Void Filling – Residental

House void filling is a ground engineering process used to fill empty spaces or gaps beneath foundations, footings, floors and concrete slabs.

The treatment restores contact between the structure and the supporting ground, helping reduce movement caused by a loss of support. Depending on the void’s size, location and purpose, resin-based or cementitious materials may be used.

Possible signs of a void beneath a house include:

• Hollow sounds beneath concrete floors
• Sinking or uneven floor slabs
• Cracks in floors, walls or brickwork
• Sections of concrete moving underfoot
• Doors or windows that begin sticking
• Gaps beneath footings or slab edges
• Localised settlement around the home
• Soil washout near foundations
• Reappearing cracks after cosmetic repairs

These symptoms can also be caused by other foundation or soil problems, so an assessment is needed to confirm whether a void is present.

Voids can develop when soil beneath a house is displaced, washed away, poorly compacted or allowed to settle.

Common causes include:

• Leaking water or sewer pipes
• Broken stormwater drains
• Poor surface drainage
• Soil erosion or washout
• Poorly compacted fill
• Changes in soil moisture
• Tree roots
• Decomposing organic material
• Previous excavation or trenching
• Settlement following construction
• Flooding or prolonged heavy rainfall

Australian building guidance identifies soil settlement, erosion, tree roots and changing moisture conditions as common contributors to foundation movement.

Yes. A void can result in part of a footing, foundation or slab losing contact with the supporting ground.

Without consistent support, the structure above may begin to settle, sink or move unevenly. This can contribute to cracked walls, uneven floors, sticking doors and other signs of foundation movement.

The National Construction Code notes that foundations and slabs rely on soil with sufficient bearing capacity to support building loads with minimal movement.

The appropriate void-filling process depends on the void’s location, depth, size and surrounding ground conditions.

Small injection points are generally created near or through the affected area. A selected resin or cementitious grout is then introduced into the void in a controlled manner until the required area has been filled and support restored.

Mainmark selects the material according to the property’s specific ground and structural requirements.

Yes. Engineered expanding resin can be injected beneath a concrete slab to fill voids and restore support.

As the resin expands, it can flow into gaps and help compact loose surrounding soil. Where the slab has also sunk, further controlled injection may be used to assist with slab lifting or level correction when conditions allow.

Void filling restores support by filling the empty space beneath a concrete slab or foundation.

Slab lifting goes a step further by carefully raising concrete that has already sunk. A project may require void filling only, or it may combine void filling with controlled slab re-levelling.

The correct approach depends on whether the slab has lost support, moved out of level or experienced both problems.

Void filling alone is intended to restore support beneath the affected area. However, resin injection can sometimes be continued in a controlled way to lift a sinking floor or concrete slab towards the required level.

The amount of level correction that can safely be achieved depends on the slab condition, surrounding structure, ground conditions and extent of settlement.

Not every small gap presents the same level of risk, but a void beneath a load-bearing footing, foundation or slab should be assessed.

Repair may be recommended when the void is affecting support, contributing to movement, allowing further erosion or placing the overlying concrete at risk of cracking or sinking.

The priority is to determine the void’s size, cause and impact on the structure.

Not every small gap presents the same level of risk, but a void beneath a load-bearing footing, foundation or slab should be assessed.

Repair may be recommended when the void is affecting support, contributing to movement, allowing further erosion or placing the overlying concrete at risk of cracking or sinking.

The priority is to determine the void’s size, cause and impact on the structure.

Yes, but the leaking pipe should generally be located and repaired before the void is permanently filled.

Filling the void without fixing the water source may allow erosion, soil softening or washout to continue. Once the leak has been addressed, the ground and structure can be assessed to determine whether void filling, ground improvement or re-levelling is required.

Drainage problems contributing to the void should be corrected as part of the overall repair plan.

Leaking gutters, blocked downpipes, damaged stormwater pipes and water flowing towards the house can alter soil moisture or wash soil away from the foundations. Australian homeowner guidance recommends maintaining drainage and directing surface water away from the building.

Void filling beneath foundations or structural concrete should not be treated as a do-it-yourself repair.

Using the wrong material, pressure or volume can fail to fill the void properly, trap water, place pressure on services or cause uncontrolled slab movement. A professional assessment helps determine the void’s extent, the reason it formed and the appropriate filling method.

The cost of house void filling depends on:

• The size and volume of the void
• Its depth and location
• The material required
• Site access
• Ground conditions
• The number of injection locations
• Whether the slab or foundation has moved
• Whether ground improvement or re-levelling is also required
• Any plumbing or drainage repairs needed beforehand

A site assessment is required to confirm the treatment area and provide an accurate quote.

The timeframe depends on the void’s size, location and the material being used.

Targeted resin injection can generally be completed with less excavation and disruption than traditional removal and replacement methods. Larger voids or projects requiring cementitious fill, drainage work or foundation repair may take longer.

Mainmark will confirm the expected timeframe after assessing the property.

Homeowners can often remain in the property during targeted void-filling work.

The treatment is generally carried out through small injection points, although temporary access restrictions may apply around the work area. Mainmark will advise you of any property-specific safety or access requirements before work begins.

Void filling addresses the loss of support beneath the structure rather than the visible crack itself.

Restoring support may help prevent movement-related damage from continuing. Some cracks may partially close if the slab or foundation is also re-levelled, but remaining damage may require separate concrete, brickwork, plaster or structural repairs.

Concrete replacement may not always be necessary when the slab itself remains structurally suitable.

Void filling can restore support beneath an existing slab without removing and rebuilding the entire area. Replacement may be required where the concrete is extensively cracked, deteriorated or otherwise unsuitable for re-support or lifting.

A site assessment will determine which option is appropriate.

A residential ground engineering or foundation repair specialist can assess the visible symptoms, likely ground conditions and treatment options.

A structural or civil engineer may also be recommended where there is significant foundation movement, substantial cracking, complex structural damage or uncertainty about the cause.