ThermoWood Ash Thermo-D

About ThermoWood Ash Thermo-D

ThermoWood Ash Thermo-D is European ash (Fraxinus excelsior) modified at about 212°C. At 1,320 lbf (5,871 N) and 0.66 g/cm³ it is the hardest and densest board in the ThermoWood range, and the darkest: treatment takes ash from near-white to a deep chocolate brown that runs right through the section rather than sitting on the surface.

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.

Untreated ash is rated not durable under EN 350, which is why you almost never see it outside. Modification is transformative here in a way it is not for a timber that was half decent to begin with. Radial shrinkage of 1.7% and tangential of 3.2% are roughly a third of the natural figures.

At 554 kg/m³ a cubic metre of ThermoWood Ash Thermo-D weighs about 554 kg air-dry — that figure, the 3.2% tangential and 1.7% radial movement and the 1,320 lbf hardness are the three inputs the calculators below actually consume. Select ThermoWood Ash Thermo-D in any of them to get species-specific results for weight, moisture, shrinkage and hardness.

Working properties

Everything ash was famous for, thermal modification takes away, and it gives something else back. This is the one page in the range where the honest headline is a loss. 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

Ash appears in Table 1 of the ITWA ThermoWood Handbook in both Thermo-S and Thermo-D, so genuine marked ThermoWood Ash is available. As a Thermo-D product it is treated near 212°C, which the ITWA classifies as durable, Class 2 under EN 113. That is a jump of three full classes from untreated ash, and it is the reason the product exists. The durability comes from the treatment rather than from natural extractives, and it applies through the whole board, so there is no sapwood to cut away. ITWA states that ThermoWood is not intended for load-bearing structures, and ThermoWood is a registered trademark; the generic term under CEN/TS 15679 is Thermally Modified Timber. Our guide to how ThermoWood is made has the detail.

Ash is the timber of tool handles, hurleys and steam-bent chair backs because it is tough and it absorbs shock. Thermal modification removes that. The board that comes out is harder on the surface but brittle underneath, and it will snap where natural ash would flex. If you are specifying it, you are specifying it for colour, stability and decay resistance, and you are accepting a timber that no longer bends.

Typical applications & suitability

Indoor Outdoor — Ash is the classic case for thermal modification: naturally it is non-durable (EN 350 Class 5), so the process transforms what it can be used for — it is the reason you see ash decking and garden furniture at all.

Documented end-uses: decking (a signature application), exterior cladding, garden furniture, interior flooring and panelling, and joinery.

Comparable species

Species closest to ThermoWood Ash Thermo-D by density, hardness, stiffness and bending strength, computed directly from the Vriksai species dataset rather than hand-picked.

Trade alternatives. The untreated original is White Ash — tough and shock-resistant, but strictly interior. Compare ThermoWood Birch, and note that thermal modification sacrifices exactly the springy toughness ash is otherwise prized for.

What modification did to White Ash

ThermoWood Ash Thermo-D is not a species — it is White Ash 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.

On this species the process buys a 59% cut in tangential movement and a 3-class gain in durability, paid for with 15% 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: White Ash.

Compare ThermoWood Ash Thermo-D 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 ThermoWood Ash Thermo-D

Includes the thermal-modification tools, since ThermoWood Ash Thermo-D behaves by treatment class as much as by species. Full set in the tool index.

Understand the figures

Thermal conductivity — k-value and insulation behaviour · ThermoWood species guide — Thermo-D/Thermo-S treatment classes · Janka hardness scale — how the lbf figure is measured · Wood density reference — oven-dry vs air-dry basis.

Further reading. What is ThermoWood? · ThermoWood vs Teak · Wood moisture content explained.