Seattle sits in a seismically active region, the Cascadia Subduction Zone, nearby crustal faults, and the Seattle Fault all pose meaningful earthquake risk. Older homes built before modern seismic codes have specific vulnerabilities that can be addressed with targeted retrofits.
The Most Common Vulnerability: Cripple Walls
Most pre-1970 Seattle homes have a short wood-framed “cripple wall” between the foundation and the first floor. In an earthquake, these walls can collapse sideways, causing the house to slide off its foundation. Plywood sheathing bracing of cripple walls is the most cost-effective retrofit available.
Cost: $5,000-$20,000 depending on perimeter length and access
Permit required: Yes in most Seattle jurisdictions
Anchor Bolts
Many older homes are not bolted to their foundations, they simply rest on them. Adding anchor bolts and mudsill plates prevents the house from sliding during ground motion.
Planning a project?
Working out what your lot will allow, and what it will cost?
Send David the address and a few lines about what you want. The first feasibility check is free and usually takes about a day: what the site and the code are likely to allow, and what to check first.
Architect since 1996, member of the AIA and NCARB. Three Masonry Excellence Awards for commercial projects; the Carr residence was featured in Above The Lake and Lux magazines.
Hillside Homes
Homes on Seattle’s many hillsides with post-and-pier construction (common in Ravenna, Montlake, Queen Anne, and Bellevue hillside areas) are particularly vulnerable. Hillside bracing involves adding diagonal bracing and connection hardware between posts and the structure above.
When Renovations Trigger Seismic Upgrades
Major renovations in Seattle can trigger seismic upgrade requirements. Projects that increase floor area by more than 50% or that involve structural changes above the foundation may require seismic upgrades to the existing structure. An architect and structural engineer working together will determine what is required.
Piper Cole Architects coordinates seismic engineering for renovation and addition projects. free feasibility check, call 425-753-6452.
