Why ThermoWood® Decking Thickness Matters- And Why 20mm Could Be Putting Lives At Risk

Thermally modified timber has transformed the UK decking market, and

ThermoWood ® knotty Pine softwood and ThermoWood® hardwood products offer genuine advantages, improved dimensional stability, enhanced resistance to moisture, and decay and have a beautiful, rich colouring. It is little wonder they have become popular choices for homeowners, contractors, and developers alike. 

But a worrying trend has emerged in recent years. A growing number of 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 ThermoWood ® or 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. 

The Science Behind Thermal Modification 

Thermal modification is a process that heats timber to temperatures typically between 180°C and 230°C in a low-oxygen environment. This permanently alters the structure of the wood, reducing its ability to absorb moisture and increasing its natural durability. 

However, the very process that confers these benefits also reduces the timber's structural strength. Thermal modification breaks down hemicellulose in the cell walls, which is directly linked to bending strength, stiffness, and resistance to impact loads. Published research and industry data consistently show that thermally modified wood 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. The board carries no meaningful structural load. But a decking board has a horizontal, load-bearing surface. Every footstep, every garden chair, every hosted BBQ area place 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 requires appropriate material choices for external applications. 

• The 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 timber decking specifies boards of 25mm or 26mm thickness as the minimum for spanning joist centres of up to 400mm. 

📐  JOIST SPACING

Building regulations and recognized industry guidance recommend joist centres  of no more than 400mm for decking , not the 500mm centres that some suppliers suggest. At 500mm centres with a 20mm ThermoWood ® board, the risk of deflection, cracking, and catastrophic failure underload is significantly elevated.

The 500mm joist spacing figure is not derived from any recognized standard for thermally modified decking, it appears to have migrated from guidance intended for heavier, denser hardwood or full-thickness softwood boards. Applying it to a 20mm thermally modified 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. If a 20mm ThermoWood ® board is installed over 500mm joist centres and 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 cascade. 

Physics is straightforward. Board deflection underload increases with the span. Increasing the joist's spacing from 400mm to 500mm plus does not increase deflection by 25%; it increases it by more than 50%. Combined with the reduced bending strength of thermally modified timber, the safety margin is not just reduced; it is effectively eliminated. 

⚠ SAFETY NOTE

An elevated deck built with 20mm thermally modified boards 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 ® Decking Correctly 

If you are specifying or purchasing thermally modified decking, ThermoWood ®, thermally modified pine decking product, here is what you should insist upon: 

• Board thickness: 25mm minimum, 26mm preferred, for all spans up to 400mm. When using knotty Pine softwood. If using ThermoWood® clear grade Hardwood Ash, Iroko or Movingui then Board thickness: 21mm minimum, 25mm if joist spans are wider.

• Joist centres: 400mm maximum for domestic applications, not 500mm, and 300mm for high footfall commercial applications. 

• Product origin and process: Look for ITWA-accredited ThermoWood® certification, or equivalent third-party verified thermal modification process documentation. 

• Species and modification of 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, 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 board is cheaper to produce than a 26mm board. That cost 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 is a liability. 

When a deck fails, the consequences fall on the homeowner, the installer, and potentially the specifier or supplier. Building regulations for non-compliance is not indemnified by a low invoice

💡  OUR COMMITMENT

At QTD Group, our ThermoWood ® softwood decking boards are supplied at 25/26mm thickness as standard, and 21/25 for hardwood and 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 landscape contractor, or a building merchant, these are the questions you should be putting to any supplier of thermally modified decking: 

• Is this board dimensioned for decking or cladding? 

• What thickness are these boards, and what joist spacing are they rated for? 

• What is the thermal modification process, and what certification does the product carry? 

• Can you provide documentation confirming compliance with the relevant UK structural and building standards for decking applications? 

• What is your liability position if boards fail in a properly constructed elevated deck? 

If your supplier cannot answer these questions clearly and in writing, that is the answer you need. 

Summary 

Thermally modified timber 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. 

The correct specification for ThermoWood ® and thermally modified softwood pine decking is straightforward: 25/26mm thick boards, 400mm joist centres maximum. Hardwoods 21/25mm thick boards. Anything less represents a compromise that the load bearing, potentially elevated nature of decking cannot accommodate. 

Safety on elevated 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.