Two different fixes for two different foundations

People often use restumping and underpinning as if they mean the same thing. They don't. Both are about stabilising a house that has moved, but they apply to different types of foundation. Which one you need depends almost entirely on how your house was built in the first place.

The simple version is this:

Restumping (also called reblocking) replaces the stumps under a house with a suspended timber floor.
Underpinning strengthens or deepens the footings under a house built on strip footings or a concrete slab.

How to tell what your house sits on

Start by looking at the floor and the base of the walls.

Timber floor on stumps. If your floors are timber boards and there are subfloor vents in the external walls, the house almost certainly sits on stumps. You may be able to see the stumps through a vent or a subfloor access hatch. This is common in Melbourne weatherboard homes, Edwardian and interwar houses, and many post-war brick veneer homes in the middle and outer suburbs.

Brick on strip footings. Some older double-brick houses, particularly Victorian and Edwardian homes in the inner suburbs, sit on continuous footings under the walls. They might have a timber floor inside, carried on stumps or on low internal brick walls, while the external brick walls sit on their own strip footings.

Concrete slab. Many newer homes are built on a concrete slab. There is no subfloor and no vents, and the floor is the slab itself, usually covered in tiles, carpet or floating floorboards.

Some houses are a mix. A period home might sit on stumps with a later brick extension on a slab at the back. In those cases, different parts of the house can need different repairs.

What restumping involves

On a house with a timber floor, the stumps carry the bearers, the bearers carry the joists, and the joists carry the floor. When stumps rot, sink or lean, the floor goes out of level.

Restumping means jacking the house up in sections, removing the old stumps, digging new footings, and installing new concrete or steel stumps. The bearers are then re-levelled and fixed to the new stumps. Damaged bearers or joists are usually replaced at the same time.

Signs that point towards restumping include:

Sloping or springy timber floors
Doors that stick or swing on their own
Gaps opening under skirting boards
Visible rot, leaning or sinking stumps under the house

What underpinning involves

Underpinning is used where the walls themselves sit on footings that have moved. That can happen when reactive clay shrinks away from the footings in a dry spell, when a leaking pipe or poor drainage softens the soil, or when the original footings were too shallow for the ground.

There are several underpinning methods, and the right one depends on the soil, the house and the engineer's design. In broad terms, the work involves digging beneath sections of the existing footing and placing new concrete or piers to transfer the load down to more stable ground. It is usually done in stages so the wall is never left unsupported.

Signs that point towards underpinning include:

Stepped cracks in brickwork, following the mortar joints
Cracks that are wider at the top than the bottom, or the reverse
Brick walls that are visibly leaning or bulging
Cracks in a concrete slab, or a slab floor that has dropped in one area

Why the cause matters as much as the fix

Neither restumping nor underpinning will last if the underlying cause is ignored. In Melbourne, that cause is very often water. A leaking stormwater drain, a downpipe that discharges straight onto the ground, a garden bed built up against the wall, or a large tree drawing moisture from the soil can all make the ground under one part of the house behave differently to the rest.

Before any foundation work, it is worth checking:

That downpipes connect to the stormwater system
That surface water drains away from the house rather than towards it
Whether trees or large shrubs are close to the footings
Whether there are any plumbing leaks under or near the house

Fixing these issues can slow down or stop movement, and it protects the money you spend on the foundations.

When you need an engineer

A straightforward restump on a timber-floored house is usually handled by a builder under a building permit. Underpinning almost always needs a structural engineer's design, because the footing sizes, depths and spacing have to suit the soil and the loads involved. For houses with significant cracking, an engineer's report is also a good way to separate cosmetic cracking from real structural movement.

Getting the diagnosis right

Because the two fixes are so different, a correct diagnosis comes first. A house with sloping floors and a cracked brick extension might need restumping at the front and underpinning at the back, or it might only need its drainage fixed and some re-packing. Paying for the wrong repair is an expensive mistake.

Melbourne Restumping Co can inspect your house, work out what kind of foundation it sits on, and explain what is causing the movement. If the job calls for an engineer, we'll tell you so and explain why.

Restumping or Underpinning? Working Out What Your Melbourne House Needs