Oak is one of the most widely used hardwoods in British building, joinery and furniture making. It is strong, durable, attractive and full of natural character. But not all oak is graded in the same way, and not every grade is suitable for every job.
When choosing oak, it helps to understand two different types of grading:
– Appearance grading, which describes how the oak looks
– Structural grading, which confirms whether the oak is suitable for load-bearing use
This guide explains the main oak grades, how European oak grading works, what Prime, FAS, Joinery and Character oak mean, and why moisture content, kiln drying and D-grade strength classes matter.
What Does Oak Grading Mean?
Oak grading is the process of assessing each board, beam or pack of oak according to its quality, appearance, strength and suitability for use.
For appearance-led projects, such as furniture, flooring, staircases, panelling or interior joinery, grading usually focuses on visual features such as knots, colour variation, sapwood, grain, splits and other natural characteristics.
For structural projects, such as oak beams, frames, posts and load-bearing components, grading must focus on strength. This is where structural grading and hardwood strength classes become important.
In simple terms, appearance grades help you choose oak that looks right. Structural grades help ensure the oak can safely do the job it is being asked to do.
How Are European Oak Visual Grades Written?
European oak visual grades often use a three-part classification. For example, a grade might appear as Q-BA, Q-B1, Q-FA or Q-P1.
Each part of the code tells you something about the oak.
What Does the “Q” Mean in Oak Grading?
The first letter is usually Q, which stands for Quercus, the Latin name for oak.
European oak commonly refers to species such as Quercus robur and Quercus petraea.
What Do the Middle Letters Mean?
The middle letter describes the product type or how the oak has been sawn and supplied.
Common examples include:
– B for boules
– S for selected boards
– F for square-edged boards or strips
– P for beams
What Does the Final Letter or Number Mean?
The final letter or number indicates the quality grade.
– A is the highest visual quality
– 1 is good quality
– 2 has more natural character
– 3 has more knots, sapwood and variation
– 4 is the lowest quality and is generally avoided for higher-quality joinery or visible work
As a broad guide, oak visual grades may be described commercially as Prime, FAS, Joinery or Character, although terminology can vary between suppliers.
How Is Oak Structurally Graded?
Strength grading can be carried out by visual inspection according to strictly defined rules, or by a grading machine.
Once the strength grade is known, together with the species, a strength class can be assigned, which leads to the characteristic values needed for structural design.
Visual strength-grading uses the grader’s experience to assess each piece of timber according to rules that define the size, type and number of strength-reducing characteristics allowed in each grade.
Natural strength-reducing features include knots, wane (the uneven edge caused by a residue of bark) and the slope of the grain. Other strength reducing features, such as splits or shakes (the splitting of the wood fibres along the grain) may have developed as a result of drying.
The strength of a piece of timber depends on a combination of the strength of the timber species and the grade of the individual piece. For example, a high grade of timber from a weaker species may be just as strong as a low grade timber from a strong species.
To make specifying easier, species and grades are grouped into classes or similar strength for softwoods and hardwoods. In both cases, higher numbers indicate stronger timber. Hardwoods range from D18 to D70.
The table below gives the grades practically available in European Oak.
| BS 5756 2007 strength class | Sections (mm²) | Corresponding strength class |
| THA | 20,000+ | D40 |
| THB | 20,000+ | D30 |
| TH1 | >20,000 | D30 |
| TH2 | >20,000 | D24 |
How Does Moisture Content Affect Oak Grading?
Moisture content plays an important role in oak grading because oak changes as it dries.
Fresh sawn, or green, oak contains a high level of moisture. As it dries, it can shrink, move, split, check or twist. Some of these changes are normal and expected, especially in large-section oak beams.
Moisture content matters because it affects:
– Stability
– Shrinkage
– Movement
– Machining quality
– Structural performance
– Suitability for internal or external use
For structural oak, moisture content can also affect grading, marking and specification. Structural hardwoods may be identified as dry, kiln dried or green depending on their condition and intended use.
Which Oak Grade Should You Choose?
Which Oak Grade Should You Choose for Oak Framing and Structural Oak?
For oak framing and structural oak, the most important consideration is not whether the timber is Prime, FAS, Joinery or Character grade. Those are appearance-led grades. For beams, posts, trusses, rafters, braces and other load-bearing components, the oak must be suitable for structural use.
Structural oak should be strength graded and specified according to the requirements of the project. This may include a hardwood strength class, such as a D-grade, where required by the engineer or design specification. The correct grade will depend on the span, section size, load, jointing method, moisture content and whether the oak is being used internally, externally or as part of a traditional green oak frame.
Green oak is commonly used for traditional oak frames because it is easier to work when freshly sawn and will naturally dry, shrink and develop surface checking over time. Air-dried or kiln-dried oak may be more suitable where greater dimensional stability is needed, particularly for internal structural features, exposed beams in finished spaces or projects where movement needs to be reduced.
As a general guide:
– Use structurally graded oak for beams, posts, trusses, rafters, braces and other load-bearing elements.
– Use green oak for traditional oak framing where natural movement, shrinkage and checking are expected.
– Use air-dried or kiln-dried oak where stability, machining accuracy or interior finish is more important.
If the oak is carrying load, the grade should always be confirmed against the structural design. The safest approach is to work from the engineer’s specification first, then choose the visual character and finish of the oak around those structural requirements.
Oak grading matters because it helps ensure the right timber is used for the right purpose.
For oak framing, appearance grade alone is not enough. A beam may look clean and substantial, but if it is carrying load, it also needs to be suitable for structural use. This is why it is important to understand both the visual grade of the oak and its structural strength class.
At Vestra Oak, all oak frames are supplied using QP1 / A-grade oak or structurally graded oak to D30 / D40 strength classes, depending on the project specification. This means the oak is selected not only for its appearance, but also for its suitability in structural framing applications.
For beams, posts, trusses, rafters, braces and other load-bearing elements, the final specification should always align with the structural design. However, by using high-quality structurally graded oak where required, Vestra Oak frames are designed and supplied with structural performance in mind from the outset.
If you are planning an oak frame building and are unsure which grade is right for your project, our team can advise on the most suitable oak specification for your design, finish and structural requirements.