What Is Underpinning? How It Works & When Your Home Needs It
The short answer
Underpinning is the process of strengthening and extending a building’s foundation so it reaches firm, stable ground. It stops a home sinking or cracking by transferring the load past weak soil, using concrete piers, screw piles or injected resin.
If a builder or engineer has told you your home needs underpinning, this is what that means in plain English, and what it involves.
Underpinning strengthens the foundation beneath an existing building. Your home was built on footings sized for the soil at the time. When that soil moves, dries out or was never firm enough, the footing loses its support and the structure drops with it, cracking walls and jamming doors. Underpinning puts new, deeper support underneath so the load bypasses the bad soil and the house stops moving.
How does underpinning work?
Underpinning works by carrying the weight of the house down to a soil layer strong enough to hold it. Rather than rebuilding the foundation, it adds support beneath the existing one. Depending on the method, that support is concrete poured under the footing, steel piles screwed deep into the ground, or resin injected to firm up the soil directly.
The house is supported throughout, so it never sits unpropped. Once the new support is in place, the footing bears on stable ground instead of the soil that was letting it drop.
The main underpinning methods
There are three methods in common use, and the right one depends on your soil, your access and how deep firm ground sits.
- mass concrete underpinning — concrete piers dug in sequence beneath the footing; the proven, traditional method
- screw pile underpinning — steel helical piles driven to deep, stable ground for soft or reactive soils
- resin injection underpinning — expanding resin injected to lift and firm the soil with no digging
When does a home need underpinning?
Underpinning is needed when a foundation has lost support and the movement is structural, not cosmetic. The tell-tale signs are stair-step cracks in brickwork that keep growing, doors and windows that suddenly stick, and floors that slope toward one corner. In Melbourne, the usual trigger is reactive clay soil melbourne shrinking in a dry spell.
Not every crack means underpinning. Our guide to the signs you need underpinning helps you tell a warning sign from a harmless one.
Underpinning vs restumping — are they the same?
No. Underpinning rebuilds or extends a footing, and is used on homes built on strip footings or slabs. Restumping replaces the timber or concrete stumps under a home that sits on stumps. Many older Melbourne homes are on stumps, so the fix is often restumping melbourne, not underpinning. We explain the difference fully in restumping vs underpinning.
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