I was running the soaker hoses around the foundation a few days ago and noticed a pretty bad crack just above the southwest corner of the foundation. Tapping at it with the toe of my shoe caused a chunk of concrete to fall away. I had no idea I possessed such power in my shoes.
I hadn’t paid much attention to this crack because it had been hiding behind shrubs up until a few weeks ago. So I had to wonder if this corner thing means my foundation is critically ill and I should be calling Dr. McPiers.
Not to worry, says R. Michael Gray, P.E. and Matthew T. Gray who have a structural engineering and home inspection firm in Houston.
It’s called a wedge crack.
What is a corner or wedge crack?
The large majority of slab-on-ground foundations will develop what are called corner cracks or wedge cracks. The name comes from the fact that these cracks develop at or very close to the outside corners of the foundation and frequently form a wedge at the corner.
These cracks develop as a result of the expansion of the brick veneer when it is warmed by the sun. When the temperature of the brick veneer rises, the brick veneer wall expands in length and pushes or slides against the slab surface. At the end of a brick veneer wall at an outside corner of the slab, there is nothing to push back and the concrete cracks at each side of the corner, forming a wedge.
Builders will usually place a piece of plastic between the bottom of the first course of brick and the slab; this reduces the friction force when the brick expands and slides against the slab. This has the practical effect of reducing the cracking on the slab at the corners but it by no means eliminates the corner cracking.
These cracks do not indicate anything unusual about the foundation. If the cracking at a corner becomes very bad, the concrete wedge may become loose and even fall off. In extreme cases, the wedge will no longer adequately support the brick veneer; when this happens, the corner will need to be repaired. This is a concrete repair and not a foundation repair.
That’s good to know. What is also good to know is the Gray’s have an fine website that although is targeted to the Houston Texas area still has lots of information that is applicable to anyone who has a concrete slab foundation perched on expansive clay soils.