Timber Board sizes explained.

Material Thickness, Width and Length.

When it comes to timber, size isn’t just a detail—it’s a crucial factor that affects strength, stability, and suitability for your project. Whether you're cladding a building or crafting bespoke joinery, knowing the right dimensions can make all the difference. In this guide, we break down timber board sizes and explain why ThermoWood® offers enhanced dimensional stability compared to standard pressure-treated softwood.

Cladding

Thickness.

Timber Cladding:

Also known as rainscreen cladding, timber cladding is commonly installed on residential and commercial buildings and must comply with specific building regulations. Most hardwood and softwood cladding is installed at a minimum thickness of 19–20mm and fixed to battens spaced at 400mm centres.

Why?

Because this thickness provides the ideal balance of weather resistance, durability, and structural stability, particularly on exposed elevations that are difficult to maintain or access.

While some softwood options are as thin as 12–18mm, these should only be used for non-habitable structures such as sheds, garages, small workshops, or the sheltered sides of garden studios. They are not suitable for buildings intended for residential use.

Widths.

Timber Cladding:

The appearance you’re aiming for will guide your cladding choices, but timber cladding offers far more flexibility than composite alternatives, which are limited by moulds and extrusion machinery.

With timber, you can mix and match different board widths within the same profile to create visual interest or striking feature areas. Machine-finished timber cladding is typically available in widths from 40mm up to 186mm when working with 20mm thick boards. However, the maximum usable width depends on the species and how the timber has been processed. For example, standard pressure-treated softwood should not exceed 140mm in width.

As a general rule, the wider the board, the thicker it needs to be, assuming the species is suitable, because Wider boards are more prone to movement.

If not properly seasoned, they can cup across the grain, putting pressure on fixings, which may eventually pull out or snap. This is especially problematic on occupied buildings where durability and long-term performance are essential.

Scottish-grown timbers like Larch, Douglas Fir, or Western Red Cedar—especially those harvested in the east of Scotland—benefit from favourable growing conditions. Once kiln-dried, they’re well adapted to the UK climate and ideal for exterior use.

ThermoWood® goes a step further. Thanks to its enhanced durability and outstanding dimensional stability, it allows for increased design freedom, including the use of wider boards, without the common risks of warping or distortion, making it perfect for both cladding and decking applications.

Lengths.

Timber Cladding:

Choosing the correct lengths for timber cladding can be a challenge. External cladding must often be cut to suit different building elevations, including obstacles like windows and doors. While boards can typically be supplied in lengths ranging from 1.2m to 5.4m, availability depends heavily on the timber species and whether it has been ThermoWood® modified or naturally harvested.

In the UK, there’s a long-standing habit of ordering specific lengths for each elevation, dating back to when only natural softwoods like Pine, Spruce, Larch, Douglas Fir, and Cedar were commonly used. These are often harvested at standard lengths of around 4.8m, making them easy to cut down for use across various parts of a project.

However, the growing use of environmentally friendly ThermoWood® technology has opened up new possibilities. This process enhances the durability of both softwood and hardwood species, including those not traditionally suitable for exterior use. But it also introduces new considerations—particularly when it comes to length.

Due to natural defects or end splits that can occur during processing, not all species can reliably provide long, defect-free lengths. For example:

• Some hardwoods like Ash and Tulipwood (Poplar) are typically available up to 3.6m.

• Others, such as Ayous and Fraké, may be available in slightly longer lengths.

• Clear-grade and knotty Pine can still be sourced in lengths up to 4.8m or even 5.4m.

👉 Why does this matter?

When customers request fixed lengths, they often expect every board to arrive in perfect condition. But if, after ThermoWood® processing or profiling, boards develop natural splits or defects, these areas must be cut out, leaving boards shorter than the ordered length. This causes delays and added waste.

That’s why we always recommend ordering random-length boards. Not only are they more cost-effective, but they also reduce waste onsite and significantly speed up procurement—keeping your project on schedule and budget.

Composite/UPVC  Cladding:

In many cases, board lengths are restricted by the machinery used during the manufacturing process. For certain products, particularly those made from engineered or composite materials, standard lengths of around 3.6 metres are the norm, with little to no flexibility in size. Unlike natural timber, these materials can’t be easily customised beyond what the moulds and extrusion machinery allow.

Decking

Thickness.

Timber Decking: 

When building with softwood decking, board thickness is critical for both safety and performance. For standard joist spacings of 400mm, the recommended minimum board thickness is 25mm. However, if your joists are spaced further apart—say 600mm—then the deckboards must be at least 32–34mm thick.

Why does thickness matter?

If the boards flex between joists, especially when made from knotty timber, the area around the knots becomes a natural weak point. Over time, exposure to weather or heavy loads could cause boards to crack or fail, posing a serious safety risk, particularly on elevated decks.

👉 The rule is simple: The wider the joist spacing, the thicker your deckboards need to be.

Conversely, if you reduce the joist spacing to 300mm, you can safely use slightly thinner boards, potentially saving on material costs. The small additional expense of extra joists is often outweighed by the reduced cost of thinner decking, provided you’re using a high-quality, well-graded timber free from large or excessive knots.

Composite Decking:

Composite decking boards typically range between 25mm and 34mm in thickness. This is because, unlike solid timber, composite materials have less inherent structural strength due to their engineered composition. As a result, they require increased thickness to ensure adequate support, reduce flexing, and maintain long-term durability under load.

Widths.

Timber Decking:

Pressure-treated softwood decking is typically produced at a maximum width of 140mm. However, ThermoWood® softwood and hardwood options can be supplied in wider dimensions thanks to their enhanced stability and resistance to movement.

It’s important to remember that decking is exposed to much harsher conditions than vertical cladding. Being laid horizontally endures more intense weathering, including prolonged UV exposure and standing water, which can accelerate wear over time.

To add visual interest, decking boards don’t all need to be the same width. Mixing different widths can create striking patterns and a more bespoke, high-end finish. So don’t be afraid to get creative with your design.

Composite Decking:

This type of product is typically only available in a single width, as its dimensions are limited by the moulds or extrusion machinery used in the manufacturing process.

Lengths.

Timber Decking:

Timber decking is commonly available in standard lengths of 2.4m, 3.6m, and 4.8m. Other lengths may be possible upon request, but this depends on the availability of raw material that is free from defects such as end splits or knots.

It’s also important to consider the practicality of longer boards; not only are they harder to handle on site, but they can also incur higher haulage costs. Choosing lengths that balance usability, material quality, and transport efficiency is key to a successful decking project.

Composite Decking:

These boards are typically manufactured in fixed lengths, most commonly 3.6m, and occasionally 4.8m, depending on whether they are moulded or extruded. Custom lengths are generally not available, as the production process does not allow for variation.