Improperly backfilled soil is the biggest culprit for house settlement.

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Why Houses Settle

So do houses settle or is it just ghost skeptics' hooey? Actually, houses do settle. When they begin a downward plunge, joints and floorboards can creak, which accounts for the mysterious sounds in settling homes. And given a long enough time line, all houses eventually will settle. When a house does, it can cause serious problems. The severity of the settlement is based on a couple of factors, including how well the house was designed and constructed and the amount of attention paid to the foundation and the ground atop which the house was built.

By far, the most common reason for a house settling is improperly backfilled soil. If you've ever puttered around in a garden after spreading fresh topsoil, you've probably noticed that even after compacting the new layer of soil, you leave footprints behind. This is much the same with a house. If a builder excavates too much soil for a basement or crawlspace and must backfill it, the process requires more attention than if the basement had been dug to the correct depth.

That's because the soil -- called non-virgin soil -- has been disturbed [source: HouseMaster]. The work of centuries of compression that created naturally compacted soil, which can provide a stable place to build a house, has been undone. To recreate a suitable base, the excavated and replaced soil has to be tamped down. If it's not compacted enough again, then the house built on top of the soil will settle.

Even if the soil is well-compacted, other construction conditions can cause house settling. An excavated area over which a house will be built isn't a good place to bury construction materials like broken studs or pieces of drywall. These materials degrade over time, creating pockets that eventually collapse. This destabilizes the soil above and can lead to a house settling. The same goes for organic material (like tree stumps or limbs) which also degrades over time. Even if the buried debris is metal or another material that won't degrade while the house stands, it can still leave pockets with the same effect.

The type of soil a house is built upon also can play a role in accelerating its settlement. Clay soil is subject to changes in expansion and contraction based on the amount of water found in it at any given time. Clay expands when saturated. This causes upheaval -- the opposite of settling -- a process that produces the same problematic results for the homeowner [source: HouseMaster]. When the clay dries, it contracts and sinks, causing settling.

Water is the enemy of your home's foundation in other ways. Tree roots that grow close to a house's foundation can create cracks by searching for water beneath it. The roots disrupt the soil compaction, leading to settlement. Tree roots also invite rainwater under the house, which can further disrupt the integrity of the soil. A house without eave troughs to ferry water away from the foundation will likely settle much more quickly than one with troughs, since water can trickle beneath the foundation.

So now you know why your house settles. But how can you tell when it's settling? Read the next page to find out.