GL24 has become the go-to glulam grade for many projects. But as UK timber supply and manufacturing capability develop, the case for questioning that default is becoming harder to ignore.
For many designers, engineers and buyers, glulam specification still starts from a familiar default. GL24 is often selected almost automatically – a hangover from decades of reliance on imported C24 timber and established European supply chains.
But as UK timber supply, manufacturing capability and sustainability priorities evolve, that default is being questioned. In practice, the difference between GL20 and GL24 is often far less significant than assumed, particularly once deflection, availability and sourcing strategy are properly considered.
Understanding when higher grades genuinely add value can unlock cost, carbon and procurement benefits without compromising performance.
Strength, stiffness – and what really governs beam size
Glulam grades are defined by characteristic bending strength, expressed in N/mm2. GL20, GL24, GL28 and GL32 simply indicate increasing strength. On paper, that can imply that higher grades automatically lead to more efficient structures.
In reality, beam sizing is frequently governed not by strength but by deflection – the requirement to limit visible movement under load.
Stiffness increases with the cube of beam depth, while strength increases with the square. The result is that relatively small increases in depth can deliver large gains in stiffness. Where a design is deflection-limited, upgrading the grade may have little or no impact on overall beam dimensions.
In many scenarios, a GL20 beam with a modest increase in depth can perform equivalently to a slimmer GL24 section. In some cases, the same section size can be retained.
What the grades actually mean
Beyond the headline numbers, glulam grades also include construction type:
- h (homogeneous) – the same grade of lamellae are used throughout the section
- c (combined) – lamellae are placed in the outer zones, with lower grades in the core
For higher grades such as GL28 and above, combined construction is generally preferred. It reflects how stresses are distributed within the beam, avoids unnecessary use of high-grade material, and is typically more cost-effective. For GL20 and GL24, homogeneous construction is more common and readily available, particularly for UK-grown material.
Availability matters more than many specifications acknowledge
A key issue raised by manufacturers is that specification often ignores what different species and supply chains can realistically deliver.
British-grown softwood typically aligns with C16 which, when laminated, produces GL20 glulam. Achieving C24 from UK timber is possible, but not straightforward at commercial scale, and availability remains limited.
Imported spruce glulam is widely available at GL24 and above. That historic pattern has shaped specification habits, sometimes without engineers stopping to ask whether the higher grade is actually required. Species availability also plays a role:
- Spruce offers the widest range of glulam grades, particularly from continental Europe
- Larch and Douglas fir, especially UK-grown, are generally limited to GL20, with GL24 more achievable for imported material
- Oak glulam is currently assigned conservatively to GL24 due to Eurocode limitations
- Pine glulam is most commonly supplied as GL24
Specifying beyond what is realistically stocked can lead to longer lead times, higher costs or late-stage design changes.
The size difference is often smaller than expected
When grades are matched for stiffness rather than strength, section sizes often change less than many specifiers expect.
In practical terms, a GL24 beam at 100 x 440mm may be matched by a GL20 beam at 120 x 440mm – or, depending on the case, the same depth and width. Across a structure, those marginal differences rarely alter architectural intent, but they can significantly affect sourcing flexibility.
Solid timber grades vs glulam grades
The relationship between solid timber grades and glulam performance is not linear. Laminating timber improves structural consistency by distributing loads across multiple lamellae and reducing the influence of weaker spots in individual boards.
That is why C16 timber can reliably produce GL20 glulam. At higher grades, however, glue lines and finger joints become increasingly critical, and strength gains depend as much on manufacturing quality as on timber strength alone.
One grade does not have to fit an entire project
There is no requirement to specify a single glulam grade across all elements of a structure. Strategic sourcing – using different grades and species where appropriate – often delivers better outcomes.
Internal beams, curved elements or visually exposed features may suit locally manufactured GL20, while longer spans or highly loaded elements may justify imported higher grades. The visual finish is controlled by visual grading, not strength grade, meaning exposed GL20 glulam can be finished to the same aesthetic standard as GL24.
Transport emissions also matter. Manufacturing large or curved elements locally can significantly reduce carbon associated with haulage, particularly when compared to importing oversized sections.
Engage early, question defaults
The common thread running through successful projects is early engagement. Bringing manufacturers and suppliers into the conversation while designs are still flexible allows grades, species and sourcing strategies to be tested properly.
Rather than defaulting to GL24, specifiers who ask whether a beam is strength-limited or deflection-limited often discover more options than expected.
A supplier-informed perspective
This article is informed by manufacturer experience, including analysis shared by Buckland Timber, which produces glulam from both UK-grown and imported material.
The takeaway is not that GL20 should replace GL24, but that better questions tend to produce better outcomes. Grade is only one variable among many, and when considered alongside deflection, availability and sourcing strategy, it can support performance, sustainability and a more resilient domestic timber supply chain.
More at www.bucklandtimber.co.uk
