What does thermal modification do to wood?

Wood is heated to 400-500°F in an oxygen-free kiln, permanently changing its cell structure. This reduces moisture absorption and increases decay resistance without chemicals.

Is thermo ash brittle?

Thermal modification does reduce flexibility somewhat. Thermo ash can be more prone to splitting at fastener points, so pre-drilling is recommended.

How long does thermo ash last?

Thermo ash typically lasts 25-30 years in decking applications, compared to 15-20 years for cedar.

Does the dark color fade?

Thermo ash will lighten somewhat with UV exposure. Regular UV-protective oil maintains the deep chocolate tone.

Thermo-Treated Ash vs Cedar wood grain comparison

decking

Thermally Modified Ash vs Cedar: Innovation Meets Tradition (2026)

Prepared by Lumberama's Wood Comparison Team

|Last updated June 2, 2026|Sources & methodology

Thermally modified ash (thermo ash) is a European-originated technology that uses high heat to transform domestic ash into a durable outdoor wood. Cedar is the traditional go-to for natural outdoor applications. This comparison examines whether thermal modification offers a meaningful upgrade over time-tested cedar.

Thermo-Treated Ash

Cedar

Key Takeaways

Thermally modified ash (thermo ash) is a European-originated technology that uses high heat to transform domestic ash into a durable outdoor wood.
Cedar is the traditional go-to for natural outdoor applications.
This comparison examines whether thermal modification offers a meaningful upgrade over time-tested cedar.
Thermo ash is an excellent choice for homeowners who want domestic hardwood performance with an eco-friendly profile.
Cedar remains the practical pick for budget-conscious builders who prioritize ease of use.

Property	Thermo-Treated Ash	Cedar
Hardness Janka	1050	350
Density Lbft3	30	23
Price Range	$4.00-$6.00/lf	$2.50-$4.50/lf
Durability Class	Class 1-2 (modified)	Naturally durable
Color	Rich chocolate brown	Warm reddish-brown

Thermo-Treated Ash

Pros

Three times harder than cedar
Chemical-free modification process
Domestic wood — no tropical sourcing
Beautiful uniform dark chocolate color

Cons

More brittle than untreated wood
Higher cost than cedar
Can be more difficult to fasten without splitting

Cedar

Pros

Lower cost and wide availability
Flexible and forgiving to work with
Proven performance over many decades
Natural aromatic insect deterrent

Cons

Much softer surface
Shorter lifespan in harsh conditions
Requires more frequent maintenance

Best Use Cases

Choose thermo ash for a harder, more durable deck surface made from domestic wood without chemicals. Choose cedar for proven, affordable performance with easy workability.

Verdict

Thermo ash is an excellent choice for homeowners who want domestic hardwood performance with an eco-friendly profile. Cedar remains the practical pick for budget-conscious builders who prioritize ease of use.