What is the maximum spacing for anchor bolts?

Under the International Residential Code (IRC R403.1.6), anchor bolts must be spaced no more than 6 feet (72 inches) on center. In high seismic or wind zones, this spacing may be reduced to 4 feet or less. Always check your local amendments.

How close to the corner must a bolt be?

There must be an anchor bolt within 12 inches (but not closer than 7 bolt diameters) of each end of each plate section. This means every corner needs a bolt within 12" on both sides.

What size anchor bolts should I use?

The minimum requirement is typically 1/2-inch diameter bolts embedded at least 7 inches into the concrete. In high-load areas, 5/8-inch bolts are often specified. Large square plate washers (3"x3"x1/4") are required in high seismic zones.

How many bolts do I need for a 20ft wall?

For a 20ft wall with standard 6ft spacing: You need one at the start (within 12"), one at the end (within 12"), and intermediate bolts max 6ft apart. 20ft / 6ft = 3.33 spaces -> 4 spaces -> 3 intermediate bolts. Total: 2 ends + 3 intermediate = 5 bolts minimum.

Can I use retrofit bolts?

Yes, if you missed placing wet-set bolts, you can drill and install wedge anchors or epoxy anchors (like Titan HD or Red Head) that are approved for sill plate applications. They must meet the same spacing and load requirements.

🔩 Anchor Bolt Spacing Calculator

IRC Code Compliance for Sill Plate Anchors

Wall Length (ft)

Max Spacing (Code)

Plate Splices

Each splice needs 2 bolts (one each side)

📜 IRC Code Requirements (R403.1.6)

Proper anchor bolt placement is critical for securing your home to its foundation. Here are the key rules:

1. Maximum Spacing

Bolts must be spaced no more than 6 feet (72 inches) on center. In high seismic categories (D0, D1, D2) or high wind zones, this is often reduced to 4 feet or less.

2. Corner & End Placement

There must be a bolt within 12 inches (but not closer than 7 bolt diameters, ~3.5") of each end of each plate section. This effectively means every corner gets two bolts (one on each wall).

3. Bolt Specifications

Minimum 1/2-inch diameter bolts. Minimum 7-inch embedment into concrete. Must use a nut and washer (3"x3" square washers required in Seismic Zones).

💡 From Rob Chen, Framing Contractor (16 years):

"The biggest mistake I see is forgetting the bolts at the splices. If you have a 20ft wall and you're using 16ft lumber, you have a splice at 16ft. You need a bolt within 12 inches on the LEFT of that cut, and another within 12 inches on the RIGHT. Don't just rely on the 6ft spacing pattern; mark your splices first!"

⚠️ Common Mistakes

1. Missing the 12" Rule: Inspectors measure this first. If your last bolt is 14" from the end of the wall, you fail.

2. Bolts hitting studs: A bolt directly under a stud is useless because you can't get the nut on. Layout your studs (16" OC) first, then mark your bolts to land in the bays (center of the 14.5" space).

3. Too close to edge: Bolts must be centered in the sill plate. If they are too close to the edge of the concrete, they can blow out the foundation.

Reviewed by Rob Chen

Licensed Framing Contractor, 16 years experience in residential framing and foundation work.

📜 IRC Code Requirements (R403.1.6)

1. Maximum Spacing

2. Corner & End Placement

3. Bolt Specifications

⚠️ Common Mistakes

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