House Underpinning Explained: Cracks, Clay and Costs

Underpinning is one of those home repair terms most people hear only after they spot a worrying crack. A diagonal split above a window, a door that suddenly sticks, a gap opening between the wall and ceiling: all can be signs that the ground under your house is moving.

This guide explains what underpinning is, what causes foundation movement in Australian homes, and how to approach the problem calmly without rushing into an expensive fix.

What underpinning actually means

Underpinning is a method of strengthening or stabilising the foundations of an existing building. Instead of replacing the whole footing, a specialist extends or supports it so the load of the house is carried by more stable ground or deeper supports. Methods vary, including concrete extensions, steel or concrete piers and resin injection, and the right one depends on soil type, the structure and the cause of movement.

It is not the same as restumping, which replaces or adjusts the stumps under a raised timber floor. Both deal with movement, but they are different jobs.

Why foundations move

Many parts of Australia have reactive clay soils, which swell when wet and shrink as they dry out. Long dry spells followed by heavy rain can lead to uneven movement across a home. Other common causes include:

• Leaking pipes or poor drainage softening the ground near the footings
• Large trees whose roots draw moisture from the soil
• Poor site preparation or footings that were not designed for the soil
• Nearby excavation or building work
• Changes in soil moisture from gardens, paving or downpipes dumping water next to the house

Warning signs to look for

• Cracks wider than a hairline, especially diagonal or stair-stepped cracks in brick
• Doors and windows that stick, jam or no longer close properly
• Gaps between walls and floors, ceilings or door frames
• Sloping or uneven floors
• Cracks in tiles or cornices that keep returning after repair

Not every crack means a structural problem. Hairline cracks are common as a house settles and as materials expand and contract. What matters is the size, the pattern and whether the crack is growing.

What to do first

1. Monitor. Take dated photos of cracks with a ruler beside them, and check them every few weeks.
2. Check water. Look for leaking taps, pipes and blocked gutters or drains near the house. Fix them through a licensed plumber where required.
3. Get an independent opinion. A structural or geotechnical engineer can assess the cause and recommend whether underpinning is needed, without a sales interest in the outcome.
4. Then get quotes. If underpinning is recommended, use the engineer's report to compare like with like.

Fixing the cause, such as a leak or a thirsty tree, can sometimes stop the movement without major work, although cracks may still need repair afterwards.

Choosing an underpinning specialist

This is structural work on your biggest asset, so take your time. Rules on licensing, engineering sign-off and council or private certifier approval differ by state and sometimes by council, so ask what applies to your job and make sure the contractor holds the right licences and insurance.

Questions to ask

• Do you work from an engineer's design, and will you provide documentation?
• What method do you propose, and why is it right for my soil and house?
• Who handles approvals and inspections where required?
• What guarantees or warranties come with the work?
• How will you protect the garden, pipes and the rest of the property?

Red flags

• High-pressure sales or a demand for a large upfront payment
• A diagnosis without any inspection or investigation of the cause
• No written quote, scope or engineer involvement
• Unwillingness to show licences, insurance or references

What affects the cost

Costs depend on how much of the house needs support, the method used, soil conditions, access, the depth required and any repairs after the work. Quotes vary widely, so compare scope as well as price. A cheaper quote that covers less work may not be cheaper at all.

Key takeaways

• Monitor cracks before panicking, and look at size, pattern and growth.
• Fix water and drainage issues first.
• Get an independent engineer's report before agreeing to underpinning.
• Check licences, approvals and warranties for your state.