Underpinning necessarily involves excavating or drilling beneath a footing that’s currently carrying the weight of your home, and it’s a fair question to ask whether that’s actually safe. In most residential underpinning, the honest answer is yes, provided the work is properly engineered and sequenced, and temporary propping is a core part of how that safety is maintained. This guide explains what propping actually is, when it’s needed, the common methods used, and what a homeowner can reasonably expect while it’s happening.
What is temporary propping and why does underpinning need it?
Temporary propping (also called shoring) is the practice of temporarily supporting a structure, or a section of it, while the permanent footing beneath it is being worked on. When a section of footing is excavated to install a concrete underpin, or a slab is being lifted with resin injection, that section briefly loses some or all of its normal support. Propping bridges that gap, transferring the structure’s load to a temporary support system until the permanent underpin or repair is in place and able to carry the load itself.
Not every underpinning job needs propping. Screw pile underpinning, for instance, often transfers load to the new pier via a bracket without ever fully removing the existing footing’s support at any one moment, so many screw pile jobs proceed without dedicated propping beyond normal site bracing. Traditional mass concrete underpinning, by contrast, genuinely does remove a section of the load path while a pit is excavated and poured, which is where propping becomes a standard and necessary part of the sequence. Our comparison of screw pile vs concrete underpinning covers this difference in more detail.
How engineers sequence excavation and propping to keep the structure safe
The core safety principle behind any underpinning sequence is straightforward: never remove support from more of the footing than the structure above can safely span across at any one time. In practice this means:
- The footing is divided into short sections (typically 1-1.5 metres) called pins or bays, rather than excavated as one continuous trench.
- Only one pin, or a small number of non-adjacent pins, is excavated at a time, so the majority of the footing continues bearing normally while a small section is temporarily unsupported.
- Where the unsupported span is longer, or the load above is heavier, propping is installed before excavation begins, transferring that section’s load to a temporary support so the excavation can proceed without relying on the remaining footing to span the gap unaided.
- The engineer’s design specifies the maximum unsupported length, the pin sequence, and whether propping is required for that specific pin, based on the load above, the footing condition, and the soil.
- Each pin is backfilled and cured (or, for screw piles, loaded onto the new pier) before the adjacent pin is opened, so the structure is never relying on more than the designed unsupported length at once.
This staged, engineer-specified sequence is the primary reason underpinning is considered a safe, well-established repair method rather than a risky undertaking, even though it involves temporarily removing support from part of a load-bearing footing.
Common propping methods used in residential underpinning
Several propping and shoring techniques are used in Australian residential underpinning, chosen to suit the specific structure and the excavation sequence:
Acrow props (adjustable steel props). The most familiar and widely used temporary support: a telescoping steel prop, adjusted to length and locked off under load, placed beneath a beam, joist or bearer to carry load down to solid ground while the footing beneath is worked on. Common in subfloor areas of stumped or suspended-floor sections of a home.
Needle beams. A steel or timber beam threaded through a wall (through a small opening cut for the purpose) at a point above the footing being excavated, resting on props or blocking on either side of the excavation. The needle beam picks up the wall’s load across the gap and transfers it to solid ground either side, allowing the footing directly beneath the needle to be safely excavated.
Dead shoring. A broader system of vertical props, horizontal needles and bracing timbers or steel used where a larger section of structure needs temporary support, more typical of substantial renovation or larger commercial underpinning than a routine residential corner underpin.
Raking shores. Angled props braced against the ground or an adjacent structure, used less frequently in residential underpinning but sometimes specified where vertical propping isn’t practical against a particular wall or feature.
The choice between these methods, and whether propping is needed at all for a given job, comes from the engineer’s assessment of the specific structure, load and excavation sequence, not a default applied to every underpinning job regardless of scope.
What homeowners can expect while propping is in place
For most residential jobs, propping is installed inside roof spaces, subfloor areas or a small opening cut into an internal wall, rather than through the middle of a lived-in room, so the visible disruption is often less than homeowners expect. That said, a few practical things are worth knowing:
- Props may be visible in a garage, subfloor or roof space for the duration of the relevant pin’s excavation and cure, then removed once that section is backfilled or loaded onto the new underpin.
- A needle beam installation may require a small, targeted opening in a wall, reinstated as part of the project once the beam is removed.
- Most households can continue living in the home normally while propping is in place; the areas affected are typically confined to where the active excavation is happening, and your contractor will flag any specific area to avoid during that stage.
- Props are not a sign something has gone wrong. Seeing a temporary prop in your subfloor or garage during an underpinning job is a normal, engineered part of a properly sequenced project, not an indication of a problem beyond what the inspection already identified.
Is propping itself something to worry about?
No, provided the propping and excavation sequence has been properly designed and installed by a licensed contractor following an engineer’s specification. The entire purpose of propping is to prevent the kind of uncontrolled load transfer that could otherwise cause additional cracking or movement during the works. Problems arise not from propping itself but from skipping it, or under-designing the unsupported span, which is precisely why a proper engineering design and a licensed, experienced contractor matter for underpinning work generally, not just for the propping component of it.
Does propping cost extra, or is it part of the underpinning price?
Where an engineer’s design calls for propping as part of the sequence, it’s normally included in the underpinning quote as part of the specified scope of works, rather than billed as a separate line item, because it’s an integral part of executing that pin safely rather than an optional extra. Where a job unexpectedly requires more extensive shoring than anticipated, for example because excavation reveals a footing in worse condition than assumed, that’s the kind of variation a contractor should flag and discuss with you before proceeding, rather than after the fact.
Temporary propping FAQs
Do I need to move out of my house while propping is installed?
Almost never for a standard residential underpinning job. Propping is typically confined to subfloor, roof space or garage areas, and most households remain in the home throughout. Your contractor will tell you in advance if any specific area needs to be avoided during a particular stage.
How long does propping stay in place?
Typically for the duration of excavating, pouring and curing that specific pin, commonly a matter of days per pin, before the load is transferred to the completed underpin and the prop is removed. On a job with multiple pins done in sequence, different props may be installed and removed at different times as the work progresses along the footing.
Can propping itself cause new cracks?
Properly designed and installed propping is intended to prevent additional movement, not cause it. New cracking during underpinning work is more likely to indicate an under-designed unsupported span or an installation issue, which is why the engineering design and the contractor’s experience with the specific propping method matter.
Is propping used for resin injection or only concrete underpinning?
Resin injection and screw pile underpinning generally involve less or no dedicated propping compared to traditional concrete underpinning, since neither method requires fully excavating and temporarily removing the existing footing’s support in the same way. Our resin injection underpinning guide covers how that method’s installation sequence differs.
Who decides whether my job needs propping?
The structural engineer who designs the underpinning specification decides, based on the load above, the footing’s condition, the unsupported span at each pin, and the excavation method proposed. It’s part of the same engineering process that determines pin depth, spacing and the underpinning method itself.
Talk to someone who can explain your specific job
If you’re weighing up an underpinning quote and want to understand what propping, if any, your job will involve, get a free quote: an independent licensed contractor (and, where needed, an engineer) can walk you through exactly how your home will be supported throughout the works.