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CRAFT Hardwood Guides

Understanding Moisture and Wood Floors:
What You Need to Know

A practical guide to moisture, wood movement, and protecting your hardwood floor - from specialists with over a decade of installations across Lancashire and Greater Manchester.

Written by Wojciech, founder of CRAFT Hardwood | Updated February 2025 | 15 min read

Wood is a natural, hygroscopic material. This means it absorbs and releases moisture in response to its environment, expanding when it gains moisture and contracting when it dries out.

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This isn't a flaw - it's simply how wood behaves. Understanding this relationship between moisture and wood is crucial for anyone installing, maintaining, or living with wooden floors.

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Over our years installing herringbone and parquet floors across Lancashire and Greater Manchester, we've seen moisture-related issues cause more problems than any other single factor. The good news? Almost all of these problems are preventable with proper preparation, installation, and understanding.

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This guide explains everything you need to know about moisture and wood floors, from why it matters to how to prevent problems.

Why Moisture Matters for Wood Floors

Wood floors are affected by moisture in three main ways:

1. Movement (Expansion and Contraction)

As wood absorbs moisture from the air, it expands. As it loses moisture, it contracts. This is entirely normal and happens throughout the life of your floor.

Seasonal movement:

  • Winter: Heating dries the air → wood loses moisture → boards contract → small gaps appear

  • Summer: Higher humidity → wood absorbs moisture → boards expand → gaps close
     

In the North West of England, we typically see 2-4mm of movement across a 5-metre span of oak flooring throughout the year. This is normal and expected.

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Excessive movement causes:

  • Large gaps between boards

  • Cupping (edges of boards higher than centres)

  • Crowning (centres of boards higher than edges)

  • Buckling or lifting

  • Squeaking or creaking

2. Structural Damage

Excessive moisture exposure can cause permanent damage:

  • Cupping: Boards curl upward at the edges (moisture absorbed from underneath)

  • Crowning: Boards dome in the centre (often from sanding cupped boards)

  • Warping: Boards twist or distort

  • Delamination: Engineered floors separate at the layers

  • Mould and rot: Prolonged moisture exposure enables biological growth

3. Finish Damage

Moisture affects floor finishes differently:

  • Oil finishes: Generally handle moisture well, but need re-oiling if compromised

  • Lacquer finishes: Can lift, peel, or cloud if moisture gets underneath

  • Stains: Can cause discolouration or water marks

The Science: Moisture Content and Equilibrium

This sounds technical, but it's worth understanding.

Moisture Content (MC)

This is the amount of water in wood expressed as a percentage of the wood's dry weight.

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For oak flooring in UK homes:

  • Ideal installation MC: 8-10%

  • Acceptable range: 7-11%

  • Service life range: 6-12% (normal seasonal variation)

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Wood at 10% MC means the water in the wood equals 10% of the wood's dry weight.

Relative Humidity (RH) and Equilibrium Moisture Content (EMC)

Wood adjusts its moisture content to match the relative humidity of its environment. This target moisture content is called the Equilibrium Moisture Content (EMC).

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Typical UK residential environment:

  • Winter (heated home): 30-40% RH → EMC of 6-8%

  • Summer (natural humidity): 50-70% RH → EMC of 9-12%

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This is why your wood floor naturally expands and contracts seasonally. It's responding to changing humidity levels in your home.

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The key principle: Wood flooring should be installed at a moisture content close to the average EMC it will experience in service.

Before Installation: Critical Moisture Checks

Proper moisture management starts before we lay a single board.

1. Subfloor Moisture Testing

This is absolutely critical and non-negotiable.

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Concrete subfloors must be tested because concrete takes months to dry fully after construction. We've visited new builds where concrete poured 6 months earlier was still too wet for flooring.

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Testing methods:

  • Hygrometer test: Measures relative humidity within the concrete slab

  • Moisture meter test: Measures surface and subsurface moisture

  • Calcium chloride test: Measures moisture vapour emission rate

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Acceptable levels for wood floor installation:

  • Concrete RH: Below 75% (ideally below 70%)

  • Concrete MC: Below 5% (ideally below 4%)

  • Moisture vapour emission: Below 3 lbs/1000 sq ft/24 hours

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What happens if concrete is too wet?

  • Moisture migrates up into wood floor

  • Cupping and warping occur

  • Adhesive may fail

  • Permanent damage within months

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We never install over wet concrete, regardless of client timelines. The consequences are too severe.

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Wooden subfloors should also be checked, especially in older properties or after water damage.
 

Acceptable levels:

  • Wooden subfloor MC: Within 2-4% of the flooring to be installed

  • Typical range: 8-12% MC

2. Damp-Proof Membranes (DPM)

Concrete subfloors require a damp-proof membrane to prevent ground moisture migrating up into the floor.

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Types we use:

  • Built-in DPM: Should exist under concrete in buildings post-1960s

  • Surface DPM: Liquid-applied membranes (painted onto concrete)

  • Sheet DPM: Polythene or bitumen sheets laid under floor

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Signs a DPM is missing or failed:

  • Concrete surface feels damp

  • Salt deposits (efflorescence) on concrete

  • Moisture meter readings above acceptable levels

  • Moisture testing shows high RH

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Missing or failed DPMs must be remedied before installation. This might mean:

  • Applying liquid DPM

  • Installing sheet membrane

  • In severe cases, removing concrete and installing proper DPM

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Wooden suspended floors should have adequate underfloor ventilation (airbricks clear and functioning) to prevent moisture build-up beneath.

3. Acclimatisation of Flooring

Once we've confirmed the subfloor is suitable, the timber itself must acclimatise.

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What is acclimatisation? Allowing the flooring timber to adjust to the moisture conditions in the room where it will be installed.

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Our process:

  • Deliver flooring to the property

  • Store in the room where it will be installed (or similar conditions)

  • Leave for minimum 7-14 days before installation

  • Heating should be on at normal living temperatures

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Why this matters: Timber delivered at 10% MC to a dry winter house (EMC 7%) will lose moisture. If installed immediately, it will shrink after installation, causing gaps. If allowed to acclimatise, it reaches 7-8% MC before installation and won't shrink afterward.

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How long to acclimatise:

  • Minimum: 7 days

  • Recommended: 10-14 days

  • Ideal: 2-3 weeks

  • Large format or solid timber: Up to 3-4 weeks

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Environmental conditions during acclimatisation:

  • Heating on at normal living temperature (18-22°C)

  • Windows and doors closed

  • Relative humidity 40-60%

  • Building work (plastering, painting) completed and dried
     

We measure the moisture content of the flooring before installation to confirm it has acclimatised properly.

During Installation: Getting It Right

Expansion Gaps

Because wood expands and contracts, we leave expansion gaps around the perimeter of every floor.

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Standard expansion gaps:

  • Perimeter walls: 10-15mm (depending on floor size)

  • Fixed obstacles (pipes, columns): 10-12mm

  • Doorways and transitions: 8-10mm

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Why expansion gaps matter: Without adequate gaps, expanding wood has nowhere to go. This causes:

  • Buckling or lifting in the centre of the floor

  • Pressure damage against walls or fixtures

  • Squeaking and movement

  • Potential structural damage

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The expansion gap is hidden beneath skirting boards, architrave, or threshold strips, so it's invisible in the finished floor.

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Room size affects expansion gap size:

  • Rooms under 8m in any direction: 10mm gaps

  • Rooms 8-12m: 12-15mm gaps

  • Rooms over 12m: 15mm+ gaps or expansion joints required

Adhesive Moisture Cure

When we glue floors down, the adhesive releases moisture as it cures.

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Our process:

  • Use low-moisture adhesives where possible

  • Allow adequate drying time between gluing and finishing

  • Ensure good ventilation during cure period

  • Don't apply finish until adhesive fully cured (typically 48-72 hours)

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Applying finish over wet adhesive can trap moisture, causing finish problems and slow drying.

Moisture Barriers with Engineered Floors

When installing engineered floors over concrete, we often use:

  • Separate moisture barrier layer beneath adhesive

  • Acoustic underlay with moisture barrier properties
     

These provide additional insurance against moisture migration from concrete.

After Installation: Living with Your Floor

Managing Seasonal Movement

Some movement is normal and inevitable. Here's what to expect:

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Winter gaps (normal):

  • Gaps of 1-2mm between boards

  • More noticeable in wider planks

  • Close up again in summer

  • Completely normal and expected

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Excessive gaps (problem):

  • Gaps of 3-5mm+ between boards

  • Gaps that don't close in summer

  • Usually indicate installation moisture content was too high

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Summer expansion (normal):

  • Gaps close completely

  • Floor feels slightly tighter

  • Entirely expected seasonal behaviour

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Excessive expansion (problem):

  • Buckling or lifting

  • Pressure against walls

  • Usually indicates insufficient expansion gaps or extremely high humidity

Controlling Indoor Humidity

You can minimise seasonal movement by maintaining stable humidity levels.

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Ideal indoor relative humidity:

  • Target: 45-55% RH year-round

  • Acceptable range: 40-60% RH

  • Problems below 35% RH: Excessive shrinkage, gaps, potential splitting

  • Problems above 65% RH: Excessive expansion, cupping, potential buckling

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How to manage humidity:

In winter (low humidity):

  • Use humidifiers in very dry homes

  • Don't over-heat (18-21°C is adequate)

  • Houseplants add moisture naturally

  • Avoid wood-burning stoves directly in rooms with wood floors (very drying)

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In summer (high humidity):

  • Use dehumidifiers if consistently above 60% RH

  • Ensure adequate ventilation

  • Fix any leaks or dampness sources promptly

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Monitoring humidity:

  • Inexpensive hygrometers (£10-20) let you track RH

  • Placement: Away from windows, radiators, or direct moisture sources

  • Check regularly to understand your home's patterns

Underfloor Heating Considerations

Underfloor heating (UFH) creates specific moisture challenges.

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Why UFH affects moisture:

  • Heating from below dries the wood from underneath

  • Can cause faster moisture loss than traditional heating

  • Creates potential for cupping if not managed properly

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Safe UFH practices:

  • Maximum UFH surface temperature: 27°C

  • Gradual temperature changes (increase/decrease 2°C per day maximum)

  • Never switch UFH off completely in winter (maintain minimum 15°C)

  • Choose engineered over solid oak for better stability

  • Use narrower planks

 

Commissioning UFH correctly:

Before flooring installation:

  1. Run UFH at full temperature for several weeks

  2. Dry out screed/concrete completely

  3. Reduce to 15°C for installation

  4. After installation, increase gradually (2°C per day) to operating temperature

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Many UFH problems arise from incorrect commissioning or rapid temperature changes.

Common Moisture Problems and Solutions

Problem 1: Cupping (Edges Higher Than Centres)

Cause: Moisture absorbed from underneath the floor.

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Common sources:

  • Wet concrete subfloor

  • Failed or missing DPM

  • Plumbing leak

  • Ground moisture (suspended timber floors without ventilation)

Solution:

  • Identify and fix moisture source

  • Allow floor to dry naturally (can take months)

  • Don't sand cupped floors immediately - they may flatten as they dry

  • If problem persists after drying, professional sanding may be needed

Problem 2: Crowning (Centres Higher Than Edges)

Cause: Usually from sanding cupped boards before they've dried.

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Common scenario: Floor cups due to moisture → inexperienced person sands the high edges → floor dries and flattens → the sanded edges are now lower than unsanded centres

Solution:

  • Professional assessment required

  • May need re-sanding if severe

  • Prevention is key: never sand cupped floors until moisture source resolved and floor dried

Problem 3: Large Gaps Between Boards

Cause: Excessive moisture loss after installation.

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Common sources:

  • Flooring installed too wet (didn't acclimatise properly)

  • Dramatic change in environment (e.g., installation in summer, first winter reveals gaps)

  • Extremely dry heating (wood-burning stoves, very high temperatures)

  • Underfloor heating run too hot

Solution:

  • If seasonal (gaps close in summer): Normal behaviour, no action needed

  • If permanent: Professional assessment needed - may require repair or re-installation

Problem 4: Buckling or Lifting

Cause: Expansion with nowhere to go.

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Common sources:

  • Insufficient expansion gaps

  • Floor installed in very dry conditions, expanded when humidity increased

  • Water leak causing rapid expansion

  • Floor glued or nailed to something preventing movement

Solution:

  • Immediate professional assessment required

  • May need to remove skirting and cut back floor

  • Address moisture source if applicable

  • Severe cases may require partial or full replacement

Problem 5: Black Staining or Mould

Cause: Prolonged moisture exposure.

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Common sources:

  • Serious water leak

  • Failed DPM with persistent ground moisture

  • Flood damage

  • Condensation from plumbing

Solution:

  • Address moisture source urgently

  • Professional assessment of extent of damage

  • Affected boards likely need replacement

  • Subfloor treatment may be required

Water Spills and Flooding: What to Do

Minor Spills (Glass of Water, Pet Bowl)

Action:

  • Wipe up immediately with dry cloth

  • No panic if cleaned within 30 minutes

  • Both oil and lacquer finishes handle brief exposure well

Prevention:

  • Rugs under pet water bowls

  • Coasters for drinks

  • Wipe spills promptly

Major Spills (Bucket Tipped Over)

Action:

  • Mop up immediately

  • Dry area thoroughly

  • Check for water between boards

  • Monitor area for 24-48 hours for signs of cupping

  • If cupping appears, use fans to accelerate drying

Flooding (Burst Pipe, Appliance Leak)

Action:

  • Stop water source immediately

  • Remove standing water

  • Contact professional immediately

  • Remove any furniture from wet areas

  • Fans and dehumidifiers to accelerate drying

  • Don't walk on floor excessively (may cause delamination in engineered floors)

  • Professional moisture assessment essential

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Prognosis depends on:

  • How quickly addressed

  • Volume and duration of water exposure

  • Whether water got beneath floor

  • Solid vs engineered flooring

  • Subfloor type

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Solid oak floors can often be dried and saved. Engineered floors may delaminate if saturated. Quick action is critical.

New Build Considerations

New builds present specific moisture challenges.

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Issues we frequently see:

  • Concrete not fully cured/dried

  • Plastering and painting still releasing moisture

  • Building "breathing out" construction moisture for first 1-2 years

  • UFH not properly commissioned

  • Overly dry winter air (new central heating, well-insulated)

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Our recommendations for new builds:

  • Delay floor installation as long as possible (last trades in)

  • Extensive moisture testing of all subfloors

  • Extended acclimatisation period (3-4 weeks minimum)

  • Consider engineered over solid

  • Inform client about typical new-build behaviour (movement in year 1-2)

  • Install after plastering/painting completed and dried

Period Property Considerations

Victorian and Edwardian properties have different challenges.

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Common issues:

  • Suspended timber floors over earth/rubble (can be damp)

  • No DPM under ground floors (pre-date DPM requirements)

  • Blocked or insufficient airbricks

  • Variable humidity (poor insulation, draughty)

  • Solid floors directly on earth (Victorian tiles or quarry tiles)

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Our approach:

  • Thorough underfloor inspection (remove boards to check)

  • Clear airbricks, ensure adequate ventilation

  • Check for rising damp or standing water under floor

  • May need to install ventilation or DPM

  • Solid oak often more appropriate

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Interestingly, many original Victorian parquet floors have survived 100+ years. The houses breathed naturally, humidity was variable but not extreme, and suspended timber floors with good ventilation managed moisture effectively.

Tools for Moisture Management

For homeowners:

  • Hygrometer: Measures relative humidity (£10-30 for basic model)

  • Monitor and maintain 40-60% RH year-round

  • Relatively simple and worthwhile investment

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For professionals (our toolkit):

  • Pin moisture meter: Quick surface MC readings for targeted readings within boards and subfloors

  • Pinless moisture meter: Non-invasive scanning for fast, non-destructive moisture mapping across larger areas

  • Thermo-hygrometer: To confirm site temperature and relative humidity conditions

  • Concrete moisture test kits: For accurate slab moisture assessment before flooring goes down

  • Supplementary vapour emission testing where required – to verify moisture risk in new or suspect subfloors

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We test moisture extensively before every installation. It's the best insurance against future problems.

The Bottom Line

Moisture and wood floors will always have a relationship. Wood is natural, hygroscopic, and responds to its environment. This isn't a design flaw - it's inherent to the material.

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The key principles:

  1. Test subfloors thoroughly before installation - non-negotiable

  2. Ensure proper DPM on concrete - essential in all modern installations

  3. Acclimatise timber properly - worth the wait

  4. Install with adequate expansion gaps - allow for movement

  5. Maintain stable humidity year-round - 40-60% RH ideal

  6. Address water issues immediately - speed matters

  7. Understand seasonal movement is normal - small winter gaps are expected

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With proper installation and reasonable care, moisture issues are rare. We've installed hundreds of floors over wet Lancashire winters and dry heated homes, and problems are uncommon, not typical.

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When moisture issues do occur, they almost always trace back to:

  • Wet subfloors that weren't properly tested

  • Missing or failed DPM

  • Inadequate acclimatisation

  • Insufficient expansion gaps

  • Water leaks not promptly addressed

All of these are preventable.

Frequently Asked Questions

Q: How much does a wood floor move seasonally?

A: In the North West of England, you can expect 2-4mm of movement across a 5-metre span of oak flooring throughout the year. Small winter gaps are completely normal and close again in summer.

Q: How long does wood flooring need to acclimatise before installation?

A: Minimum 7 days, ideally 10-14 days. Solid timber and large format boards may need up to 3-4 weeks. Heating should be on at normal living temperature throughout.

Q: Can I install wood flooring over underfloor heating?

A: Yes, but engineered oak is more suitable than solid. [Read: Solid Oak vs Engineered Oak — which is right for your home?]
Maximum surface temperature should never exceed 27°C and changes should be gradual - no more than 2°C per day.

Q: What humidity level should I maintain for my wood floor?

A: The ideal range is 45-55% relative humidity year-round. Problems occur below 35% RH causing gaps and shrinkage, and above 65% RH causing expansion and cupping.

Q: What should I do immediately if my wood floor gets flooded?

A: Act immediately- remove standing water, use fans and dehumidifiers, and call a professional. Solid oak can often be saved if treated quickly. Engineered floors may delaminate if saturated.

Q: What is the difference between cupping and crowning?

A: Cupping is when board edges curl upward, caused by moisture from underneath. Crowning is when the centre of a board is higher than the edges, usually caused by sanding cupped boards before they have fully dried.

Questions About Your Project?

Moisture management is one of the most important aspects of successful floor installation. If you're planning a project and have concerns about:

  • Concrete subfloor moisture levels

  • Damp-proof membrane requirements

  • Acclimatisation timing for your specific situation

  • Managing humidity in your property type

  • Previous moisture problems in your home

We're happy to discuss these during a free site visit.

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Concerned about moisture and your flooring project?
Call us on: 07856 308 208
or email: contact@crafthardwood.co.uk

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We serve Chorley, Lancashire, Greater Manchester, Liverpool, and throughout the North West. We conduct thorough moisture testing on every project and never compromise on proper preparation.

Our experience with hundreds of installations across the North West's varied property types - from damp Victorian cellars to bone-dry new builds - means we understand the specific challenges of our region's properties.

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