TL;DR (generated, confidence: high)

Portland's residential seismic strengthening page outlines prescriptive criteria for qualifying single- and two-family dwellings and explains why older and newer homes remain vulnerable to earthquake damage without proper foundation anchoring and cripple wall bracing.

Key points:

Prescriptive method applies to one- and two-family dwellings, buildings not more than three stories, with cripple wall height not exceeding 14 inches in three-story buildings and four feet in one- and two-story buildings (Residential Seismic Strengthening ↗)
Requires continuous concrete foundation around entire building perimeter with foundation subgrade slope not steeper than 3:1 ratio (Residential Seismic Strengthening ↗)
Homes built before 1960 were commonly not bolted to foundations, making them vulnerable to ground movement during earthquakes (Residential Seismic Strengthening ↗)
Homes from 1970s onward can fail during earthquakes despite foundation bolting if cripple walls lack proper bracing with stronger materials like plywood (Residential Seismic Strengthening ↗)
Weak bracing materials including cement plaster (stucco) and wood siding cannot withstand moderate to strong earthquake forces on cripple walls (Residential Seismic Strengthening ↗)

Entities: method: Prescriptive method | building_component: Cripple wall · Concrete foundation | material: Plywood · Cement plaster/stucco | year: 1960

Residential Seismic Strengthening

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Source: Residential Seismic Strengthening ↗ Last modified: Sun, 19 Apr 2026 20:05:54 GMT

Prescriptive method criteria

You may use this prescriptive method if your project meets the following criteria:

One- and two-family dwellings
Buildings not more than three stories
In three-story buildings, cripple wall stud height is not more than 14 inches.
Cripple walls do not exceed four feet in height in one- and two-story buildings.
Continuous concrete foundation around the entire building perimeter.
The foundation subgrade is not steeper than three horizontal to one vertical at any point.

Common construction problems

Older homes are prone to earthquake damage

Many homes built before 1960 were not bolted to their foundations. Without this anchorage, a large earthquake can move the ground and the foundation right out from under the house. Older homes have been damaged or destroyed in every major earthquake on the West Coast.

Newer, anchored homes can also be at risk in an earthquake

Some wood frame homes built as recently as the 1970s were shaken off their foundations in recent earthquakes. Even though these modern homes were bolted down, they failed because of weak bracing materials on the cripple walls.

The cripple wall is a short wall that connects the foundation to the first floor of the house and encloses the “crawl space.” Weak bracing materials, like cement plaster (stucco) or wood siding, are not strong enough to survive the force of moderate to strong earthquakes. The cripple wall must be braced with stronger materials like plywood. If not, the next earthquake may damage the cripple wall and move your home off its foundation, even if you’ve bolted it down.