Restumping vs underpinning

The distinction in one line

Restumping replaces supports that have failed. Underpinning fixes ground that has failed.

They are not alternatives you choose between on price. Which one applies is determined by how the house is built and what has actually moved.

Which do you have

Restumping applies if your house is a raised timber home sitting on stumps, with a subfloor space you can get into. The floor frame is carried on posts, and if those posts rot, get eaten or sink, the floor follows. Replacing them is restumping.

Underpinning applies if your house sits on a concrete slab or on strip footings at ground level. There are no stumps. The footing bears directly on soil, and if that soil subsides or loses capacity, the footing goes with it. Extending the footing down to competent ground is underpinning.

Most of the time it is obvious from looking at the house. Where it gets less obvious is on properties that have both, which is common on the Gold Coast: an original timber house on stumps with a slab on ground extension added later. Those two parts move independently and the joint between them is often exactly where the cracking shows up.

How the symptoms differ

Failed stumps typically show as

• A floor that slopes, often worst toward one corner
• Bounce or movement underfoot
• Gaps between skirting and floorboards
• Doors sticking, particularly seasonally
• Visible deterioration of the stumps themselves when you look

Footing or ground movement typically shows as

• Diagonal cracking stepping through brickwork, out from door and window corners
• Cracks that are wider at one end than the other, indicating rotation
• Windows and door frames visibly out of square
• Cracking in a slab floor or in tiled areas
• Separation between an extension and the original house

The overlap is that both can produce sticking doors and cracked cornices, which is why the building type matters more than the symptom for the initial sort.

How the work compares

Why underpinning needs an engineer and restumping often does not

This trips people up when they compare quotes, because one process looks much heavier than the other.

With restumping, the failure mode is visible. You can get under the house, probe every stump, take floor levels and see exactly what has gone. A competent licensed builder can scope that from evidence.

With underpinning, the failure is underground. You cannot see the soil profile at depth by looking at a crack. Whether the competent layer is one metre down or four, what the reactive zone is doing, and how load should be transferred are geotechnical and structural questions. That is why a proper underpinning job involves soil investigation and a structural engineer's design, with the contractor building to that design.

If someone offers to underpin your house after a ten minute look at a crack, with no investigation and no engineering, that is the point to get a second opinion.

Sometimes it is neither

Worth saying plainly. A meaningful share of the cracking and movement people worry about turns out to be one of the following.

• Normal settlement. Newer houses shrink and settle in their early years, and hairline plaster cracking is routine.
• A drainage problem. A downpipe discharging beside a footing, ground falling toward the house, or a leaking service. Fixing that can arrest the movement, and it costs a fraction of either remedy.
• Seasonal clay movement that reverses. Cracks that open in the dry and close in the wet are telling you about moisture, not about progressive failure.
• Floor frame issues. Bouncy or dipped floors caused by joists rather than supports.
• How the house was built. Older homes often have a small consistent fall that has never changed.

Monitoring for a period before committing is often the cheapest thing you can do, particularly where nothing is getting visibly worse.