Foundation Engineering / Technical Guide
Underpinning can stabilise a house affected by foundation movement, but it is not the right solution for every crack or moving slab.
Structural engineer Matthew Cornell explains how underpinning works, when it may be necessary and why identifying the cause of movement must come before choosing a repair.
Underpinning is a method of strengthening or stabilising an existing building’s foundations. House underpinning commonly involves constructing additional concrete footings or engineered piers beneath existing foundations to support the building on suitable ground. This guide explains how underpinning works, when it is necessary and when another repair may be more appropriate.
These supports may be reinforced concrete footings or engineered piling systems that transfer building loads to suitable ground below.
Underpinning can be an effective solution when a building has suffered foundation settlement. But it is not the correct response to every cracked wall, sloping floor or moving slab.
The Most Important Thing to Understand
Before deciding to underpin a house, establish why its foundations are moving. Some movement is caused by settlement that is unlikely to reverse. Other movement is associated with changing moisture conditions in reactive clay soils. These problems may require very different solutions.
The starting point
Cracks in Your Home: Do You Need Underpinning?
Finding cracks in your house can be unsettling, particularly when they appear to be getting wider or are accompanied by doors that stick or floors that no longer seem level.
Homeowners often contact us believing their house needs underpinning because a contractor, builder or previous consultant has suggested it.
Sometimes underpinning is appropriate. Sometimes it would be an expensive way of treating the wrong problem.
Crack width alone does not establish whether underpinning is needed. An engineer should also consider the cracking pattern, floor levels, foundation construction, drainage, ground conditions and history of movement.
The engineering explanation
What Is Underpinning?
Underpinning is the construction of additional foundation support for an existing building.
The new support is designed to transfer some or all of the relevant building loads to ground or foundation elements capable of carrying those loads with acceptable movement.
In residential construction, underpinning commonly involves reinforced concrete piers and supporting connections beneath existing footings. Other projects may use steel screw piles or alternative engineered systems.
An individual new support is sometimes called an underpin. The overall construction process is called underpinning.
Underpinning may be designed to arrest further settlement, improve support or, in some circumstances, allow controlled lifting of part of a building.
It does not automatically restore a house to its original level, remove every crack or eliminate every future source of movement.
Choosing the correct solution
When Does a House Need Underpinning?
Underpinning may be appropriate where an existing footing has settled because the supporting ground is inadequate and the cause of settlement cannot reasonably be addressed through simpler measures.
Examples include:
- footings bearing on loose or inadequately compacted fill;
- settlement of compressible or weak founding material;
- inadequate foundation depth or support for the actual site conditions;
- localised loss of foundation support; and
- situations where a damaged building needs additional structural support before repairs can proceed.
Whether underpinning is appropriate depends on the cause of movement, building construction, geotechnical conditions, structural loads and practical repair options.
The presence of settlement does not, by itself, prove that underpinning is the best solution.
An important distinction
When Is Underpinning Not the Best Solution?
One of the most important distinctions in Australian residential engineering is the difference between foundation settlement and movement associated with reactive clay soils.
Foundation Settlement
A footing moves downward because the supporting ground compresses, consolidates or otherwise loses support. Depending on the cause, the movement may be largely permanent.
Underpinning may provide an effective means of transferring loads to more suitable ground.
Reactive Clay Movement
Clay soils expand and shrink as their moisture content changes. This can cause different parts of a building to rise or fall over time.
Managing the source of moisture variation may be more appropriate than installing deep supports beneath selected footings.
Underpinning a structure affected by reactive soil movement requires particular care. Supporting one part of a building differently from the remainder can introduce new differential movement and distress.
Do Not Assume a Cracked Slab Is Subsiding
Seasonal cracking, changes in door clearances and variations in floor levels may indicate moisture-related ground movement rather than permanent settlement.
Read our related article: What Is Slab Heave?
Construction methods
How Is Concrete Underpinning Performed?
A common form of concrete underpinning involves excavating new footing locations beside an existing building and connecting the new supports to its existing foundations.
The size, depth, location and spacing of the new footings must be designed for the particular building and ground conditions. There is no universal underpin spacing or depth suitable for every house.
Excavation and Reinforcement
The contractor excavates to the required founding material using a construction sequence that maintains adequate support to the existing structure.
Depending on the design, reinforcement is installed in the excavation and the new concrete footing is constructed.
Transferring the Building Load
The new support must be connected to the existing foundation so that building loads can be transferred as intended.
This may involve an engineered concrete connection, a support arrangement beneath the existing footing, controlled jacking or another detail suited to the structure.
Excavation beneath existing footings must be carefully sequenced and controlled. Unplanned undermining can destabilise a building.
Can Underpinning Lift a House?
In some projects, hydraulic jacks are used to lift settled portions of a structure before the new supports are permanently engaged.
Jacking is not always necessary or appropriate. Lifting a building can affect walls, finishes, plumbing, connections and other parts of the structure.
Some settlement cracks may close during lifting, but others will remain and require separate repairs.
Access and location
Can Internal Footings Be Underpinned?
Yes. Underpinning is not limited to the external walls of a house.
Where an internal footing requires additional support, access may involve removing floor finishes and cutting through a concrete slab to excavate beneath or beside the footing.
Internal work can be more disruptive because of restricted access, dust, services, finishes and the need to protect occupied areas.
Project planning
How Long Does Underpinning Take?
The construction period depends on the number and depth of supports, ground conditions, access, excavation methods, concrete curing, jacking requirements and associated repair work.
A relatively small underpinning project may take several weeks, while more extensive or complicated projects can take considerably longer.
Some work can be completed while the property remains occupied. Other projects require temporary relocation or restricted access to affected rooms.
Ask the contractor to provide a project-specific program rather than relying on a standard duration.
Homeowner guide
How to Arrange Underpinning in Five Steps
If you suspect your house needs underpinning, the following sequence will help you avoid committing to a major repair before the problem has been properly diagnosed.
Step 01
Engage an Independent Structural Engineer
Start with an investigation of the movement and damage, not a quotation for a predetermined repair.
Describe the symptoms: cracking, changes in floor levels, sticking doors, movement history and any previous repairs.
The engineer may need to:
- inspect and document the damage;
- examine the building and its surroundings;
- measure floor levels where relevant;
- review original structural drawings and soil reports;
- consider drainage, vegetation and moisture conditions;
- review relevant underground service information; and
- recommend geotechnical or other specialist investigations.
The extent of investigation should be proportionate to the problem. Not every property needs every test.
In Queensland, check the engineer’s registration through the Board of Professional Engineers Queensland.
Before recommending underpinning, the engineer should consider whether drainage correction, moisture management, monitoring or another form of repair would be more suitable.
Step 02
Investigate the Ground Conditions
If underpinning remains a possible solution, an appropriate geotechnical investigation will usually be needed.
This is different from assuming that an existing residential site classification report contains everything required for underpinning design.
The investigation may need to establish:
- soil and rock types beneath the building;
- the depth and properties of suitable founding material;
- groundwater or moisture conditions;
- evidence of fill, soft material or other ground-related problems; and
- the information needed to design the proposed foundation system.
The investigation depth depends on the site and proposed system. It should be specified for the project rather than assumed to be a fixed number of metres.
See also: Soil Tests: What They Mean.
Step 03
Obtain the Engineering Design
If underpinning is justified, a suitably qualified and registered engineer prepares the design or arranges the necessary engineering design services.
The documentation should address the location and capacity of supports, connections to the existing structure, relevant construction sequencing, inspection requirements and any proposed lifting.
The original diagnostic investigation and the detailed underpinning design are separate tasks and may be undertaken by different engineering practices.
Cornell Engineers’ role: Our current practice focuses on investigating structural movement, determining likely causes, assessing repair options and providing independent engineering reports. Detailed underpinning design and construction documentation may be referred to an appropriately experienced design engineer.
Step 04
Obtain Contractor Quotations and Approvals
Ask experienced underpinning contractors to quote against a defined scope and engineering design so that the quotations can be meaningfully compared.
Clarify:
- excavation and ground-condition allowances;
- access, demolition and reinstatement;
- temporary support and construction sequencing;
- jacking, if specified;
- engineering inspections and certification;
- plumbing checks and service protection;
- building approvals and certifier arrangements; and
- potential variations and exclusions.
Confirm approval and certification requirements with a building certifier before work begins. Substantial underpinning work in Queensland will ordinarily require building approval, but the approval pathway should be checked for the particular project.
Step 05
Construct, Inspect and Document the Work
The contractor undertakes the work in accordance with the approved design and required construction sequence.
Engineering inspections should occur at the stages specified for the project, including before relevant excavations or reinforcement are concealed by concrete.
If jacking is required, it should be undertaken under an appropriate engineered procedure with monitoring of the building’s response.
At completion, obtain the relevant inspection and certification records, final measurements where applicable, and details of any outstanding repair or maintenance requirements.
Queensland requirements
Does Underpinning Require Building Approval?
Underpinning is structural work to an existing building. In Queensland, substantial underpinning projects will ordinarily require building approval and appropriate engineering documentation.
The exact requirements depend on the nature and extent of the proposed work. A building certifier should confirm the applicable approval process and required inspections before construction starts.
The Queensland Building and Construction Commission provides information about building approvals, certification and the responsibilities of owners and contractors.
Alternative repair methods
Concrete Underpinning vs Resin Injection
Resin injection and conventional underpinning are different methods of treating ground or foundation problems.
Resin injection generally involves introducing expanding material into the ground beneath or near a structure. Depending on the system and conditions, the objective may be to fill voids, improve support or produce controlled lifting.
Conventional underpinning instead constructs additional structural foundation elements to transfer building loads to suitable supporting ground.
Neither method should be selected simply because a house has cracks.
Be Careful About Choosing a Repair Before Diagnosing the Cause
Resin injection may be appropriate for particular ground conditions, but it does not solve every form of foundation movement. It may also complicate later investigation or alternative repair work.
Before proceeding, obtain an independent engineering assessment of the cause of movement and the suitability of the proposed treatment.
We are particularly cautious about repair proposals based only on visible cracking, without adequate consideration of soil conditions, building construction and movement history.
Experience in practice
Underpinning Investigations in Queensland
Cornell Engineers has experience investigating foundation movement and assessing underpinning and other structural repair methods.
Our involvement has included evaluating cracking and movement, reviewing ground conditions, assessing proposed repairs and inspecting underpinning work.
Today, our emphasis is on difficult structural investigations, independent causation assessments and practical recommendations. Where specialist design or construction services are needed, those can be undertaken by appropriately experienced practitioners.
Common questions
Underpinning FAQs
What does underpinning mean?
Underpinning means strengthening or stabilising an existing building’s foundations by adding support beneath or alongside the original footings.
What is an underpin?
An underpin is an individual foundation support installed as part of underpinning works. Its construction depends on the selected engineering system.
Does every cracked house need underpinning?
No. Cracking can have several causes, including reactive clay movement, settlement, construction details and other building-related effects. An investigation is needed to establish whether underpinning is justified.
How much does underpinning cost?
Costs vary substantially with the number and type of supports, ground conditions, access, temporary works, lifting requirements and reinstatement. An engineering investigation and any necessary geotechnical work should precede a reliable construction quotation.
How long does underpinning take?
Smaller projects may take several weeks. More complex projects can take longer, particularly where access is restricted or extensive structural and finishing repairs are required.
Can underpinning lift a house back to level?
Sometimes. Controlled jacking can be incorporated into certain underpinning systems, but complete re-levelling is not always possible, necessary or advisable.
Can internal footings be underpinned?
Yes. Internal footings can be underpinned, although access may require removal of finishes and excavation through the existing floor slab.
Is resin injection the same as underpinning?
No. Resin injection treats ground conditions using an injected material. Conventional underpinning adds engineered foundation supports. The suitability of either method depends on the diagnosed problem.
Should I call an underpinning contractor or an engineer first?
For unexplained cracking or movement, engage an independent structural engineer first. Establishing the cause helps determine whether underpinning, another repair or no immediate structural work is appropriate.
Further reading
Related Structural Engineering Guides
Cornell Engineers
Worried Your House Might Need Underpinning?
If your building has significant cracking, unusual movement or suspected foundation settlement, the first step is to understand the cause.
Cornell Engineers investigates complex structural movement and foundation problems, including cases where underpinning has been recommended and an independent engineering opinion is needed.
Send us the property address, photographs, a description of the movement and any existing engineering or soil reports. We will assess whether the matter is suitable for our practice before preparing a fee proposal.
11 replies on “What Is Underpinning? When Does a House Need It?”
Thank you for your introduction. I have read some materials about resin injection, and my level of acceptance is probably quite similar to yours overall.
Generally speaking, I acknowledge that resin injection construction is fast and clean, and it can indeed relevel many footings effectively. However, in my opinion, its primary function is still high-pressure filling, somewhat similar to mudjacking. If the underlying foundation soil later experiences swelling, softening or collapsible settlement due to water ingress or moisture changes, this may eventually lead to a practical failure of the underpinning system. On the other hand, if the soil is reasonably firm during resin injection, the relevelling is carefully controlled, and future moisture variation can be prevented, then the strengthening should perform well, as the mechanical properties of the resin itself should generally be reliable.
By comparison, methods such as reblocking and piling, to varying degrees, usually involve some level of soil replacement and deeper footing embedment into more stable bearing layers. These deeper foundation soils are generally less affected by external temperature changes, surface water or tree root activity, and therefore tend to provide a more stable long-term system. However, these methods are also more expensive and considerably more difficult to carry out in practice.
Great article thank you, anyone in Perth West Australia that you would recommend
I am a contractor in Mechanicsville Va. Thank you for the detailed and very helpful website info.
Hi, thanks for “What is underpinning”, which gives me related in-depth knowledge. But I have a question. An underpin provides a solid platform for jacking up a concrete foundation at the time of constructing the pin and also later when further jacking is needed. The jack looks like air pump driven on the ground level. What sort of a jack is applicable please? Thanks, Michael
I faced a similar issue with my property in Vic Metro, but underpinning and restumping services came to my aid. Before receiving any quotes, I was strongly advised to consult with an engineer to ensure accuracy.
Thanks for your comment Zu. Thanks for stopping by.
Hi, my name is Ray Clough & i enjoyed reading your underpinning article
My situation is that i am a retired ( of 15 years ) all hands on builder of 40 years in Queensland ( Hervey Bay ) My daughter has a highset building of 3 owners in southwest Sydney that has a corner vertical crack ( 12 – 15 mm wide ) in brickwork at highset level only, after the concrete slab that goes out to face of the bricks around perimeter of building . This is the only serious crack i want to deal with at this time. I was wondering if you had any associates in this Sydney area that you could recommend. This is an unusual request but Sydney`s full of sharks & shonky work, so i was hoping to work with someone i could be more comfortable with there.
If this is something of no interest to you, that’s ok.
Regards Ray Clough 0408256844
I really want to have vest knowledge on underpinning. Why is it done and what can be done to less the cost of it in and already existing building. Thank you
Nice and comprehensive guide to underpinning. thanks.
I like that you mentioned how you can determine if an underpinning contractor is reputable based on what their previous clients have to say about them. My brother is thinking about opening a bakery this fall but worried that the flooring in his building may be unstable since frequent storms cause the footing to shift over time. Maybe he should rely on an experienced professional that can get the job done right.
I was never aware that prior to underpinning services, it is important to get a structural engineer’s evaluation as they will determine the extent of the restoration and will be able to predict the duration it takes. My mother is concerned as she noticed some cracks on the walls of her home. I will advise her to hire a structural engineer so that she will know how to proceed.