- What it is: Thermally modified pine decking — heat-treated at 190–212°C without chemicals to achieve Durability Class 2 and zero maintenance requirement.
- Service life: 30 years BRE endorsed for above-ground use — realistic 25–40 years. No treatment, no oiling, no staining ever required.
- Profiles: Standard 26x117mm, secret fix grooved 26x117mm, secret fix smooth 26x140mm, secret fix brushed 26x140mm.
- Secret fix system: Brofix hidden fixing clips — completely fixing-free deck face, consistent board gaps, no screw holes visible.
- Cost: £55–£80 per m² boards. Full 15m² deck materials approximately £950–£1,400.
- Also known as: Thermo pine decking, thermally modified timber decking, modified pine decking — all the same product.
ThermoWood decking — thermally modified pine produced without chemicals, capable of lasting 30+ years with zero maintenance — is the strongest natural timber decking specification available in the UK in 2026. It outperforms standard softwood decking on every measure that matters: durability, dimensional stability, service life, and whole-life cost. Our range includes standard face-fix boards and three secret fix profiles using the Brofix hidden fixing system, giving a completely clean deck surface with no visible screws. This guide covers everything — what thermally modified timber decking is, all four products in detail, how the Brofix secret fix system works, profiles compared, costs, and installation guidance.
ThermoWood decking installed — the rich honey-brown of fresh thermally modified pine gradually weathers to a warm silver-grey over 12–18 months without any treatment. The natural weathering process is uniform and attractive — and requires zero intervention throughout the 30-year service life.
What Is Thermally Modified Timber Decking?
ThermoWood decking is Scots pine (Pinus sylvestris) that has been thermally modified — heat-treated at 190–212°C in a controlled steam atmosphere, without any chemical additives. The process permanently and irreversibly alters the cellular structure of the wood at a molecular level: hemicellulose — the primary food source for wood decay fungi — is broken down; equilibrium moisture content is reduced from approximately 12% to 4–6%; and dimensional stability is improved by approximately 60% compared to unmodified softwood.
The result is a timber that has been upgraded from Durability Class 4–5 (standard softwood — not durable) to Durability Class 2 (Durable) without any chemical treatment. It requires zero maintenance throughout its 30-year BRE endorsed service life. It will not rot, will not decay biologically, will not require annual oiling or periodic preservative treatment — it simply performs and ages. The term "ThermoWood" is a registered trademark of the International ThermoWood Association (ITWA), produced to the SFS 5765 standard. We supply Lunawood ThermoWood from Lunawood Oy, Finland's largest thermally modified timber producer.
You may see the same product described as thermo pine decking, thermally modified pine decking, or modified timber decking — these all refer to the same heat treatment process applied to Scots pine. For the complete technical picture see our ThermoWood technical guide.
ThermoWood Decking — All Products
ThermoWood decking boards on pallet — kiln dried, thermally modified, and ready for installation. The consistent honey-brown colour across the full pallet reflects the uniformity of the thermal modification process. All boards are FSC and PEFC certified.
Standard face-fix profile. Smooth surface, fixed with A4 stainless screws through the face. Most affordable entry point into ThermoWood decking. Suitable for any deck construction where hidden fixings are not required.
Grooved underside profile accepts Brofix secret fixing clips — completely fixing-free face. Narrower 117mm board width creates more defined board rhythm. Ideal for contemporary deck designs where a clean surface is the priority.
Wider 140mm board with smooth face and Brofix secret fixing system. Fewer boards per m², cleaner wider-board aesthetic. The most popular ThermoWood decking specification for contemporary garden projects.
Wire-brushed surface that opens the grain for enhanced texture, slip resistance, and a more rustic character. Hidden fixing system. The brushed finish also helps the board weather more evenly to silver-grey over time.
The Brofix Secret Fix System — How It Works
ThermoWood decking secret fixing — the Brofix clip system conceals all fixings beneath the deck surface. Each clip engages with the groove profile on the underside of the board, securing it to the joist while maintaining a consistent drainage gap between boards.
Brofix installation — each clip is screwed to the joist and engages with the board groove simultaneously, pulling the board tight while maintaining the correct inter-board gap. The system is fast to install — significantly quicker than pre-drilling and face-screwing individual boards.
The Brofix hidden fixing system is a stainless steel clip that simultaneously fixes the decking board to the joist and maintains a consistent drainage gap between boards — with no screws, nails, or fixing marks visible on the deck surface. Each clip engages with the groove profile machined along the underside edge of the decking board, securing it to the joist with a single screw through the clip itself. The result is a completely clean deck face with consistent 5mm gaps between boards for drainage and ventilation.
Benefits of the Brofix secret fix system over conventional face-fixing:
- No visible fixings — clean, contemporary deck face with no screw holes or marks
- Consistent gaps — each clip sets the same inter-board gap automatically, eliminating the spacing errors common in face-fixed decks
- Faster installation — one clip per joist fixing point versus drilling and screwing through each board face
- No pre-drilling required — eliminates the risk of splitting boards near ends during installation
- Board replacement — individual boards can be removed and replaced by disengaging the clips, without disturbing adjacent boards
Download the full Brofix installation guide PDF for complete technical specifications, fixing centres, joist spacing requirements, and step-by-step installation detail. The Brofix fixing system is available from our UK stock alongside all ThermoWood decking profiles.
Profile Comparison — Smooth vs Grooved vs Brushed
ThermoWood decking close-up — the characteristic rich honey-brown colour and dense, fine grain of thermally modified pine. The modification process darkens and densifies the surface compared to standard pine, giving ThermoWood decking a visual quality closer to a hardwood than a softwood.
ThermoWood brushed decking — the wire-brushed surface opens the grain to create enhanced texture and slip resistance. The brushed finish also produces more even greying as the board weathers, and gives a more characterful, tactile surface compared to the smooth profile.
| Profile | Size | Fixing method | Surface | Best for |
|---|---|---|---|---|
| Standard smooth | 26x117mm | Face fix — A4 stainless screws | Smooth, flat | Budget-conscious ThermoWood spec, replacement decks |
| Secret fix grooved | 26x117mm | Brofix hidden clips | Smooth, flat | Contemporary designs, narrower board rhythm |
| Secret fix smooth | 26x140mm | Brofix hidden clips | Smooth, flat | Most popular — clean wide-board contemporary deck |
| Secret fix brushed | 26x140mm | Brofix hidden clips | Wire-brushed grain | Maximum grip, rustic character, even weathering |
ThermoWood Decking vs the Alternatives
| Material | Service life | Maintenance | Cost/m² | 30-yr total/m² |
|---|---|---|---|---|
| ThermoWood decking | 25–40 yrs | Zero — ever | £55–£80 | £55–£80 |
| Siberian larch decking | 20–30 yrs | Minimal — oil optional | £45–£70 | £55–£90 |
| Composite decking | 15–25 yrs | Annual clean | £60–£120 | £70–£130 |
| Treated softwood decking | 10–15 yrs | Oil every 1–2 yrs | £15–£30 | £60–£120 (inc. treatment + replacement) |
| Hardwood decking (Ipe etc.) | 25–40 yrs | Oil annually for colour | £80–£150 | £90–£180 |
Treated softwood decking looks cheapest upfront at £15–£30 per m² — but factor in annual oiling, periodic sanding, and replacement every 10–15 years and the 30-year cost exceeds ThermoWood at £55–£80 per m² once. Composite decking costs more upfront and still requires annual cleaning. ThermoWood's 30-year whole-life cost is the lowest of any premium decking specification — because the cost of the boards is the entire cost of ownership. No oil, no treatment, no replacement within a normal building's service life. For the full comparison see our ThermoWood vs composite decking guide.
How ThermoWood Decking Weathers
ThermoWood decking weathered to silver-grey — left untreated, ThermoWood weathers from honey-brown to warm silver-grey over 12–18 months. The greying is uniform, attractive, and structurally harmless — the timber remains fully durable throughout. Many homeowners prefer the weathered look and choose to leave it completely untreated from installation.
ThermoWood decking weathers in the same way as ThermoWood cladding — a gradual photochemical process driven by UV exposure that transitions the colour from honey-brown through golden to silver-grey over approximately 12–18 months. This is not decay — it is a surface colour change that does not affect the timber's structural integrity or durability in any way. The greying is self-maintaining once established and requires no intervention.
If you want to maintain the initial honey-brown colour, apply a coat of UV-stable decking oil such as Koopmans Houtolie before installation and every 3–5 years thereafter. This slows the greying process significantly but does not stop it entirely. The oil does not affect durability — it is purely a colour maintenance choice. Many owners of ThermoWood decks prefer to leave it untreated and enjoy the natural silver-grey patina that develops over time.
ThermoWood Decking Installation — Key Points
ThermoWood 26mm decking boards require joists at maximum 400mm centres for residential foot traffic. Use pressure-treated C16 softwood or Siberian larch joists — never untreated softwood joists which will fail before the decking boards. Ensure adequate air gap below the deck (minimum 50mm) for ventilation and drainage.
Allow ThermoWood decking to acclimatise on site for 24–48 hours before installation — stacked with spacers to allow air circulation. This allows the boards to reach equilibrium with the ambient conditions before fixing, minimising post-installation movement.
Place the first board and fix with a Brofix start clip at each joist. Insert standard Brofix clips into the groove of each subsequent board and screw through the clip into the joist — one clip per joist crossing per board. The clip simultaneously fixes the board and sets the 5mm drainage gap. Download the Brofix installation guide for full fixing centre specifications.
Use A4 stainless steel decking screws throughout — never galvanised which will rust within 2–3 years. Pre-drill 5mm pilot holes within 50mm of board ends to prevent splitting. Two screws per joist per board. Countersink slightly so screw heads are flush with the board surface.
Seal all cut ends on site immediately after sawing with an end grain sealer or exterior wood oil. ThermoWood's modification process does not fully penetrate to cut ends — unprotected cut end grain absorbs moisture faster than the face and can cause localised cracking if left unsealed.
Install ThermoWood decking boards with the heart side (the side with the tighter, more curved growth rings) facing down. This is standard practice for all decking — boards cup away from the heart side as they dry, meaning heart-side-down boards cup upward at the edges (a bathtub shape) which drains water. Heart-side-up boards cup downward (a saucer shape) which retains water.
1 — Galvanised fixings: ThermoWood's acidity is slightly higher than standard softwood due to the thermal modification process. Galvanised fixings corrode faster in contact with ThermoWood than with standard timber. Always use A4 stainless steel — for both Brofix clips and any face fixings. 2 — Direct ground contact: ThermoWood is rated Class 2 for above-ground use. For in-ground applications (posts, ground-contact joists) use a minimum Class 1 species or pressure-treated timber. The decking boards themselves should never be in direct soil contact.
How long does ThermoWood decking last in the UK?
ThermoWood Thermo-D decking carries a BRE endorsed expected service life of 30 years for above-ground applications. Realistic service life in UK above-ground deck construction is 25–40 years with zero maintenance. No preservative treatment, no oiling, no staining is ever required.
Does ThermoWood decking need treating or oiling?
No — ThermoWood decking requires zero maintenance throughout its service life. Oiling every 3–5 years is optional if you want to slow the natural greying process and maintain the honey-brown colour — but it has no effect on durability. The deck will last just as long untreated as oiled.
What is the difference between ThermoWood decking profiles?
Standard smooth 26x117mm is face-fixed with stainless screws. Secret fix grooved 26x117mm and secret fix smooth 26x140mm use the Brofix hidden fixing system for a fixing-free face, with smooth surfaces. Secret fix brushed 26x140mm also uses Brofix hidden fixings but has a wire-brushed surface for enhanced texture and grip. All four are the same thermally modified pine — the difference is board width, surface finish, and fixing method.
What is a secret fix decking system?
A secret fix or hidden fix decking system uses concealed stainless steel clips that engage with a groove on the side of each board, fixing it to the joist below without any visible screws on the deck face. The Brofix system maintains a consistent 5mm drainage gap between boards automatically. The result is a completely clean deck surface with no fixing marks.
How much does ThermoWood decking cost in the UK?
ThermoWood decking costs £55–£80 per m² for boards depending on profile. Add £8–£12 per m² for Brofix fixings or stainless screws and hardware. Professional installation adds £25–£45 per m². For a typical 15m² garden deck, total material cost is approximately £950–£1,400.
ThermoWood Decking — All Profiles from UK Stock
We supply ThermoWood decking in standard smooth, secret fix grooved, secret fix smooth, and secret fix brushed profiles from UK stock. Brofix hidden fixing system available alongside all decking boards. FSC and PEFC certified. Nationwide delivery in 7–14 days. Contact our team with your deck dimensions for a precise quantity and quote.