What to Do When Your Garage Slab Has Moisture Problems

The Hidden Problem Beneath Your Garage Floor

Your garage floor looks dry. You can park on it, store boxes on it, walk across it without getting your feet wet. So when someone tells you the slab has a moisture problem, it sounds unlikely. How can a dry-looking floor have moisture issues?

The answer lies in how concrete works. Concrete is porous. Even when the surface appears dry, moisture migrates upward through the slab from the soil below. In a garage, this is not a problem because the space is ventilated and unfinished. But the moment you seal that slab with insulation, flooring, and conditioned air, you trap that moisture inside the building envelope. What was invisible becomes destructive: flooring peels, mold grows, odors develop, and materials deteriorate.

This guide covers everything you need to know about slab moisture before converting your garage into living space.

Signs of Slab Moisture Problems

Some signs are obvious. Others require closer inspection.

Visible Indicators

• Dark or wet spots on the slab surface that appear without any obvious water source
• White crystalline deposits (efflorescence) on the concrete, caused by mineral salts carried to the surface by evaporating moisture
• Peeling paint or coating on the slab surface, where adhesion has failed due to moisture pushing up from below
• Puddles after rain that persist longer than 24 hours, suggesting water is entering from beneath or from exterior sources
• Musty or damp odor in the garage, especially when the door has been closed for extended periods

Less Obvious Indicators

• Condensation on cool objects placed on the slab (metal tools, boxes, plastic bins) suggests that slab temperature and moisture levels create a condensation environment
• Rust on metal objects stored directly on the floor
• Mold on cardboard boxes or other organic materials stored on the slab
• Previous flooring failure if someone installed floor covering that later buckled, warped, or developed mold

How to Test for Slab Moisture

Before spending money on solutions, quantify the problem. Three testing methods are available, ranging from a free DIY screen to professional laboratory analysis.

The Plastic Sheet Test (DIY Screening)

This is a simple, free test that gives you a rough indication of whether moisture is present.

1. Clean a 2-foot by 2-foot area of the slab surface
2. Tape a piece of clear plastic sheeting (4-mil or heavier) tightly to the slab on all four sides
3. Wait 24 to 48 hours
4. Peel back the plastic and check for condensation on the underside

If you see moisture droplets or the concrete beneath the plastic is darker than the surrounding slab, moisture is migrating through the concrete. This test does not tell you how much moisture is present, but it confirms that a problem exists and warrants professional testing.

Calcium Chloride Test (ASTM F1869)

This is the traditional professional test for measuring moisture vapor emission rate (MVER) from a concrete slab.

How it works: A small container of calcium chloride (a moisture-absorbing salt) is placed on the clean slab surface and sealed under a plastic dome. After 60 to 72 hours, the container is weighed. The weight gain tells you how much moisture vapor passed through the slab during the test period.

Results: Expressed in pounds of moisture per 1,000 square feet per 24 hours.

• Below 3 lbs: Generally acceptable for most flooring installations
• 3 to 5 lbs: Moderate moisture. Some flooring types require mitigation at this level
• 5 to 8 lbs: Significant moisture. Mitigation required before any flooring
• Above 8 lbs: Severe moisture. Evaluate whether the conversion is feasible

Cost: $200 to $500 for a professional test with multiple sample locations.

Limitations: This test measures surface moisture emission, which can be affected by recent cleaning, weather conditions, and whether the slab was sealed or coated. It should be conducted on a bare, uncoated slab that has been exposed to ambient conditions for at least 48 hours.

Relative Humidity Probe Test (ASTM F2170)

This is the newer, more accurate method preferred by many flooring manufacturers and industry professionals.

How it works: Holes are drilled into the slab to a depth equal to 40% of the slab thickness. Humidity probes are inserted and allowed to equilibrate for a minimum of 72 hours. The probes measure the relative humidity (RH) within the concrete.

Results: Expressed as a percentage of relative humidity.

• Below 75% RH: Acceptable for most flooring systems
• 75% to 85% RH: Moderate moisture. Mitigation likely required
• Above 85% RH: Severe moisture. Significant mitigation or project reconsideration needed

Cost: $300 to $800 for a professional test with multiple probes.

Advantages: This method measures moisture throughout the slab depth, not just at the surface. It is less affected by ambient conditions and provides a more reliable prediction of long-term moisture behavior.

What Causes Garage Slab Moisture

Understanding the cause determines the solution.

No Vapor Barrier Beneath the Slab

This is the most common cause. Garage slabs, especially those poured before the 1990s, were often placed directly on soil or a gravel base without a polyethylene vapor barrier. Without that barrier, ground moisture has a direct path upward through the concrete.

Poor Exterior Drainage

If the grading around your garage slopes toward the structure rather than away from it, surface water collects against the slab edges and foundation. This water migrates under and through the slab over time. Downspouts that dump roof runoff near the garage foundation compound the problem.

High Water Table

In low-lying areas and neighborhoods near San Francisco Bay, the water table can sit close to the surface, especially during the wet season. Hydrostatic pressure from groundwater pushes moisture upward through any concrete slab, regardless of the slab’s condition.

Slab Cracks

Cracks in the garage slab create preferential pathways for moisture. Even hairline cracks allow moisture vapor to pass through at higher rates than intact concrete. Larger cracks may allow liquid water to enter during periods of high groundwater.

Subsurface Drainage Issues

Underground springs, old creek beds, broken irrigation lines, or failed drainage pipes can create localized areas of saturated soil beneath the garage. These conditions may not be apparent on the surface but produce concentrated moisture problems in the slab above.

Solutions by Severity

Mild Moisture (MVER Below 3 lbs, RH Below 75%)

Topical sealers and moisture barriers

For mild moisture conditions, a penetrating concrete sealer or an epoxy moisture barrier applied directly to the slab surface is often sufficient. These products chemically react with the concrete or create a film that reduces moisture vapor transmission.

• Cost: $2 to $5 per square foot, applied
• Application: Clean slab, apply one to two coats, allow to cure (typically 24 to 48 hours)
• Longevity: 5 to 10 years for penetrating sealers; longer for epoxy systems
• Limitations: Topical treatments reduce moisture transmission but do not eliminate it. They are appropriate only for mild conditions

Moderate Moisture (MVER 3-5 lbs, RH 75%-85%)

Dimple mat drainage membranes

A dimple mat (also called a drainage membrane or subfloor membrane) creates an air gap between the slab and the new floor assembly. Moisture vapor passes through the concrete and into the air gap, where it evaporates or drains to the perimeter.

• Cost: $3 to $6 per square foot, installed
• How it works: The dimpled plastic sheet is laid over the slab, dimple side down, creating small air channels. A subfloor panel or plywood layer is installed on top. Flooring goes over the subfloor
• Advantages: No slab preparation needed beyond cleaning. Provides thermal insulation. Allows the slab to breathe. Relatively quick to install
• Considerations: Reduces available ceiling height by approximately 1 to 1.5 inches

Exterior drainage corrections

If the moisture source is exterior water infiltration rather than rising ground moisture, correcting the drainage may solve the problem at its source.

• Regrade the soil around the garage to slope away from the structure (minimum 6 inches of fall over the first 10 feet)

• Extend downspouts to discharge at least 6 feet from the foundation

• Install a French drain along the exterior perimeter of the garage to intercept subsurface water

• Cost: $2,000 to $8,000 depending on the scope of drainage work

Severe Moisture (MVER Above 5 lbs, RH Above 85%)

Vapor barrier overlay with topping slab

For severe moisture conditions, a new vapor barrier and concrete topping slab may be necessary. This effectively creates a new slab on top of the old one, with a proper moisture barrier in between.

• Cost: $8 to $15 per square foot
• How it works: A 10-mil or 15-mil vapor barrier is laid over the existing slab with sealed seams. A new 2 to 3-inch reinforced concrete topping slab is poured on top. The new slab has the vapor barrier protection that the original slab lacks
• Considerations: This approach adds 2 to 3 inches of floor height, which reduces available ceiling height. It also adds significant weight to the existing foundation

Interior perimeter drainage system

For slabs with active water infiltration (liquid water, not just vapor), an interior perimeter drainage system collects and removes water before it reaches the living space.

• Cost: $5,000 to $15,000
• How it works: A trench is cut around the interior perimeter of the garage slab. A perforated drain pipe is installed in gravel. Water collected by the drain is routed to a sump pit with an automatic pump that discharges to the exterior
• Considerations: This is the same system used for basement waterproofing. It is effective but adds ongoing maintenance (the sump pump must be kept operational) and reduces usable floor area

When to Walk Away

Not every garage is a good candidate for conversion, and severe moisture problems are one of the clearest deal-breakers. Consider an alternative to conversion when:

• The water table is at or above slab elevation during the wet season. No practical amount of mitigation will overcome persistent hydrostatic pressure at this level.
• Active water infiltration persists even after exterior drainage corrections. If liquid water continues to enter through the slab after you have corrected grading, extended downspouts, and installed perimeter drains, the source may be beyond practical control.
• Moisture remediation costs exceed 30% to 40% of the total conversion budget. At this point, you may be spending more on fixing the slab than it would cost to pour a new one. A detached ADU built on a properly prepared foundation with a code-compliant vapor barrier eliminates the moisture issue entirely.
• Ceiling height is already borderline. Moisture solutions like topping slabs and dimple mats reduce available ceiling height. If your garage is already close to the 7-foot minimum, adding 2 to 3 inches of moisture remediation on the floor may push you below code.

In these situations, a detached ADU or a different conversion project is usually a better investment.

How Design-Build Teams Evaluate Slab Moisture

At Custom Home Design and Build, slab moisture testing is a standard part of our garage conversion feasibility assessment. Before we begin design work, we:

1. Visually inspect the slab for signs of moisture, efflorescence, cracking, and previous coatings
2. Conduct moisture testing using professional MVER or RH methods
3. Evaluate exterior drainage conditions around the garage
4. Research the site for known water table levels and soil conditions
5. Present findings to the homeowner with a clear recommendation: proceed with a specific moisture solution, or consider an alternative project

This upfront assessment costs far less than discovering moisture problems after flooring is installed or walls are closed up. It protects your investment and ensures that the conversion will perform well for decades.

Test Before You Build

Slab moisture is invisible, but its consequences are not. A few hundred dollars spent on professional moisture testing can save you thousands in failed flooring, mold remediation, and rework. Whether your slab needs a simple sealer or a complete drainage system, the right solution starts with accurate data.

If you are considering a garage conversion in the Bay Area, contact Custom Home Design and Build to schedule a feasibility assessment. We will test your slab, evaluate your garage’s conversion potential, and give you a clear path forward.