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What to do if your home is subsiding

March 2019

Foundations are one of the most structurally significant parts of any home. While solid, stable foundations can help to support a home for years to come, buildings can and will move. A fundamental reason for this movement is changes affecting the foundation ground.

James O’Grady from ground engineering specialist Mainmark said, “Many homes experience foundation movement due to changing ground conditions; this can be known as subsidence. When the ground can no longer support the building, the foundations gradually move downwards, causing the home to sink in one area or across the entire foundation footprint.”

While seasonal or extreme weather conditions, such as drought or floods, poor drainage, leaking pipes and invasive tree roots can all affect a home’s foundations, so too can nearby construction.

“Whether it’s a neighbour adding an extension or inground pool, new building developments or a major infrastructure project in the area, nearby excavation, tunnelling and drilling activity can cause vibration and ground movement, which can lead to subsidence and impact the home’s foundation stability.”

Understanding what’s happening in the ground beneath the foundations, including the type of soil the house is built on, is important when identifying the cause and possible solutions for the subsidence, to help homeowners protect their biggest asset.

Five tips for protecting your home from subsidence

1. Know the signs of subsidence
It is vitally important to identify the signs of subsidence early and act without delay. Every home design is different and the signs may not be immediately obvious, so it helps to know what to look for.

“Window frames and doors becoming jammed or misaligned in timber homes, or cracking walls in brick construction, may be the first signs of structural issues caused by subsidence. Skirting boards separating from the wall or the formation of puddles around the perimeter of your home may also indicate foundation ground issues,” said O’Grady.

2. When to worry about cracks in walls
While smaller hairline cracks in walls are fairly common and not usually cause for concern, large cracks may appear because the property’s foundation has shrunk or lost its strength, causing all or a part of a building to sink. Problematic wall cracks typically start at windows, doorways or corners of buildings, and are often zig-zag/stepped cracks in brickwork which usually follow the mortar lines. They are typically wider than 5mm (or half a centimetre) or big enough to insert your little finger into.

3. Be aware of soil conditions beneath your home
Reactive clay soil is prevalent in parts of the North Island, particularly Auckland and surrounds. These soils can shrink, or crack and shift during hot weather, and then expand during wetter seasons.

4. Consider the different solutions available
Modern remediation solutions can be applied with minimal impact. They are also non-invasive, fast and cost effective when compared to traditional underpinningmethods, with homes often re-levelled within a day, without any need for occupants to vacate the property or move furniture. When a glass sliding door in an Upper Hutt home became noticeably misaligned, poor soil bearing capacity was the likely cause. Mainmark used its proprietary Teretek® engineered resin injection to improve the foundation ground while raising and re-levelling the slab, without any need for excavation. The ground was stabilised quickly, helping the homeowner to avoid the time and expense of alternative underpinning methods.

5. Always seek expert advice
If signs of subsidence have appeared, consult structural and geotechnical engineers or ground engineering experts. Mainmark has treated more than 11,000 sites throughout Australasia, from single-storey homes to large commercial buildings.

For more information and advice about ground issues and remediation solutions, visit Mainmark or call 0800 873 835.


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