What does a sinking floor mean?

Sinking concrete floors are an indication of building subsidence (when the ground sinks due to movement of underground material).

A building can sink at various points or the collapse can be spread across the entire footprint of the foundation, depending on where the affected ground is and how large an area it is. This can be very serious, and if left untended is likely to worsen, creating further building subsidence and other consequent damage.

Before

This heritage-listed school in Sydney had lost ground support at one end, so the engineers called in Mainmark

After

The heritage-listed school, re-supported and raised back to level, plus some sandstone replaced and joints re-pointed

Sinking floors in heritage buildings

Mainmark has delivered successful results to hundreds of heritage buildings. Our concrete floor levelling methods are a great solution, for many reasons, not least of all raising, re-levelling and re-supporting the building gives it a new lease of life, allowing an historical structure to stand strong into the future.

Sinking floors can be common in heritage buildings, often due to old footings as well as subsidence. As the building and its floors are brought back to level, any internal and external wall cracks generally close up too. Most importantly, structural damage is resolved, and with a stable foundation the future of the building is sound.

What causes subsidence?

There are many different causes of subsidence and various contributing factors. All involve some sort of change in the ground, which, in turn, generates movement of the soil. For example, droughts dry the ground, resulting in the soil (especially clay) contracting. Seismic activity shakes the ground, often resulting in liquefaction which ‘squeezes’ liquid up from the ground. Excavation and construction, even heavy traffic and machinery vibration, can move, displace and alter the condition of the ground.

As well, different types of ground are more affected by different conditions. For example; clay is particularly prone to contracting in drought conditions, gravel and stony grounds can be more affected by nearby excavation, and excess water has a softening effect on dirt. Subsidence can happen over large areas of land or in a small targeted area. It can occur over a lengthy period of time, or can be an immediate reaction to nearby activity or natural events. When ground has been affected, building subsidence is common. Foundations or footings are no longer ably supported, sinking due to weak ground.

Likely causes:

Water flooding the ground

Drought drying out soil

Washaways from broken pipes (such as water, sewer, stormwater drainage)

Poorly compacted fill

Liquid, gas or mineral resources being removed from the ground

Erosion

Earthquake and seismic activity

Tree roots can suck moisture from the ground

Vibrations caused by heavy road traffic or by machinery

Absence of an organised footing system – in very old buildings or in buildings with additions or alterations

Nearby excavation

Heavy loading

Deterioration of retaining walls

How to fix sinking floors?

The only way to fix sinking concrete floors is to raise foundation, re-level and re-support the building. Mainmark specialises in concrete levelling. Level correction not only re-levels a building, but also strengthens any weak ground under and around the perimeter of the foundations, ensuring a stable base and helping to avoid any recurring sinking issues. Traditional level correction or “underpinning” methods involve excavating or digging up parts of the foundation, pouring in concrete, waiting for it to set and then jacking up the building off the concrete blocks. This method can be:

  • time consuming (often requiring weeks or months)
  • messy
  • usually means you’ll have to vacate part or all of the building while tradespeople work

Mainmark methods are much friendlier, both to the building and to those that occupy it. Our non-invasive concrete re-levelling techniques are cost and time efficient.

Various product warranties and Building Code requirements apply (please contact us to see which apply in your region or country).

Key benefits of Mainmark’s level correction methods include:

Floors and buildings are brought back to level

Non-invasive techniques keep further damage to the building, and the landscaping around it, to an absolute minimum. We don’t tear up floors and excavate ground, so historical integrity remains unaffected

Internal and external wall cracks generally close up, leaving only cosmetic treatment (plastering, painting and re-pointing brickwork)

Jammed doors and windows can usually operate properly again

Methods are effective on various foundations, such as, bluestone, brick and sandstone

The methods we use strengthen weak ground, so the building becomes level and more stable.

Mainmark level correction methods used on heritage buildings:

Read about how we helped the Heritage listed Bank of New Zealand building in Christchurch stand strong.