Moisture Testing Floors Before Coverings: Hygrometers and BS 8203/8204
Quick Answer: A new screed or concrete subfloor must be dry enough before floor coverings go down, and the standard test in the UK is the surface (insulated box) hygrometer measuring equilibrium relative humidity. The widely used threshold is 75% RH or below before laying most resilient and timber coverings, in line with BS 8203 (resilient floorings) and BS 8204 (screeds/in-situ floorings). Always check the adhesive and covering manufacturer's own limit — some require 75% RH, and stricter products or wood floors may need lower. If the floor cannot reach the threshold in time, a surface damp-proof membrane (e.g. a two-part epoxy DPM) is used to suppress residual moisture.
Summary
Laying a floor covering onto a subfloor that is still drying out is one of the most common — and most expensive — failures in the flooring trade. Trapped moisture has nowhere to go once a vinyl, LVT, resin or wood floor seals the surface. It re-emerges as bond failure, blistering, lifting, cupping of timber, alkali attack on adhesives, and dark staining. By the time it shows, the only fix is usually to take the floor up and start again. Moisture testing exists to stop that happening.
Moisture testing matters to floor layers, tilers, screeders and main contractors managing a programme. The key idea is that you are not measuring "is it damp" with a quick stab of a pin meter — you are measuring the equilibrium relative humidity of the air in equilibrium with the subfloor, which tells you how much moisture would be available to the floor covering once it is sealed in. That is what the surface hygrometer (or in-situ RH probe) does, and it is the method specified in the British Standards.
The biggest misconceptions are: that electronic pin/conductivity meters can pass a floor for covering (they cannot — they only indicate and are useful for monitoring trends, not for the go/no-go decision); that a screed dries at a fixed rate regardless of conditions (it does not — drying depends on temperature, humidity and air movement); and that a surface DPM is a routine shortcut (it is a remedy for residual construction moisture, not a substitute for letting a floor dry where time allows, and it does not cure rising damp through a failed structural DPM).
Key Facts
- Primary test — equilibrium relative humidity (ERH) measured with a surface insulated-box hygrometer or an in-situ RH probe in a sleeved hole.
- Standard threshold — ≤75% RH before laying most resilient/timber coverings (BS 8203 / BS 8204).
- Wood floors — many timber and engineered wood systems require a lower limit; check the manufacturer (some specify ≤65–75% RH or a corresponding moisture content).
- Equilibration time — a surface box hygrometer must be sealed to the floor and left to reach equilibrium, commonly a minimum of 72 hours (some methods/instruments specify different periods — follow the instrument and BS).
- Pin/conductivity meters — for indication and trend monitoring only; never the basis for a go/no-go decision to lay.
- Sand/cement screed drying — rule of thumb ~1mm per day for the first 50mm, then slower for depth beyond 50mm, under good drying conditions (≈20°C, ≤65% RH, moving air).
- A 75mm screed can therefore take well over two months to reach 75% RH in ideal conditions — and much longer if it is cold, humid or unventilated.
- Calcium sulfate (anhydrite) screeds dry differently and need laitance removal; follow the screed manufacturer's drying and testing guidance — they often require lower RH for some coverings.
- Structural DPM — a damp-proof membrane below the slab (or a DPC in the structure) deals with ground moisture; the RH test deals with construction/drying moisture, not a failed structural DPM.
- Surface DPM — a liquid-applied (commonly two-part epoxy) membrane bonded to the prepared subfloor that suppresses residual construction moisture so coverings can be laid earlier; used where drying time is unavailable.
- Underfloor heating — the screed must be commissioned (a controlled heat-up/cool-down cycle) before testing and laying; never test a screed over UFH that has not been through its commissioning cycle. See underfloor heating screed.
- Number of tests — test in several representative locations, including the slowest-drying areas (centre of a large bay, near external walls, low/cold spots), not just one convenient spot.
- Site conditions for testing — the building should be at near-normal service temperature and humidity; testing a cold shell gives a falsely reassuring or unrepresentative reading.
- Records — record location, instrument, RH%, temperature and date for each test; this is your evidence if a covering fails later.
Quick Reference Table
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Try squote free →| Reading | Interpretation | Action |
|---|---|---|
| ≤65% RH | Dry for almost all coverings | Lay (confirm against manufacturer) |
| ≤75% RH | Acceptable for most resilient/timber coverings | Lay if covering/adhesive allows ≤75% RH |
| 76–84% RH | Too wet for most coverings | Continue drying, or apply surface DPM |
| ≥85% RH | Wet | Do not lay; investigate / dry / surface DPM |
| Pin meter "high" | Indicative only | Confirm with box hygrometer / RH probe |
| Subfloor type | Typical pre-lay limit | Notes |
|---|---|---|
| Sand/cement screed | ≤75% RH | ~1mm/day drying to 50mm depth |
| Concrete slab | ≤75% RH | Slow; large bays test the centre |
| Anhydrite (calcium sulfate) screed | Per manufacturer (often lower for some coverings) | Remove laitance; follow maker's guidance |
| Screed over UFH | Per manufacturer, after commissioning | Commission heat cycle first |
Thresholds are guidance. The covering and adhesive manufacturer's stated RH limit always takes precedence.
Detailed Guidance
Why relative humidity, not "is it damp"
A floor covering fails because of the moisture that can still move out of the subfloor after the covering seals it in. The equilibrium relative humidity of the subfloor is the best practical predictor of that. A surface insulated-box hygrometer traps a pocket of air against the floor; left to equilibrate, the air reaches the same RH as the subfloor surface, and you read it off the sensor. An in-situ probe does the same thing deeper in a sleeved hole and is generally regarded as the more rigorous method for thick or slow-drying slabs. Conductivity/pin meters measure surface electrical resistance, which is affected by salts, finish and surface dampness — fine for spotting wet patches and tracking drying, useless as the final decision.
Reading the box hygrometer correctly
1. Clean and lightly abrade the test area (no laitance, dust, curing membrane).
2. Seal the insulated box/hood to the floor (butyl/sealant) — airtight.
3. Leave to equilibrate (commonly >=72h; follow the instrument/BS).
4. Read the RH at the sensor WITHOUT breaking the seal for long.
5. Record RH%, temperature, location, date. Repeat at multiple points.
6. Decision: lay only if RH <= covering/adhesive limit AT EVERY point.
If one point fails, the floor fails — you do not average. Test the slow spots on purpose.
Drying times and how to speed them up safely
Sand/cement screed dries at roughly 1mm/day for the first 50mm of depth in good conditions, then progressively slower beyond that. "Good conditions" means a warm, ventilated, not-too-humid building — roughly 20°C, 65% RH or below, with air movement. A cold, sealed shell can barely dry at all. You can help by getting the building watertight and warm, running dehumidifiers, and keeping air moving — but do not force-dry so hard that the screed cracks or curls. For screed selection and depth, see screed types, screed types comparison and screed depth.
Surface DPMs — when and how
Where the programme will not allow a screed to reach the threshold naturally, a surface damp-proof membrane suppresses residual construction moisture so coverings can be laid earlier. Two-part epoxy DPMs are the common choice; they are applied to a sound, prepared, contaminant-free subfloor, often followed by a smoothing/levelling compound. Key points: a surface DPM is a remedy for residual construction moisture, it has a stated maximum RH it can be applied over (check the data sheet), and it is NOT a cure for rising damp through a missing or failed structural DPM. If ground moisture is the problem, fix the structure — see damp proof membrane and internal wall insulation damp risk for the wider moisture picture, and epoxy coatings/resin flooring guide for resin systems that have their own subfloor-moisture limits.
Preparation and levelling before testing
Test the subfloor in the condition it will receive the covering. Remove laitance, curing compounds and contaminants first, because they affect both the reading and the bond. Levelling/smoothing compounds add their own (small) moisture and have a recoat/dry time — see floor levelling, levelling compounds and subfloor preparation. For tiled finishes the moisture tolerances differ (cementitious adhesives are far more forgiving than vinyl), but anhydrite screeds still need priming and laitance removal — see subfloor preparation guide.
Frequently Asked Questions
Can I just use a pin moisture meter to decide?
No. Pin/conductivity meters are for indication and monitoring the drying trend only. The go/no-go decision must be made on an equilibrium RH test (surface box hygrometer or in-situ probe) against the covering manufacturer's limit. Using a pin meter to sign off a floor is the classic cause of bond failures and warranty disputes.
What RH do I actually lay at — 75% or lower?
Default to ≤75% RH for most resilient and timber coverings per BS 8203/BS 8204, but the adhesive and covering manufacturer's stated limit overrides the general figure. Some wood floors and some moisture-sensitive products require lower. Read the data sheet for the specific products you are using and test to that number.
How long does a screed take to dry?
For sand/cement, roughly 1mm per day for the first 50mm in good conditions, slowing after that — so a 75mm screed can easily take two to three months or more, and far longer in a cold, unventilated building. Anhydrite screeds dry on a different basis. If you do not have the time, plan for a surface DPM from the start rather than discovering the problem on lay day.
Does a surface DPM fix rising damp?
No. A surface DPM suppresses residual construction (drying) moisture so you can lay coverings sooner. It does not replace a failed or missing structural damp-proof membrane and is not a treatment for rising damp from the ground. If you suspect ground moisture, investigate the structure first. See rising damp.
Do I test a screed over underfloor heating?
Yes, but only after the UFH has been through its commissioning heat-up/cool-down cycle, which drives out a lot of moisture. Test (and lay) after commissioning, with the system off and the floor returned to ambient, following the screed and UFH manufacturer's procedure. See underfloor heating screed.
Regulations & Standards
BS 8203 — Code of practice for installation of resilient floor coverings; subfloor moisture testing and the RH threshold for resilient coverings.
BS 8204 (series) — Screeds, bases and in-situ floorings; tolerances, soundness and moisture/drying of screeds and bases.
BS 8201 — Code of practice for installation of flooring of wood and wood-based panels; moisture requirements for timber floors.
BS 5325 — Code of practice for installation of textile (carpet) floor coverings; subfloor conditions.
BS EN ISO methods — relative-humidity test methods for hardened concrete/screed (in-situ probe / hygrometer procedures) referenced for ERH measurement.
Contract Flooring Association (CFA) guidance — industry good-practice notes on subfloor moisture testing and surface DPMs.
Contract Flooring Association (CFA) — subfloor preparation and moisture-testing guidance
BSI — BS 8203 (resilient floor coverings) — installation code including moisture testing
BSI — BS 8204 (screeds and in-situ floorings) — screed soundness and drying
NBS / TRADA timber flooring guidance — moisture content limits for wood floors
subfloor preparation — preparing the base before testing and laying
screed types — sand/cement vs anhydrite screed drying behaviour
floor levelling — levelling compounds, dry times and recoat
underfloor heating screed — commissioning a screed over UFH before testing