How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What are the common problems of different types of house foundations?
Wednesday, October 24, 2018
There is no single, perfect foundation type. Each is chosen by the architect or builder based on a particular site’s topography and soil, along with kind of structure the foundation will support. Also, every region has a customary local foundation type. A coastal pile foundation, for example, is well suited and frequently used for waterfront locations that require an elevated floor level due to a flood-prone site. The piles are often milled tree trunks that rely on the friction between the soil and buried surface area of the pile for stability. Certain types of wood, like cypress, have a surprisingly long lifespan when set in the ground as a foundation and can remain structurally sound for 100 years or more. But sometimes rot sets in early, with a curious result. When we were working in the Florida Keys years ago we occasionally came across homes constructed entirely of cypress lumber, named “Moody Mansions” by their builder during the 1980s. Fast-forward 25 years, and the outside layer of buried cypress log piles had rotted away and loosened, although the interior wood was still sound. So the piles had a little wiggle room around them and, when you walked across the house from side to side, there was a definite feeling of being on a ship at sea. The floor shifted ever-so-slightly under you—a disturbing sensation. Many of the Moody Mansions ended up with a replacement foundation of concrete piers, the next type reviewed, while some homeowners just found it an amusing idiosyncrasy. The defect negatively affected the value of the homes, however.
Concrete and masonry piers, with concrete and steel reinforcement in the center, or solid reinforced concrete are another popular foundation choice in coastal or flood-prone areas. This type is also called “pier and beam” and was frequently used for inland homes in the early 20th century, when constructed with wood beams and frame over a crawl space and using brick for the piers. The piers are built over a concrete pad below ground, so any movement of the soil underneath the pads will cause tilting of the piers and settlement of the structure above. A concrete pile and grade beam combination is sometimes used as an alternative when the soil is determined to have inadequate bearing capacity for concrete pads.
The masonry pier foundation is sturdy and hurricane-resistant, but can suffer from “concrete spalling” if moisture is allowed to reach the steel reinforcing bars at the center of the piers, which causes the steel to begin to corrode. Rust generates a slow, but powerful expansion of the surface of the steel, which pops crack lines in the concrete. This, in turn, allows more moisture to get inside and the spalling picks up speed. Homes near the coast that are exposed to continuous salt mist spray are especially likely to develop this problem over time.
Spalling also occurs in concrete beams and elevated floor slabs, as shown in the photos below. When it becomes as advanced as in the second photo, chunks of concrete begin to fall out and structural integrity is threatened, which is why temporary steel shoring is visible at the left side of the foreground. To read more about concrete spalling, go to our blog There's cracks running along the home's concrete tie beam. What's wrong?
A stem wall foundation is ideal for a sloping site because the area inside the wall can be filled with dirt to create a level surface to pour a concrete floor slab, or the top of the stem wall will provide a bearing surface for an elevated wood floor with a crawl space underneath. Movement of the ground under a stem wall can cause the bottom part to rotate slightly and/or settle unevenly, with cracks in the base of the wall telling the story of the distress below. Many mid-century homes in our area combined a stem wall perimeter foundation with center rows of masonry piers and wood beams. A basement foundation is essentially a stem wall foundation extended further below ground, with excavation in the center. Though they have problems particular the foundation type, such as water intrusion, we have none of this type in our area.
The monolithic slab foundation largely replaced stem walls in the 1960s, due to ease and simplicity of construction. It is also often called a thickened edge foundation. Concrete floor slabs had already become a popular alternative to wood floors, so thickening the edge of the slab and adding some reinforcing steel to create support for the walls made economic sense to builders. It is still the most popular foundation type today in most areas of the country. Soil movement under a monolithic slab foundation produces crack patterns similar to stem walls, but they are more often observed in the wall above the slab. Some builders in our North Florida area have begun to use post-tensioned steel running through the entire floor slab to help it resist concrete slab cracking due to minor settlement or concrete shrinkage.
Also, multiple different conditions can cause structural distress in any foundation:
1) Soil erosion can be caused by surface rainwater drainage or underground water flow. We see it most often in homes built on the side of a hill or on ground sloping down to a river or lake. The photos below show two examples: a concrete footing that has become exposed above ground due to rainwater running downhill around the house and scouring the soil, and a concrete stoop and that is leaning away from the house with a rainwater drainage gully running under it. A gutter system deposits water safely distant from a home’s foundation is one solution to this problem.
2) Clay soil runs in veins under some neighborhoods in Gainesville and other areas of the country. It expands when wet, then shrinks as it dries out, causing the house to heave up and down. Homes can slowly self-destruct with the repeated wet/dry cycles, and a long drought causes the worst problems.
3) Sinkholes are another problem in our Alachua county area, along with other Florida counties, due to dissolving pockets of karst rock underlayment in the ground, by the underground aquifer flowing through North Florida. The occasional dramatic sinkhole collapse that swallows up houses and cars makes the evening newscasts, but most sinkholes collapse slowly, giving plenty warning to residents that a problem is developing under their home. To read more about both sinkholes and clay soil, see our blog “What’s my chance of buying a Gainesville home over a sinkhole?”
4) Trees near a house can heave a foundation with their roots. Just as often, though, a tree next to a home can cause soil settlement foundation problems by removing too much moisture in the soil in the area. Trees near a house provide shade, a windbreak, and pleasant views; but too close and they can be destructive. See our blog “How can trees damage a house?” for more on this subject.
Visit our blog post How do I recognize serious structural problems in a house? to learn the signs to look for, and for advice on buying a home with known structural issues check out our Should I buy a house with structural problems? blog.
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To learn more about exterior walls and structures, see these other blog posts:
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