Concrete moisture, before the floor
Know what the slab is holding before the floor goes down.
Concrete can look bone-dry in the Las Vegas air and still carry enough moisture to undo a glue-down floor, an adhesive bond or a coating. This guide explains how slab moisture is tested, what the results mean, and how to get a slab ready for testing day.
- Published by Vegas Flooring Outlet
- Licensed C-16 / C-20
- 14+ years in Las Vegas
Why it matters
The slab and the floor have to agree
Every moisture-sensitive flooring product, adhesive, underlayment and coating is designed to work within the limits its manufacturer writes down. A moisture test is how a project finds out where the slab actually stands against those limits, before anything is glued, troweled or sealed over it.
Limits live in the paperwork
The acceptable reading is set by the specific flooring, adhesive and prep products chosen, plus the project spec. There is no single number that clears every floor.
Surface dry is not slab dry
The top of a slab gives up moisture fastest, especially in dry desert air. Deeper concrete can stay wetter, and once covered, that moisture redistributes upward.
A test is a snapshot
Results describe the tested spots, at the time of testing, under the conditions recorded. Weather, HVAC, leaks and schedule changes can move the picture.
The methods
Two standard tests, two different questions
Most flooring instructions in the US point to one or both of these ASTM test methods. They measure different things, report in different units and cannot be converted into each other.
ASTM F2170
Relative humidity inside the slab
A sensor sits in a sleeved hole at a set depth and reports the humidity of the air in equilibrium with the concrete around it.
Read the RH probe guide →ASTM F1869
Vapor emission from the surface
A sealed dome over a dish of calcium chloride collects moisture leaving the bare surface over a timed period.
Read the calcium chloride guide →Comparing
RH vs MVER
Why a percentage and a weight-per-area rate are not interchangeable, and which one your products ask for.
Compare the two →Interactive
The Slab Lab
Open a cut-away slab, insert an in-situ probe to 40% of the slab depth, and watch the readout settle as the air in the hole comes to equilibrium with the concrete around it. In a real ASTM F2170 test that settling period is measured in hours; here it is sped up to a few seconds.
What the Slab Lab shows (text version)
- A slab-on-grade sits on a sub-base and vapor retarder. Moisture moves upward through it and leaves through the top surface.
- Concrete near the top dries first, so it can feel dry while concrete lower down is still wetter. The blue tint shows that gradient.
- For a slab drying only through its top, ASTM F2170 places the sensor at 40% of the slab thickness, a depth intended to represent the moisture the slab can redistribute once it is covered.
- After the hole is drilled and sleeved, the probe needs time for the air in the hole to reach equilibrium before a reading is recorded. The animation compresses that time.
- The readout bar is an illustration only. It has no units and is not a project reading or a pass/fail limit.
Who it is for
Written for everyone who touches the slab schedule
- General contractors
- When to schedule testing, what the site has to look like on the day, and how to read the report you are handed.
- Property managers
- Why a unit turn on an older slab can still need a test, and what to ask before approving glue-down work.
- Builders
- How fast-track schedules, HVAC start-up and slab-on-grade construction interact with flooring readiness.
- Flooring installers
- Matching the requested method to the product instructions, and documenting conditions before you trowel.
- Homeowners
- Whether your project likely needs a test, and why a “waterproof” plank does not make the slab question go away.
Las Vegas
Desert air, but not a dry slab
The valley averages only about 4 inches of precipitation a year, which makes it easy to assume concrete dries fast here. The surface often does. But a slab-on-grade sits on soil that can hold moisture from irrigation, plumbing and the occasional storm, and new concrete starts out holding its own mix water. Low indoor humidity and big daytime heat can also make a surface read drier than the slab underneath it.
How Las Vegas conditions affect slab moisture →
Rainfall: NWS Las Vegas 1991–2020 normals. See sources.
All guides
Start where your project is
Do I need a moisture test?
Answer five questions about your slab, flooring, adhesive and spec. Get a plain suggestion and what to ask next.
Open →ASTM F2170: in-situ RH probes
Why the probe goes 40% deep, how long it sits, and what the number does and does not tell you.
Open →ASTM F1869: calcium chloride
The dome-and-dish surface test, the conditions it needs, and the slabs it is not meant for.
Open →RH vs MVER
Two different measurements, two different units, and why the manufacturer’s written limit decides.
Open →Las Vegas slab conditions
Desert air, slab-on-grade, fast-track schedules and HVAC: what changes when the slab is in the valley.
Open →Testing-day checklist for GCs
A printable site-prep list: access, HVAC, bare concrete, locations, and who keeps the probes safe.
Open →What a test report includes
The parts of a useful written record, and the questions to ask when one lands in your inbox.
Open →Moisture mitigation basics
An overview of the usual paths when results exceed a product limit. Not instructions.
Open →Slab moisture FAQ
Short answers on timing, which test to use, meters, DIY kits, waterproof floors and old slabs.
Open →By Vegas Flooring Outlet · Updated
Planning flooring over a Las Vegas slab?
Vegas Flooring Outlet offers project-related concrete moisture testing before flooring, adhesive or coating work. Share the project address, slab area, selected flooring system, requested method and schedule; the team reviews the scope before confirming a method or an appointment.