How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What is the average lifespan of a house foundation?
Wednesday, June 13, 2018
Determining the life expectancy of something that is buried in the ground is tricky business. The variables include the amount of rain that soaks the ground, fluctuations of the water table below it, creeping tree roots, severity of temperature fluctuations—and specifically freeze/thaw cycles, movement of soil due to geological or site drainage conditions, concentrated loads at points along it, and the composition of both the soil and foundation materials.
Also, quick-fix repairs like the one shown below are an indication of a structural problem that may, or may not, be related to aging.
All these things make a lifespan generalization risky, so a range rather than a specific number is the best we can do. These numbers are based on estimates published by the International Association of Certified Home Inspectors (InterNACHI), a study by the National Association of Home Builders (NAHB), and our own experience over the years.
FOUNDATION TYPE AVERAGE LIFESPAN YEARS
• Concrete Footing - 100+ years
• Thickened Edge Concrete Slab - 100+ years
• Concrete Piers and Grade Beam - 70 to 100 years, average 80
• Treated or Cypress Wood Piles - 40 to 75 years, average 60
• Concrete or Brick Pier on Grade - 50 to 80 years, average 65
• Stacked Concrete Block on Pad (mobile home) - 25 to 50 years, average 40
Here’s a bar graph comparison of foundation life expectancies.
The Variables That Affect Foundation Lifespan
Here are some problems that can shorten the life of a foundation:
• Sinkholes - There are three major types of sinkholes: 1) solution sinkholes, 2) cover subsidence sinkholes, and 3) cover collapse sinkholes. The first two types sink slowly over time. It’s the cover collapse sinkholes, which usually fall down abruptly and dramatically, that are on the evening TV news. Our blog post What are the warning signs of a sinkhole? outlines what to look for.
• Clay soil - Although a sinkhole is the most feared underground defect, clay soil is the more common one. The elasticity of clay causes it to shrink during dry spells, then swell during seasons with heavy rains. The swell/shrink cycle causes the ground under a home built over a layer of clay soil to heave up and down. Several of these wet/dry cycles, if they are extreme, can fracture a foundation, with corresponding cracks running up the walls.
Determining whether structural cracks in a home are due to sinkhole activity, clay soil, or erosion, is complicated. Engineering experts sometimes reach different conclusions about the cause of a structural problem when examining the same home. To learn more, see our blog post What is the difference between soil subsidence, heave, creep, and settlement?
• Trees near the foundation - Most people know that tree roots growing under a home can heave the foundation but, because roots remove so much water from the ground, they can also cause soil shrinkage and foundation settlement. See our blog post How can trees damage a house? for more.
• Concrete spalling - Spalling is caused by moisture penetrating the layer of concrete covering the reinforcing steel in concrete footing, foundation pier, or grade beam. The example shown at left is at the underside of a cantilever floor slab. Over time, the steel begins to rust inside the beam and, because rust is slow but very powerful expansive process, cracks appear at the surface over the reinforcing steel. It‘s a progressive deterioration, because the open crack allows more moisture to accelerate the corrosion--formally known as ferrous oxide scale--which opens the crack further. Eventually small chunks of concrete begin falling out and, if left without repair long enough, structural failure follows.
• Poor site drainage - Rainwater draining around a house or under it on a sloped site can erode and expose the foundation, eventually undercutting it. The ground around the foundation should slope away from it on all sides for best drainage. A long-term plumbing leak in the ground under a home can also cause foundation problems.
• Wood rot - 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 foot—a disturbing sensation. Many of the Moody Mansions ended up with a replacement foundation of concrete piers, while some homeowners just found it an amusing idiosyncrasy. The defect negatively affected the value of the homes, however.
• Brick pier deterioration - Freeze damage of wet brick and a problem called “rising damp” can cause brick foundation piers to begin to crumble. For more details, see our blog post What causes the surface of old bricks to erode away into sandy powder?
To Learn More
Go to our blog post What are the common problems of different types of house foundations? for more on foundation defects.
Go to our blog post What is the average lifespan of the parts of a house? for rating of other house components. To understand the basis, potential use, and limitations of lifespan ratings, see our blog post ”How accurate are the average life expectancy ratings of home components? Are they actually useful?”
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To learn more about exterior walls and structures, see these other blog posts:
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