Should You Specify GL20 or GL24? Understanding Glulam Grades

Part of the reason why we’re talking about timber grading is that Britain’s got used to using C24 graded 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 it will get specified without even considering whether it needs to be C24.

By doing this, we kind of bypass the British market almost unintentionally.

While you can go to C24 in Britain, and there’s been of research projects to show that you 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.

A curved glulam beam being made in the Buckland Timber factory

Why grade matters (and when it doesn’t)

When specifying glulam, there are two main considerations:

1. Structural strength

2. 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 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 quite a small increase in depth can result in a large increase in stiffness.

This means for most beams which 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.

Timber stacked ready for the glulam process

What’s available – species and grades

Here’s what to expect from the most commonly used timber species:

Spruce

Spruce is the most commonly used species 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.

Glued timber ready to go into the clamps

The surprising 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 or even 440mm 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.

If you’d like some advice on the feasibility of a project during its early stages, or you have a question around grade requirements for your project, do get in touch and one of the team will be happy to help.