The point: moisture is the most common cause of failure and the easiest to test. Test before you start, write down the results, and don't start if the substrate is above the limit.
Where moisture comes from: water left from construction (the mix water still drying out), moisture rising from the ground where there is no or a damaged membrane, and leaks, spills or wet cleaning.
The main test methods
| Method | What it measures | How it works | Typical use |
|---|---|---|---|
| In-situ relative humidity probe (e.g. ASTM F2170) | Humidity inside the slab | Drill holes to a set depth (often 40% of the slab depth for slabs drying from the top only), insert sleeves and probes, wait for equilibrium, read %RH | Widely used for resilient flooring and many resin systems |
| Surface hood / hygrometer test (e.g. BS 8203 / BS 8201 annex methods) | Humidity of air trapped over the surface | Seal an insulated box over the floor, leave it to reach equilibrium (typically at least 72 hours), read %RH | Common in UK-influenced markets |
| Calcium carbide (CM) method | Moisture content by weight (%) | Break out a sample, crush it, react it with carbide in a pressure vessel | Common in Europe and many other markets; destructive |
| Calcium chloride test (e.g. ASTM F1869) | Moisture vapour emission rate | Seal a dish of calcium chloride on the surface for 60–72 hours and weigh it | Older US method; still named in some specifications |
| Plastic sheet test (e.g. ASTM D4263) | Whether moisture is present (yes/no) | Tape a plastic sheet to the floor for 16+ hours; check for condensation or darkening | Quick indicator only, not for acceptance |
| Electronic moisture meter | Comparative reading | Surface contact or radio-frequency reading | Mapping wet and dry areas; not for acceptance |
Typical limits (always check the data sheet):
- Many resin and resilient systems: about 75% RH (probe or hood test).
- Some systems allow higher values; moisture-tolerant epoxy primers and barriers can be used at much higher readings when the data sheet says so.
- CM method: cement-based substrates often around 2–4% depending on the product and country; calcium sulphate screeds much lower, often around 0.5%.
- Calcium chloride: limits are commonly stated around 3–5 lb per 1,000 ft² per 24 hours, depending on the product.
Doing the tests properly:
- Bring the building to its normal service conditions (temperature and humidity) for at least 48 hours before testing where possible.
- Test enough locations: one test is never enough. A common rule is three tests for the first 1,000 ft² (about 100 m²) and one more for each additional 1,000 ft², plus extra tests in suspect areas (near walls, drains, columns, recently repaired areas).
- Record date, time, location, method, depth, readings, air temperature and humidity.
- Photograph each test location with its label.
If the substrate is too wet:
- Wait and retest, with dehumidifiers or heating if the programme allows.
- Use a moisture-tolerant primer or barrier system, priced as a variation and agreed in writing.
- Never start "at the client's request" without that request and the risk being in writing.