Thermally Modified Larch

About Thermally Modified Larch

Thermally Modified Larch is European larch (Larix decidua) modified at around 212°C. At 530 lbf (2,358 N) and 0.55 g/cm³ it is soft for its weight, which is a fair summary of larch generally: fairly dense, not especially hard.

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.

Radial shrinkage of 1.3% and tangential of 2.8% after treatment are roughly a third of natural larch. Untreated European larch is rated only slightly to moderately durable under EN 350 and it is resinous and prone to cupping, so there is a real gain here on both counts.

At 550 kg/m³ a cubic metre of Thermally Modified Larch weighs about 550 kg air-dry — that figure, the 2.8% tangential and 1.3% radial movement and the 530 lbf hardness are the three inputs the calculators below actually consume. Select Thermally Modified Larch in any of them to get species-specific results for weight, moisture, shrinkage and hardness.

Working properties

Larch is resinous and it has hard, tight, awkward knots. The process deals with the first permanently. The knots stay, and treatment makes them more brittle. 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

Check what you are being sold. European larch does not appear in Table 1 of the ITWA ThermoWood Handbook. Thermally modified larch is a real and widely traded product, but it sits under the generic CEN/TS 15679 description Thermally Modified Timber rather than under the ITWA ThermoWood mark, and names like "Thermo-D" applied to it are a producer's designation rather than an ITWA classification. That matters because the ITWA mark is what guarantees an audited process and a consistent result; without it, quality varies by producer. For reference, ITWA gives Class 2, durable under EN 113 for genuine Thermo-D at around 212°C, and states that ThermoWood is not intended for load-bearing structures. Treat the Class 2 on this page as indicative and ask your supplier for their own test declaration. Our guide to how ThermoWood is made explains what the mark covers.

Where larch does win is grain. It has a stronger, more marked figure than pine or spruce, and treatment darkens it without flattening it, which is why it gets specified for cladding where the boards are meant to be looked at rather than just to keep the rain out.

Typical applications & suitability

Indoor Outdoor — Larch is a revealing case. It is widely believed to be naturally durable, but the technical data puts its heartwood at only Class 3–4 — and its heartwood is practically untreatable with preservative. Thermal modification is therefore one of the few ways to genuinely lift larch's durability rather than rely on its reputation.

Documented end-uses: exterior cladding and rain screens, decking, garden structures, interior panelling and flooring.

Comparable species

Species closest to Thermally Modified Larch 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 Larch; the denser, slower-grown variant is ThermoWood Siberian Larch. Note the process also drives off larch's troublesome resin.

What modification did to European Larch

Thermally Modified Larch is not a species — it is European Larch 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 66% cut in tangential movement and a 1-class gain in durability, paid for with 17% of its bending strength and 28% of its surface hardness. 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 Larch.

Compare Thermally Modified Larch 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 Larch

Includes the thermal-modification tools, since Thermally Modified Larch behaves by treatment class as much as by species. Full set in the tool index.

Understand the figures

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 · Moisture content — EMC, FSP and service conditions.

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