Hygrometer Box Test: The Only BS-Compliant Way to Confirm a Screed Is Dry
What a hygrometer box test is, why concrete moisture meters do not satisfy BS 8203/8204, and the 75% RH threshold every floor finisher checks for. Hire from £25/day.
A homeowner installs engineered oak flooring two months after the liquid screed went down in their new kitchen extension. The floor looks fitted. Three months later, individual boards start cupping at the edges. By six months, the entire floor has lifted enough to require taking up and replacing. The screed was not dry. Nobody tested it; the floor finisher took the homeowner's word that the screed was old enough, and the manufacturer's warranty on the floor was voided because no moisture test was on file.
A hygrometer box test, hired for 25 pounds for the day, would have given a numerical relative humidity reading. Below 75% RH, the screed is dry enough for engineered wood. Above, it isn't. Without that number, the floor finisher is relying on the homeowner's guess about screed age, and the warranty doesn't protect against guesses.
What a hygrometer box test actually is
A hygrometer box test is the BS-compliant procedure for measuring the moisture content of a screed before flooring is laid. The test method comes from British Standards BS 8203 (Resilient floor coverings) and BS 8204 (Concrete bases and screeds). It's the only test method that any major UK flooring manufacturer accepts as evidence that a screed was dry enough at the time of fitting.
The procedure: an insulated plastic or rubber box (typically 200mm to 300mm square) is sealed to the screed surface with a self-adhesive flange or with edge mastic. The box traps the air immediately above the screed surface. Over 72 hours, moisture from the screed equilibrates with the air inside the box. After 72 hours, a small access port is opened, a digital relative humidity probe is inserted, and the RH of the trapped air is read.
The reading represents the equilibrium relative humidity of the screed surface. Different floor finishes specify different RH thresholds, but the headline numbers most UK installers care about are:
| Floor finish | Maximum acceptable RH | Source / standard |
|---|---|---|
| Engineered hardwood | 75% RH | BS 8203 + most manufacturer specs |
| Solid hardwood | 65% RH | Stricter than engineered; many specs require even lower |
| LVT (luxury vinyl tile) | 75% RH | BS 8203 |
| Vinyl sheet flooring | 75% RH | BS 8203 |
| Carpet (foam-backed) | 65% RH | Foam backing fails earlier than wood with high moisture |
| Carpet (hessian-backed) | 75% RH | Less sensitive than foam |
| Tile / stone (cement-based) | Tolerant; usually no RH limit | Cement adhesives are tolerant of residual moisture |
For a kitchen extension with engineered wood or LVT in the kitchen, 75% RH is the threshold. For solid wood in adjacent rooms, 65% RH. The hygrometer box test is the only test that gives you that number with a level of accuracy any manufacturer accepts.
Why concrete moisture meters do not pass the BS test
This is the single most important fact in this guide. Walk into any builders' merchant and you can buy a concrete moisture meter (the small device with two pins or a rubber pad sensor) for 40 to 200 pounds. The meter gives a reading in percentage that looks like a moisture content. It is not the same as a hygrometer box test, and no major UK flooring manufacturer accepts a moisture meter reading as evidence that a screed was within specification.
Concrete moisture meters measure the moisture content of the very top millimetre or two of the surface. The reading depends on the type of screed, the surface finish, the temperature, and the meter calibration. Different meters give different readings on the same screed. The numbers are useful as a quick indication but not as an absolute measure.
Hygrometer box tests measure equilibrium relative humidity, which is a more reliable proxy for the moisture available to migrate up into the floor finish over time. Three days of equilibration give the system time to settle to a stable reading that reflects the moisture deeper in the screed, not just the surface skin.
The flooring industry settled on RH testing decades ago because the surface meter readings were unreliable, and warranty claims based on surface readings were impossible to defend. RH testing is the only method that produces a defensible record of screed moisture state at the time of installation.
Warning
If your floor finisher offers to test the screed with a concrete moisture meter and call it acceptable, that test does not satisfy the manufacturer's warranty terms. Check the warranty document for any installed wood, vinyl, or LVT floor; almost without exception they specify "BS 8203 hygrometer test" or equivalent. If the meter reading is the only test on file and the floor fails, the warranty claim will be denied.
How the test works in practice
The procedure is mechanical and the timing is the only awkward part.
Choose the test location
Pick a representative area of the screed, away from external walls (which dry faster) and away from heat sources. For a kitchen extension, a position in the centre of the room is typical. Multiple test points are recommended for floors over 50 square metres; one in the centre and one near (but not next to) a wall.
Clean the test area
Wipe the screed surface clean of dust and any residual curing compound. The box flange must seal cleanly to the surface; a dusty surface leaks air around the seal and gives a false reading.
Apply the box and seal it to the screed
Most rental hygrometer boxes have a self-adhesive flange. Press the box firmly onto the screed; the flange seals against the surface. Some kits use a separate edge mastic; apply a continuous bead around the box base before pressing it down.
Leave the box undisturbed for 72 hours
The 72-hour equilibration period is the most important part. Shorter periods give readings biased toward surface moisture, which is less useful. Longer periods are fine but unnecessary. Mark the start time on the box with a marker pen.
Insert the RH probe and read
At 72 hours, peel the small port cover on the box, insert the calibrated RH probe through the port (some kits use a different rubber stopper system), and wait one minute for the probe to equilibrate. Read the displayed relative humidity.
Document the reading
Photograph the probe display showing the RH reading and the date/time. Record the test location on a floor plan. The photo and floor plan are the warranty record; without them, the test result is just a number a homeowner remembers.
If the reading is below the manufacturer's threshold, fitting can proceed. If above, additional drying time is needed. Active drying with a dehumidifier (typically 14 to 21 additional days for a borderline-fail reading) brings most screeds down to passable.
How long does a screed take to reach 75% RH
The general industry rule of thumb for sand-and-cement screed is one millimetre per day for the first 50mm, and two days per millimetre beyond 50mm. A 75mm screed takes approximately 50 + (25 × 2) = 100 days to reach a passable RH under typical UK conditions. That's the published guidance from screed industry bodies like the Floor Fitting Industry Federation.
For liquid screed (calcium sulphate / anhydrite-based), the published drying time is similar but the surface finish (laitance removal) and the screed protocol matter more. A 50mm liquid screed dries in roughly 40 to 60 days under controlled conditions.
Real-world drying times vary widely depending on:
- Ambient humidity: A screed in a sealed extension during winter dries slowly because the surrounding air is already humid. Active drying with a dehumidifier accelerates this.
- Temperature: Cold screed surfaces transfer moisture slowly. The screed manufacturer's drying tables assume around 20°C. Cold rooms in winter need either heating or longer waits.
- Underfloor heating commissioning: Running UFH at low temperature for 14 days, then ramping up over 14 days, accelerates drying. The "force-drying" sequence is part of UFH commissioning and is also a moisture-removal step.
- Screed type: Modern fast-drying screeds (Cemex Cemfaster, Tarmac TopFlow Soundbar Rapid, similar) reach 75% RH in 14 to 28 days. They cost more per cubic metre but pay back in shortened programme time.
For an extension build, plan on screed drying taking the headline guidance times unless you've used a fast-drying screed and confirmed the spec with the supplier. Don't trust the screed contractor's verbal "should be fine in three weeks" without a test result.
What to buy versus what to hire
For a homeowner doing one extension, hire is almost always the right answer.
| Option | Cost | Recommendation |
|---|---|---|
| Hire a hygrometer box kit for one test | £25-50/day, plus 72-hour wait so usually 4-day hire | Right for a single screed test on one floor area. The trade hire shops have calibrated kits with current calibration certificates. |
| Buy a hygrometer box kit | £250-450 for a Tramex CMExpert or Protimeter Hygromaster | Right only if you're commissioning multiple screeds (multiple extensions, builder doing several jobs). Single-build use cannot justify the price. |
| Engage a flooring contractor with their own kit | Usually included in the floor fitting quote | Right if the floor fitter does the test as part of their service. Confirm in writing that the test is included and the result will be on file. |
| Engage an independent moisture surveyor | £150-250 per visit | Right if the project is delicate (expensive solid wood floor, manufacturer demanding multiple test points), or for warranty disputes |
For a single recommendation: hire from a major hire shop like Brandon Hire or HSS. 35 pounds for a 4-day hire (3 days of the kit being sealed to the screed plus 1 day of pickup/dropoff) gets you a calibrated trade-grade kit. Confirm the calibration certificate is current and the kit is the BS 8203-compliant box-and-probe type, not a surface meter.
If your floor finisher includes the test in their installation quote, that's the cleanest option. The test result becomes part of their installation file, the result is on the manufacturer's warranty record, and you don't handle the kit yourself.
Where you'll use it
The hygrometer test comes out before flooring goes down:
- Screeding at the end of the screed cure cycle to confirm readiness for flooring
- Flooring immediately before any wood, LVT, vinyl, or foam-backed carpet installation
- Underfloor heating after the UFH commissioning sequence completes, to confirm the floor is dry enough for the final finish
The test is a one-time event per floor area. Once the floor is laid, the kit goes back to the hire shop and you don't see it again unless there's a moisture-related warranty claim later.
What the reading actually tells you
Beyond the pass/fail comparison against a manufacturer threshold, the actual RH number gives diagnostic information.
Below 65% RH: The screed is fully dry by any standard. Any floor finish is safe to lay. Warranty risk is minimal.
65 to 75% RH: The screed is dry enough for engineered wood, LVT, and most other modern floor finishes, but is borderline for solid wood, foam-backed carpet, and any hygroscopic finish. Wait further or move ahead depending on the finish.
75 to 85% RH: The screed is too wet for almost any sensitive floor finish. Active drying needed (dehumidifier, ventilation, heating) for a further period of weeks before retesting.
Above 85% RH: The screed is far from ready. Either the screed is genuinely fresh (recently laid) or there's a moisture source somewhere (rising damp, a leak, condensation). Investigate the cause; if the screed has been laid for the expected drying time and is still showing 85%+ RH, something is wrong.
Common mistakes
Skipping the test entirely. The single biggest mistake. Many homeowner-managed builds rely on screed age and verbal reassurance from the contractor rather than a documented test. The result is the floor failure described in the opening anecdote, with no warranty cover.
Accepting a concrete moisture meter reading as the test. Already covered. The meter is not the BS-compliant test method.
Testing too early in the screed cure cycle. A test at 14 days on a screed that needs 60 days will fail dramatically. You can't speed up the screed's drying by testing it sooner; the test is for confirming readiness, not for measuring progress.
Testing in a single position only. For floors over 50 square metres, multiple test points catch local variations. A screed that's dry in the middle can still be wet near walls or near floor-to-floor service penetrations.
Sealing the box poorly. Air leakage around the box edge gives a low RH reading because the trapped air mixes with room air. Inspect the seal carefully; check for any visible gap before the 72-hour wait starts.
Not photographing the result. Without a dated photograph of the probe reading, the test exists only in memory. The warranty claim later asks for evidence; provide the photo plus floor-plan record of test position.