Our guide to Thermowood

Heat has long been used to improve and enhance the natural properties of wood. In ancient times, tribes in Africa heat-treated their spears on the fire in order to harden them, while the Vikings would burn the outside of their ships to make them fire retardant and water resistant.

As @AccessScience explains, “In the early part of the twentieth century, it was found that drying wood at high temperature increased dimensional stability (resistance to swelling and shrinkage) and reduced hygroscopicity (ability to take up and retain moisture).”

Thermo modified wood is a modern take on these ancient techniques. Beautiful, sustainable and offering a range of benefits over untreated timber, Thermo treated wood is a popular choice for a wide range of wooden construction projects, especially decking and cladding. Here, we take a closer look at this innovative material.

What is Thermowood®?

Thermo modified wood is made by heating a range of timber species, including Frake, Ash, Pine, and Spruce, to temperatures of 180? – 230° in specially designed kilns. The wood is kept at this temperature for up to 72 hours. The Finnish Thermowood association has its own patented process, with clearly defined steps, which we source from, but we also source similar products in a greater range of species with comparable from several other mills. In this article we’re using the terms interchangeably – but can of course deliver you with registered Thermowood® to match your specifier’s demands.

Steam is used to protect the wood and prevent splitting and damage to the timber. The heat treatment causes a number of chemical and structural changes in the wood – enhancing its qualities.

Key benefits of Thermo woods

– Stability 

One of the most important advantages Thermo wood has over other types of untreated timber is that it’s very stable. The intensive heat used during the treatment process dries the wood out deep into its core. This removes all moisture and resin from the timber.

When Thermowood® is installed on site, it won’t swell, crack, or shrink as much as untreated timber. It also won’t secrete sap, even in extreme temperatures.

This stability is a very important quality for many people using heat treated timber, including designers looking for long straight lines.

– Longevity 

Heat treating wood in this way helps to extend the lifespan of the timber. The atmospheric and environmental conditions the wood is exposed to will still play an important role in determining how long the timber will last.

However, thermodified timber still has a longer average lifespan compared to standard wood as some of the natural sugars have been eliminated thus increasing its resistance to rot and fungi. This is a major advantage for those who want their construction projects to last as long as possible.

At the end of its life, chemical-free heat treated wood can simply be discarded or recycled – an added benefit over chemically treated wood.

Industrial timber kiln:

– Insulation 

When wood is heated to a high temperature, it loses water and becomes more porous. This can have a big impact on its powers of insulation and means heat treated wood is much better at insulating buildings compared to non-treated timber.

– Sustainably sourced

Thermo modified woods are sourced from well-managed, FSC and PEFC-certified forests. No chemicals are used during its production and the trees it’s made from contribute to improving air quality in the local environment as they grow.

Find out more about Thermo woods, and the benefits of heat treated timber by exploring our site or getting in touch with a member of our team.

Kiln Photo by Roman Zaiets / Shutterstock.com

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Guide to thermowood

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Frequently asked questions

Do you supply timber for domestic uses?

Yes, we supply timber for domestic applications as well as commercial and public constructions. From garden decking to shed cladding, we will be happy to supply smaller deliveries for home use.

Is your timber FSC-certified?

Yes, all timber we supply is FSC or PEFC certified and sourced from independently verified responsibly managed forests. If we can’t find your request in FSC/PEFC certified timber we’ll suggest an alternative that is certified or clearly note that the offer is minimum EUTR.

Where does EcoChoice deliver to?

EcoChoice delivers to construction sites, warehouses and homes across the UK, as long as there’s access and unloading for our trucks.

What are the best timber species for exterior use?

There are many different timber species suitable for exterior use and which is the best for your project depends on your requirements. In general, though, we recommend EkkiGreenheartOpepe and Oak for heavier civil/marine uses. For intermediary external use such as decking, where the timber needs to be both strong and durable but not on the same level as civil engineering (bridgespiling, etc) we usually supply CumaruIpe, Jatoba, and Balau. And for the lighter exterior use where the timber is non-structural such as cladding, we deliver CedarLarchThermowood and others.

Which timber is the most durable?

Durability Class 1 timbers are the most durable. This of course depends on the use (hazard) class they’ll be in service for. For direct ground and water contact such as sleepers, fenders, marine piling, etc, Ekki and Greenheart are known to be the most durable timbers available. For decking and cladding where there’s no direct ground contact (use class 3) there’s a much wider range of timbers such Larch, Cedar, Thermo modified Pine or Ash and others.

What’s the best timber for internal cladding?

Internal cladding, where your timber is not in ground and not exposed to the elements can be classed as either use-class 1 or 2 – in which case most timbers will meet this durability criteria. That is not to say they’ll be the best, since internal environments have lower humidity (moisture) levels than outside. So in these cases, it’s best to go for a kiln-dried timber cladding board as these are more stable and delivered already in lower MC (moisture content) than the usual fresh sawn timbers sent to external applications. A word of caution – as timber reacts to variations in moisture, it’s best to avoid cladding any areas inside your bathroom.

How can I make decking less slippery?

The first thing is to ensure your deck boards are kept clean from leaves and other debris which will, over time, cover the surface and make it more slippery. Grooved boards can be used to improve grip if installed and used perpendicular to traffic, so especially useful in narrow passages and footbridges. Lastly, you can add our Grip-Choice strips: these are a mix of epoxy with aggregates that add gripping ridges to your deck area. Gripchoice is an easy way to make decking less slippery and safer, especially over large areas or where there’s high traffic. We can pre-install Grip-Choice strips for your decking before delivery or ship the strips separately to add to your existing deck’s grooves as a retro-fit solution.

What wood machining services do you offer?

We can offer planing, sawing, drilling and profiling of rough-sawn wood to any profile you require: from handrails to large port fenders.

Can you supply pre-drilled timber?

Yes, we will be happy to provide pre-drilled timber which is tailored to your needs for quick installation, thus saving you valuable time on site.

What cladding profiles do you supply?

We provide a wide range of cladding profiles, including tongue and groove, featheredge and shiplap. You can check the standard profiles here – but we can also match or custom make profiles to suit your specific needs.

Does timber change colour?

Yes, timber is well-known for changing colour over time. Some species can darken quickly right after installation, and like most untreated wood will eventually weather to a beautiful silver-grey. Interior timber is less likely to change colour (depending on conditions), and timber can be stained, painted or treated with UV-resistant oils to reduce colour change.

Do I need to treat exterior timber?

Some timbers such as Oak and Ekki, can withstand the strains of exterior use without any treatment. However, to extend the lifespan and maintain the aesthetic of your wood, we do recommend treating your timber with stains and oils. Pre-treated timber, such as ThermoWood and OrganoWood, are low-maintenance options which are more durable, but can have their life service extended by applying a clean and stain maintenance schedule.

Do I need to treat interior timber?

Depends on what area the timber is being used. For floor joists and timber rafters, it is common to see timber treated, or least very well kiln-dried. Other uses such as flooring may benefit from varnishing while internal timber cladding for decorative purposes shouldn’t require any treatment other than if you want to change its colour with different types of stains or oils.

Where are you based?

We’re based in Cambridge, UK, and coordinate multiple sawmills all over the world to manufacture your order. We then organise delivery with a range of hauliers so that your orders gets to your site on time.

What are the usual decking sizes?

Timber decking is usually 21x140mm or similar. For heavier applications where there’s more load and traffic, we deliver 35 or 45mm thick boards (such as Hastings Pier). Narrower boards are also possible: 95-90mm is a standard width – but we can manufacture whatever size you require.

What are the usual cladding sizes?

Typical cladding section sizes are 18×95 and 18x145mm although we can always custom make a section to suit your project’s design requirements. That includes waney edge boards in Oak or Chestnut as well as extra wide 195mm boards.

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Having needed the supply of cladding, decking boards and other specified timbers through out the year Eco choice have been very helpful in sourcing the specified timbers and quantities required at a very competitive price.

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