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Building Code Reference

Got a CRC R506 correction?

Slab-on-grade requirements

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 R506 is the California Residential Code section governing concrete slab-on-grade floors in residential construction. It requires interior concrete floors to be at least 3.5 inches thick, placed over a minimum 4-inch base of clean aggregate to break capillary moisture, and covered with a 6-mil polyethylene vapor retarder with seams lapped at least 6 inches. Permits for new homes, room additions, and ADUs commonly cite R506 when the structural or architectural sheets are missing one of these three details.

How to fix it

  1. Add a slab section detail to the structural sheets

    If the plans show the slab on a floor plan or foundation plan without a cross-section detail, add one. The detail needs to show the four layers from bottom to top: native or compacted subgrade, 4-inch aggregate base, 6-mil vapor retarder, and 3.5-inch minimum concrete. Label each layer with the material spec. A scale of 1 inch = 1 foot or larger is typical. This one detail is what plan-checkers look for first.

  2. Specify the aggregate base

    Show a minimum 4-inch base course of clean graded sand, gravel, crushed stone, crushed concrete or crushed blast-furnace slag passing a 2-inch sieve between the vapor retarder and the subgrade. Add a note in the detail: Min. 4-inch clean crushed aggregate base, compacted. Without this call-out, the plan-checker has nothing to verify.

  3. Specify the vapor retarder correctly

    The vapor retarder must be 6-mil polyethylene minimum. Show it in the slab section detail and add a note: 6-mil poly vapor retarder, seams lapped min. 6 inches. Also note how it terminates at the wall. It should turn up at least 6 inches and be taped or tucked behind the concrete at the foundation wall. If it just ends at the edge of the slab, call out the lap and termination detail.

  4. Confirm slab thickness on the detail

    The slab must be at least 3.5 inches (nominally 4 inches in most contractor practice) of concrete. Show this dimension in the section cut with a clear arrow. Calling it a 4-inch slab is fine as long as the dimension is on the detail. If you are doing a garage conversion or existing slab work, check whether the existing slab gets removed entirely. If so, the new slab needs a full R506-compliant detail. If the existing slab is kept, note that in the plan and call out the existing dimensions.

  5. Note reinforcement requirements or confirmed exception

    CRC R506 does not require reinforcement in slabs on grade for residential use, but plan-checkers sometimes flag this if the detail is silent on the topic. Add a brief note: Reinforcement not required per CRC R506; provide per structural engineer if specified. If your structural engineer has called out rebar or welded wire mesh for your project conditions, reference the structural sheets.

  6. Cross-check with CALGreen 4.505.2

    CALGreen Section 4.505.2 has its own capillary break requirement that applies to new residential construction. The aggregate base required under CALGreen may need to match a specific gradation. If your project is subject to CALGreen (all new construction and most ADUs are), confirm the aggregate base spec on your slab detail also meets 4.505.2. One detail can satisfy both codes if the material call-out is complete.

  7. Recheck the sheet index and revisions

    After updating the detail, make sure the revision cloud and delta tag are on the updated sheet. If you are resubmitting after a corrections letter, the plan-checker needs to find your change fast. Add a note near the revision: R506 slab detail added per correction. Label it clearly and reference it in the resubmittal cover letter so the plan-checker does not spend time hunting for your fix.

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Why you got flagged

Plan-checkers flag R506 most often when the slab detail on the structural sheets is incomplete. The three required elements (thickness, aggregate base, vapor retarder) need to appear together, and leaving out even one triggers a correction. If the section detail just calls out the slab thickness and says nothing about the base or moisture barrier, the plan-checker will circle it and send it back.

The vapor retarder specification is the single most common miss. A lot of plans note "6-mil poly" on the foundation plan without calling out the 6-inch lap requirement at seams and the detail showing how it turns up at the wall. The vapor retarder has to be a continuous membrane, not two separate pieces butted together. When the lap is not dimensioned, the plan-checker cannot approve it.

Aggregate base omissions come in second. The code requires a 4-inch minimum layer of clean aggregate (typically 3/4-inch crushed rock) between the subgrade and the vapor retarder. Plans that specify the concrete thickness but skip the aggregate base detail leave the plan-checker nothing to verify. This shows up on ADU garage conversions frequently, where the existing slab often gets demolished and replaced, and the new slab detail gets shortchanged.

A fourth scenario is calling out material requirements in the spec sheet but not showing them in the slab section detail. The plan-checker reviews the drawings, not the spec book, when doing an initial check. If the info lives only in a spec section and the slab cross-section detail is silent, you will get a correction asking you to add the call-out to the detail itself.

The official code

California Residential Code R506, Concrete Floors (On Ground), California Building Standards Commission

Read the source →

Common questions

Does CRC R506 apply to ADU garage conversions?
Yes, if the garage floor slab is demolished and replaced as part of the conversion. If the existing slab is kept, R506 still applies to any new concrete work, and the plan-checker may ask you to verify the existing slab meets the minimum 3.5-inch thickness or note it as non-conforming existing construction. For a full garage conversion where the slab is replaced, submit a slab section detail showing all three layers: 4-inch aggregate base, 6-mil vapor retarder, and 3.5-inch slab. If the existing slab is staying, document its thickness and condition in the plan notes. Either way, plan-checkers pay close attention to garage conversion slabs because they are frequently thin (2.5 to 3 inches in many older OC homes) and lack a vapor retarder entirely.
Is reinforcement required by CRC R506?
No. CRC R506 does not mandate rebar or welded wire mesh in residential slab-on-grade floors. Reinforcement is the structural engineer's call based on soil conditions, loads, and project specifics. If your soils report or structural engineer specifies reinforcing, that goes on the structural sheets. The plain-language answer is: R506 itself does not require it, but your project conditions might. Note this in your slab detail so the plan-checker is not wondering. If you are adding reinforcing voluntarily, call it out with bar size and spacing on the slab detail to avoid a correction asking you to reconcile the structural and architectural sheets.
What thickness does CRC R506 require for a concrete floor slab?
The minimum is 3.5 inches (just under 4 inches nominal). Most projects call out a 4-inch slab in practice, which exceeds the minimum. Show the dimension on your slab section detail with an arrow callout. If you are matching an existing slab or constrained by finished floor height, confirm the minimum 3.5 inches is achievable before you pour. Going below 3.5 inches on any portion of the slab (at doorway transitions or where a recessed shower pan sits lower) is a code violation. Anything below grade or at transitions should be noted and detailed separately.
What size aggregate is required under the slab?
CRC R506 requires a minimum 4-inch layer of clean granular material. The most common spec is 3/4-inch crushed rock (clean crushed aggregate). Clean means it is free of soil, sand fines, and organic material that could hold moisture. The aggregate base sits between the vapor retarder and the compacted subgrade, not on top of the poly. Get the layer order right in your section detail or you will get another correction. In some municipalities, inspectors will probe the aggregate layer during rough inspection to verify the 4-inch minimum depth, so do not shortchange it during construction.
Does the vapor retarder go under or over the aggregate base?
The vapor retarder goes on top of the aggregate base, directly under the concrete slab. Layer order from bottom: compacted subgrade, 4-inch aggregate base, 6-mil vapor retarder, concrete slab. Some contractors have put the poly under the rock, which defeats the purpose. The aggregate base is there to break capillary action before moisture reaches the membrane. Show this order in your slab section detail. A cross-section callout with arrows labeling each layer takes about 30 minutes to add and prevents this common back-and-forth.
Can I use a thicker vapor retarder instead of 6-mil?
Yes. 6-mil polyethylene is the minimum; you can use thicker material (10-mil or 15-mil are common upgrades on projects in areas with high moisture or near the water table). Some vapor retarder products marketed for slab use are multi-ply composites rated at 10-mil or higher. Any product that meets or exceeds the permeance rating of 6-mil poly satisfies R506. Note the thickness and product on your detail. If you are in an area with standing water or high groundwater, your soils engineer may specify a heavier barrier anyway. Coordinate the spec between structural and architectural sheets so there is no conflict.
What happens if the existing subgrade is not compacted?
CRC R506 does not specifically address subgrade compaction, but plan-checkers often reference R401.3 (site drainage and grading) and the soils engineer report when the subgrade condition is suspect. If your soils report flags low bearing capacity or expansive soils, the structural engineer may call for additional subgrade prep before the aggregate base goes down. For a standard residential slab in Orange County without a soils flag, compacted subgrade plus the R506 aggregate base and vapor retarder is typically sufficient. If you are unsure, ask your soils engineer before pouring. Fixing a failed slab after the pour is far more expensive than doing the prep right the first time.

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