Underpinning & Restumping in Adelaide
When your home's foundations start to sink, shift, or deteriorate, underpinning and restumping are the two primary methods for restoring structural stability. Adelaide Foundation Repair connects you with licensed Adelaide specialists who assess your specific foundation problem and apply the correct solution — underpinning for concrete foundations, restumping for suspended timber floors.
What Is Underpinning?
Underpinning is the process of strengthening and stabilising an existing foundation by extending it deeper into more stable soil or by widening its footprint to better distribute the building's weight. It's used when the original foundation is no longer adequate — typically because the soil beneath has shifted, compacted, or been eroded.
In Adelaide, underpinning is most commonly needed in suburbs built on reactive clay soils — such as Salisbury, Elizabeth, and the north-eastern foothills. These clay soils expand when wet during winter rains and shrink when dry during summer, creating a cycle of movement that can crack and destabilise concrete slab and strip footing foundations over time. Homes built before the introduction of modern footing standards (AS 2870) in the 1980s are particularly vulnerable.
Common Underpinning Methods
The specialists we refer employ several underpinning techniques, selected based on your soil conditions, foundation type, access, and budget:
- Mass concrete underpinning (traditional pit method): Sections of the existing footing are excavated in sequence and filled with concrete to create a deeper, wider foundation bearing on stable soil. This is a proven method used for over a century and is well suited to accessible strip footings.
- Screw pile underpinning: Steel screw piles are hydraulically driven deep into the ground until they reach stable strata, then connected to the existing foundation with steel brackets. This method is faster than mass concrete, creates less mess, and is ideal for sites with limited access or where deep bearing is needed to bypass reactive surface soils.
- Resin injection underpinning: Expanding polyurethane resin is injected beneath the foundation through small holes. The resin expands to fill voids and compact loose soil, lifting and stabilising the slab. This is a minimally invasive method suitable for remediating voided or poorly compacted ground beneath concrete slabs. It can be completed in a day and allows immediate use of the building.
What Is Restumping?
Restumping — also known as reblocking or restumping and relevelling — is the replacement of deteriorated timber stumps that support a suspended timber floor. This is a distinctly Adelaide problem: tens of thousands of homes built between the 1920s and 1970s across suburbs like Norwood, Prospect, Unley, and the inner west sit on timber stumps that are now reaching the end of their service life.
Timber stumps deteriorate through a combination of moisture (from soil contact and poor sub-floor ventilation), termite attack, and wood rot. As stumps fail, floors become uneven, doors and windows jam, and walls develop cracks. In severe cases, stumps can collapse entirely. The replacement options are:
- Concrete stumps: Precast reinforced concrete stumps are the most common replacement. They are rot-proof, termite-proof, and come with a long service life. They are installed on concrete pads at the base of each stump hole.
- Adjustable steel stumps: Galvanised steel stumps with threaded tops can be fine-tuned to precisely level the floor. They are ideal where future adjustment may be needed — for example, on reactive clay soils where some ongoing movement is expected.
Signs You Need Underpinning or Restumping
If you recognise any of the following signs, it's worth having a specialist assess your foundations:
- Floors that slope, bounce, or feel spongy underfoot
- Doors and windows that stick, jam, or won't latch — especially if the problem is seasonal
- Cracks in walls, particularly around door frames and window openings, or stair-step cracks in brickwork
- Gaps appearing between walls and ceilings, or between skirting boards and floorboards
- Visible cracks in concrete slabs or exterior brickwork
- A musty smell or visible rot in the sub-floor area (indicates stump decay)
The Underpinning and Restumping Process
While every job is different, a typical underpinning or restumping project follows these stages:
- Structural inspection: The specialist inspects your property, takes level readings, checks sub-floor conditions, and may arrange soil testing. They'll determine whether underpinning or restumping is needed (or both — some homes need restumping of the timber floor area plus underpinning of perimeter footings).
- Engineering specification: For underpinning, an engineer may be engaged to specify the depth, size, and spacing of underpins. For restumping, the specialist can usually specify the work directly.
- Site preparation: Access is created — this may involve removing floorboards for internal restumping, or excavating externally for underpinning. Services are located and protected.
- Installation: Underpins are poured or piles driven; stumps are installed and the floor is relevelled.
- Finishing: Floorboards are reinstated, external excavations backfilled, and any affected landscaping restored. Cracks in walls can be patched (though it's usually recommended to wait a season or two to allow the building to stabilise before cosmetic repairs).
Why Adelaide Conditions Demand Local Expertise
Adelaide's foundation repair needs differ from other Australian cities. The prevalence of reactive clay soils — classified as Class H (highly reactive) or Class E (extremely reactive) under AS 2870 — across much of the Adelaide Plains means seasonal foundation movement is common. The northern suburbs in particular sit on deep clay deposits that can expand by up to 60 mm or more between wet and dry seasons. A specialist who understands local soil behaviour will specify underpinning depths and stump designs appropriate for your specific site conditions, not just apply a generic solution.
The specialists we refer are experienced with Adelaide's conditions and housing types — from bluestone cottages in the city fringe to 1960s brick veneer in the north-east, from beachside homes on sandy soils to split-level homes on hillside blocks in the foothills.
Frequently Asked Questions
Underpinning costs depend on the number of underpins needed, access, depth, and method. As a guide, traditional mass concrete underpinning for a single corner of a home might cost $5,000–$10,000. Full perimeter underpinning can be $20,000–$50,000. Screw pile underpinning is typically comparable per lineal metre but may be faster. Resin injection for slab void filling may be $3,000–$8,000 depending on the area. The only way to get an accurate price is a site inspection.
Restumping a typical 3-bedroom home with concrete stumps typically ranges from $8,000 to $20,000 depending on the number of stumps, access, and whether floorboards need removal. Adjustable steel stumps are slightly more expensive per stump but allow precise levelling. Costs include building permits, stump installation, floor relevelling, and making good. Always get a detailed written quote that specifies the number of stumps and the type being used.
Restumping a typical home takes 1–2 weeks. Underpinning can take anywhere from 3 days for a small section to 3–4 weeks for full perimeter underpinning. Resin injection underpinning can be completed in a single day. The specialist will give you a timeline estimate after the inspection. Note that you may need to vacate the home during major restumping or underpinning work.
Yes, in most cases. Underpinning and restumping are structural works that typically require a development approval or building consent from your local council. The specialists we refer handle the approval process as part of their service. They'll advise you on what's required for your specific council area — requirements vary between councils such as City of Norwood Payneham & St Peters, City of Unley, City of Salisbury, and others.