ASTM F2170 is the standard test method for determining relative humidity in concrete floor slabs using in-situ probes. Instead of measuring what leaves the surface, it measures the humidity of a small pocket of air sealed inside the concrete at a specified depth, once that air has come into balance with the concrete around it.
How the test works, step by step
- Condition the building. The slab and the air above it are brought to the temperature and humidity the space will have in service, and held there for 48 hours before readings are taken.[1]
- Lay out the locations. The method calls for three tests in the first 1,000 square feet and at least one more for each additional 1,000 square feet,[1] typically one near the middle of the area and the rest spread toward the edges.
- Drill to depth. For a slab that dries only through its top surface, the hole goes to 40% of the slab thickness.[1] Slabs that dry from both faces use a shallower depth set out in the standard.
- Clean, sleeve and seal. Dust is removed and a sleeve or liner isolates the measurement pocket from the room air, so the sensor only sees the concrete at depth.
- Wait for equilibrium. The drilled hole is left to come to moisture equilibrium, 24 hours in the current method, before a reading counts.[1]
- Read and record. Relative humidity and temperature are recorded for each location, along with the ambient conditions, times and any limitations.
Why 40% deep?
A slab-on-grade loses moisture through its top. That leaves a gradient: drier near the surface, wetter lower down. When the slab is covered with flooring, the moisture inside evens out and the top of the slab gets wetter than it was while exposed. Reading at a set fraction of the depth gives a number that better reflects what the underside of the floor covering will experience after installation than a surface reading would. The Slab Lab below shows that gradient.
- What it reports
- Relative humidity (%) and temperature at each probe location.
- What it measures
- Moisture condition inside the concrete at a defined depth.
- What it needs
- Service conditions in the building, drilling access, protected probe locations, time.
- What it does not do
- Set a pass/fail number. The selected products’ written limits do that.
What an RH reading can and cannot tell you
A reading tells you the relative humidity at that hole, at that depth, at that time, under the conditions recorded. It does not describe every square foot between holes, and it does not predict next month if the HVAC is switched off, irrigation changes or a pipe starts leaking. It also is not a verdict by itself: the same reading can be within one adhesive’s limit and outside another’s.
Practical notes for the site
- Get drilling approval and confirm utility clearance and any post-tension or radiant tubing before anyone drills.
- Protect probe locations from trades, carts and cleaning water until the final reading.
- Note anything that changes conditions during the test window: doors propped open, temporary heaters, wet work nearby.
- Use the current edition of the standard named in your spec. Requirements have changed between editions.
Vegas Flooring Outlet’s own page explains how it scopes project-related slab moisture testing, including how the method is confirmed during scope review.