Moisture testing a Florida concrete slab before coating

A moisture test tells you whether the slab will let a coating stay stuck. In Florida it is worth doing on almost every garage, because the conditions that cause moisture failures are common here, and a dry-looking floor can still be pushing vapor up from below.

Why Florida slabs hold moisture

The water table is close

Across much of Florida the ground stays wet close to the surface. A USGS study of Osceola County found the water table usually sits about 2 to 20 feet below the land surface, and in low, flat, poorly drained areas it commonly stands at or within a few feet of the surface, rising above it after heavy rain. A garage slab poured on that ground is in contact with moist soil for its whole life. Conditions vary a lot from one lot to the next, which is why a test on your floor beats any rule of thumb.

Garage slabs often have no plastic underneath

Under a house, Florida's residential code requires a 6-mil polyethylene sheet or other approved vapor retarder between the slab and the ground. The same section lists garages as an exception, along with carports and other unheated spaces. So even a garage in a newer house, built to code, may have no vapor retarder under it, and you cannot see one from the top of the slab.

Humidity and rain add to it

Concrete also takes on water from the top. Wet tires, wind-driven rain through an open door and the summer rainy season all put water on a Florida garage floor. A Tampa Bay installer's pricing page makes the useful distinction: water carried in on tires is a surface issue a topcoat can handle, while vapor rising from under the slab is something no topcoat can stop on its own.

The common tests, in plain words

Plastic sheet test (ASTM D4263)

Tape a square of clear plastic to the floor, leave it, then look for condensation or darkened concrete underneath. ASTM describes it as a method that indicates the presence of capillary moisture. It gives a yes or no, not a number, and the coating data sheets we read name the other two tests as their measure. Treat it as a cheap first look.

Calcium chloride test (ASTM F1869)

A small dish of calcium chloride sits under a sealed dome on the cleaned floor and absorbs the vapor coming out of the slab. After the test period it is weighed. The result is the weight of water that would evaporate from 1,000 square feet of floor in 24 hours, written as pounds per 1,000 square feet per 24 hours.

ASTM notes the result reflects the floor at the time of the test and may not predict later conditions, and that the test should not be run over existing coatings. So on a garage with an old coating, the old coating comes off before this test means anything.

In-slab relative humidity probes (ASTM F2170)

Holes are drilled into the slab, sleeves and probes go in, and after a waiting period the probe reads the relative humidity inside the concrete as a percentage. It measures inside the slab rather than at the surface. Ask the crew how long their probes sit before the reading, since the standard sets a waiting period.

What the results mean

A result only means something when you compare it to the limit on the data sheet of the product going on your floor. Here is what three sheets we read allow:

Product (data sheet)	Calcium chloride limit	In-slab RH limit
Leggari Polyaspartic	3 lb	80%
CHEMPROOF SC (a moisture-mitigation primer, stating the limit for standard coatings)	3 lb	85%
Polytek Floor Coating Epoxy, used as a primer	4.5 lb	not stated

Limits are per 1,000 square feet per 24 hours. The CHEMPROOF sheet says that above its limits, blistering and delamination of the coating may occur. Different products allow different readings, so always ask which product the crew is using and what its sheet says.

Ask the crew for the reading in writing, with the test used, the date, and where on the floor the test sat. One test near the door and one at the back tells you more than one test in the middle.

Mitigation options

If the slab fails, the usual fix is a moisture-control primer or membrane that goes down before the decorative layers. These products are rated for higher readings. CHEMPROOF SC's sheet, for example, rates it for slabs up to 15 lb per 1,000 square feet per 24 hours. Polytek's sheet says readings between 4.5 and 15 lb call for its moisture-seal product.

Cost, according to two Florida installer price pages:

• One allows $1 to $3 per square foot for mitigation
• The other says $2 to $6 per square foot in difficult cases

Not every garage needs it, and the Tampa Bay installer says adding it to every job would waste money. That is the point of testing first.

Other things that help, alongside the primer:

• Fixing drainage so water does not run toward the garage
• Keeping the door closed during heavy rain in the days before the install
• Scheduling the install after a dry stretch

None of these replace the primer when a slab reads high.

Questions to ask before you sign

• Which moisture test will you run, and where on the floor?
• What reading does the product sheet allow?
• What did my slab read? Can I have it in writing?
• If it fails, which mitigation product will you use, what reading is it rated for, and what does it add to the price?
• Does the warranty exclude moisture failures? If it does, does it still exclude them when you tested and installed mitigation?

The last question matters most in Florida, where slab moisture is the failure a warranty is most likely to be asked about. Our contractor checklist and peeling guide cover the rest.