Garage Slab Prep & Repair.
Make the existing slab livable — or know when to pour over.
An existing garage slab is the single biggest cost-saver in a garage-to-ADU conversion — but only if it can actually serve as a finished floor. We inspect every slab against four criteria (thickness, level, moisture barrier, structural integrity) and recommend preserve, patch-and-pour-over, or full demo. The decision shapes the entire project budget.
Typical range
$1,500 – $15,000 for typical 400 sqft garage
Per unit
$1.50 – $28 / sqft of slab
Timeline
1–2 weeks for assessment + pour-over; 2–3 weeks for full demo and re-pour.
The short version.
Most California garage slabs were poured between 1940 and 1985 to a 4 in nominal thickness with little to no vapor retarder and minimal rebar. The slab usually slopes 1–2% toward the garage door — fine for parking, problematic for finished flooring. The first step in any conversion is a slab assessment: thickness probe, level survey, moisture test, and crack mapping.
If the slab is 4 in thick, within ±3/8 in of level, has functional rebar, and tests under 4 lbs MVER on a moisture test, it's preserved as-is with a topical moisture seal. About 40% of garage slabs in our LA and Bay Area conversions fall into this category.
When the slab fails one or two criteria, the fix is a self-leveling pour-over (3/4 to 2 in of leveler) plus a sheet vapor retarder. About 45% of conversions use this approach. When the slab is too thin, too cracked, or sits over uncompacted soil, full demo and re-pour is required — adds $8K–$14K but eliminates risk.
What you can actually pick.
Preserve existing slab + topical seal
Pros — Cheapest option, no added floor height, fastest schedule.
Cons — Only works on slabs in good condition.
$1.50–$3 / sqftMatches existing slab lifeSelf-leveling pour-over (3/4–2 in)
Pros — Fixes most level and moisture issues, fast install.
Cons — Adds floor height, requires door jamb adjustment.
$4–$8 / sqft50+ yearsFull demo + re-pour
Pros — Resets every variable, integrates new under-slab plumbing.
Cons — Most expensive, breaks the cost advantage of conversion.
$18–$28 / sqft50+ years
What we deliver.
- Slab assessment — thickness core, level survey, moisture test, crack map
- Existing-conditions report with photographs and recommendations
- Crack repair with epoxy injection or polyurethane
- Self-leveling pour-over with sheet vapor retarder if needed
- Door-jamb and door-undercut adjustments for new floor height
- New under-slab plumbing trenches cut into existing slab
- Patch and seal trenches with bonding agent and color-matched concrete
- Final moisture test and floor-prep sign-off before flooring install
The code parts most owners miss.
- Living-space slabs require a vapor retarder per CBC §1907.6 — existing garage slabs almost never have one.
- Slab must be within ±1/4 in over 10 ft for hardwood and LVP; ±3/8 in over 10 ft for tile.
- Moisture vapor emission rate (MVER) must be < 3 lbs/1,000 sqft/24 hr for most flooring.
- Trenching that exposes rebar at < 1 in cover triggers structural review.
- Garage-floor drains (if present) must be capped and abandoned per plumbing code.
Why getting this right pays off.
The garage slab decision is where conversion projects either deliver the promised cost advantage or quietly turn into expensive remodels. A correct preserve-as-is call saves $6K–$12K versus pour-over. A wrong preserve-as-is call leads to flooring failure inside 18 months and a tear-out / re-pour that costs $25K and weeks of work.
The decision tree is not optional. We do the assessment on every garage conversion, document it in writing, and price the project against the actual slab condition.
What goes wrong — and how to avoid it.
- Assuming the slab is fine because it looks fine — moisture issues are invisible until flooring fails
- Pouring leveler without a vapor retarder — locks moisture under the floor
- Forgetting to drop the door jamb for the new floor height — door won't close
- Cutting trenches through unmarked rebar — weakens slab, expensive structural fix
- Skipping the moisture test before flooring — voids manufacturer warranty
After we hand you the keys.
- Re-seal topical moisture coating every 7–10 years if exposed
- Monitor any new cracks for movement over time
- Keep landscape grading sloped away from the converted garage perimeter
- Inspect door-undercut clearances seasonally for swelling or settling
Slab prep — the part of concrete nobody photographs.
A slab is only as good as what's under it. Base rock specification, vapor barrier choice, and rebar layout are decided in 30 minutes by the framer-foreman and decide whether your floor cracks, your radiant heat works, or your moisture wicks up into hardwood in year three.
US market size
US aggregate market: ~$32B / year. CA aggregate: ~$3.5B — among the largest state markets.
California reality
Cal/OSHA dust standards (silica) apply during slab prep + grinding. CARB requires vapor retarder on every slab under living space per CALGreen.
The manufacturers behind the spec sheet.
- Our default
Stego Industries
San Clemente, CA — privately held.
Market — Largest US vapor retarder manufacturer.
Product — Stego Wrap 15-mil Class A, Stego Crawl, StegoCrete.
In California — California-based; Class A vapor retarder is the CALGreen-compliant default.
Class A 15-mil exceeds CALGreen minimum and lasts 25+ years vs 6-mil poly that fails in 3-5.
- Spec on request
W.R. Meadows
Hampshire, IL.
Market — Major concrete curing + vapor retarder supplier.
Product — Perminator vapor retarder, Hydralastic 836.
In California — Strong distributor presence at Ganahl + Dixieline.
Perminator 15-mil is direct alternative to Stego Wrap; same performance class.
- Our default
Vulcan Materials
Birmingham, AL — NYSE: VMC.
Market — Largest US aggregate producer.
Product — 3/4" Class 2 base rock, sand, crushed concrete recycle.
In California — Major CA quarries — Irwindale (LA), Pleasanton (Bay).
Class 2 base rock from Vulcan or CalPortland is the engineer-spec'd CA standard.
- Our default
Wirsbo / Uponor
Apple Valley, MN.
Market — Largest US PEX manufacturer.
Product — AquaPEX, Wirsbo radiant tubing.
In California — Default radiant floor heat tubing in CA ADUs and remodels.
30+ year warranty, ASTM F876/F877 compliant, sold through every Ferguson + Pace Supply branch.
Tier-by-tier — what you actually get.
Standard prep
$3–$5 / sqft
e.g. 4" base rock + 10-mil poly + #4 rebar 18" OC
Garage slabs, non-conditioned space.
CALGreen compliant
$5–$8 / sqft
e.g. 4-6" base rock + Stego Class A + #4 rebar 16" OC
Conditioned living space, code-default.
Premium with radiant
$12–$18 / sqft
e.g. 6" base rock + Stego + Uponor PEX + rigid foam
Bay Area + cold-climate, premium finish floors.
California distributors.
Vulcan Materials, CalPortland
Statewide quarries.
Base rock, sand, aggregate.
Ferguson, Pace Supply
Statewide plumbing/MEP.
Uponor PEX, Stego vapor retarder, manifolds.
What it costs this year.
3/4" Class 2 base rock (delivered)
+9% YTD
≈$48 / ton
Trucking + quarry costs both up.
Stego Wrap 15-mil 14x210 roll
+4% YTD
≈$420
Specialty plastic resin pricing.
Uponor AquaPEX 1/2" 500 ft roll
+6% YTD
≈$285
Copper alternative pricing tracks copper.
What we tell owners — off the record.
Most CA contractors still default to 6-mil polyethylene as their 'vapor barrier' even though it fails at every penetration and within 5 years. CALGreen specifies Class A (15-mil Stego or equivalent). The cost delta is $0.40/sf — never accept 6-mil under a conditioned slab.
Base rock thickness drives slab performance more than concrete PSI. 4" compacted Class 2 + capillary break + vapor retarder + 4" slab is the floor for any CA living space. Cut the base to 2" and you get bowing, cracking, and moisture wicking within 5 years.
Radiant floor heat (Uponor PEX in slab) is the most overlooked upgrade in California ADU construction. Adds ~$5-8/sf and pairs with a 60-gallon heat pump water heater for primary heat — eliminates ducting, cuts heating energy use by 30%, and is invisible.
Rebar tying is the labor cost — not the rebar itself. A skilled team will hand-tie a 600-sf slab in 4-6 hours. A budget crew will skip every third tie and your inspector will catch it before pour.
What the brand reps won't tell you.
- Sand-bedded slab? Don't. Sand under a vapor retarder defeats the entire system because it stores moisture. Always use clean angular base rock — Class 2 or 3/4" minus.
- Concrete pumped to a remote pour is 12-25% more expensive than direct chute pour. Worth it for hillside or backyard ADUs where the truck can't reach.
- Curing compound vs water-cure: water-cure (wet burlap, plastic sheeting) yields 20-30% stronger concrete vs spray-on curing compounds. Skipped by every production contractor.
Our default spec
Default: 4-6" Vulcan Class 2 base, compacted to 95%, Stego Wrap 15-mil Class A vapor retarder taped at all seams + boots at penetrations, #4 rebar 16" OC each way, water-cured 7 days. Radiant upgrade: Uponor AquaPEX 1/2" tubing on 12" centers on chairs above vapor retarder.
In short.
- Can I keep my existing garage slab for an ADU conversion?
- Roughly 40% of the time, yes. The slab needs to be 4 in thick, within ±3/8 in of level, have functional reinforcement, and test under 4 lbs MVER on a moisture test. We do the assessment before any contract.
- How much does it cost to prep a garage slab for conversion?
- $1,500–$15,000 for a typical 400 sqft garage. Preserve + topical seal is $1.50–$3/sqft. Self-leveling pour-over is $4–$8/sqft. Full demo and re-pour is $18–$28/sqft.
- Do I need a vapor retarder under the garage slab?
- Yes — required by CBC §1907.6 for any living space. Existing garage slabs rarely have one, so we add either a sheet retarder under a self-leveling pour-over or a topical epoxy moisture-vapor coating.
- What if my garage slab is sloped?
- Garage slabs slope 1–2% toward the door for drainage. A self-leveling pour-over (3/4 to 2 in) corrects the slope and gives you a flat finished floor. Adds $4–$8/sqft.
- Can I cut the existing slab for new plumbing?
- Yes, with care. We mark existing rebar with ground-penetrating radar before cutting, trench only as needed, and patch with bonding agent and color-matched concrete after plumbing rough.
- Will the new floor height match the rest of the house?
- Usually not — the garage slab typically sits 4–8 in below the main house's floor. We address this with a transition threshold, a step, or a leveling pour-over depending on the layout.
- How long does slab prep take?
- 1–2 weeks for assessment plus a self-leveling pour-over. 2–3 weeks for full demo, plumbing trenches, and a fresh slab pour with proper cure time.
- Will my flooring warranty be honored over a converted garage slab?
- Only if the slab is documented to meet the manufacturer's moisture and flatness specs. We provide the moisture test reading and flatness measurement in writing before the flooring installer starts.
Keep reading.
Planning garage slab prep & repair?
Send us the address and the scope. We'll come back with a line-item budget, a permit path, and a realistic schedule — before you spend on drawings.
Start a project →Slab Prep done right — priced from the last three we built.
- We show you the actual line items, not a marketing range
- Materials sourced from our standing trade accounts
- Site visit before any number gets sent
Free site visit · Free estimate