Moisture is the most common reason resin floors, coatings and flooring adhesives fail, and it is also one of the easiest problems to test for. Water vapour moving up through a slab has nowhere to go under an impermeable coating. It builds pressure, and the floor blisters, softens or lifts.
This guide covers where the moisture comes from, which concrete moisture test to use, how many tests you need and what to do when the result is too high. The manufacturer's data sheet always sets the limit for the product you are installing. The figures below are typical values for guidance.
Where the moisture comes from
Three sources account for almost every problem:
- Construction water. New concrete and screeds hold a lot of water that takes weeks or months to leave. New slabs usually need at least 28 days of curing before you even test, and often much longer before they are dry enough to coat.
- Ground moisture. Slabs on ground with no damp-proof membrane, or a damaged one, keep drawing water up from below.
- Leaks, spills and wet cleaning. Plumbing leaks, washdown and open roofs add water after the slab has dried.
Concrete moisture test methods compared
Different countries and specifications name different tests. Use the method your specification or the product data sheet names.
| Method | What it measures | How it works | Typical use |
|---|---|---|---|
| In-situ relative humidity probe (for example ASTM F2170) | Humidity inside the slab | Drill holes to a set depth, insert sleeves and probes, wait for equilibrium, read %RH | Widely used for resilient flooring and many resin systems |
| Hood or hygrometer test (for example BS 8203 methods) | Humidity of air trapped over the surface | Seal an insulated box over the floor, leave it to equalise (often at least 72 hours), read %RH | Common in UK-influenced markets |
| Calcium carbide (CM) method | Moisture content by weight (%) | Crush a sample and react it with carbide in a pressure vessel | Common in Europe and elsewhere; destructive |
| Calcium chloride test (for example ASTM F1869) | Moisture vapour emission rate | Seal a dish of calcium chloride on the surface for 60 to 72 hours and weigh it | Older US method, still named in some specifications |
| Plastic sheet test (for example ASTM D4263) | Whether moisture is present | Tape a sheet to the floor for 16 hours or more and look for condensation | Quick indicator only, not for acceptance |
| Electronic moisture meter | Comparative reading | Surface contact or radio-frequency reading | Finding wet and dry areas, not for acceptance |
The last two are useful for screening a floor. Neither should be used to accept or reject a slab.
Typical moisture limits
These are common values, not rules. Always check the data sheet.
- Many resin and resilient floor systems: about 75% RH by probe or hood test.
- Moisture-tolerant epoxy primers and barriers: some are rated for much higher readings, but only where the data sheet says so.
- CM method: cement-based substrates are often around 2 to 4%, depending on the product and country. Calcium sulphate (anhydrite) screeds are much lower, often around 0.5%.
- Calcium chloride: commonly stated at about 3 to 5 lb per 1,000 ft² per 24 hours, depending on the product.
How to carry out the test properly
- Bring the building to its normal service temperature and humidity for at least 48 hours before testing where you can.
- Test enough locations. One test is never enough. A common rule is three tests for the first 1,000 ft² (about 100 m²), one more for each additional 1,000 ft², and extra tests in suspect areas: near walls, drains, columns and recent repairs.
- Record the date, time, location, method, depth, readings, and air temperature and humidity.
- Photograph each test location with its label.
What to do if the slab is too wet
- Wait and retest. Dehumidifiers and heating can speed drying if the programme allows.
- Use a moisture-tolerant primer or barrier system. Price it as a variation and agree it in writing.
- Never start on a failed slab at the client's request without that request, and the risk, in writing.
Frequently asked questions
What moisture level is acceptable for epoxy flooring?
It depends on the product. Many epoxy and resilient floor data sheets set a limit around 75% RH, and some moisture-tolerant primers allow more. The manufacturer's current data sheet decides.
How long should new concrete cure before an epoxy coating?
New slabs usually need at least 28 days of curing before testing, and often longer before they are dry enough. Cement screeds are sometimes estimated at about a day per millimetre for the first 50 mm in good drying conditions. That is only a guide. Test the slab rather than counting days.
Is a plastic sheet test good enough?
No. It only shows whether moisture is present. It is a quick check, not a basis for accepting a slab.
How many moisture tests do I need?
A common rule is three tests for the first 1,000 ft² (about 100 m²) and one for each additional 1,000 ft², plus extra tests in suspect areas. Follow your specification if it asks for more.
Which test is best?
The one your specification and the product data sheet call for. In-situ RH probes are widely used for resin and resilient flooring, hood tests are common in UK-influenced markets, and CM tests are common in Europe.
Test limits and methods vary by product, country and specification. This guide is general information, not a substitute for the manufacturer's data sheet or the project specification.