Why choose thermally treated timber for your garden joinery?

When designing for the outdoors, choosing the right timber is more than a question of aesthetics; it’s about performance, sustainability, and long-term satisfaction.

At The Garden Trellis Company, we’ve carefully selected thermally treated timber to complement our other timber options for our Prestige range and bespoke joinery projects because it offers a rare combination of beauty, stability, and environmental responsibility.

What is thermally treated timber?

Thermally treated timber, also known as thermowood, results from a chemical-free heat and steam process that naturally enhances the durability and stability of the timber being treated. The process reduces the wood’s moisture content, minimising its ability to absorb water and dramatically reducing the movement commonly seen in exterior timber.

As Julian, our founder, puts it:

“The thermal modification process offers the best way yet to naturally enhance both the durability and stability of timber boards, creating a long-lasting, attractive material ideal for painted or natural exterior joinery. We use both thermally modified Scandinavian pine and New Zealand Radiata pine. Left natural, both will weather from dark to mid-brown to an attractive silver-grey.”

This transformation makes thermally treated timber a smart, forward-thinking choice, particularly for UK gardens, where changeable weather can negatively impact the appearance and longevity of timber products.

1. Exceptional stability: less movement, longer life

Perhaps the most important reason we’ve adopted thermally treated timber, especially in our garden gates, is its outstanding dimensional stability. Unlike standard pressure-treated timber, it doesn’t shrink, swell, or warp to the same degree when exposed to moisture and temperature changes.

This means your trellis panels, gates or fencing will:

• Stay square and aligned.
• Maintain clean lines and tight joints.
• Look newer for longer.

In short, it preserves the integrity and appearance of your investment with minimal maintenance.

2. Chemical-free durability

One of the most attractive features of thermally treated timber is its improved performance without harsh chemicals. Instead, it relies on heat and steam alone, making it a more environmentally responsible choice.

This means:

• Safer for gardens with children and pets
• Better for the soil and the surrounding planting
• A more sustainable option for eco-conscious homeowners

It’s a natural upgrade from traditional pressure-treated timber in appearance and environmental credentials.

3. Sustainably grown and easier to source

We are committed to sourcing timber responsibly and always look for FSC or PEFC certified options wherever possible. Fortunately, thermally treated Scandinavian and New Zealand pines are high-performing and more readily available in certified sustainable grades than many tropical hardwoods.

This ensures our supply chain remains consistent, ethical, and transparent, whilst also helping to reduce the pressure on endangered hardwood species.

4. A natural hardwood look, without the cost or delay

While thermally treated timber starts with a rich brown hue, all our timber products, whether thermally treated or not, will mellow to a beautiful silvery grey within a few months outdoors. That means:

• A more consistent long-term aesthetic across garden features
• No need to choose timber based on its fresh colour alone

It also has a far more refined look than pressure-treated softwood, making it an excellent choice for customers looking for a more premium finish at a lower cost than hardwoods.

5. Excellent paint retention

As part of our bespoke joinery service, we offer a full professional paint finish. The increased dimensional stability of thermally treated timber makes it perfect for painting. It:

• Holds paint better
• Is less prone to peeling and flaking
• Requires fewer touch-ups over time

That makes it ideal for garden joinery that must perform as well as it looks, especially in high-use areas like gates or architectural features.

A thoughtful upgrade for long-lasting style

From its impressive stability to its chemical-free treatment and beautiful finish, thermally treated timber represents the next generation of exterior joinery materials. Whether you’re specifying for a bespoke project or choosing a gate or panel from our Prestige range, this timber offers a blend of form, function, and sustainability that truly elevates any outdoor space.

Want to see it for yourself?

Explore our Prestige Range or get in touch to discuss your bespoke joinery project today.