These two terms are often confused, and tradespeople use them loosely, sometimes interchangeably. They are not the same thing. Using the wrong repair method wastes money and may not fix the actual problem.
The short answer: reblocking (restumping) replaces the individual vertical supports (stumps) under an older timber-frame home. Underpinning extends or strengthens the foundation of any home, usually by adding new load-bearing elements that reach below the reactive zone. If you have an older home with a suspended timber floor, you probably need reblocking. If you have a concrete slab home with a cracked or settled foundation, you need underpinning.
What Is Reblocking / Restumping?
Reblocking and restumping are interchangeable terms for the same process: replacing the stumps (short vertical posts) that support a suspended timber floor. These stumps sit on pads in the clay and carry the load of the bearers, joists, and floor above.
Older Wagga homes built between approximately 1900 and the 1960s typically use this construction method. The original stumps were most often timber, hardwood or treated pine. Over decades, timber stumps in Wagga’s cyclically wet-dry clay:
- Rot at or below ground level where they are exposed to moisture
- Compress under load over time
- Tilt as the clay moves beneath them
- Are attacked by termites
The visible signs are floors that slope, bounce, or squeak; doors and windows that stick; gaps between skirting boards and floors; and, in advanced cases, visible lean in the structure.
Reblocking replaces the old stumps with new ones, typically precast concrete stumps or, on more reactive sites, steel screw piers. The house is temporarily jacked up, old stumps are removed, new ones are set, and the house is lowered back onto them. In most cases, house relevelling is carried out at the same time to correct any slope that has developed.
What Is Underpinning?
Underpinning is a broader category of work that strengthens or deepens the foundation of a building. It is most commonly used for:
- Concrete slab homes where the slab has settled or cracked
- Brick or masonry homes where perimeter footings have moved
- Any structure where the existing foundation is inadequate for the soil conditions
- Adjacent excavation that has undermined existing footings
The classic underpinning method is mass concrete: excavating beneath existing footings in alternating sections and pouring concrete to extend the footing to greater depth. The more modern approach, increasingly common in Wagga, uses steel screw piers driven or screwed below the reactive zone, with a load transfer mechanism connecting the existing slab or footing to the new pier.
Underpinning is almost always designed by a structural engineer, involves a building approval process, and is priced per pier or per linear metre of footing stabilised. It is typically more expensive than reblocking but addresses deep structural settlement that reblocking is not designed for.
Which Does Your Wagga Home Need?
The answer depends primarily on your floor and foundation type:
| Home Type | Foundation | Likely Repair |
|---|---|---|
| Pre-1960s timber frame, suspended floor | Timber stumps | Reblocking/restumping |
| Post-1960s single brick, suspended floor | Concrete stumps | Restumping or underpinning |
| Post-1970s slab home | Concrete slab (waffle or raft) | Underpinning |
| Brick veneer on slab | Concrete slab | Underpinning |
| Heritage home, brick perimeter walls | Brick pier foundations | Underpinning (specialist) |
| Commercial tilt-panel building | Concrete slab or strip footing | Underpinning |
If you are uncertain of your foundation type, the simplest way to check a suspended floor is to look under the house from a subfloor access point. If you can see ground beneath the floor, you have a suspended floor system with stumps. If the floor is directly on concrete, you have a slab.
Signs You Need Reblocking
- Timber-frame home with uneven, sloping or bouncy floors
- Stumps visible under the house that show rot, compression, or tilt
- Timber stumps with termite damage (you may see mud tubes or hollow sections)
- Perimeter stumps lower than internal stumps (floors slope to the outside)
- The house is more than 50 years old and stumps have never been replaced
- Previous concrete stumps are cracking or tilting
In Wagga suburbs like Ashmont, Tolland, Lake Albert, and Kooringal, there are large numbers of 1950s, 1970s homes with original or once-replaced timber stumps that are now due for their second replacement.
Signs You Need Underpinning
- Concrete slab home with cracked walls, especially diagonal cracks at door/window openings
- Stepped cracking in brick veneer
- Slab visibly dipped or humped in zones
- Floor level difference measurable by spirit level across the floor plan
- Structural engineer report recommending deep foundation support
- Settlement concentrated at the perimeter, lifting slab interior away from perimeter beam
- Previous patch repairs to cracks that keep re-opening
Newer estates like Estella, Bourkelands, and Glenfield Park that were built in the 1990s and 2000s on H-class clay are increasingly seeing underpinning claims on homes that are 20-30 years old.
Can You Need Both?
Yes, though it is unusual in a single home. In some cases, an older property has been extended. The original part of the house has a suspended floor on stumps (needing reblocking), while the newer extension has a slab (potentially needing underpinning). Each part of the structure is assessed and treated differently.
More commonly, a home that has been restumped but continues to move may need underpinning on specific pier positions that are in areas of deep reactive clay that the stumps cannot reach.
Approval and Certification
Reblocking: Straightforward restumping of a like-for-like nature is often exempt development in NSW, but this depends on the scope, location, and whether the home is heritage listed. Always confirm with Wagga Wagga City Council. See our detailed guide to council approval for underpinning and restumping.
Underpinning: Almost always requires a structural engineer’s design and a building approval. The engineer produces a footing design, the work is carried out, and the engineer certifies the completed work. See our guide to structural engineer reports for foundation repair in NSW.
Cost Comparison
| Work Type | Typical Wagga Range (AUD) | Council Approval |
|---|---|---|
| Partial restumping (subset of stumps) | $5,000 to $15,000 | Often not required |
| Full restumping (all stumps) | $12,000 to $35,000 | Check with council |
| Underpinning, slab home (6-12 piers) | $15,000 to $40,000 | Usually required |
| Underpinning, major works | $40,000 to $100,000+ | Always required |
See the 2026 foundation repair cost guide for full breakdowns.
Choosing the Right Contractor
Reblocking is typically carried out by restumping specialists or building contractors with sub-floor access experience. Underpinning for slab homes is more specialised, look for contractors who work with structural engineers and use screw pier or mass concrete underpinning systems.
Wagga Foundation Repair handles both restumping and underpinning, and we work with local structural engineers for any project requiring certification. If you are not sure which applies, an inspection will confirm the foundation type and what repair is needed. See our guide to comparing foundation repair quotes before signing anything.
Frequently Asked Questions
Q: My builder said I need reblocking but the structural engineer said underpinning, who is right? A: The structural engineer’s assessment takes precedence on a question like this. Engineers are liable for their recommendations and base them on calculations, not just observation. That said, get the engineer’s report in writing and ask them to explain the reasoning.
Q: Can a restumping job be done without a structural engineer? A: Straightforward like-for-like stump replacement often does not require an engineer. However, if the scope involves changing footing depth, using a different structural system, or is in a flood-prone area (such as North Wagga), an engineer should be involved.
Q: How long does each process take? A: Full restumping of a typical Wagga home takes 3-7 days. Underpinning timelines vary from 1 day (minor screw pier work) to several weeks (major mass concrete projects). Neither typically requires you to vacate the house, though you may not be able to use the affected rooms during work.
Q: Is one option more durable than the other? A: Underpinning with deep screw piers provides the most durable long-term solution because it reaches below the reactive zone. Restumping with concrete stumps is also long-lasting on appropriate sites. See our concrete stumps vs steel piers comparison.
Q: My home has both a concrete slab extension and original timber floor, can one contractor handle both? A: Yes, provided the contractor has experience with both systems. Make sure the scope of work clearly addresses both areas, with appropriate specifications for each.
Not sure whether you need reblocking or underpinning? Get a free on-site assessment; if a contractor is confirmed for your area, they’ll carry out an inspection and you’ll get an honest recommendation.