Key Takeaways
- Foundation bolting anchors your home's wooden sill plate to the concrete foundation with steel bolts, preventing the house from sliding off during an earthquake — the single most common failure mode in older California homes.
- Cripple wall bracing reinforces the short stud walls in your crawl space with structural plywood, preventing them from collapsing and dropping the house onto the foundation.
- Together, bolt-and-brace retrofits cost $5,000–$10,000 for most single-family homes and can be completed in one to three days with minimal disruption.
- California allows homeowner-performed retrofits under prescriptive standard plans (EQ1, EQ2, EQ3) for qualifying homes — no contractor required, though a building permit and final inspection are still mandatory.
- The CEA Brace+Bolt program offers up to $3,000 in grant funding toward standard bolt-and-brace retrofits for eligible homeowners in high-risk ZIP codes.
- Every pre-1980 raised-foundation home in a seismic zone should be evaluated for this retrofit — it's the highest-impact, lowest-cost earthquake protection available for residential structures.
What Is Foundation Bolting?
Foundation bolting — also called anchor bolting or sill bolting — is the process of mechanically connecting your home's wooden frame to the concrete foundation beneath it. In most wood-frame homes, the lowest piece of the wall framing is a horizontal board called the sill plate (or mudsill), which sits directly on top of the concrete foundation. Foundation bolting drills through this sill plate and into the concrete, then secures the connection with a steel bolt, plate washer, and nut.
This sounds simple, and mechanically it is. But the consequences of not having this connection are severe. During an earthquake, the ground moves laterally — side to side — and the concrete foundation moves with it. If the wooden house frame isn't bolted to the foundation, the house stays behind due to inertia while the foundation moves out from under it. The result: the house slides off its foundation, severing all utility connections (gas, water, electrical, sewer) and creating a structure that's often a total loss even though the house itself may be structurally intact.
This exact failure mode has occurred in nearly every major California earthquake. In the 1994 Northridge earthquake, thousands of homes slid off their foundations because they lacked adequate anchor bolts. Many of these homes were built before the 1950s, when anchor bolts weren't required by building codes. Others were built with bolts that were too few, too widely spaced, or improperly installed.
How Anchor Bolts Work
Modern residential anchor bolts are typically ½-inch or 5/8-inch diameter steel bolts embedded in or attached to the concrete foundation. There are two main types used in retrofit work:
Expansion bolts (also called wedge anchors): These bolts are inserted into a hole drilled through the sill plate and into the concrete. When the nut is tightened, a wedge at the bottom of the bolt expands against the sides of the hole in the concrete, creating a friction-based connection. Expansion bolts are the most common type used in residential retrofits because they're faster to install and less expensive than epoxy bolts.
Epoxy-set bolts: These bolts use a two-part structural adhesive (epoxy) to bond the bolt into the drilled hole in the concrete. The epoxy fills the gap between the bolt and the concrete, creating a chemical bond that's stronger than expansion bolts in some conditions. Epoxy bolts are required when the drilled hole is too close to the edge of the concrete (where expansion bolts could crack the foundation) or when the concrete is in poor condition.
The California Existing Building Code specifies bolt spacing, edge distances, and installation requirements. For standard residential retrofits, bolts are typically installed every four to six feet along the sill plate, with bolts required within 12 inches of each end of each sill plate section and at each side of openings.
Signs Your Home Needs Foundation Bolting
If your home was built before 1950, it almost certainly needs foundation bolting. Homes from this era were typically built with the sill plate simply resting on the concrete by gravity, or with minimal nails or drift pins that provide little to no lateral resistance.
If your home was built between 1950 and 1979, it may have some anchor bolts, but they often don't meet current code requirements for spacing, size, or installation quality. It's worth a crawl space inspection to check.
You can inspect for anchor bolts yourself if you have crawl space access. Look along the top of the concrete foundation where the wooden sill plate sits. You should see bolts with nuts and square plate washers at regular intervals — roughly every four to six feet. If you see no bolts, or only a few bolts widely spaced, your home is a strong candidate for foundation bolting.
What Is Cripple Wall Bracing?
Cripple walls are the short wood-framed walls that exist between the top of the concrete foundation and the underside of the first floor in homes with raised foundations. These walls create the crawl space beneath the house, and they typically range from one to four feet tall — though in hillside homes, they can be much taller on the downhill side.
The name "cripple wall" comes from traditional construction terminology — a "cripple" stud is any stud that doesn't run the full height of a wall, either because it's above a header or below a window, or because it's in these short sub-floor walls. The name has nothing to do with the walls' structural performance, though in earthquake terms, the name has turned out to be unfortunately apt.
Why Cripple Walls Are Vulnerable
Cripple walls are vulnerable for a straightforward reason: they're short, they support the full weight of the house above them, and in older homes, they're typically framed with only studs and horizontal plates — no sheathing or bracing to resist lateral (sideways) forces.
Think of it this way: stand a row of pencils on end on a table, lay a book on top of them, and push the table sideways. The pencils fold and the book drops. That's essentially what happens to an unbraced cripple wall during an earthquake: the studs have no lateral resistance, they fold over, and the house drops.
This failure mode was dramatically illustrated in the 1989 Loma Prieta earthquake, particularly in San Francisco's Marina District. Dozens of homes with unbraced cripple walls suffered partial or complete collapse when the cripple walls folded during shaking. The houses above dropped one to four feet onto their foundations, causing massive structural damage, severing utilities, and creating fire hazards.
How Cripple Wall Bracing Works
Cripple wall bracing is the application of structural plywood sheathing to the inside face of the cripple walls. The plywood transforms the cripple wall from a collection of individual studs (which have no lateral resistance) into a shear wall (which resists lateral forces through the panel's diagonal stiffness).
The key elements of cripple wall bracing are:
Structural plywood panels: Typically ½-inch or 15/32-inch structural-grade plywood (CDX or structural-1 grade), nailed directly to the cripple wall studs and plates. The plywood must extend from the bottom plate (on top of the foundation) to the top plate (supporting the first floor) to create a complete load path.
Nailing pattern: The plywood is nailed with 8d common nails at specific spacing — typically 4 inches on center at panel edges and 12 inches on center in the field (interior of the panel). This nailing pattern is critical to the shear wall's strength and is one of the primary things building inspectors check during the final inspection.
Framing connections: The cripple wall framing must be properly connected at all points in the load path. This includes the connection between the cripple wall's bottom plate and the sill plate (which sits on the foundation), and the connection between the cripple wall's top plate and the first-floor framing above. Framing clips, straps, and holdowns are used at these connections.
Ventilation: Crawl space ventilation is required by building code to prevent moisture buildup. When plywood is installed on cripple walls, it may cover existing ventilation openings. These must be relocated or new vents must be cut into areas where plywood isn't installed. The standard plans specify minimum ventilation requirements.
DIY vs. Professional Retrofit
California's Prescriptive Standard Plans
One of the most significant developments in residential seismic retrofitting is the availability of prescriptive standard plans — pre-engineered retrofit designs that qualifying homeowners can use without hiring a structural engineer. In California, these plans are published as part of the California Existing Building Code and are available through the CEA Brace+Bolt program.
The current standard plans, updated in the 2022 California Existing Building Code, are designated as:
| Plan | Official Designation | What It Covers | Applicable Homes |
|---|---|---|---|
| EQ1 | Standard Plan A | Foundation bolting only | Homes needing only anchor bolt installation |
| EQ2 | Standard Plan B | Cripple wall bracing only | Homes with existing adequate bolting but unbraced cripple walls |
| EQ3 | Standard Plan C | Foundation bolting + cripple wall bracing | Most common — covers both retrofit scopes |
These plans specify exactly what materials to use, how to install them, and what the finished product should look like. They include bolt spacing requirements, plywood nailing patterns, hardware specifications, and all the details a contractor or homeowner needs to complete the work.
Standard Plan Eligibility Requirements
Not every home qualifies for the prescriptive standard plans. Your home must meet all of the following criteria:
| Eligibility Criterion | Requirement |
|---|---|
| Structure type | One or two-family dwelling (single-family home or duplex) |
| Number of stories | One or two stories of living space above the cripple wall |
| Foundation type | Continuous concrete perimeter foundation with cripple wall |
| Cripple wall height | Maximum 4 feet (48 inches) |
| Foundation condition | Concrete foundation in serviceable condition (no major cracking or deterioration) |
| Slope | Generally level site (not a hillside with significant grade change) |
| Post-and-pier | Home does not rely primarily on post-and-pier foundation system |
If your home doesn't meet these criteria — for example, if it has cripple walls taller than four feet, a hillside site, or a deteriorated foundation — you'll need a custom engineered retrofit designed by a licensed structural engineer. This adds $3,000–$10,000 in engineering costs but ensures the retrofit is designed for your home's specific conditions.
Doing It Yourself
California law allows homeowners to perform their own construction work on their own homes without a contractor's license, including seismic retrofitting. Under this provision, you can pull a building permit, perform the bolt-and-brace work yourself following the standard plans, and have it inspected — all without hiring a licensed contractor.
What you'll need:
Tools include a rotary hammer drill with ½-inch or 5/8-inch masonry bit, a standard drill for plywood nailing, a circular saw for cutting plywood, safety glasses, dust mask or respirator, hearing protection, and appropriate work clothing for crawl space conditions. You'll also need a work light and extension cord for the crawl space.
Materials include expansion bolts or epoxy bolts (as specified in the plan), plate washers and nuts, structural plywood, 8d common nails, framing clips and connectors (Simpson Strong-Tie L70 or equivalent), and any required holdown hardware.
Realistic assessment of DIY difficulty:
The actual retrofit work is straightforward carpentry — drilling holes, tightening bolts, cutting plywood, and nailing it to the framing. The difficulty lies almost entirely in the working conditions. Crawl spaces are typically 18 to 36 inches tall, dark, dusty, and possibly home to spiders, rodents, and standing water. You'll be working on your back or stomach for hours, maneuvering plywood sheets, and operating power tools in confined spaces.
If you're physically fit, comfortable in tight spaces, and have basic carpentry experience, a DIY retrofit is very feasible. Budget two to four full days of work for a typical home. Material costs alone are typically $1,500–$3,000, compared to $5,000–$10,000 with a contractor — so the labor savings can be $3,000–$7,000.
If you're not comfortable with crawl space work, or if your crawl space has particularly challenging conditions (very low clearance, standing water, extensive ductwork), hiring a professional is money well spent.
Hiring a Professional
For most homeowners, hiring a licensed contractor is the preferred approach. Look for contractors who specialize in seismic retrofitting — this is a niche trade, and general contractors may not be as familiar with the specific requirements and techniques involved.
How to find a qualified contractor:
The CEA Brace+Bolt contractor list is the best starting point. These contractors have been specifically vetted for residential retrofit work and are familiar with the standard plans and program requirements. Even if you're not participating in the Brace+Bolt program, these contractors are a reliable starting point.
You can also search the California Contractors State License Board (CSLB) database at www.cslb.ca.gov. Look for contractors with a B (General Building) or C-8 (Concrete) license. Verify that their license is active and in good standing, and check for any complaints or disciplinary actions.
Get at least three bids. Retrofit pricing varies significantly — it's not unusual to see a 50% difference between the lowest and highest bids for the same scope of work. Make sure you're comparing apples to apples: each bid should specify the number of anchor bolts, linear footage of cripple wall bracing, specific materials, permit costs, and whether the price includes permit fees and final inspection.
The Retrofit Process: Step by Step
Whether you're doing the work yourself or hiring a contractor, here's what the bolt-and-brace retrofit process looks like from start to finish:
1. Assessment and Planning (1–2 weeks)
Inspect the crawl space (or have a contractor do it) to determine the current condition: Are there existing anchor bolts? What spacing? Are the cripple walls braced? What are the crawl space conditions? Determine which standard plan applies (EQ1, EQ2, or EQ3), or whether a custom engineering solution is needed.
2. Permitting (1–3 weeks)
Apply for a building permit from your local building department. For standard plan retrofits, this is usually an over-the-counter or expedited process — many jurisdictions have streamlined permitting for residential seismic retrofits. Bring a copy of the appropriate standard plan and a basic site plan showing your home's footprint and foundation layout.
Permit fees vary by jurisdiction but are typically $200–$500 for a standard residential retrofit.
3. Material Procurement (1–3 days)
Purchase the materials specified in the standard plan. Most materials are available at standard building supply stores. You'll need:
- Anchor bolts (expansion or epoxy) with plate washers and nuts
- Structural plywood (½-inch or 15/32-inch CDX or structural-1)
- 8d common nails (for plywood)
- Framing clips and connectors (Simpson Strong-Tie or equivalent)
- Holdown hardware (if specified in the plan)
- Concrete patch material (for any foundation repairs needed)
4. Foundation Bolting (½–1 day)
Drill through the sill plate and into the concrete foundation at the specified spacing (typically every 4–6 feet, within 12 inches of sill plate ends, and at each side of openings). Install anchor bolts and tighten plate washers and nuts to specified torque.
For expansion bolts, drill a hole slightly larger than the bolt diameter through the wood and into the concrete (minimum 7 inches embedment in concrete is typical), insert the bolt, and tighten. For epoxy bolts, drill the concrete hole oversized, inject epoxy, insert the bolt, and allow the specified cure time before loading.
5. Cripple Wall Bracing (1–2 days)
Cut structural plywood panels to fit the cripple wall dimensions. Install panels on the inside face of the cripple walls, nailing to all studs and plates with 8d nails at the specified pattern (typically 4 inches on center at edges, 12 inches on center in the field).
Install framing clips at the connections between the cripple wall bottom plate and sill plate, and between the cripple wall top plate and the first-floor framing above. If the standard plan specifies holdown hardware at certain locations (typically at the ends of braced wall sections), install those as well.
Ensure that existing crawl space ventilation is maintained. If plywood panels cover existing vents, either cut new vents in non-braced areas or install vent openings in the plywood panels per the standard plan specifications.
6. Final Inspection (scheduled after completion)
Contact your local building department to schedule a final inspection. The inspector will enter the crawl space and verify:
- Bolt spacing and installation quality
- Plywood grade, thickness, and nailing pattern
- Framing connections and hardware
- Ventilation compliance
- General conformance with the approved plan
If the work passes inspection, you'll receive a final sign-off on the permit. Keep this documentation — it's proof of a code-compliant retrofit, which is valuable for insurance, property value, and real estate transactions.
The CEA Brace+Bolt Program
The California Earthquake Authority's Brace+Bolt program is the primary financial assistance program for residential bolt-and-brace retrofits. Since 2014, the program has provided grants to over 20,000 homeowners, making it the largest residential seismic retrofit incentive program in the United States.
How the Program Works
Each year, the CEA opens registration for new participants (typically in the spring). Homeowners check their eligibility online, register, and — if selected — receive a commitment for up to $3,000 toward the cost of a standard bolt-and-brace retrofit. The homeowner then selects a contractor from the program's approved list, completes the retrofit following the program's specifications, and submits documentation for reimbursement.
What the Program Covers
The Brace+Bolt grant covers a portion of the contractor's fee for a standard bolt-and-brace retrofit. The program requires that the work follow the standard prescriptive plans and be performed by a contractor from their approved list (for grant-funded work — you can still participate with your own contractor for a reduced grant in some cases).
The $3,000 grant typically covers 30–60% of the total cost for a standard retrofit, depending on your home's size and local pricing. You're responsible for the balance.
Important Dates and Tips
Registration is competitive — the program receives more applications than it can fund in most years. Register as soon as enrollment opens to maximize your chances. Check the program website for current dates and eligibility requirements, as these can change from year to year.
Even if you don't receive a Brace+Bolt grant, the program's resources are valuable. The standard plans, contractor lists, and educational materials are available to all homeowners regardless of grant status.
See the full breakdown of retrofit costs and financial assistance →
Foundation Bolting and Cripple Wall Bracing for Different Home Types
Single-Story Home on Flat Ground
This is the simplest and most affordable retrofit scenario. A typical single-story home on a flat lot with a standard crawl space can be retrofitted in one to two days for $4,000–$8,000. The standard plans are designed primarily for this home type, and the work is straightforward.
Two-Story Home on Flat Ground
Two-story homes are heavier, which means the cripple walls and foundation connections must resist greater forces. The standard plans account for this by requiring more bracing and closer bolt spacing for two-story homes. Expect costs to be 20–40% higher than a comparable single-story home.
Homes with Garages
Garages often represent a weak point because the large door opening means less wall area available for bracing on that side of the house. The standard plans address this by requiring additional bracing on adjacent walls or by specifying that certain areas must be fully braced to compensate. If your garage is integral to the house (not a detached structure), make sure the retrofit plan accounts for this vulnerability.
Homes with Additions
Homes that have been expanded over the years often have multiple foundation types, varying cripple wall heights, and different structural configurations. The addition may not connect well to the original structure, creating a seismic weak point at the junction. Some additions may not qualify for the prescriptive standard plans and may require custom engineering.
Hillside Homes
As noted in the eligibility table, homes on hillsides with tall cripple walls (over four feet) generally don't qualify for the prescriptive standard plans. These homes need custom engineering, and the retrofit costs are typically $10,000–$40,000 or more depending on the site conditions and structural requirements. The tall, slender cripple walls on the downhill side are particularly vulnerable and may require steel bracing or moment frames in addition to plywood sheathing.
Return to our complete seismic retrofitting guide →
Sources
- California Existing Building Code, 2022 Edition, Chapter A3 — Prescriptive Provisions for Seismic Strengthening of Cripple Walls and Sill Plate Anchorage of Light, Wood-Frame Residential Buildings. Available at: https://codes.iccsafe.org/content/CEBC2022P6
- California Earthquake Authority — Brace+Bolt Program. Available at: https://www.earthquakebracebolt.com
- FEMA P-1100 — Vulnerability-Based Seismic Assessment and Retrofit of One- and Two-Family Dwellings, 2019. Applied Technology Council.
- Simpson Strong-Tie — Seismic Retrofit Solutions for Residential Structures. Available at: https://www.strongtie.com
- California Contractors State License Board — Homeowner Information. Available at: https://www.cslb.ca.gov
- USGS — The Loma Prieta, California, Earthquake of October 17, 1989. Professional Paper 1551.
- USGS — The Northridge, California, Earthquake of January 17, 1994. Professional Paper 1553.
- California Existing Building Code, 2022 Edition — Chapter A3 and Appendix Chapter A3.