Kebony vs Thermally Modified Timber for Decking: Are They Really the Same?

What is the difference between Kebony and thermally modified timber?

The simplest distinction lies in how the wood is modified.

Kebony Clear begins as Radiata pine. During manufacture, a bio-based modification liquid penetrates the timber before a curing process permanently changes the cell walls. The finished material becomes considerably denser than the original pine and gains greater dimensional stability and biological durability. Kebony publishes a finished density of approximately 670 kg/m³ for Clear decking.

Thermally modified timber follows a different route. Manufacturers expose timber to high temperatures in a controlled environment, which changes the chemical structure of its cellulose, hemicellulose and lignin.

Those changes reduce the wood’s ability to absorb moisture. As a result, heat-treated timber usually becomes more dimensionally stable and more resistant to fungal decay. However, the same thermal degradation that produces these benefits can also affect strength and toughness.

Why does thermal modification improve durability?

Ordinary timber constantly exchanges moisture with the surrounding air. As its moisture content rises and falls, the wood swells and shrinks.

For decking, that movement can contribute to cupping, checking, opening joints and movement around fixings. Therefore, improved dimensional stability has obvious value.

Heat treatment changes some of the chemical groups within timber that readily bind with water. Consequently, thermally modified wood normally reaches a lower equilibrium moisture content and shows less swelling and shrinkage than the untreated material. Independent research consistently identifies improved dimensional stability as one of the principal benefits of thermal treatment.

Thermal modification can also improve resistance to wood-destroying fungi. Again, the treatment severity matters. Higher temperatures and greater levels of chemical modification tend to produce better biological durability, although species and process remain important.

These are genuine advantages for an exterior decking board. The difficulty is that improved resistance to moisture and biological decay does not necessarily mean improved mechanical performance.

What does the science say about strength loss in thermo wood?

One of the most useful papers for understanding this subject is Homan and Jorissen’s 2004 review, Wood Modification Developments, published in HERON.

The paper considers several methods of wood modification and explains the relationship between heat treatment, biological durability and mechanical performance. It identifies reduced strength and increasing brittleness as important issues associated with thermal modification.

The research is particularly useful because it demonstrates that there is no universal percentage loss that applies to every thermally modified timber.

For the Finnish thermal process reviewed in the paper, bending strength reductions reached around 30% at higher treatment temperatures. Other thermal processes discussed in the same review produced bending-strength reductions approaching 40%.

Independent research into thermal wood modification has reported bending-strength reductions approaching 40% for some processes, although the actual change depends upon species, temperature, treatment duration and manufacturing method.

It would therefore be inaccurate to say that every thermo wood product loses 40% of its strength. The figure describes the upper end reported for some thermal processes, not a universal property.

Why does heating timber affect its strength?

Wood is a natural composite material. Its mechanical behaviour depends on the relationship between cellulose, hemicellulose and lignin within its cellular structure.

As treatment temperature rises, thermal degradation changes those components.

Research by Boonstra and colleagues examined the strength properties of thermally modified softwoods. The study found that tensile strength parallel to the grain decreased substantially after treatment. Bending strength also reduced, while impact strength showed a particularly marked reduction.

This matters because impact strength and tensile performance tell us something about the timber’s toughness, rather than simply its ability to survive a laboratory bending test.

A strong material is not necessarily a tough material. Strength describes the load a material can resist before failure, while toughness describes its ability to absorb energy and resist fracture.

For decking, both can matter.

Does thermal modification make timber brittle?

Thermal modification can make wood more brittle, particularly as treatment severity increases.

This does not mean every thermo board will snap during installation. Nevertheless, reduced toughness becomes relevant at locations where stress concentrates.

Boonstra’s research found a significant reduction in impact strength following thermal modification. Wider scientific reviews also identify strength reduction as one of the limitations of heat-treated wood.

This does not prohibit decking use. Instead, it makes product selection, support centres and fixing details more important.

So is thermally modified timber suitable for decking?

Yes, provided the particular product has suitable properties for that application.

A decking board generally spans a relatively short distance between joists. Therefore, a manufacturer can accommodate reduced material strength through board thickness, profile design, closer joist centres and an appropriate fixing system.

The problem starts when somebody treats the words thermally modified as though they describe a structural strength class.

They do not.

Thermal modification describes a manufacturing process. It does not tell us, by itself, the board’s characteristic bending strength, stiffness or permitted structural span.

For that information, the designer needs product-specific technical data. Where reliable mechanical data cannot be obtained for an unbranded thermo board, I would be cautious about making structural assumptions simply because the product has been sold as exterior decking.

Kebony Clear takes a different approach

Kebony Clear is also modified timber, although its manufacturing process does not depend solely upon heat degradation.

According to Kebony’s Environmental Product Declaration, Clear decking uses Radiata pine together with bio-based modification material. The treatment penetrates the timber and permanently changes the cell structure.

The finished product has a declared density of approximately 670 kg/m³. Kebony also publishes typical mechanical values of approximately 36.1 MPa bending strength and 12.4 GPa modulus of elasticity for Clear Radiata material.

There is an important qualification here. These are typical material properties rather than universal engineering design values. A structural designer must still use the appropriate product information and design methodology for the proposed application.

That qualification strengthens the comparison. Neither Kebony nor thermo wood should bypass proper structural design simply because it performs well against decay.

Density and weight are noticeably different

The modification processes also move timber density in different directions.

Kebony Clear has a published density of approximately 670 kg/m³. The original Radiata pine is considerably lighter, so the modification process creates a denser finished material.

Thermal treatment normally causes some mass loss because heat degrades and removes parts of the wood’s original constituents. The degree varies according to species and treatment conditions. Therefore, there is no useful universal density figure for generic thermo wood.

Anyone who handles both materials may notice the difference. Kebony Clear feels relatively substantial for a modified softwood. Thermally modified pine will normally feel lighter, although the original species and board dimensions remain important.

Weight alone does not determine quality. However, it provides another physical indication that these materials have undergone very different modification processes.

What about dimensional stability?

Both materials offer benefits here.

Thermal modification reduces the timber’s affinity for moisture. Consequently, heat-treated boards generally swell and shrink less than untreated equivalents.

Kebony Clear also shows substantially improved moisture movement compared with ordinary Radiata pine. Its published technical information records low tangential swelling compared with untreated material.

For homeowners, this matters because stable decking tends to maintain more consistent gaps and geometry through seasonal changes.

Nevertheless, modified timber still remains timber. Correct gaps, drainage, ventilation and fixing details remain essential. No modification process compensates for a deck that traps moisture or prevents boards from drying.

Structural spans need proper engineering

A common mistake in decking discussions is to confuse deck-board support centres with structural joist spans.

They are different calculations.

Each element must transfer its load safely to the next.

For structural timber design in the UK, Eurocode 5 provides the relevant design framework. The second-generation BS EN 1995-1-1:2025 has now been published, while the first-generation Eurocodes remain within the UK transition period before withdrawal.

The Eurocode approach considers both strength and serviceability. Therefore, designers assess whether a timber member can safely resist load and whether its deflection remains acceptable during use.

That distinction matters enormously with decking. A joist can be strong enough not to break but still deflect enough to produce an uncomfortable or poorly performing deck.

What happened under BS 5268-7.1?

Many experienced UK decking installers will recognise spans derived from the older BS 5268-7.1 methodology.

The legacy domestic floor-joist approach used an imposed load of 1.5 kN/m² for effective spans of 2.4 metres or greater. For shorter spans it applied a more onerous loading arrangement, partly to prevent very small joist sections being selected.

BS 5268 has now been superseded for current structural design. Nevertheless, the legacy span tables remain useful when understanding where many traditional domestic decking span practices originated.

For a new deck, I would design the structure using current standards and actual loading conditions rather than blindly copying an old span table.

This becomes particularly important where the deck carries concentrated or unusually heavy loads.

• large planters;
• glazed balustrades;
• outdoor kitchens;
• hot tubs; and
• substantial raised structures.

A domestic 1.5 kN/m² loading assumption should never become an excuse to ignore loads that clearly fall outside normal domestic use.

The decking board also needs adequate support

Although Eurocode 5 governs the structural timber design, the decking board itself still needs suitable support centres.

Board thickness, width, material properties and profile all influence how the surface behaves under load.

A thermally modified board that has lost some bending strength may require different joist centres from an untreated board of the same dimensions. Similarly, a heavily grooved board has less material at its thinnest section than a full rectangular board.

For this reason, I would always check the manufacturer’s declared maximum support centres for the specific decking product.

Those figures should complement the structural design rather than replace it. The supporting joists still require a proper span calculation.

Fixings matter just as much as timber spans

A deck does not work as a collection of separate timber members. Its connections hold the load path together.

Eurocode 5 addresses the durability of timber structures and the design of mechanically fastened connections. It also requires designers to consider corrosion protection for metal fasteners according to their exposure.

Exterior decking normally experiences conditions associated with Service Class 3, because members may be exposed to weather and elevated moisture.

Therefore, fixing specification is not simply about choosing a screw that looks substantial enough.

The material, coating, diameter, penetration, edge distance and fixing arrangement all matter.

For Kebony Clear, the manufacturer specifies corrosion-resistant fasteners and recommends stainless steel for external use, with higher corrosion resistance required in particularly aggressive environments.

In my professional view, high-quality external decking should use fixings selected for the expected exposure rather than ordinary internal or lightly plated screws.

With thermo wood, fixing details deserve additional care because lower toughness may increase sensitivity around screw holes and board edges.

Pre-drilling, correct edge distances and the specified fixing system may therefore form an essential part of the product’s performance.

Durability Class is not the same as lifespan

One of the most common misunderstandings around decking concerns the word durability.

Several separate concepts tend to become mixed together.

Durability Class

Durability Class describes resistance to biological attack under the relevant classification system.

Kebony Clear carries EN 350 Durability Class 1, placing it within the highest durability category used by that standard.

That does not mean every Kebony deck will automatically last a particular number of years.

Use Class

Use Class describes the conditions in which timber will operate, including its exposure to moisture and ground contact.

Again, this is not a prediction of exact service life.

Warranty

A manufacturer’s warranty is a commercial commitment governed by its terms and conditions.

Kebony currently provides a 30-year rot and decay warranty for Clear decking.

A 30-year warranty is useful information, but it should not be presented as proof that every deck will last exactly 30 years.

Design, construction and exposure still influence the finished structure.

What does an EPD tell us?

Kebony Clear also has an Environmental Product Declaration, usually abbreviated to EPD.

An EPD is not a durability certificate. Nor is it a product warranty.

Instead, it presents standardised environmental information based upon a life-cycle assessment. For a construction product, that assessment can consider raw materials, manufacturing, transport, installation, replacement and eventual end of life.

Kebony’s current Clear decking EPD follows the relevant environmental declaration framework and provides information about the composition and life-cycle impact of the product.

Within that life-cycle model, the declaration uses a 30-year reference service life for Kebony Clear decking.

That needs careful interpretation.

Actual service life depends upon exposure, detailing, structure, ventilation, drainage, fixings, maintenance and use.

How long should thermally modified decking last?

There is no responsible single answer for generic thermo wood.

A thermally modified product’s likely durability depends upon the original species and the severity of the treatment. The finished durability classification also matters.

Therefore, phrases such as “thermo decking lasts 30 years” should always prompt another question:

Which thermo decking?

A product backed by recognised test data, a declared durability class and proper installation guidance is very different from an unbranded board described merely as thermally modified pine.

If those documents do not exist, price alone should not fill the information gap.

What happens to the colour?

Kebony Clear begins with a deep brown appearance before naturally weathering towards silver-grey.

The change comes from exposure to ultraviolet light and weather rather than biological failure. Homeowners can allow the boards to develop this patina or use a compatible coating if they prefer to retain more of the original brown colour.

Thermally modified timber behaves in a broadly similar visual way.

Heat produces its characteristic brown tone, while exterior exposure gradually greys an unfinished surface.

Therefore, colour retention should not be confused with timber durability. A grey board is not necessarily deteriorating.

Likewise, applying oil primarily to maintain colour is different from applying a preservative treatment to stop untreated timber decaying.

What maintenance do these materials need?

Kebony Clear requires normal cleaning rather than repeated preservative treatment. The manufacturer describes the modification as permanent throughout the treated timber.

Thermally modified timber can also require relatively little preservative maintenance when the treatment has delivered adequate durability.

However, every timber deck still benefits from housekeeping.

Leaves, soil and organic debris should not remain trapped between boards or against walls. Ventilation beneath the deck must remain open, and drainage paths should continue to work.

Cleaning also reduces the accumulation of algae and surface contamination.

Consequently, low maintenance should never be interpreted as no maintenance whatsoever.

How do costs compare?

Kebony Clear generally sits within the premium timber-decking market.

Generic thermally modified softwood is usually cheaper, although board dimensions, grade, profile and fixing system make direct comparisons difficult.

Kebony Clear generally commands a higher material price than basic thermally modified pine. However, comparing board prices alone can be misleading.

A proper decking cost should consider:

• board price;
• wastage;
• support centres;
• substructure quantities;
• clips or screws;
• installation time;
• finishing requirements;
• maintenance; and
• expected replacement cycle.

If one product requires closer joist spacing, more fixings or slower installation, the apparent saving in board price may reduce once the complete deck has been costed.

Equally, where a thermo board has sound technical data and suits the proposed structure, its lower initial price may make it a perfectly rational choice.

Kebony vs thermally modified timber: which would I choose?

I would not dismiss thermally modified timber.

It offers genuine improvements over ordinary timber, particularly in dimensional stability and biological durability. Independent scientific research supports those benefits.

However, the same research also shows why the words modified timber need qualification.

Heat treatment can reduce bending strength, tensile performance and impact resistance. Some thermal processes have produced bending-strength reductions approaching 40%, although that figure does not apply universally.

For that reason, I would want product-specific technical information before specifying a generic thermally modified board for an important decking project.

Kebony Clear gives the designer a different proposition. It combines real timber with high biological durability, increased density and published technical data for the finished material. Its EPD also provides transparent environmental information about the product and its life-cycle assumptions.

It costs more, so the specification still needs justification.

Nevertheless, for a premium natural timber deck where durability, stability, predictable material properties and appearance all matter, Kebony Clear presents a particularly strong case.

The more important question is how the whole deck is designed

The choice between Kebony vs thermally modified timber should not end with a discussion about rot.

A good decking material must work as part of a complete system.

The boards need suitable support. Joists need appropriate spans. Beams and posts need adequate capacity, while fixings must transfer loads without premature corrosion or splitting the timber.

Ventilation and drainage also remain essential.

From inspecting and designing decking over many years, I regard these details as more important than a material’s marketing description.

A premium timber installed onto an inadequate frame can still produce a poor deck. Conversely, a carefully selected thermally modified board can perform very well when the designer understands its limitations and follows the relevant technical guidance.

That is the real distinction.

Kebony and thermally modified timber can both be suitable for decking, but they are not technically equivalent. The modification process matters because it changes not only how timber resists decay, but also how it behaves mechanically.

For a homeowner, the sensible next question is therefore not simply, “Which board lasts longest?”

It is:

“What are the documented properties of this board, and has the deck beneath it been designed properly?”

Technical references

Technical note: Do not interpret the reported 40% bending-strength reduction as applying universally to all thermally modified timber. Likewise, an EPD reference service life is an environmental life-cycle modelling parameter and not a guarantee of actual deck lifespan.

Written by Karl Harrison, who has has specified, designed and worked with Kebony and thermally modified decking products on UK decking projects. Karl Harrison is a landscape consultant, decking specialist and independent expert witness with more than 20 years’ experience in the design, specification and construction of timber and composite decking.

He is the founder of The Decking Network and author of The Definitive Guide to Decking.