Seismic Retrofit for Older Seattle Homes: Weak Points and Priorities

Why are wood houses mostly safe in earthquakes, and where are they not?

Wood-frame houses perform well in earthquakes compared with many other building types. They are light, which keeps earthquake forces small, and the many nailed connections in walls, floors and roof let the structure flex without breaking. Most damage to older wood houses in past West Coast earthquakes did not come from walls or roofs collapsing; it came from a small number of predictable weak points, almost all near the ground.

The reason is how earthquake force travels. The ground moves, the foundation moves with it, and the house above must be pulled along by whatever connects it to the foundation. If that connection is weak, the foundation moves and the house does not, which means the house slides or the short walls beneath it fold. Once that happens, damage to plumbing, gas lines and everything inside follows. A retrofit strengthens that connection and the short walls, so the house moves with its foundation.

The Puget Sound region's earthquake sources, including the Cascadia subduction zone, deep intraslab earthquakes and shallow faults such as the Seattle Fault, are mapped and described by the Washington Department of Natural Resources.

Which weak points belong to which kinds of houses?

Many Seattle neighborhoods developed before the Second World War, including Wallingford, Ballard, Ravenna, Queen Anne and Capitol Hill, are full of the first and fifth types, often on the same house.

What can you look for in your own crawlspace?

With a flashlight and a few minutes, most homeowners can answer the first questions an engineer will ask.

1. Find the sill. The sill is the horizontal piece of wood sitting directly on top of the concrete foundation.
2. Look for anchors. Bolts with nuts and washers coming up through the sill from the concrete, or steel plates bolted to the side of the concrete and the sill, are anchors. Note how far apart they are, if you find any.
3. Measure the cripple walls. If there is a short wood wall between the sill and the floor above, measure its height and look at its inside face. Bare studs with no plywood are unbraced.
4. Check the concrete. Crumbling, powdery or heavily cracked concrete affects how anchors are installed and may need repair first.
5. Look up at the floor. Note whether the floor joists or blocking are connected to the sill with metal clips or just toe-nailed.

Photos of each of these, sent ahead of a site visit, help the engineer arrive prepared.

In what order should retrofit work be done?

When budget is limited, a retrofit can be phased. The order follows risk: the failure that is most likely and most damaging comes first.

1. Anchor the house to the foundation. Without anchorage, nothing else matters much.
2. Brace cripple walls. On houses that have them, unbraced cripple walls are the most common cause of collapse onto the foundation.
3. Address rooms over garages. The soft first story is a well-documented failure mode and costs more to fix, so it is often planned as its own phase.
4. Deal with the chimney. A falling chimney threatens people more than the house; bracing or removing the top portion is relatively inexpensive.
5. Consider upper-level and veneer issues. These matter in specific houses and come after the basics.

When does a retrofit need an engineer instead of a prescriptive plan?

The Seattle Department of Construction and Inspections accepts a prescriptive retrofit plan for simple houses, which specifies anchor spacing and bracing lengths without calculations. It is designed for one- or two-story houses on reasonably level lots with short cripple walls and sound concrete. Engineering is needed when the house falls outside those limits: tall or stepped cripple walls, hillside foundations, living space over a garage, irregular floor plans, poor concrete or a finished basement. Nationally, FEMA publishes vulnerability-based guidance for one- and two-family houses that engineers use alongside local requirements.

Which nonstructural steps reduce earthquake damage?

• Strap the water heater top and bottom to the wall framing, as the plumbing code requires for new installations. A falling water heater can break gas and water lines.
• Use flexible gas connectors on appliances, and know where the gas shutoff is and how to close it.
• Secure tall furniture such as bookcases and dressers to wall studs.
• Latch cabinets in kitchens and above beds.

What does a retrofit not do?

A residential retrofit is designed to keep the house on its foundation and reduce the chance of collapse. It does not make a house earthquake-proof. Cracked plaster, broken windows and damaged chimneys can still occur. Houses on steep slopes or liquefaction-prone soils face ground hazards that structural work alone cannot remove; those need geotechnical evaluation. And a retrofit addresses the house's structure, not the contents inside it.

Next steps toward a retrofit for your house