How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manufactured and modular homes
Why is the concrete window sill cracking?
Wednesday, June 27, 2018
The exterior window sills in concrete block homes are often precast in a factory and set in place by masons when the blocks are laid. Steel reinforcing bars are cast into the center of the sill to strengthen it. Concrete has high compression strength but low resistance to bending (tensile strength), so the steel reinforcement is necessary.
Water is sometimes able to penetrate the concrete because of damage to the surface or just deterioration over time. Once the moisture reaches the steel inside, rust begins. Because corrosion is an expansive process, the rusty steel bar eventually begins to open a crack, and later starts pushing off small chunks of concrete. The process is called “spalling,” and you can learn more about it at our blog post There's cracks running along the home's concrete tie beam. What's wrong?
Here’s another, more advanced, example of concrete spalling at a window sill.
Also, see our other posts How do I recognize serious structural problems in a house? and What causes a horizontal crack in a block or brick wall? and What are the places to look for structural cracks in a house?
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To learn more about exterior walls and structures, see these other blog posts:
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