When specifying timber decking in the UK, durability is usually the first priority. The key comparison most homeowners and specifiers want answered is straightforward: which lasts longer — Siberian larch decking or ThermoWood decking?
Both are premium natural timber options widely used in residential gardens, roof terraces and commercial landscape projects. However, their performance characteristics differ significantly, and those differences directly affect lifespan, maintenance and long-term cost efficiency.
If you are reviewing timber options more broadly, see our full decking guide for a detailed technical overview of installation standards, profiles and structural considerations.
Quick Comparison: Larch vs ThermoWood Decking
| Factor | Siberian Larch Decking | ThermoWood Decking |
|---|---|---|
| Timber Type | Slow-grown softwood (Larix sibirica) | Thermally modified Scandinavian pine |
| Durability Class (EN 350) | Class 3 (Moderately Durable) | Class 2–3 (Improved via thermal modification) |
| Expected Lifespan (UK, above ground) | 15–25 years | 20–30+ years |
| Dimensional Stability | Moderate seasonal movement | High stability, reduced moisture absorption |
| Resistance to Rot | Good | Very good |
| Maintenance Requirement | Medium | Low–Medium |
| Weathering Appearance | Silver-grey over time | Silver-grey over time (slower movement) |
Durability & Service Life in UK Conditions
Siberian Larch Decking Lifespan
Siberian larch is naturally dense and resin-rich compared to standard softwoods. When correctly installed with adequate airflow, drainage and stainless steel fixings, it typically achieves:
- 15–20 years in exposed garden environments
- Up to 25 years with proper maintenance and favourable site conditions
However, larch remains untreated natural timber. It absorbs and releases moisture seasonally, which can lead to surface checking, minor cupping and gradual weathering.
Correct spacing, substructure design and ventilation are critical to prevent premature movement or decay.
ThermoWood Decking Lifespan
ThermoWood decking undergoes a controlled thermal modification process (typically 180–212°C using heat and steam). This alters the timber’s cell structure, reducing its ability to absorb moisture and improving biological durability.
In UK external decking applications, ThermoWood commonly achieves:
- 20–30+ years service life
- Improved fungal resistance
- Reduced expansion and contraction cycles
Because thermal modification lowers equilibrium moisture content, boards remain significantly more dimensionally stable throughout seasonal humidity changes.
Dimensional Stability & Movement
Movement is one of the main causes of decking failure.
| Performance Area | Larch | ThermoWood |
|---|---|---|
| Moisture Absorption | Moderate | Low |
| Expansion / Contraction | Visible seasonal movement | Reduced movement |
| Risk of Cupping | Possible if poorly installed | Lower risk |
| Board Stability Over Time | Good | Very Good |
ThermoWood generally offers superior dimensional stability, which directly contributes to longer structural lifespan and reduced maintenance over time.
Maintenance & Weathering
Larch Maintenance
Larch decking can be left untreated to weather naturally. However, periodic cleaning and optional oiling will:
- Reduce surface checking
- Maintain colour longer
- Extend overall service life
ThermoWood Maintenance
ThermoWood decking can also be left untreated. Its lower moisture uptake reduces the frequency of maintenance cycles. Oil treatments are optional for colour retention but not required for structural durability.
Long-Term Cost Efficiency
Initial material cost does not equal lifecycle cost. When evaluating longevity:
- Larch may have a lower upfront cost.
- ThermoWood often provides longer intervals between repairs.
- Reduced movement lowers fixings stress and board replacement frequency.
Over a 25–30 year period, ThermoWood can offer improved cost stability due to its reduced maintenance and extended service life.
To view available profiles and board options, see our full range of decking boards.
Installation & Delivery Considerations
Correct installation has a greater impact on lifespan than timber species alone. Always ensure:
- Proper subframe ventilation
- Minimum ground clearance
- Stainless steel fixings
- Correct expansion gaps
For lead times, payment details and logistics, review our delivery options page.
FAQ: Larch vs ThermoWood Decking
Which decking lasts longer in the UK climate?
ThermoWood typically lasts longer due to its reduced moisture absorption and improved biological durability.
Is Siberian larch suitable for exposed coastal areas?
Yes, but it may require more frequent maintenance in high-moisture or salt-air environments.
Does ThermoWood require treatment?
No structural treatment is required. Oil can be applied for aesthetic colour retention only.
Will both timbers turn grey?
Yes. Both species will weather to a silver-grey tone if left untreated.
Is one stronger than the other?
Larch is naturally dense and strong. ThermoWood slightly reduces bending strength due to thermal modification but remains structurally suitable for decking applications.
Final Comparison: Which Should You Choose?
If maximum lifespan and dimensional stability are the priority, ThermoWood decking generally performs better over extended periods.
If natural character, density and traditional timber performance are preferred — with proper maintenance — Siberian larch remains a robust and widely specified option.
The correct choice depends on site exposure, budget, maintenance expectations and aesthetic preference.