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Foundation forms and rebar set on a cleared hillside lot before a concrete pour

Seismic Retrofitting

Foundation Bolting

Many older houses rest on their foundations with little holding them there. Foundation bolting anchors the wood sill to the concrete so the house cannot slide off in an earthquake.

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Foundation Bolting

The sill plate is the wood board that sits on top of a concrete foundation and supports the rest of the house. Modern code requires it to be anchored with bolts at set spacing. Many houses built before modern codes were anchored with few bolts, widely spaced, or none. In a strong earthquake, the ground moves and the house, held only by its weight and a few nails, can slide off.

Foundation bolting adds anchors through the sill into the concrete at the spacing a retrofit plan requires. It is quick work compared with most construction, and it addresses one of the most damaging failures an older house can have in an earthquake.

Bolts or plates

When there is room above the sill, holes are drilled through the sill into the concrete and anchors are installed: expansion bolts, or threaded rods set in epoxy. Square plate washers on top spread the load across the sill. When the floor framing sits too close to the sill to drill from above, steel foundation plates attach to the side of the sill and the side of the foundation instead.

  • Expansion anchors

    Mechanical anchors that grip the concrete as they are tightened.

  • Epoxy anchors

    Threaded rods set in adhesive, often used in older or weaker concrete.

  • Foundation plates

    Steel plates on the side of the sill and foundation where clearance is tight.

The concrete and the sill

An anchor only holds if the concrete around it is sound and the sill is solid wood. Some very old foundations are made of weak or crumbling concrete, or unreinforced brick or stone, and bolting into them is not enough; those need an engineer. Rotted or termite-damaged sill sections are replaced before anchors go in.

Houses without cripple walls

Some older raised-floor houses have no cripple walls; the floor framing sits directly on the sill on top of the foundation. Those houses need bolting but not bracing. Houses with cripple walls need both, because a bolted house can still fail if the short walls above the bolts collapse.

From our clients

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5.0 out of 5, based on 17 Google reviews

  • Taylor Wilson

    October 2022 · Google review

    "He was easy to schedule and very timely working with the ebb program which is very strict with its rules to qualify. He was able to complete the job quickly, answered all questions, offered up suggestions for future projects, and cleaned up all material and equipment before leaving the job site."
  • Diana White

    February 2022 · Google review

    "Very professional , responsive, and knowledgeable- installed earthquake retrofit for my house."

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Anchor the house to the ground

We will count your existing bolts, check the sill and the concrete, and give you a written price.

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Foundation Bolting: FAQs

How many anchor bolts does my house need?

That depends on the length of the foundation, the house size and the retrofit plan being followed, which sets maximum spacing between anchors and minimum numbers near corners and ends. We measure the perimeter, count existing anchors and show you what needs to be added.

My house already has some bolts. Is that enough?

Not necessarily. Older houses often have bolts that are too far apart, too small, missing plate washers or rusted. We check the existing anchors and add what is needed to meet the plan. Existing bolts in good condition can count toward the total.

Is foundation bolting messy?

Most of the work happens in the crawlspace, with drilling into concrete. Dust stays mostly under the house, and the rooms above are not disturbed. We clean up the crawlspace and the access area when we finish, which retrofit clients mention in their reviews.

Do I need a permit for foundation bolting?

Yes. Seismic retrofit work is permitted and inspected, and the inspector checks anchors, plates and bracing before signing off. If you are using the Earthquake Brace + Bolt program, the permit must be for the retrofit alone and reference an accepted plan.

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