Thermally Modified Wood Decking: Pros, Cons, and Lifespan

Last Updated on September 2, 2026 by Sam Wood Worker

Thermally modified wood decking is real wood changed with controlled heat and steam. The process reduces how much moisture the wood absorbs. It can also improve dimensional stability and resistance to fungal decay.

This makes it an interesting middle ground. You get real wood grain without using a conventional pressure-treatment process in the deck boards. You also get better stability than the untreated version of the same species.

There are tradeoffs. Thermal modification can reduce some strength properties and make boards more sensitive to rough handling. The material usually costs more than pressure-treated pine. Its brown color also fades to gray unless you maintain a suitable UV-protective finish.

Quick answer: Thermally modified decking can be a good choice for homeowners who want natural wood, improved stability and lower moisture absorption. Quality exterior products may provide decades of service when installed above ground with good drainage. However, performance depends on the wood species, treatment class, board profile, installation and maintenance.

Research note: This guide is based on manufacturer documentation, technical data sheets, installation instructions and comparisons with other decking materials.

Always follow the instructions for the exact decking product. Fasteners, board gaps, joist spacing, ventilation and approved uses vary. Local building requirements still apply to the deck frame, stairs, guards, footings and connections.

Thermally Modified Decking at a Glance

What Is Thermally Modified Wood?

Thermally modified wood begins as ordinary lumber. It is placed in a controlled chamber and heated to a high temperature. Steam is commonly used as part of the process, while oxygen is carefully controlled to prevent the wood from burning.

The International ThermoWood Association defines thermal modification as a process that permanently changes the chemical and physical properties of wood using heat and steam. Its ThermoWood process uses a minimum modification temperature of 160°C, while the exact schedule depends on the species and product class.

Heating changes parts of the wood that readily interact with moisture. The finished material normally reaches a lower equilibrium moisture content and moves less as humidity changes. Its biological durability can also improve.

The modification goes through the board rather than forming a thin surface coating. Cutting the wood does not reveal an ordinary untreated core. Even so, cut ends may need protection or sealing when the manufacturer requires it.

Thermally modified wood is not the same as:

• Pressure-treated lumber, which receives chemical preservatives.
• Kiln-dried lumber, which is dried but not modified to the same degree.
• Heat-treated packaging wood, which may be heated only to control pests.
• Composite decking, which normally combines wood fiber and plastic.
• PVC decking, which contains no wood fiber.

ThermoWood, Thermory and Lunawood: Are They the Same?

No. These names are related, but they should not be used as if every board were identical.

ThermoWood® is a registered trademark connected to the International ThermoWood Association and its controlled process and quality system.

Thermory is a manufacturer that offers thermally modified decking in products such as ash and pine. Its profiles, clips and installation requirements are specific to its systems.

Lunawood produces ThermoWood products, including exterior decking made from thermally modified Nordic softwood.

Other manufacturers use different processes, temperatures, species and quality controls. The phrase “thermally modified” alone does not tell you the durability class, structural properties, warranty or approved exposure.

Ask for the technical data sheet rather than judging only by the material name.

Benefits of Thermally Modified Wood Decking

1. Better dimensional stability

Ordinary wood absorbs and releases moisture as weather changes. This causes swelling and shrinking. Over time, movement can contribute to cupping, twisting and uneven gaps.

Thermal modification reduces moisture uptake. As a result, the boards normally move less than untreated wood of the same species.

This does not make them movement-free. Correct spacing, ventilation and fastening are still necessary. A stable board can still cup when one face stays wet and the other dries quickly.

2. Improved resistance to fungal decay

The process makes the wood less suitable for organisms that cause decay. Exterior-grade products can achieve improved biological durability without a conventional preservative treatment.

For example, Lunawood states that its LunaThermo-D product meets durability class 2 under EN 350 and is intended for above-ground exterior uses such as decking. Its documentation also references an expected service life of 30 years for appropriate decking and cladding applications.

That figure applies to a specific product and use class. It should not be copied to every thermally modified board.

Decay-resistant also does not mean waterproof or maintenance-free. Leaves, blocked gaps, poor ventilation and failed flashing can shorten deck life.

3. Real wood grain and variation

Every board is natural wood. The grain, knots and color variation depend on the original species.

Thermally modified ash often has a strong hardwood grain and a deeper brown appearance. Modified pine usually has more visible knots and a softer look.

This natural variation is a major advantage for buyers who do not want a repeated manufactured pattern.

Lay several boards beside each other before fastening them. This lets you distribute light, dark and strongly patterned pieces across the deck.

4. No conventional preservative chemicals in the modification process

The ThermoWood process uses heat and steam rather than conventional wood preservatives. This may appeal to buyers who prefer a modified natural material.

Do not turn that fact into a broad safety promise. A finished decking product may include oil, pigment or another factory-applied coating. Check the complete safety and product documents before using the boards near gardens, pools, play areas or outdoor dining spaces.

5. Consistent color through the board

Heat darkens the wood throughout its thickness. A cut edge normally shows a similar tone rather than revealing a pale untreated center.

Sunlight still changes the exposed surface. If the boards are left unfinished, they gradually weather toward gray. Fresh cuts and protected areas may then look different from the exposed face.

6. Lower resin content

Thermal modification removes or changes resin in suitable softwoods. This can reduce resin bleeding and make the material cleaner to handle in warm weather.

Low resin does not remove the need for correct surface preparation. Follow the coating manufacturer’s instructions before applying oil.

7. An alternative to tropical hardwood

Thermally modified ash or pine can provide a rich brown appearance without choosing a tropical species such as ipe or cumaru.

Environmental performance depends on more than species. Look for certified forest sourcing, product life, shipping distance, manufacturing information and end-of-life options.

Disadvantages of Thermally Modified Wood Decking

1. Boards can split during handling or fastening

Thermal modification improves stability but can reduce flexibility and some strength properties. Boards may be more sensitive to impact, bending and poor fastening.

Do not drop long boards or force a bowed piece into place. Support them during carrying and storage.

Pre-drilling requirements vary. Thermory specifically requires pre-drilled screw holes for its thermally modified ash when visible screws are used. Its guidance says the drill bit should be larger than the screw diameter for that application.

Follow the exact manufacturer detail. Do not use one pre-drilling rule for every species and fastening system.

2. It costs more than pressure-treated wood

Thermally modified decking needs controlled processing. Some products are imported or available only through specialist suppliers.

The total cost can include:

• Deck boards
• Hidden clips or stainless-steel screws
• Delivery
• Extra waste for damaged or unsuitable boards
• Matching fascia
• Factory or site-applied finish
• Labor
• Replacement-board storage

Pressure-treated pine is normally the lower-cost starting option. Thermally modified wood competes more directly with cedar, premium composite and some hardwood decking.

3. The original brown color fades

UV exposure gradually turns unfinished thermally modified wood silver-gray. This is normal weathering, not automatic proof of rot.

Gray boards can still develop decay if water remains trapped, so inspect the wood rather than judging it only by color.

If you want to preserve the brown appearance, use a pigmented, UV-resistant finish approved by the decking manufacturer. Clear oil may change the appearance but usually provides less UV protection than a suitable pigmented product.

4. Most deck boards are not structural framing

Thermally modified decking is usually sold as a surface material. Do not use it for joists, beams, posts, ledgers or guard framing unless the exact product has structural design values and approval for that use.

Thermal treatment can reduce some mechanical properties. The 2025 ThermoWood Handbook advises that final design decisions should follow product-specific guidance.

Use code-approved framing below the boards. A durable surface cannot correct an unsafe deck structure.

5. Availability can be limited

The local lumber yard may not stock the species, profile or clip you need. Special orders can increase delivery time and freight cost.

Matching a board several years later may also be difficult. Consider buying a few extra full-length boards from the original order. Store them flat, supported, dry and out of direct sun according to supplier instructions.

6. Maintenance is still required

Thermally modified wood is not a no-maintenance material.

Leaves, pollen, grease and dirt still collect on it. Surface mold can grow on organic debris. Fasteners and the supporting frame still need inspection.

The gray appearance may be acceptable without regular oiling, but the deck still needs cleaning and drainage care.

7. The material may dent or wear

Hardness depends on the original species. Thermally modified pine remains a softwood and may dent more easily than modified ash or dense tropical hardwood.

Thermal treatment should not be confused with surface hardening. Ask for species-specific hardness and wear information if the deck will receive heavy furniture or high foot traffic.

How Long Does Thermally Modified Decking Last ?

There is no honest lifespan number for every thermally modified deck.

Some exterior-grade products have published durability classifications and expected service-life information. Lunawood documentation, for example, identifies LunaThermo-D as durability class 2 under EN 350 and refers to an expected service life of 30 years in suitable above-ground decking and cladding applications.

Use this as an example of product-specific evidence, not a promise for another brand.

Actual life depends on:

• Wood species
• Treatment process and class
• Board thickness and profile
• Above-ground or ground-contact exposure
• Climate and rainfall
• Airflow below the deck
• Drainage between boards
• Joist-top moisture
• Fastener compatibility
• Cleaning and finish maintenance
• Installation quality

These terms are not interchangeable. A 30-year service-life statement is not automatically a 30-year full replacement warranty.

What Can Shorten Its Lifespan?

Thermally modified boards can fail early when water is trapped or installation rules are ignored.

Common risks include:

• Gaps too narrow for drainage
• Poor airflow below a low deck
• Ground contact when the product is rated only for above-ground use
• Leaves and soil packed between boards
• Sprinklers repeatedly soaking one area
• Missing ledger flashing
• Water trapped at joist tops
• Unapproved fasteners
• Screws placed too close to board ends
• Overdriven screws or clips
• Wrong joist spacing
• Rough handling that cracks the boards

The aim is simple: let water leave and let every part dry.

Thermally Modified Ash vs. Pine

Ash and pine can both be thermally modified, but they do not become the same material.

Treatment quality and board profile matter as much as the species name. Compare technical sheets from the products available in your area.

Thermally Modified Wood vs. Other Decking

Compared with pressure-treated pine

Choose pressure-treated pine when the starting price and local availability matter most. Consider thermally modified wood when you want lower moisture movement, a more finished appearance and boards produced without conventional preservative treatment.

Both need a sound frame, good drainage and suitable fasteners.

Compared with cedar

Cedar is easy to cut and has a familiar natural look. Thermally modified exterior products may provide better stability than ordinary cedar, but the result depends on the exact species and treatment.

Cedar and modified pine are both relatively soft. Furniture and dropped objects can dent them.

Compared with composite decking

Thermally modified decking is real wood and has natural grain variation. Capped composite normally needs less work to maintain its manufactured color and does not require wood oil.

Thermally modified wood can be sanded and refinished with an approved system. Composite caps generally cannot be sanded in the same way.

Compared with ipe

Ipe is much denser and harder. It can resist impact and wear well, but its weight, drilling requirements and price can make installation demanding.

Thermally modified ash offers a dark hardwood appearance at a lower density. It should not be described as equal to ipe in hardness or durability.

Is Thermally Modified Decking Good in a Wet Climate?

It can work well in rainy and humid regions when the exact product is approved for exterior decking.

Lower moisture absorption and better stability are useful advantages. However, the product should not remain wet continuously.

In a wet climate:

• Provide good airflow below the deck.
• Follow the required board gap.
• Keep the surface clear of leaves.
• Protect the ledger correctly.
• Avoid direct soil contact unless the product allows it.
• Use corrosion-resistant hardware suitable for the location.
• Keep joist tops and beam connections able to dry.

An exterior durability class does not excuse poor drainage.

Does Thermally Modified Wood Get Hot?

All deck surfaces become warm in direct sunlight. Surface temperature changes with color, wood density, airflow and weather.

Thermally modified boards are dark when new, so they may feel warm in strong sun. They may still feel different from dark composite or PVC products, but broad claims about which material stays coolest require product-specific testing.

Order samples. Place them where the deck will be built and compare them on a hot, clear day. Test the exact colors and finishes you plan to buy.

Does It Need Oil or Sealer?

Thermal modification creates the basic stability and decay resistance. A surface finish does not create those properties.

Oil is mainly used to:

• Slow UV-driven color change
• Maintain a brown appearance
• Reduce surface wetting
• Help control checking and roughness

If you prefer a silver-gray patina, some products can be left to weather naturally. Cleaning and inspection are still required.

Use only a finish recommended for thermally modified wood. Follow its surface-preparation, coat, temperature and drying instructions. Too much oil can remain sticky or create uneven color.

How to Maintain Thermally Modified Decking

Sweep away debris

Remove leaves, pollen and soil before they hold moisture against the boards.

Wash gently

Use water, a soft brush and a cleaner approved by the decking manufacturer. Test it in a hidden area.

High pressure can raise the grain and leave permanent lines. If pressure washing is allowed, follow the stated nozzle, distance and pressure limit.

Maintain the chosen appearance

Renew an approved pigmented oil when needed if you want to preserve the brown color. Weather, traffic and sun exposure affect the schedule.

Inspect the whole deck

Check boards, fasteners, joists, beams, ledger flashing, posts, guards and stairs. A durable deck board does not make the hidden frame maintenance-free.

Correct moisture sources

Redirect sprinklers and roof runoff. Move planters occasionally and use supports that allow air below them.

Thermally Modified Decking Installation Basics

Installation details vary widely, especially with proprietary clips and profiles.

Store the boards correctly

Keep packages flat, supported and protected as the supplier directs. Do not bend long boards or drag finished faces.

Plan the frame for the product

Confirm maximum joist spacing before building or resurfacing. Diagonal boards, stairs and special patterns may require closer support.

Use approved fasteners

Stainless steel may be required or recommended in many systems, but the grade and fastener type depend on the product and environment. Use the listed clip or screw.

Pre-drill when required

Thermally modified ash is particularly sensitive to splitting. Follow the manufacturer’s pilot-hole diameter and edge-distance rules.

Maintain drainage gaps

Do not invent the spacing. Hidden clips may set the side gap automatically, while end gaps and clearances still need attention.

Protect cut ends if required

Some systems specify end sealer or finish on field cuts. Others may not. Check before installation.

Avoid unapproved ground contact

Many thermally modified decking products are intended for above-ground use. Lunawood, for example, says its decking is not recommended for direct ground contact.

Follow local building requirements

Decking boards are normally non-structural. Joists, beams, ledgers, posts, footings, stairs and guards need an approved design and code-compliant connections.

PVC Decking Guide: Pros, Cons and Buying Tips

Common Buying Mistakes

Buying by the phrase “thermally modified” alone

Ask for the species, treatment class, durability class, intended use and quality documentation.

Assuming every product lasts 30 years

Service-life statements apply to specific products and exposure conditions. Read the evidence and warranty.

Treating brown color as permanent

Unfinished boards turn gray outdoors. Maintaining the brown look requires an approved UV-resistant finish.

Choosing boards before checking clips

Proprietary fasteners affect price, installation and future board replacement. Confirm local availability.

Using decking as structural lumber

Most thermally modified deck boards are not approved as joists, beams or posts.

Skipping spare boards

Colors, profiles and suppliers can change. A few properly stored replacement boards may be valuable later.

Ignoring installation waste

Natural defects and handling damage can increase waste. Confirm the supplier’s grading and return policy.

Believing no chemicals means no precautions

Cutting creates fine wood dust. Factory finishes may have separate safety instructions. Use suitable dust collection and personal protection.

Is Thermally Modified Wood Decking Worth It?

It is more likely to be worth the price when:

• You want real wood rather than plastic-based decking.
• Natural grain and variation are important.
• You want better stability than untreated wood of the same species.
• The deck is above ground with good airflow.
• You accept natural graying or will maintain a finish.
• You can buy matching fasteners and replacement boards locally.
• The product has clear technical data and installation instructions.

It may not be the best choice when:

• The lowest initial price is the priority.
• You want almost no surface maintenance.
• The deck will be in unapproved ground contact.
• The boards will face heavy impact or abuse.
• You cannot obtain product-specific clips or spare boards.
• The installer does not plan to follow pre-drilling and spacing rules.

Questions to Ask Before Buying

1. Which wood species is used?
2. Which thermal-modification process and class apply?
3. What durability class has the product achieved?
4. Is it approved for exterior decking and the planned exposure?
5. Is ground or water contact allowed?
6. What service-life evidence and warranty are provided?
7. What joist spacing is required?
8. Which fasteners or hidden clips are approved?
9. Is pre-drilling required?
10. What board gaps and ventilation are needed?
11. Must cut ends be sealed?
12. Which cleaner and finish should be used?
13. How often should the finish be renewed?
14. Are matching fascia and replacement boards available?
15. Is the wood independently certified for responsible sourcing?

Frequently Asked Questions

Does thermally modified decking need to be sealed?

It normally does not need sealer to create its basic decay resistance because that comes from the modification process. An approved pigmented oil can slow graying and maintain the brown color.

Does thermally modified wood turn gray?

Yes. Unfinished boards normally weather from brown to silver-gray in sunlight. This is a surface color change and is not, by itself, proof of rot.

Is thermally modified wood waterproof?

No. It generally absorbs less moisture than untreated wood, but it is not waterproof. Board gaps, drainage and ventilation remain important.

Can it be used for deck framing?

Most thermally modified decking is non-structural. Do not use it for joists, beams, ledgers or posts unless the exact product has approved structural values for that use.

Why does thermally modified ash need pre-drilling?

The treatment can make the wood more sensitive to splitting. Pre-drilling reduces stress around the screw. Follow the product-specific drill size and edge-distance instructions.

Can thermally modified decking grow mold?

Surface mold can grow on dirt and organic debris. Better decay resistance does not keep the surface permanently clean. Regular sweeping, washing and drainage help.

Is thermally modified wood safe around children and pets?

Thermal modification normally uses heat and steam instead of conventional preservatives. However, the complete product may include a factory finish. Review its safety documentation and approved uses rather than relying only on the treatment method.

Can thermally modified decking be sanded?

Many products can be lightly sanded when required, but sanding changes the surface and may affect a factory finish. Follow the manufacturer’s grit and refinishing guidance.

Is thermally modified decking suitable around a pool?

It may be suitable when the product is approved for that exposure. Check wet-slip information, fastener corrosion requirements, cleaning guidance and any restrictions involving pool chemicals.

Is thermally modified decking worth the extra cost?

It can be worth it when you value natural wood, improved stability and decay resistance. It offers less value when the lowest price or the least possible maintenance matters most.

Final Thoughts

Thermally modified wood decking offers something many alternatives cannot: real wood with permanently changed moisture and durability properties.

Its strengths are natural appearance, improved stability and better decay resistance than untreated wood of the same species. Its weaknesses are higher cost, limited availability, color change and the need for careful handling and fastening.

Do not buy from a broad lifespan claim alone. Compare the species, treatment class, durability rating, approved exposure, fastening system and warranty. Order samples and read the complete installation guide.

For the right above-ground deck, thermally modified wood can provide a long-lasting natural surface. Its performance still depends on good drainage, a safe frame and careful installation.

Research Sources

• International ThermoWood Association: What is ThermoWood?
• International ThermoWood Association: ThermoWood Handbook 2025
• Thermory: Decking installation and care
• Thermory USA: Installation guides and product downloads
• Lunawood: ThermoWood decking technical information
• American Wood Council: Prescriptive Residential Wood Deck Construction Guide