level removed early because the programme needed the props elsewhere.