Settling refers to the downward movement of a house from its original position. While all houses experience some settling, uneven downward movement is what creates problems that catch the attention of homeowners. Symptoms of uneven and unnatural settling include doors and windows becoming misaligned, porches sagging and walls separating or cracking. There are various causes for such problems, including poor preparation of the land on which the house rests.
Houses are constructed from a variety of materials, all of which expand and contract differently when they absorb or lose moisture. Tim Carter, in an article on the website Ask the Builder, says that not all cracks signify settling. Some, he says, are related to natural expansion and contraction. He describes the joining points of materials, such as where columns meet beams, as being similar to the joints in the human body. They move slightly to relieve pressure. When corners separate, however, or the 90-degree angles between floor and wall shift noticeably, excessive settling is occurring. Often the cause is related to a soil problem, such as poor compaction, bad backfill and uneven moisture in expansive soils.
When contracting to purchase a home that hasn't yet been built, the future homeowners should be aware of whether their subdivision is being graded and constructed using cut-and-fill techniques. In this situation, contractors remove dirt excavated from one part of the subdivision to even out the grading in another area. If not compacted properly, this can create excessive settling.
Back Fill Problems
One example of a backfill problem is a house built on slab over a site that was smoothed out with backfill during grading. If the contractor is careless, the fill dirt may contain topsoil full of decomposing organic matter that makes it spongy and easy to compress over time. Or the contractor may use an appropriately gritty backfill, but fail to compact it enough. In either case, the site won't evenly support the slab and excessive settling will occur. This may result in the need for slabjacking, a process in which a gravelly fill material is pumped under the slab.
Bad backfill procedures may also cause uneven settling in homes with basements or with exterior details, such as columns. A roof eave may sag if supported by a column anchored in poorly compacted fill dirt.
Prospective owners of either new or old homes need to consider expansive soils — those that either swell or contract excessively based on changing moisture levels under and around a house. It is necessary to obtain a soil report that identifies any locations of expansive soil. New construction may prevent the problem through the addition of perimeter drains or foundation piers that stabilize movement in the house's foundation. Older homes may be retrofitted with these materials.