ThermoWood Secret Fix Decking Boards Smooth 26×140mm – BROfix

These ThermoWood smooth decking boards are machined with an edge groove for secret fix installation using BROfix hidden decking fasteners. The board face is completely smooth and flat — no grooves, no texture, no visible fixings. Just a clean, unbroken deck surface with a contemporary finish.

At 26x140mm, these boards offer a wider face than our grooved 26x117mm secret fix boards, covering more area per board run and creating a bolder, more open deck aesthetic. The BROfix clip fits into the edge groove at each joist position and is fixed with a single stainless steel screw, which is then fully concealed beneath the next board as you work across the deck.

Product Specifications

Smooth Face vs Grooved Face Secret Fix Decking

Both our ThermoWood secret fix boards use BROfix hidden fasteners and produce a completely screw-free deck surface. The difference is in the board face profile:

The grooved 26x117mm board has a grooved face profile — the grooves add texture and can improve grip underfoot, making it well suited to areas where slip resistance is a consideration.

This smooth 26x140mm board has a flat, uninterrupted face. It gives a more refined, architectural look and is particularly well suited to contemporary garden spaces, raised decks, and projects where a premium finish is the priority. The wider 140mm face also means fewer board runs per square metre, reducing installation time.

Why ThermoWood for Secret Fix Decking?

Secret fix systems rely on consistent board geometry. If boards warp or cup after installation, the hidden clips are placed under lateral stress and the flat surface is lost. ThermoWood is treated above 180°C without chemicals, which permanently reduces the timber's ability to absorb and release moisture. The boards move far less across the seasons than untreated softwood — keeping the deck flat and the BROfix system performing as designed year after year.

ThermoWood achieves a natural durability of Class 1–2 without chemical preservatives, and requires no annual retreatment to perform in full UK outdoor exposure.

Pair With BROfix Hidden Decking Fasteners

These boards are designed for use with BROfix hidden decking fasteners — sold separately in packs of 300, used at 2.2 clips per linear metre. For 26x140mm boards, divide your deck area (m²) by 0.140 to get total linear metres, then multiply by 2.2 and add 10% for wastage.

Use stainless steel A2 screws — 4.2 x 42 mm, Torx 20 head — to fix BROfix clips to the joists. Do not use zinc-plated or galvanised screws with ThermoWood as they can cause surface staining.

Download the full installation guide: BROfix Hidden Decking Fasteners — Installation Guide (PDF)

Finishing and Maintenance

ThermoWood requires no treatment to maintain its structural integrity outdoors. Left untreated the boards will silver to a uniform grey over time. To retain the warm brown tone of new ThermoWood, apply a UV-stable decking oil before installation and reapply periodically. We stock Remmers and Koopmans wood oils and stains that are well suited to ThermoWood decking. Always seal cut ends with an end-grain sealer or decking oil immediately after cutting.

Frequently Asked Questions