Cracks in a California house: when they are normal and when to call a structural engineer

“Is it going to come down?”

Few words scare a first-time buyer like “crack” in a report. Inspector Aguiar says he hears the same question all the time — and that the answer, the vast majority of the time, is no.

A state that shakes every day

The seismic network records dozens of small earthquakes a day in Southern California alone, most far too small to be felt. Add clay soil that expands and contracts with moisture, tree roots, the natural shrinkage of concrete and the age of the house, and hairline cracks in stucco, slabs and driveways become almost universal.

Two kinds of foundation, two readings

To read cracks, it helps to know what the house sits on:

• Concrete slab: the house is built directly on a concrete pad. There is no space underneath, so the inspector cannot see the foundation — only indirect signs.
• Raised foundation: the wood framing sits on a concrete base with a crawl space underneath. The inspector can get in and look at the base, the supports, moisture and plumbing.

When it is normal

• In one house on a slab, pulling up the carpet revealed a floor full of wide cracks. But doors and windows opened and closed normally and the walls had no cracks. The cause was concrete poured with too much water — a workmanship issue that causes shrinkage, with no structural damage.
• In another purchase, the inspection through the crawl space found a crack in the foundation. The repair was negotiated with the seller. An inexperienced buyer might have walked away; for someone who understood it, it was a one-time repair.
• In San Diego, Aguiar counted 15 cracks in the concrete foundation of a house with a crawl space. None of them, in his view, put the house at risk; the fix is bolted-on metal reinforcement.

“That doesn’t mean your house is going to fall down.” — inspector Aguiar

When to call a structural engineer

Doors and windows that start sticking, with cracks running out of the corners, can point to structural movement. The most widely accepted warning signs are:

• doors and windows that stopped closing properly, especially with diagonal cracks at the corners;
• wide cracks — Aguiar’s reference is an opening you can fit your hand into, down in the crawl space — or cracks that keep growing;
• a noticeable slope in the floor, or a step between the two sides of a crack;
• exterior walls that bulge or lean;
• cracks that line up with moisture, a leak or soil erosion.

The engineer decides, not the general inspector

In those cases, the general inspector should not hand down a verdict but refer you to a licensed structural engineer, who evaluates the cause and, if needed, the repair. Inside the contingency window, that report can support a repair request, a credit or walking away.

The bolts insurers want to see

In houses with a raised foundation, the wood sill that rests on the concrete base has to be bolted to it. Aguiar has gone back to a house just to photograph the bolts an insurer asked to see.

The reason is seismic. In older houses, especially those built before 1980, the framing may simply rest on the foundation and can slide off in a strong earthquake. The standard retrofit combines anchor bolts and, where there are short walls in the crawl space (cripple walls), bracing with plywood. California has a program, Earthquake Brace + Bolt, that helps pay for this retrofit in selected ZIP codes.

A buyer’s checklist

• Open and close every door and window during the showing.
• Ask whether the house has a crawl space and whether the framing is bolted to the foundation.
• Ask for photos of the relevant cracks in the report, with something for scale.
• Call a structural engineer when the warning signs are there — do not wait for closing.
• Check the history: foundation repairs and retrofits should have a city permit.