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Building Code Reference

Got a CRC R304.1 correction?

Pressure-treated wood at foundation

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CRC R304.1 requires that wood in contact with or close to the ground be either naturally durable or preservative-treated to resist decay. The rule covers sill plates, posts, columns, beams, and any framing member within 8 inches of exposed ground on the exterior. In crawl spaces, the code sets minimum clearances: 18 inches from finish grade to the bottom of floor joists, and 12 inches to the bottom of girders. Residential projects with any below-grade wood framing — including conventional foundations, ADU additions, and deck substructures — will get flagged if the plans omit treated-wood call-outs or leave the grade-to-framing clearances undimensioned.

How to fix it

  1. Add treated-wood call-outs to foundation details

    On your foundation plan and foundation detail sheet, add a note to every wood member in contact with or within 8 inches of exposed exterior grade. The note should read something like: "All sill plates and wood members within 8" of exposed ground per CRC R304.1 to be pressure-treated lumber, min. 0.40 lb/ft³ retention per AWPA standards, or naturally durable species." Put this note on the detail itself — not just in the general notes — so the plan-checker can verify it directly on the sheet.

  2. Dimension crawl space clearances on the foundation section

    On your foundation cross-section detail, add explicit vertical dimensions showing: (1) finished grade to underside of floor joists — must be 18 inches minimum, and (2) finished grade to underside of girders — must be 12 inches minimum. Do not leave these as implied dimensions. A dimension string or a labeled dimension call-out is required. If your existing detail doesn't have room, add a separate note box on the same sheet: "Crawl space clearance: 18" min. grade to bottom of joists, 12" min. to bottom of girders per CRC R304.1."

  3. Specify posts and columns bearing on concrete

    For any wood post or column bearing directly on a concrete slab, footing, or pier, add one of the following on the structural detail: (a) Note specifying pressure-treated lumber for the post, min. 0.40 lb/ft³ for ground contact, or (b) Show an ICC-listed post base connector (Simpson ABA, ABU, or equivalent) with a minimum 1-inch standoff from the concrete surface. Either solution satisfies R304.1. The connector option is often preferred on interior posts where treated lumber isn't structurally required elsewhere.

  4. Check grade elevations on sloped lots

    If your site has significant grade change around the perimeter, review the low-side of the structure separately. On your site plan or foundation plan, add a note or dimension showing the minimum grade-to-framing clearance at the lowest point of exposed grade. If the grade-to-sill distance is less than 8 inches at any point, that wood must be treated or the grade must be cut back. Mark the control point elevation and the corresponding framing elevation so the plan-checker can verify compliance without additional inquiry.

  5. Update the general notes sheet

    Add a code compliance note to your general notes or code summary sheet: "Wood in contact with or within 8 inches of exposed exterior ground to be pressure-treated per CRC R304.1. Crawl space clearance: 18" minimum from grade to bottom of joists, 12" minimum to bottom of girders." This establishes a project-wide baseline. Individual details still need their own call-outs, but the general note helps the plan-checker see that you've accounted for the requirement globally.

  6. Verify the correction letter cite and resubmit

    Before resubmitting, re-read the specific comment on your corrections letter. Plan-checker comments under R304.1 usually fall into two buckets: (a) missing treatment specification on a named member, or (b) undimensioned crawl space clearance. Confirm your revision addresses the exact language in the comment — not just the general requirement. If the comment says "sill plate at garage," make sure the garage foundation detail is updated, not just the main foundation section. Mark the revision clouds on the corrected sheets so the checker can locate your changes quickly.

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Why you got flagged

Here's why plan-checkers flag this code more often than contractors expect.

The most common trigger is a foundation detail that shows the sill plate sitting on the mudsill with no treatment specification called out. A plan-checker reviewing a single-family remodel or ADU foundation sheet is looking for a clear note — something like "PT sill per CRC R304.1" on the detail. When the detail is silent, the checker will mark it a correction regardless of whether the contractor actually intended to use pressure-treated lumber. The omission is the problem, not the material.

Crawl space clearances are the second major flag. Section R304.1 requires 18 inches from grade to the underside of floor joists and 12 inches to the underside of girders. A lot of drawings show the crawl space foundation section schematically without dimensioning those vertical clearances. The checker has no way to verify compliance from an undimensioned drawing, so it goes back as a correction.

Posts and columns in contact with concrete are another recurring miss. Wood posts bearing on a concrete slab or pier need to be treated — or separated from the concrete by an approved standoff connector — because concrete wicks moisture. Plans that show a wood post landing directly on a concrete pier cap without specifying treated lumber or a code-compliant connector will get flagged under R304.1.

The fourth scenario involves wood within 8 inches of exposed exterior grade. On hillside lots and sloped sites, the grade elevation varies around the perimeter. A framing detail drawn for the high-side of the lot may show plenty of clearance, but the same detail applied to the low side may put unprotected wood within the 8-inch threshold. When site plans don't annotate the grade-to-framing distance at the low point of the structure, plan-checkers will flag it — especially on sloped lots in Orange County where the grade variation can be significant.

The official code

2025 California Residential Code R304.1 — Protection of Wood Against Decay

Read the source →

Common questions

Does CRC R304.1 apply to ADU foundations?
Yes. R304.1 applies to all residential construction covered by the California Residential Code, including ADUs. Any wood framing in your ADU foundation — sill plates, posts, beams — that falls within 8 inches of exposed exterior grade must be pressure-treated or naturally durable species. ADU permits get scrutinized closely, and foundation details are one of the first sheets plan-checkers review. If your ADU is on a raised foundation with a crawl space, also verify the 18-inch joist clearance and 12-inch girder clearance are dimensioned on your drawings.
What lumber grade meets CRC R304.1 pressure-treatment requirements?
The code points to AWPA (American Wood Protection Association) standards for preservative-treated lumber. For sill plates and framing members above ground, the common standard is 0.25 lb/ft³ retention (UC3B use category). For wood in direct contact with the ground or concrete — like posts bearing on piers — the standard steps up to 0.40 lb/ft³ (UC4A or UC4B depending on the application). The treatment grade should be stamped on the lumber itself, and your plans should reference the applicable use category or retention level so the inspector can verify. Note that "green treated" lumber at most lumberyards meets these standards — verify the stamp before specifying or using it.
Can I use a post base connector instead of treated wood under my posts?
Yes, that's a common approach. An ICC-listed post base connector — like a Simpson Strong-Tie ABA or ABU series — with a standoff that keeps the wood at least 1 inch above the concrete surface can satisfy R304.1 for posts. The standoff eliminates the direct concrete-to-wood moisture path. Your structural detail needs to show the connector model or reference an approved equivalent, and the note should explain why treated lumber is not specified at that location. Check with your plan-checker before final design — some jurisdictions in Orange County have specific connector requirements or prefer seeing treated lumber regardless.
What is the minimum crawl space clearance under CRC R304.1?
18 inches from finish grade to the underside of floor joists, and 12 inches from finish grade to the underside of girders or beams. These are absolute minimums — your plans need to dimension these explicitly. An undimensioned crawl space section is one of the most common triggers for an R304.1 correction. If your site has sloped grade, measure at the low point of the perimeter, not the average. The code minimum applies at every point around the structure.
Does a concrete slab-on-grade project need to worry about R304.1?
Less than a raised-foundation project, but it still applies to any wood in contact with or near the slab edges. Bottom plates bearing on a slab must be pressure-treated. If a wood post is embedded in or bearing on a slab, it needs treated lumber or an approved standoff connector. Exterior walls where the bottom plate contacts the slab at grade level are the main focus. If your slab edge is at or below exterior grade and the bottom plate is within 8 inches of exposed ground, R304.1 applies.
Does R304.1 apply to deck posts and beams?
Yes. Deck posts in contact with or close to grade are a common R304.1 flag — especially posts bearing on concrete piers at grade. The post itself needs to be treated or mounted on a listed connector with standoff. Deck beams and joists are covered if they're within 8 inches of grade, which can happen on low-profile decks on sloped lots. Deck permits in cities like Anaheim and Orange frequently come back with R304.1 comments on the post-to-pier connection when the detail doesn't call out treatment or show the connector model.

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