Why ThermoWood® Softwood Decking Thickness Matters, and why 20mm could be putting lives at risk.
- Eliana Hodge
- 11 minutes ago
- 6 min read
Thermally modified timber has transformed the UK decking market, and ThermoWood® knotty pine softwood offers genuine advantages: improved dimensional stability, enhanced resistance to moisture and decay, along with its beautiful rich honey brown colouring. It is little wonder it has become a popular choice for homeowners, contractors, and developers alike.
However, a worrying trend has emerged in recent years. A growing number of softwood decking boards are being sold at 20mm thickness, the same profile commonly used for vertical wall cladding. When this dimension is applied to a horizontal decking application using thermally modified pine, it creates a product that falls short of what the application demands, both structurally and in terms of building regulation compliance.
This article explains why, and what you should be asking your supplier before any boards leave the yard.
What is Thermal Modification?
Thermal modification is a process that heats timber to temperatures typically around 212°C in a low-oxygen heat and steam environment. This permanently alters the structure of the wood, removing the natural resins, sugars and impurities. This improves durability against insects and decay and reduces its ability to absorb moisture and therefore gives it greater stability.
However, the very process that provides these benefits also reduces the timber's structural strength. The timber becomes stiffer and less flexible as the thermal modification breaks down hemicellulose in the cell walls, which reduces the bending strength and impact loads. This process is fine for external applications, provided the proper construction and installation process is adhered to. Published research and industry data consistently show that thermally modified softwood can lose between 20% and 40% of its modulus of rupture (MOR) compared with unmodified timber of the same species.
⚠ KEY POINT. | A thermally modified board is not structurally equivalent to an untreated board of the same thickness. Specifying it at the same dimensions as standard timber ignores this fundamental material difference. |
For wall cladding, a vertical, non-load-bearing application, 20mm is perfectly appropriate as the boards carry no meaningful structural load. However, some suppliers think 18mm is good enough for cladding, which it’s not, for the reasons we highlighted in another article.
Decking boards have a horizontal, load-bearing surface. Every footstep, every garden chair, and every BBQ area has a dynamic load onto that board. The structural difference matters enormously.
What Industry Standards Actually Require
There is no single piece of UK legislation that specifies decking board thickness in isolation, but that does not mean "anything goes." The relevant framework is clear when read in full:
BS 8417 (Preservation of Wood) and BS EN 335 hazard class guidance require appropriate material choices for external applications.
Building Regulations Approved Document A (Structure) requires that any element of a building, including elevated decking, must be designed to safely carry the loads placed upon it.
NHBC Standards and warranty guidance for residential decking typically require boards to be of sufficient section to span between joists without unacceptable deflection or risk of failure.
General industry best practice for softwood and modified softwood timber decking specifies boards of 25mm, 26mm and 32mm thickness as the minimum for spanning joist centres of up to 400mm.
📐 JOIST SPACING centres | Building regulations and recognised industry guidance recommend joist centers of no more than 400mm for decking, not the 500/600mm joist centres that some suppliers suggest. At 500mm centres with a 20mm ThercentresmoWood® softwood board, the risk of deflection, cracking, and catastrophic failure under load is significantly elevated. The weak points in softwood are where the knots converge across the board width. |
The 500mm joist spacing figure is not derived from any recognised standard for thermally modified decking. It seems to have migrated from guidance intended for full-thickness, untreated softwood boards of around 32/34mm thick natural unmodified material. Applying it to a 20mm thermally modified softwood pine board is a serious error of specification.
The Real-World Risk: Especially on Elevated Decks
Ground-level decking that fails is unpleasant and expensive. An elevated deck that fails can be life-changing or fatal.
Consider a typical raised garden deck: 1.5 to 2 meters above ground level, supporting several adults, furniture, and a barbecue area. Some may even have a paddling pool or hot tub, which would also require a heavy-duty structure to support these. A 20mm ThermoWood® softwood board is installed over 500mm or 600mm joist centres, and if a board cracks or snaps under load, the consequences are not limited to a broken plank. A person can fall through, or the sudden failure of one board can cause adjacent boards to give way.
Board deflection under load increases with the span. Increasing joist spacing from 400mm to 500mm plus does not increase deflection by just 25%; it increases it by more than 50%. Combined with the reduced bending strength of thermally modified timber and knot content, the safety margin is not just reduced; it is effectively eliminated.
⚠ SAFETY NOTE | An elevated deck or ground deck built with 20mm thermally modified softwood boards with over 500mm joist centres may look identical to a correctly specified deck — right up until the point that it fails. There are no visible warning signs until the break occurs. |
How to Specify ThermoWood® Softwood Decking Correctly
If you are specifying or purchasing thermally modified softwood knotty pine decking, here is what you should insist upon:
Board thickness: 25mm minimum, 26mm preferred, for all spans up to 400mm.
Joist centres: 400mm maximum for domestic applications, and 300mm for high-footfall commercial applications. Not 500mm or more,
Product origin and process: Look for ThermoWood® certification.
Species and modification temperature: Not all "thermally modified" products are equal. Higher treatment temperatures typically mean better durability but greater strength reduction. Know what you are buying.
Supplier accountability: Ask your supplier to confirm in writing that the boards they are selling are dimensioned and specified for decking applications, not cladding applications.
That last point is important. Some boards marketed as "decking" are, in profile and thickness, indistinguishable from cladding boards. The end use of the product and the load it will be asked to carry must drive the specification, not the price point.
A Note on Price and Value
We understand the commercial pressures that exist in the construction and landscaping sectors. Customers are price-conscious, and competition between suppliers is fierce. But there is a meaningful difference between competitive pricing on a correctly specified product and the sale of an undersized product at a lower price because it uses less material.
A 20mm finish thickness board is machined out of a 25mm thick board and is therefore cheaper to produce than a 26mm board that is machine finished out of a 32mm thick board. That saving is real. But it is not a saving that should be passed on to customers in a decking application, because the product is not fit for that purpose. It is not a value proposition; it becomes a liability.
💡 OUR COMMITMENT . | At QTD, our ThermoWood® softwood decking boards are supplied at 26mm thickness as standard, correctly dimensioned for decking applications and specified for joist centres of no more than 400mm. We do not sell cladding-profile boards for decking. |
Questions to Ask Your Supplier:
Whether you are a homeowner, a contractor, or a building merchant, these are the questions you should be putting to any supplier of thermally modified softwood pine decking:
1. Is this board dimensioned for decking or cladding?
2. What thickness are these boards, and what joist spacing are they rated for?
3. What is the thermal modification process, and what certification does the product carry?
If your supplier cannot answer these questions clearly and in writing, that is the answer you need.
Summary
Thermally modified softwood pine is an excellent material when correctly specified. The challenges arise not from the product itself but from the misapplication of cladding-profile dimensions to a structural decking context, and from joist spacing guidance that does not reflect the reduced strength characteristics of thermally treated wood.
Safety on elevated and ground structures is not a selling point; it is a baseline requirement. Specifying correctly is not a premium; it is the minimum standard every customer deserves.
Still Unsure?
Speak to our expert team at QTD and book a showroom visit on the website for tailored advice based on your project.
For more information, visit qtdgroup.com or contact us directly.
Comments