Thermally Modified Oak
Also known as thermo oak, thermally modified oak · Botanical Quercus spp.
Revision history
- — Data re-verified in site-wide audit (formulas, sources, durability classes)
- — Working properties and comparable-species data cross-checked against listed sources
About Thermally Modified Oak
Thermally Modified Oak is oak (Quercus) modified at about 212°C. At 1,120 lbf (4,982 N) and 0.68 g/cm³ it is the densest board in the range, though not the hardest. It is also the odd one out, because oak is the only timber here that was already durable before anyone heated it.
Note: this is a modified product, and the panel above is modelled rather than measured. The hardness figure is the base species, because thermal modification changes hardness very little. For movement, the shrinkage and the fibre saturation point both fall after treatment, and that is why modified timber moves less across the same change in moisture. Published strength reductions vary by source; the table above uses our library figures for this pair. Modification also lowers bending strength and toughness relative to the untreated timber. Use this panel to size and to compare species, and take structural, fire, durability or movement figures from your supplier's declaration for the product you are actually buying.
That changes what you are buying. Everywhere else in the range, modification is bought for decay resistance. Here it is bought for stability and colour. European oak heartwood is rated durable under EN 350 as it stands, but it has notoriously high tangential shrinkage and a reputation for moving. Radial shrinkage of 1.8% and tangential of 3.5% after treatment are roughly a third of the natural figures.
At 680 kg/m³ a cubic metre of Thermally Modified Oak weighs about 680 kg air-dry — that figure, the 3.5% tangential and 1.8% radial movement and the 1,120 lbf hardness are the three inputs the calculators below actually consume. Select Thermally Modified Oak in any of them to get species-specific results for weight, moisture, shrinkage and hardness.
Working properties
Oak was hard on edges before, and modification does not soften it. What it does do is make oak's existing chemistry more awkward. How the board behaves once it is out of the kiln is set by the process itself, which is described in full in our guide to how ThermoWood is made. What follows is specific to this timber, drawn from the International ThermoWood Association, VTT Technical Research Centre of Finland and published thermal-modification research.
Durability & use classification
A note on species. The ITWA ThermoWood Handbook Table 1 lists red oak (Quercus rubra) for Thermo-D. European oak (Quercus robur) is thermally modified by a number of producers, but under the generic CEN/TS 15679 description Thermally Modified Timber rather than as ITWA ThermoWood. If the ITWA mark matters to your specification, ask which oak you are being quoted and ask to see the declaration. On the process itself, a Thermo-D class product is treated at around 212°C, which ITWA classifies as durable, Class 2 under EN 113; Thermo-S at about 190°C reaches Class 3. ITWA states the material is not intended for load-bearing structures. Our guide to how ThermoWood is made explains the process. Note the class shown above. The spec panel rates this material Class 1 rather than the Class 2 that the ITWA quotes as the general Thermo-D benchmark — plausible, since oak already starts at Class 2 untreated and modification lifts it further. Treat Class 2 as the guaranteed floor for Thermo-D generally, and confirm the specific figure with your supplier's declaration.
Then there are the tannins, and they do not cook out. Wet modified oak will still stain steel black and streak render or masonry underneath it. Stainless fixings and a drip detail are not a nicety on this board, they are the specification. Nothing about the heat treatment changes that.
Typical applications & suitability
Indoor Outdoor — An unusual case: oak is already durable untreated, so modification here is bought mainly for stability and colour rather than durability, and it delivers: it cuts movement well below natural oak's notoriously high tangential shrinkage.
Documented end-uses: exterior cladding, decking, interior flooring and panelling, joinery and feature surfaces.
Comparable species
Species closest to Thermally Modified Oak by density, hardness, stiffness and bending strength, computed directly from the Vriksai species dataset rather than hand-picked.
Trade alternatives. The untreated original is European Oak — already Class 2 and considerably stronger, so choose thermo-oak when stability and colour, not durability, are the brief. For genuinely durable European alternatives, Sweet Chestnut.
What modification did to European Oak
Thermally Modified Oak is not a species — it is European Oak after heat treatment. Both sets of figures are in our library, so the effect of the process on this timber can be stated exactly rather than described in general terms.
| Property | European Oak | Thermally Modified Oak | Change |
|---|---|---|---|
| Tangential movement | 8.4% | 3.5% | -58% |
| Radial movement | 4.7% | 1.8% | -62% |
| EN 350 durability | Class 2 | Class 1 | 1 class better |
| Bending strength (MOR) | 97 MPa | 78 MPa | -20% |
| Stiffness (MOE) | 10.6 GPa | 11.0 GPa | +4% |
| Janka hardness | 1,120 lbf | 1,120 lbf | +0% |
| Density | 0.67 g/cm³ | 0.68 g/cm³ | +1% |
On this species the process buys a 58% cut in tangential movement and a 1-class gain in durability, paid for with 20% of its bending strength. A normal trade-off for this treatment class: fine for cladding and exterior joinery, size structural work off the treated figure, never the untreated one.
The mechanism — hemicellulose breakdown, why durability is through-section rather than surface-deep, and why the strength loss happens — is set out once in our guide to thermal modification and the ThermoWood species reference, rather than repeated on each treated species page. Untreated figures: European Oak.
Alternatives to Thermally Modified Oak
Thermally Modified Oak measures 1,120 lbf Janka at 0.68 g/cm³, EN 350 durability Class 1, movement rated Good. Thermally Modified Oak is a modified timber: its durability comes from process, its strength from the base species. Substitutes are matched on both, not on the treated figure alone. Read the numbers against the Janka scale and EN 350 durability classes.
Thermal modification lowers hardness slightly, so if the treated figure is short of what the job needs, these are the harder options at comparable service durability.
Lower-hardness options at similar treated durability — usually cheaper per m³ and faster through the shop.
Modification is already doing the stability work here. These achieve equal or lower movement by species or by deeper treatment. Size the gaps with the shrinkage predictor and EMC calculator.
If Thermally Modified Oak is unavailable and the specification cannot change, these are the nearest substitutes across hardness, density, movement, stiffness, bending strength and durability class together — not on any single property. Lower distance means a closer match.
Computed from measured properties only — hardness, density, movement, stiffness, bending strength and EN 350 class. Price, availability and appearance are never inputs. Figures are representative values, not consignment absolutes: two species within a few percent are equivalent until verified on the material. Full method and the rules the engine cannot break.
Comparing modified timber to unmodified is a comparison of process as much as species — treatment class sets the durability, the base species sets the strength. Compare directly before substituting.
Compare Thermally Modified Oak side by side
Modified against unmodified, and modification class against class — the comparisons that matter for treated timber. Every pairing shows the measured figures side by side; full matrix in the comparison index.
Calculators relevant to Thermally Modified Oak
Includes the thermal-modification tools, since Thermally Modified Oak behaves by treatment class as much as by species. Full set in the tool index.
Understand the figures
Wood density reference — oven-dry vs air-dry basis · Moisture content — EMC, FSP and service conditions · Strength properties — MOR, MOE and what they govern · Durability & fire — EN 350 classes and E84 ratings.
Further reading. What is ThermoWood? · ThermoWood vs Teak · Wood moisture content explained.
Common questions about Thermally Modified Oak
A note on the strength figures. This species is not on the ITWA ThermoWood species list, so no association-published strength or hardness figures exist for it. The values above are for the untreated base species and are marked unverified — thermal modification reduces strength, typically in the region of 10–30% depending on class and species, with bending strength affected more than hardness. Thermally modified timber is not strength-graded and must not be used for load-bearing structures. Take any design value from your supplier's declaration for the actual product.
unverified no published source for this figure · ITWA International ThermoWood Association / VTT · calc derived from the figures above