New Zealand is the land of the long, white rain-cloud, with plenty of water draining on to the land and finding its way to the sea.
The leaky house scandal of the past five years has raised our awareness of the impact that water has on a house's structure, but with the recent land subsidences and slips around houses in Eastbourne, Wellington and the Bay of Plenty, homeowners need to also be aware of the impact of water on the land around their houses.
Even in areas of bush or trees, land may not be stable. Until recently, stormwater in most urban and suburban areas drained into the ground from run-off from roofs or paved surfaces.
After years of water pouring into the ground, the land becomes saturated. A disturbance as small as chopping down a tree, moving dirt or adding buildings may set off a chain of slips and slides.
How can you be sure that your home or one you want to buy is on stable land? Fortunately there are some preliminary visual inspections and more detailed engineering work that can detect potential problems.
A house inspection should include visual inspection of the land around it for signs of movement. On sloping land, inspectors look for cracks in the ground behind retaining walls and check whether the face of the wall is beginning to bow.
Where the wall is concrete or concrete block, inspectors also look for hairline cracks that may indicate the wall or the dirt behind it is shifting.
While trees are good for holding soil together and absorbing water, they can also cause problems: pines, for example, are shallow-rooted and can quickly become unstable, while trees with large spreading roots can block drains or put pressure on retaining walls.
Inspectors also look for signs of water seepage: checking to see if there are pools of water being held in the ground that may cause it to fall or slide. They check for drainage or holes that allow water to move away and not pool.
But engineer Lou Uren, of NZ House Inspections Waikato, warns that poorly placed drains can also do damage. He has seen pipes draining uphill, or laid across the top of a gully which will erode the land and eventually force water to burst out. And drains are no good if they are blocked by roots or branches.
Inspectors also pay attention to the beach or cliff front of coastal properties, checking for early clues of subsidence.
Sandy areas are more likely to move than clay or rock, but again there are clues in the surrounding area that inspectors look for.
Have a look at what other home-owners have done with rock walls or other attempts to hold back the sea. Check with the local council about attempts to halt coastal erosion.
Where problems are detected or where retaining walls are substantial, inspectors recommend a more in-depth report by a structural or geo-technical engineer.
These include tests of materials to determine what the wall is made of, checking building history or permits (to see if the structure is up to the job), examining ground structure, and may include taking core soil samples.
While such in-depth studies may cost thousands of dollars, this may be worth it compared to the tens of thousands of dollars that may be dropped from the purchase price, or the hundreds of thousands of dollars of damage that subsidence could inflict on a property.
If your property does suffer from damage caused by earthquake, natural landslip, a storm or flood, you may be covered by the Earthquake Commission. EQCover is automatic when you take out a home or contents fire insurance policy, as your insurance includes a premium for EQCover.
But this does not include damage caused by, ``movement of the ground due to below-ground subsidence, soil expansion, soil shrinkage, soil compaction or erosion'' - in other words, subsidence is not covered.
In some circumstances, EQCover also covers the costs of protecting your property if it is not damaged but in immediate danger from a disaster.
However, EQC cannot pay for the cost of protection works against future natural disasters, that is up to property owners.
Gary Koornneef - NZ House Inspection Company