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Home / Code Guides / Anchor Bolts

Sill Plate Anchor Bolt Code — Size, Spacing & Placement

Anchor bolts are the entire connection between the house and the planet, and the IRC specs them like it: a diameter, an embedment, a spacing, and rules for where the first and last bolt must land.

RequirementModel Code StandardSection
Size & embedment1/2 in. minimum diameter bolts embedded at least 7 in. into concrete or grouted masonryIRC R403.1.6
SpacingMaximum 6 ft on center; sill plates supporting braced wall panels in higher seismic categories tighten furtherIRC R403.1.6
Per-piece minimumAt least two bolts in every plate section, with one bolt within 12 in. — but no closer than seven bolt diameters — of each endIRC R403.1.6
Nuts & washersBolts fitted with nuts and washers; seismic design categories D0–D2 require 3 in. × 3 in. × 0.229 in. plate washersIRC R602.11.1
Sill materialPlates in contact with concrete are preservative-treated or naturally durable wood, with a foam sill sealer allowed beneathIRC R317.1
AlternativesApproved anchor straps and, for interior non-braced walls, post-installed anchors per their evaluation reportIRC R403.1.6

Worth knowing

  • Wet-set bolts drifted out of position are the norm on real pours — epoxy or wedge anchors per an ICC-ES report are the standard fix, at the report’s embedment, not the wet-set number.
  • A plate splice with one bolt serving two plate ends violates the two-per-piece rule — short closet walls catch this constantly.
  • In SDC D territory (much of California, parts of the mountain West), the square plate washers are not optional and inspectors look for them specifically.
  • Bolts through the plate’s edge third instead of the middle third lose most of their lateral value — layout before the pour beats drilling after.

Common questions

What does the code say about size & embedment for Anchor Bolts?

1/2 in. minimum diameter bolts embedded at least 7 in. into concrete or grouted masonry. (IRC R403.1.6)

What does the code say about spacing for Anchor Bolts?

Maximum 6 ft on center; sill plates supporting braced wall panels in higher seismic categories tighten further. (IRC R403.1.6)

What does the code say about per-piece minimum for Anchor Bolts?

At least two bolts in every plate section, with one bolt within 12 in. — but no closer than seven bolt diameters — of each end. (IRC R403.1.6)

Adopted edition by state

Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.

Related sections and guides

This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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