Robin Nicholson from Buckland Timber, a Grown in Britain certificate holder, shares his thoughts on homegrown C16 and how there’s more scope for using UK-grown timber than you might think.
UK Timber-Despite the availability of quality, sustainable, homegrown alternatives, approximately 80% of the timber used in the UK is still imported. Across the supply chain – from timber merchants and sawmillers to manufacturers and construction firms – there’s growing recognition that local timber sources can deliver tangible benefits. These include reduced carbon footprints, lower costs, more secure supply chains, and meeting the increasing customer demand for sustainable products.
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That’s a remarkable amount of work hours for a single machine, the Norcar 600 owned by Erkki Rinne is taken well care of, it even has the original Diesel engine.
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Kieran Anders is a forestry contractor working in the lake district. His work involves hand cutting and extracting timber using a skidder and tractor-trailer forwarder.
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Arwel takes great pride in the fact that the mill has no waste whatsoever, “the peelings are used for children’s playgrounds, gardens and for farm animals in barns in the winter and the sawdust has multiple uses in gardens and farms as well.
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Timber hauliers need to encourage young blood in, and also look after the hauliers we have, we need make the sector a safe and positive place to work.
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GL20 or GL24?
Architects and engineers regularly ask us whether they can get a project in a certain grade – GL28, for example, or whether locally-grown timber will work for their design. The short answer might be no, but there’s usually a much longer conversation worth having about why, and what the alternatives might achieve.
Understanding timber grades isn’t just about knowing the numbers. It’s about getting the best outcome for your project – balancing structural performance, cost, sustainability, and aesthetics. Often, there’s more flexibility than you might realise.
Part of the reason we’re talking about timber grading is that Britain has become used to using C24-grade timber. This is because we’ve historically imported lots of our timber from Scandinavian countries and that is the standard grade produced there.
However, the standard grade produced in Britain is more like C16. What this means is, quite often, specifiers are so used to specifying in C24 that they will specify it without even considering whether the project needs C24 timber.
By doing this, we are almost unintentionally bypassing the British market, something that the recent ‘Trust UK C16’ campaign is trying to rectify.
While you can go to C24 in Britain, and research projects have shown that we can produce this grade in the UK, it is not easy on a commercial scale. Rather than only focusing on achieving C24, we think it’s more helpful to know that C16 – the standard most British sawmills can produce easily – is actually a great choice for many projects, especially as it becomes GL20 when we make it into glulam.
Why and when grade matters
When specifying glulam, there are two main considerations:
- Structural strength
- Deflection.
Structural strength is straightforward – the timber needs to be strong enough to carry the loads safely. But most of the time, beam sizing is actually limited by deflection rather than strength. The beam needs to be stiff enough that it doesn’t sag visibly or excessively under load.
This distinction matters because the increase in the stiffness of a beam depends on, or is proportional to, the cube of its depth, whereas the increase in strength of beam is proportional to the square of its depth. The end result is that quite a small increase in depth can result in a large increase in stiffness.
This means, for most beams that are deflection limited, the increase in size needed to meet design requirements is less than you would think. Understanding whether your design is strength-limited or deflection-limited opens up options you might not have considered.
Understanding glulam grades
Glulam grades follow a fairly straightforward naming system. GL20, GL24, GL28, and GL32 refer to the characteristic bending strength of the material in N/mm². Higher numbers = stronger timber.
The letters that follow the grade – h and c – describe how the beam is constructed:
- h (homogeneous) means that the same grade of timber is used throughout the depth of the beam, so all planks are high strength.
- c (combined) means high-strength planks are used only in the outer zones – the top and bottom of the beam, where stresses are highest – while lower-strength planks are used in the middle.
For grades GL28 and above, it’s best to specify c (combined) construction. This is a more efficient use of material, as manufacturers avoid wasting high-grade lamella where the stresses are low. It also reflects the way most producers prefer to manufacture higher-strength glulam sections, and it’s the best value for money for the end user.
What’s available
Here’s what to expect from the most commonly used timber species:
Spruce
Spruce is the most common species used for glulam and offers the widest range of strength grades. If glulam is made from British-grown spruce, it would typically achieve GL20h. For imported spruce, the most common grade is GL24h, although GL28h/c, GL30h/c, and GL32h/c are also available.
In the past, GL36 was more readily obtainable, but it’s now less common due to changes in grading standards when German and Austrian manufacturers transitioned from DIN standards to the Eurocodes.
Larch
UK-grown larch glulam is generally produced to GL20h, while imported material is commonly available as GL24h and sometimes GL28c. It’s best not to specify grades above GL28 for larch. In practice, GL24 is the safest and most readily available option, as larch tends to be less widely stocked than spruce.
Douglas Fir
Douglas Fir follows a similar pattern. UK-grown glulam typically achieves GL20h, and imported grades of GL24h and GL28c are possible. Again, it’s best to specify GL24 for availability, as Douglas Fir is not as commonly stocked as larch.
Oak
Oak glulam cannot currently be graded according to the Eurocode standard for glulam manufacture (BS EN 14080). As a result, a conservative approach is taken, and it is generally assigned to GL24.
At least one French manufacturer has an ETA (European Technical Approval) for oak glulam, based on extensive testing rather than direct application of the Eurocode grading rules.
Pine
Pine glulam is produced both in-house and through imports, most commonly in GL24h. While GL28 grades may be available for imported pine, the species is less frequently used in glulam manufacture, so GL24 remains the practical and reliable specification.
The truth about beam sizes
Here’s something that often surprises people – the difference in strength grade affects the beam size less than you might think. This is partly due to conservative design assumptions. The increase in depth needed to accommodate a lower grade is often very small. For example, instead of a 440mm deep beam in GL24, you might only need a 460mm beam in GL20.
In many cases, we find we can use the same beam sizes for GL20 UK-grown timber as we would for GL24 imported timber. This means specifying a lower grade, where appropriate, doesn’t necessarily result in larger or heavier structures – it can simply make better use of local material.
We’ve seen this play out in practice on projects. On the Horniman Museum project, for example, the client was keen to use locally grown timber. They had initially specified a higher grade, but through design iterations, we established the beam sizing could remain the same using UK-grown material.
What’s the difference in strength between solid timber grading and glulam grading?
The relationship between solid timber grade and resulting glulam grade isn’t always straightforward either. We use C16 British-grown timber to make GL20 beams – laminating the timber increases the overall strength because the glue lines distribute loads and reduce the impact of weaker spots in individual boards.
As you move up the grades, the situation changes. A GL24 beam typically uses C24 timber; GL28 uses C30; and at the very top, GL32 may require C40 planks. However, once you reach these higher levels, the glue line and finger joint strength start to become limiting factors. The beam’s strength is then defined less by the timber itself and more by how the laminations are bonded together.
Do you have to use one grade for your whole project?
No, you don’t have to specify one grade for everything. Strategic material sourcing – using the right material from the right place – often delivers better outcomes across the board.
Using UK-grown GL20, for example, where appropriate, brings several benefits. Cost is one consideration – locally sourced timber can be more economical, particularly in larch. Sustainability is another – reduced haulage means lower carbon emissions, while supporting British forestry strengthens the domestic timber industry.
The carbon impact of transport is more significant than many people realise. When you manufacture large, curved elements locally rather than importing them, you significantly reduce transport mileage. When you can source external timber from British-grown stock, the carbon savings add up quickly.
This approach doesn’t mean compromising on quality or aesthetics. The visual finish of glulam is determined by the visual grade specification, not the strength grade. You can achieve a beautiful exposed timber aesthetic with GL20 as with GL24.
Recent projects demonstrate this well. For the Lobslack Seed Processing Facility for Forestry England, we combined British-grown larch for the external canopy, in-house manufactured curved beams, and Austrian CLT and spruce glulam – each material sourced based on what made most sense for that particular application. The result was cost-effective, sustainable, and structurally excellent.
The key is engaging early in the design process so there’s time to explore these options properly, rather than defaulting to a single specification that might not be optimal for every element.
Top tips on grading when working with glulam
- Engage early. Bringing your timber supplier into the conversation during the design phase rather than after specifications are fixed gives you more options and can prevent costly changes later.
- Question the grade. If a higher grade is specified, it’s worth asking whether it’s actually needed for structural strength or whether the design is deflection-limited. You might find there’s flexibility you didn’t expect.
- Consider the whole picture. Grade is just one factor. Cost, availability, lead times, sustainability, and sourcing strategy all play a part in getting the best outcome.
- Strategic sourcing works. Using different materials from different sources based on what works best for each application – rather than single-source procurement – often gives better value, lower carbon impact, and wider design possibilities.
- Choosing glulam grades doesn’t have to be complicated. With the right conversation early enough in the process, you can make informed decisions that balance performance, cost, and sustainability without compromising your design vision.
Grown in Britain
Grown in Britain (GiB) has been driving demand for homegrown timber through its certification scheme for more than a decade, ensuring responsible forest management and full supply-chain traceability. UK-grown timber supports local economies, enhances nature in one of the most nature-depleted countries on earth, and offers genuine carbon and supply chain advantages.
In 2025, GiB received confirmation of Category A status under the Government’s Timber Procurement Policy. This removes longstanding barriers to certified homegrown timber in government procurement and major construction projects, directly supporting the Government’s Timber in Construction Roadmap.
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