For Builders & General Contractors
Garage-to-ADU Conversions: What to Do With the Slab
That garage floor was engineered to hold a leaking truck. Now it has to behave like a bedroom floor. Here is what actually has to happen to it — and where the callbacks come from when it doesn't.
Garage conversion is the highest-volume ADU type in Southern California, and it is the only one where you inherit a finished floor you did not pour. Every other line item on the job is new work you control. The slab is the exception — and it is the single most common source of a warranty call twelve months after final inspection.
If you build ADUs in Riverside, San Bernardino, Orange, or LA County, you have already run into some version of this: the LVP cups along one wall, the baseboard shows a tide line, or the client calls in February to say the new room smells like a basement. Nine times out of ten the framing was fine, the insulation was fine, the finish floor was installed to spec — and the slab underneath it was never evaluated.
This is the technical brief we give the contractors we supply. It covers what is actually wrong with a garage slab, the three ways to correct it, where the work belongs in your schedule, and language you can drop straight into a scope of work.
Code references are to the 2025 California Residential Code, effective January 1, 2026. Several relevant sections were renumbered from the 2022 cycle — if your plan checker is still working from the prior edition, the section numbers will differ. Always confirm with the authority having jurisdiction.
Four things are wrong with every garage slab
Not "might be." These are structural to how garages get built.
There is nothing under it
Here is the part that catches people: the code agrees with you. The 2025 California Residential Code requires a vapor retarder under slabs in conditioned space — R506.3.3, minimum 6-mil polyethylene or approved equivalent, with a CALGreen capillary break per R506.3.3.1 — and it expressly exempts garages. Your slab is bare underneath because it was allowed to be.
That exemption made sense when the space held a truck. It stops making sense the day you condition it. And because R506.3.3 governs new pours rather than existing slabs, nothing in the residential code is going to tell you when the retrofit is necessary — that call comes from ASTM F710 and the finish flooring manufacturer's data sheet.
You cannot retrofit a vapor barrier underneath an existing slab. The moment that garage becomes conditioned, the slab starts moving ground moisture up into a room that now has drywall, insulation, and a finish floor sitting on it. The barrier has to go on top, and it has to go on before anything moisture-sensitive does.
Test before you spec: ASTM F2170-19a in-situ relative humidity probes — three for the first 1,000 sq ft, one per additional 1,000, read after a minimum 24-hour equilibration. (The 72-hour figure many of us learned was shortened in the 2018 revision.)
Then check the number against the actual finish. The limits vary more than most people expect, and they track the adhesive and installation method far more than the brand: engineered and solid wood typically cap around 75% RH / 3 lb MVER, glue-down resilient with a standard acrylic adhesive lands around 75–80%, and floating rigid-core LVP is frequently rated to 90% — Shaw's SPC and Mohawk's UniDrop both publish 90% RH / 8 lb. A slab that disqualifies a wood floor may be fine under a floating plank. Pull the current data sheet for the specific product and adhesive; several manufacturers have revised these numbers.
It is not level, and that is on purpose
Garages have to drain. In the 2025 CRC that requirement sits at R317.1 — renumbered from R309.1 in the previous cycle, so check which edition your plan checker is working from — and the language is mandatory: parking areas shall be sloped to move liquids toward a drain or the main vehicle entry door. No percentage is prescribed, but in practice your framer is setting plates on a floor that drops an inch or two across a 20-foot bay, sometimes more where the apron was poured to meet a driveway.
The useful nuance: once that bay is habitable space it is no longer a garage, and R317.1 stops applying to it. The slope isn't something you have to preserve. It's something you inherit and have to flatten — and self-leveling underlayment is the standard answer.
The catch is that SLU is only as good as what it bonds to. Poured over a sealed, dusty, or oil-glazed slab it delaminates in sheets, and you find out when the finish floor starts clicking underfoot.
Thirty years of oil is in the concrete, not on it
Motor oil, ATF, brake fluid, and hydraulic fluid penetrate. Degreasing lifts what is on the surface and leaves what has wicked into the top eighth of an inch, which is exactly the zone every coating, primer, and underlayment needs to bond to. Add whatever big-box sealer a previous owner rolled on, and you have a surface that will reject adhesion in patches you cannot see.
Mechanical prep is the only reliable answer. ICRI Guideline 310.2R-2013 defines the CSP 1–10 scale everybody specs against. Grinding lands around CSP 2; shot blasting starts around CSP 3. MAPEI specifies CSP 2 by grinding to CSP 3 by shot blast under an epoxy moisture barrier, and LATICRETE calls for a minimum CSP 3 under self-leveling underlayment. Shot blast to CSP 3 and you have satisfied both, which is why it is the safer number to put on a plan.
Acid etching will not get you there. It lifts laitance but leaves curing compounds, sealers, and most bond-inhibiting contamination in place, produces a shallow and highly variable profile, drives water into a slab you are trying to dry, and the rinse is a disposal problem in most California jurisdictions. MAPEI and LATICRETE both state outright that chemical removal methods are not recommended.
Five-minute field check: drop water in ten spots across the bay, especially where the car sat. If it beads instead of darkening the concrete within about a minute, you have a sealer or contamination problem and the slab needs mechanical prep before anything else is priced.
Some of those cracks are still working
LA County's garage conversion standard notes — unincorporated county, revised March 2023 — require the existing slab to be "reasonably free of settlement or expansive soils cracks," and call for a new concrete layer over a roughened surface, or added vapor barrier protection, where damage is extensive. They also set thickness thresholds worth knowing: a 2-inch sound slab may be used with a 2-inch topping, and a 3-inch sound slab may be approved without one — at the District Office Manager's discretion, not as of right. Your city may take a different view, so confirm before you price it.
Much of the Inland Empire sits on expansive soils, which means the question is never whether a slab has cracks. It is whether they are done moving.
A static shrinkage crack gets filled and forgotten. A crack that is still opening seasonally will telegraph through underlayment, coating, and tile alike. Map the cracks, note width and any vertical offset, and treat differential lippage as a signal to price a topping slab instead of a patch.
The one that costs you money
Moisture is the failure that shows up after you are paid, after the client has moved in, and after the flooring installer has stopped answering the phone. An RH probe test costs a fraction of one percent of the conversion budget and reads in 24 hours. Nothing else in this article buys you as much protection for as little.
Three ways to correct it
Choose from the slab's condition and the client's finish, not from habit.
| Path | When it's right | What it involves | Planning range |
|---|---|---|---|
| A — Coated slab as the finish floor | Slab is sound. Client wants a modern, seamless look. Fastest schedule, no moisture-sensitive material on top. | Grind to profile, repair cracks, moisture-mitigation primer if indicated, then a flake, solid-color, or metallic system. Slope corrected with SLU only where the client wants it flat. | $7–12 / sq ft polished concrete look: $8–15+ |
| B — Mitigate, level, then finish floor | Client wants LVP, engineered wood, carpet, or tile. This is the majority of conversions. | Grind, crack repair, 100%-solids moisture vapor barrier coating, then SLU to flat. Finish flooring installs over a warrantable substrate. | $6–14 / sq ft |
| C — Topping slab | Extensive cracking, differential settlement, or a slab too thin or too damaged to correct on the surface. | Roughen the existing surface, dowel to the old slab with epoxy-set rebar per plan detail, pour a bonded topping, then finish per A or B. | $8–15 / sq ft |
Southern California planning figures for 2026, materials and labor, for carrying a number before the slab has been tested. Not a quote — condition drives price more than square footage does. Two things a per-square-foot table hides: Path B climbs toward the top of its range fast once you are past a single-coat mitigation system or a quarter inch of leveling, and on Path C a 400 sq ft garage is only about two and a half yards of concrete, so short-load fees and wheelbarrow or pump access can add several dollars a foot on their own.
Where it belongs in your schedule
This is the part most GCs get backwards, and it is pure margin.
- Before you demo the doorWater-drop test and a crack map while the bay is still empty and you can see the whole floor. Ten minutes. It tells you which of the three paths you are pricing.
- Before you frameSet RH probes and let them read for at least 24 hours. Grind the entire slab while it is one open bay with no walls in it. Grinding a framed room means hand-grinding edges at several times the labor, and you will still miss corners.
- After rough-in, before insulation and drywallCrack repair and the moisture vapor barrier coating. The slab is protected for the rest of the build and you are no longer scheduling around finish trades.
- After drywall, before finish flooringSelf-leveling underlayment. Do it after the walls are up so the SLU terminates cleanly at the plate line instead of being cut back later.
- LastFinish floor, or the decorative coating if the concrete is the finish. Protect it — the punch-list trades are the ones who damage it.
Grind before you frame
If you take one scheduling change away from this article, make it this one. Moving the grind ahead of framing on a 400 sq ft conversion typically saves more labor than the moisture testing costs, and it produces a better profile across the whole floor because the machine can actually run.
Scope language you can use
Drop this into your subcontract or plan notes so the slab is somebody's defined responsibility.
1. Assessment. Contractor shall perform in-situ relative humidity testing per ASTM F2170-19a (minimum three probes for the first 1,000 sq ft, one per additional 1,000; minimum 24-hour equilibration) after the space is enclosed and at service conditions. Results to be evaluated against the published limits of the specified floor covering and its adhesive, and submitted before finish flooring is ordered.
2. Surface preparation. Existing slab shall be mechanically prepared by diamond grinding or shot blast to a minimum ICRI CSP 3 profile per ICRI Guideline 310.2R-2013, free of coatings, sealers, curing compounds, adhesives, and hydrocarbon contamination. Acid etching shall not be used in lieu of mechanical preparation.
3. Crack and joint treatment. Non-moving cracks to be routed and filled with a rigid epoxy or polyurea repair material. Moving joints to be honored through all subsequent layers. Cracks exhibiting vertical displacement to be reported before proceeding.
4. Moisture mitigation. Where RH exceeds the finish flooring manufacturer's limit, apply a 100%-solids epoxy moisture vapor barrier at the manufacturer's specified rate, primed and broadcast as required for the subsequent layer.
5. Flatness. Existing garage slope shall be corrected with cementitious self-leveling underlayment over the manufacturer's primer, to a finished tolerance suitable for the specified floor covering.
6. Submittals. Technical data sheets and safety data sheets for all products, plus moisture test results, to be provided to the owner at closeout.
Empire supplies every layer in that scope — surface prep equipment and diamond tooling, moisture mitigation and primer systems, repair materials, self-leveling underlayment, and the decorative systems if the concrete is staying exposed. If you would rather not self-perform, we install across the Inland Empire, Orange County, and greater Los Angeles.
Common questions
Can I just put LVP over the garage slab?
Often yes — but only after you know the moisture number and the floor is flat. The limits track the install method more than the brand: floating rigid-core LVP is frequently rated to 90% RH / 8 lb (Shaw SPC and Mohawk UniDrop both publish that), while glue-down resilient with a standard acrylic adhesive commonly caps around 75–80% and wood around 75%. Installing outside the published limit voids the warranty, which is exactly the coverage you are leaning on when the callback comes. On a garage slab with no vapor retarder beneath it, the RH reading decides — not the age or appearance of the concrete.
Do I have to level the whole floor?
The floor has to be flat enough for the specified finish and has to satisfy your plan checker for habitable space. In practice that means correcting the drainage slope, which self-leveling underlayment does efficiently. If the client is keeping polished or coated concrete as the finish, a gentler correction is sometimes acceptable — but confirm it against the approved plans rather than assuming.
How long does the slab work add to the schedule?
Budget roughly a week of calendar time across the whole build, most of it passive: 24 hours minimum for RH probes to equilibrate, a day for grinding and crack repair, overnight cures on the mitigation coating, and 24 to 72 hours after self-leveling before the finish floor goes down. Sequenced as described above, almost none of it holds up another trade.
What if the slab is only three inches thick?
Thin slabs are common in older detached garages. LA County's garage conversion notes address it directly for unincorporated county: a 2-inch sound slab may be used with a 2-inch concrete topping, and a 3-inch sound slab may be approved without a topping — but that approval is at the District Office Manager's discretion, not automatic. Other jurisdictions set their own thresholds. Verify thickness by coring or at a saw cut and take the number to your plan checker before you price the finish.
Does the oil stain matter if I'm covering it anyway?
Yes. Hydrocarbon contamination sits in the top layer of the concrete, which is the exact zone a primer, mitigation coating, or underlayment has to bond to. Covering it does not neutralize it — it just moves the failure under a finished floor where it is expensive to reach. Grind it out.
Do you supply contractors, or only install?
Both. Empire Coating Solutions is a Kretus-authorized supplier, installer, and training academy in Corona, and we stock surface prep equipment, moisture mitigation systems, repair materials, and decorative coatings for contractors working across Southern California. Call 909-272-5137 and we will spec the slab with you.
Got a conversion on the board? Send us the slab.
Photos, the age of the garage, and a rough square footage are usually enough for us to tell you which of the three paths you are looking at and what to carry in the budget. No charge, and no obligation to buy the material from us.
Empire Coating Solutions · Corona, CA · Serving Riverside, San Bernardino, Orange, Los Angeles & San Diego counties