Does Merbau Decking Shrink? Essential Decking Information for 2025

Introduction to Merbau Decking Characteristics

Merbau decking, known scientifically as Intsia bijuga and alternatively called kwila or ipil, is one of Australia’s most popular hardwood decking choices due to its durability, rich coloration, and natural resistance to pests and decay . However, like all timber products, merbau is subject to dimensional changes that homeowners and builders must understand before installation.

This comprehensive guide examines merbau’s shrinkage properties, installation best practices, maintenance requirements, and how it compares to alternative decking materials in 2025.

Understanding Merbau’s Shrinkage Properties

Scientific Shrinkage Data

According to WoodSolutions Australia, merbau exhibits the following shrinkage characteristics :

• Tangential shrinkage: 2.50% (very low)
• Radial shrinkage: 1.20% (low)
• Unit movement tangential: 0.30%
• Unit movement radial: 0.19%

These measurements indicate merbau has below-average shrinkage compared to many other hardwoods, making it dimensionally stable for outdoor applications. However, proper installation must still account for movement.

Factors Affecting Shrinkage

Several variables influence how much merbau decking will shrink:

1. Moisture Content at Installation

• “Green” (freshly cut) merbau contains high moisture and will shrink significantly as it dries
• Kiln-dried merbau undergoes controlled drying to minimize post-installation shrinkage

1. Climate Conditions

• Humid environments cause expansion
• Dry conditions lead to shrinkage
• Seasonal changes create cyclical movement

1. Deck Orientation

• North-facing decks in full sun experience more extreme moisture fluctuations
• Shaded areas maintain more stable moisture levels

1. Board Width

• Wider boards show more noticeable shrinkage gaps
• Narrower boards minimize visible gaps

Installation Best Practices for Merbau Decking

Gap Spacing Recommendations

Industry experts provide conflicting advice about merbau decking gaps based on the timber’s condition:

• For kiln-dried merbau: 2-3mm gaps recommended
• For green/unseasoned merbau: Can be installed tight as shrinkage will create gaps
• General rule: Use 2.5mm spacers (like deck nails) for consistent spacing

“Merbau decking and most decking has a pencil round edge. If you butted them hard up against each other, water, dirt, and muck would accumulate in the small v shape formed. Eventually even merbau would rot and cup. Only a tiny gap is necessary – say 1.5mm.”

Installation Timing Considerations

• Install during drier months when timber is at its most contracted state
• Allow new merbau to acclimate on-site for several days before installation
• Consider tongue-and-groove systems for gap-free appearances while allowing movement

Maintenance Impact on Shrinkage

Proper maintenance significantly affects merbau’s dimensional stability:

1. Oiling Schedule

• Unsealed merbau experiences more moisture absorption and release
• Annual oiling maintains stable moisture content
• Use water-based oils for best penetration

1. Cleaning Practices

• Regular sweeping prevents moisture-trapping debris
• Annual deep cleaning removes mold/mildew that retains moisture

1. UV Protection

• UV exposure accelerates surface drying and checking
• Oil with UV inhibitors reduces surface shrinkage

Comparing Merbau to Alternative Decking Materials

vs. Composite Decking

vs. Other Hardwoods

• Shrinkage: Merbau shrinks less than spotted gum but more than blackbutt
• Movement: Merbau’s interlocked grain provides good stability
• Durability: Class 1 above ground (40+ years)

Common Problems and Solutions

Tannin Bleeding

• Cause: Natural tannins leaching when wet
• Solution: Pre-wash boards before installation
• Prevention: Proper sealing with oil

Cupping/Warping

• Cause: Uneven moisture absorption
• Solution: Ensure proper gap spacing during installation
• Prevention: Use kiln-dried merbau

Excessive Gapping

• Cause: Over-drying in hot climates
• Solution: Install during humid periods
• Prevention: Regular oiling

2025 Cost Considerations

• Merbau decking: $70-$120/m² installed
• Maintenance: $5-$10/m² annually for oiling
• Comparative: 15-25% more expensive than treated pine

Environmental Considerations

Recent studies show concerning sustainability issues with merbau:

• Classified as “near threatened” by IUCN
• Illegal logging threatens ecosystems
• 75+ years to reach maturity
• Consider FSC-certified merbau or alternatives

Professional Recommendations

Based on industry expertise:

1. For new installations:

• Use kiln-dried merbau with 2-3mm gaps
• Pre-treat boards to remove tannins
• Install perpendicular to joists for stability

1. For existing decks:

• Monitor gaps seasonally
• Re-oil when water no longer beads on surface
• Address cupping early with sanding/resealing

1. For bushfire zones:

• Merbau meets BAL 12.5-29 requirements
• Consider higher-rated composites for BAL 40+ areas

Conclusion: Is Merbau Right for Your Deck?

Merbau remains a premium decking choice in 2025 for those who:

• Prioritize natural wood aesthetics
• Can commit to annual maintenance
• Source sustainable timber
• Understand and accommodate shrinkage

For lower-maintenance alternatives with minimal movement, modern composite decking offers compelling benefits. However, properly installed and maintained merbau provides unmatched natural beauty that can last decades.

Key Takeaways:

1. Merbau has low-to-moderate shrinkage (1.2-2.5%) compared to other hardwoods
2. Proper gap spacing (2-3mm) prevents moisture-related damage
3. Annual oiling maintains dimensional stability
4. Environmental concerns warrant careful sourcing
5. New composite alternatives offer zero-shrinkage solutions

References

1. Decking Heat Resistance. Decking Perth. Retrieved from https://deckingperth.com.au/decking-heat-resistance/
2. Why Merbau is the King of Timber Decking. Narangba Timbers. Retrieved from https://narangbatimbers.com.au/merbau-king-timber-decking/
3. Time to Rethink the Use of this Unsustainable Timber. NexGEN Decking. Retrieved from https://nexgendecking.com.au/merbau-decking-time-to-rethink/
4. Merbau Decking Melbourne. WoodEvo. Retrieved from https://woodevo.com.au/merbau-decking-melbourne/
5. Merbau. WoodSolutions. Retrieved from https://www.woodsolutions.com.au/wood-species/hardwood/merbau
6. What You Need to Know About Merbau Timber Patio Decks. Great Aussie Patios. Retrieved from https://www.greataussiepatios.com.au/blog/what-you-need-to-know-about-merbau-timber-patio-decks.aspx
7. Leaving no gaps between decking boards (really that bad)? Woodwork Forums. Retrieved from https://www.woodworkforums.com/f325/leaving-gaps-decking-boards-bad-57928