All concrete foundations crack. Concrete is brittle, and shrinks after initial installation. We all live and work in buildings with foundation...

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All concrete foundations crack. Concrete is brittle, and shrinks after initial installation. We all live and work in buildings with foundation cracks. Shrinkage cracks are not a problem. But what’s a structurally significant crack and what’s not worth a second glance?

The problem

The pictured foundation under this home on Seattle’s Queen Anne Hill had some pretty obvious problems. Location, direction and patterns of cracking can tell more about the condition of the foundation than just the size of one particular crack.

It is the context of the crack, if you will, that is more important than its mere existence.

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A foundation is a rigid box. Significant settling cracks never appear in only one location. Picture an open-top cardboard box in your mind, and imagine cutting vertically down one edge with a knife. In order for settling to occur at the cut, another side of the box, either 90 or 180 degrees away, must be cut also. Of course, if there are doors or windows in a walk-out basement, that opening in the foundation will act as a surrogate second cut and allow settling to occur.

A general rule of thumb says a foundation crack larger than the width of a fingernail is worthy of a second glance, and that larger than ¼-inch is significant. However, there are many variables, including age of the foundation (to determine if it has reinforcing steel), location of the crack, weight distribution of the house and lateral separation of the foundation.

For example, the pictured foundation has three floors above it, was built on a steep hillside, had badly mismanaged external water drainage systems and — most telling — it was built 80 years ago with no footing, nor a lick of rebar.

Conversely, looking at a slightly larger-than-fingernail vertical crack on a single-level home built eight years ago on a level and stable lot would not be a concern at all.

A horizontal crack in a foundation is rare and typically means outside pressure pushing inward. This can mean foundation failure and should always be looked at. Vertical cracking, on the other hand, is typically shrinkage, and in most cases is harmless.

Cracks on a 45-degree angle, as pictured, mean settling (same is true of drywall and plaster walls inside). The size of the crack at the top versus the bottom can give clues as to which way the settling is occurring.

Generally, the cracks are larger at the top in our area, as building corners are where the majority of settling occurs due to hillsides and downspouts.

On the left side of the picture, the foundation has settled dramatically, going where gravity attempts to take everything on Queen Anne — down. The foundation to the right of the crack has remained in its original location. Out of the picture, the adjacent foundation wall 90 degrees to the left had eight or 10 smaller corresponding cracks over 30 feet, the sum of those smaller cracks being equal to the pictured larger crack (getting back to our rigid-box example).

The repair

Foundations can be repaired by bolting, pinning or strapping, or even complete replacement of damaged sections.

In this case, another engineered option was pursued, and it is partially visible. The contractor built a foundation footing parallel to the inside of the original foundation. A wood-framed wall was built on top of the new concrete footing, along two adjoining walls. The walls were then bolted to the new footing and covered with plywood to resist lateral movement. This repair left the old crack in place by simply eliminating the need for the foundation in that location, except to hold a few inches of dirt off the new wood wall.

When doing foundation repairs where significant settlement has occurred, often the home is jacked back to, or close to, original level, while in another case it may just be secured at present height, preventing further movement.

This decision depends on the amount of movement, the time period it occurred, and the risk of subsequent damage that will occur when jacking it back up.

Darrell Hay is a local home inspector and manages rental properties. Send e-mail to dhay@seattletimes.com. Sorry, no personal replies.