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Building Code Reference

Got a CALGreen 4.505.2 correction?

Capillary break slab

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

CALGreen 4.505.2 requires a capillary break under concrete slabs that the building code requires to have a vapor retarder, to stop moisture from wicking up through the slab. The standard method is a 4-inch-thick base of 1/2-inch or larger clean aggregate with the vapor retarder in direct contact with the concrete. Living areas and ADU floors on grade qualify; garages and other unheated accessory slabs are exempt from the CRC vapor retarder, so they usually fall outside it. Plan-checkers look for the aggregate spec and depth called out on the foundation detail.

How to fix it

  1. Pull the foundation detail and check for the aggregate callout

    Open your foundation section drawing — typically on the structural sheets (S1 or S2). Look for the note that specifies what sits directly beneath the vapor retarder. If you see just 'vapor barrier on compacted subgrade' with no gravel layer called out, that's the gap. You need a callout showing the capillary break aggregate between the subgrade and the vapor retarder. Check every slab-on-grade detail in the set, including ADU slabs. Garage slabs are exempt from the CRC R506.3.3 vapor retarder, so 4.505.2 reaches them only when a vapor retarder is otherwise required.

  2. Add the aggregate spec with size and depth

    On the foundation section detail, add a notation that reads: '4" thick base of 1/2" or larger clean aggregate, capillary break per CALGreen Section 4.505.2.1.' The key numbers are a 4-inch-thick base and aggregate 1/2 inch or larger. Do not use pea gravel, crusher fines, or decomposed granite — the aggregate must be clean, meaning minimal fines so water can't wick through the layer. If your structural engineer's detail doesn't include it, have them add a note or issue a delta revision to the foundation section. Most engineers can turn this around in a day.

  3. Show the correct layer sequence in the slab section

    The section must show the layers in order from bottom to top: compacted native soil or engineered fill, then the 4-inch clean aggregate layer, then the vapor retarder (minimum 6-mil polyethylene or approved retarder per CRC R506.3.3, in direct contact with the concrete), then the concrete slab. If you show the vapor retarder on top of the concrete, or omit the aggregate entirely, the plan-checker will kick it back. Label each layer in the section with a leader line and text note. The plan-checker should be able to read the section and confirm code compliance without opening the specification book.

  4. Verify the vapor retarder spec meets the companion requirement

    CALGreen 4.505.2.1 requires the vapor retarder to be in direct contact with the concrete, and CRC R506.3.3 sets the retarder itself: at least 6-mil polyethylene or an approved vapor retarder, with joints lapped not less than 6 inches. If your detail calls out thinner poly or unlapped joints, update it. Some plan-checkers will check both the capillary break aggregate spec and the vapor retarder spec in the same pass, so fix both before resubmitting. Confirm your spec section (typically Division 03 or 31) also reflects the updated thickness — inconsistencies between the drawings and specs generate their own correction.

  5. Cross-reference the detail on your architectural sheets

    On the architectural floor plan (typically A1 or A2) where you show 'concrete slab on grade,' add a note or detail reference pointing to the foundation section. Something like 'Slab: see struct. detail S1/5 for CALGreen 4.505.2 capillary break.' Plan-checkers spot missing coordination notes and will write a correction even if the structural detail is correct. The cross-reference closes that gap and typically takes under ten minutes to add. If you have a CALGreen checklist sheet in your plan set (common in Orange County jurisdictions), make sure 4.505.2 is checked off there too.

  6. Confirm your geotech report doesn't conflict with the aggregate requirement

    Some geotechnical reports specify a different subbase system — for example, a structural fill with compaction specs that they argue substitutes for the aggregate layer. CALGreen 4.505.2.1 also accepts other equivalent methods approved by the enforcing agency, or a slab design specified by a licensed design professional. If your geotech or engineer takes that position, the documents must say so explicitly. If they don't, the plan-checker defaults to the 4-inch aggregate method. Review the geotech report before you resubmit and coordinate with your structural engineer if there's a conflict. Geotech-versus-structural inconsistencies are a separate correction that can add another cycle.

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Why you got flagged

Plan-checkers flag CALGreen 4.505.2 most often because the slab detail is incomplete, not because the contractor skipped the work. Here's the breakdown.

The most common hit is a missing aggregate callout on the foundation section. The detail shows the vapor retarder sitting directly on compacted soil, with no notation for the gravel base beneath it. The code's standard method is an aggregate layer of 1/2-inch or larger clean aggregate, 4 inches thick, which needs to appear on the drawings. If your foundation section just says "poly vapor barrier on compacted subgrade," you will get this correction.

A second scenario: the aggregate is shown but the size isn't specified. A note that says "gravel layer per CALGreen" without dimensions doesn't satisfy the plan-checker. You need the nominal aggregate size and the 4-inch depth minimum called out explicitly on the detail. Some reviewers will also want a note that the aggregate is clean — free of fines that would allow capillary action to continue through the layer.

Third: the detail exists on the structural sheets but wasn't coordinated with the architectural floor plan notes. Plan-checkers review both sheets and will flag the discrepancy if the architectural sheets show "concrete slab on grade" without referencing the capillary break detail on the structural sheets. A simple cross-reference note on A1 or A2 fixes this.

A less common but expensive situation: the project uses a post-tensioned slab system, and the geotechnical report calls for a vapor barrier but the calgreen capillary break layer was omitted from the PT slab section. The PT engineer's sheets need to show the aggregate layer too, not just the structural engineer's typical section.

The official code

CALGreen Part 11 — California Green Building Standards Code (Building Standards Commission)

Read the source →

Common questions

Does CALGreen 4.505.2 apply to garage slabs?
Usually not. CALGreen 4.505.2 applies to slabs that the CBC or CRC requires to have a vapor retarder, and CRC R506.3.3 exempts garages, utility buildings and other unheated accessory structures from the vapor retarder. If your garage slab is required to have a vapor retarder anyway, for example by local amendment or because it will be enclosed and heated, the capillary break applies to it too. Plan-checkers review all slab-on-grade details in the set, so note on the garage detail which rule you followed.
What aggregate size is required for the capillary break under a slab?
CALGreen 4.505.2.1 calls for a 4-inch-thick base of clean aggregate 1/2 inch or larger. The aggregate must be clean, meaning it has minimal fines. Pea gravel, crushed fines, decomposed granite, or sand do not meet this requirement because fine particles allow capillary action to continue through the layer. Specify the size and depth explicitly on the foundation detail — a generic note referencing 'gravel' without dimensions will not pass plan check. Your supplier can provide a gradation spec sheet for the record if the inspector asks for it during foundation inspection.
Can I use a thicker vapor retarder instead of the aggregate layer?
No. The vapor retarder and the capillary break aggregate layer are separate requirements under CALGreen. A heavier vapor retarder (15-mil, 20-mil, or a Class A vapor barrier product) does not substitute for the 4-inch aggregate base. The aggregate layer physically stops water from moving by capillary action through the soil column, while the vapor retarder stops vapor transmission through the slab surface. CALGreen requires both. If you're considering a drainage mat or composite product as an alternative, it must be specified by the engineer of record with an explicit equivalency reference. A product data sheet alone won't satisfy the plan-checker.
Does CALGreen 4.505.2 apply to ADU concrete slabs?
Yes. ADUs are residential construction and are subject to all CALGreen mandatory residential measures, including 4.505.2. This applies whether the ADU is a new detached structure, a converted garage, or a basement conversion where a new slab is poured. If you're converting a garage to an ADU and the existing slab doesn't have a documented capillary break, your structural or soils engineer needs to address how the existing conditions meet or are exempted from the requirement. Conversion plans get this correction frequently because the existing slab section is often copied from older permit drawings that predate the CALGreen mandate.
Do I need to show the capillary break on both structural and architectural sheets?
The capillary break detail belongs on the structural foundation section, but the architectural sheets should cross-reference it. If your architectural floor plan shows a concrete slab on grade without any note pointing to the structural foundation detail, plan-checkers will flag the missing coordination. Add a note on the architectural plan referencing the structural detail by sheet and detail number. If your jurisdiction requires a CALGreen checklist sheet (many Orange County cities do), make sure 4.505.2 is checked off there too. Cross-referencing takes a few minutes to add and can save you a 60-day correction round.
What if my geotechnical report recommends a different subbase?
If the geotech report specifies an engineered fill or alternative subbase system that it claims satisfies the moisture protection requirement, the report must explicitly say so and reference the applicable code provision or provide an equivalency justification. The plan-checker defaults to the CALGreen 4.505.2 aggregate requirement unless the geotech report clearly documents the alternative and your structural engineer's drawings reflect that alternative. Conflicting information between the geotech report and the foundation detail is a separate correction category. Coordinate with your structural engineer before resubmitting to make sure both documents say the same thing.

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