Wind-dried oak structural timber naturally falls into fire class D according to European standard EN 13501-1. This is the standard fire class for solid wood above a certain thickness, and for most structural applications in commercial and residential construction, this is sufficient. The wind-drying process itself does not change the fire class, but the low moisture content and high density of well-dried oak do influence how the wood behaves in the event of a fire. The questions that architects and contractors ask in practice about fire class, fire resistance, and documentation are each answered below.
What fire class does solid oak naturally have?
Solid oak naturally falls into fire class D according to the European classification EN 13501-1. This is the standard class for untreated solid wood that does not contribute to rapid fire spread. Oak performs relatively well compared to other wood species, due to its high density and tough cell structure.
European fire classes range from A1 (non-combustible, like concrete) to F (no performance determined). Wood typically falls into class C, D, or E, depending on species, thickness, and treatment. Structurally, oak sits at the better end of this spectrum. This does not mean oak does not burn, but rather that it ignites relatively slowly, forms a stable charring layer, and as a result retains its load-bearing function longer than many other materials.
For structural use, fire class D is sufficient in most situations. Where higher requirements apply — such as in certain public buildings or specific fire compartments — class C or B may be required. This calls for additional measures, which are discussed further below.
How does the wind-drying process affect the fire properties of oak?
The wind-drying process does not change the formal fire class of oak, but it does influence fire behavior in practice. Wind-dried oak has a lower moisture content than green or kiln-dried timber, which means less energy is needed to ignite the wood. At the same time, the slow, natural drying process results in higher density and reduced internal stress.
Wood with a high moisture content delays ignition because evaporating moisture absorbs energy. Wind-dried wood does not have this effect. However, the high density of well-dried oak largely compensates for this: dense wood chars on the outside and protects the core for longer. This charring layer acts as an insulating shell that slows further combustion.
For calculating fire resistance — specifically the so-called charring rate — Eurocode 5 uses a value of 0.7 mm per minute for solid hardwood. This value applies regardless of moisture content within the normal range for structural timber. Wind-dried oak does not deviate significantly from this in practice. The wind-dried oak beams we supply have a moisture content that typically falls well within structural standards.
What is the difference between fire class and fire resistance in structural timber?
Fire class and fire resistance are two distinct terms that are often confused. Fire class describes how a material contributes to a fire: how quickly it ignites, how much heat it releases, and whether it produces flaming droplets. Fire resistance describes how long a structural element retains its load-bearing or separating function in the event of a fire.
In practice, this means the following:
- Fire class is a material property, expressed as a letter (A1 to F).
- Fire resistance is a structural property, expressed in minutes (such as R30, R60, or R90).
- A structural element with fire class D can still achieve a fire resistance of R60, provided the cross-section is large enough.
- Fire resistance is calculated based on the charring rate and the remaining load-bearing core.
For an architect, this distinction is critical when preparing a specification. Building regulations impose requirements on the fire resistance of structures, not on the fire class of the wood itself — unless it concerns surface coatings or cladding. For solid timber structures, fire resistance is calculated or tested, and large cross-sections of antique oak often work in their favor. This applies equally to old oak boards used visibly as flooring or wall cladding, where the position within the structure determines the applicable requirements.
Does reclaimed oak structural timber meet the fire class requirements of building regulations?
Reclaimed oak structural timber meets the fire class requirements set by building regulations for structural timber in most cases. Building regulations reference European standard EN 13501-1 for fire performance, and untreated solid oak achieves fire class D under that standard, which is sufficient for most occupancy types.
Practical application depends on the occupancy type of the building and the position of the timber within the structure:
- In residential construction, fire class requirements for structural timber are generally low.
- In commercial buildings, schools, or care facilities, higher requirements may apply to surface materials.
- If the timber remains visible as wall cladding or ceiling cladding, different requirements apply than for concealed structural elements.
- In fire compartmentation, the requirements for the separating structure are decisive, not the fire class of the wood itself.
An important consideration with reclaimed timber is documentation. New timber is supplied with CE marking and a declaration of performance. With reclaimed timber, this supply chain is typically absent. This does not mean the timber fails to meet requirements, but it does mean that the architect or structural engineer must provide additional justification. More on this in the final section of this article.
When is fire-retardant treatment required for oak structural timber?
Fire-retardant treatment is required when the fire class requirements of a project exceed fire class D. This is not the case for every project, and the need depends heavily on the specific application, occupancy type, and position of the timber within the building.
Situations in which treatment is frequently required:
- Visible timber in publicly accessible areas such as shops, restaurants, or offices, where class C or B is required.
- Timber ceilings or wall cladding in escape routes, stairwells, or corridors.
- Projects with high occupancy loads, such as schools, theaters, or care facilities.
- Situations where the structural engineer cannot demonstrate fire resistance through cross-section calculation.
A fire-retardant coating or impregnation can raise oak to class C or even class B. It is important that this treatment is compatible with the surface structure of the wood. With antique oak, which often has an open grain and a rich patina, the choice of product and application method is critical to the final result. Always consult a specialist supplier or fire safety consultant before selecting a treatment. Would you like to see how oak structural timber is used in practice? Browse our completed projects for inspiration.
How to document fire class properties in an architect’s specification
With reclaimed oak structural timber, documentation follows a different path than with new, certified timber. Standard CE marking is not available, but this does not mean fire performance cannot be substantiated. The key lies in a combination of material identification, normative reference, and structural calculation.
A practical approach for the specification:
- Material description: State the wood species (Quercus robur or Quercus petraea), density, and moisture content. This forms the basis for fire class classification.
- Normative reference: Reference EN 13501-1 and the associated fire class tables for solid hardwood. For oak with a density above 450 kg/m³, fire class D can be demonstrated without additional testing.
- Charring calculation: Have the structural engineer calculate fire resistance based on Eurocode 5 (EN 1995-1-2), using the standard charring rate for hardwood.
- Supplier declaration: Request a written statement from the supplier confirming wood species, origin, and moisture content. This is not CE marking, but it provides the structural engineer and building authority with the information needed for assessment.
- Coordination with the authority: In cases of uncertainty, contact the relevant fire safety or building control authority at an early stage. They will assess the justification and can indicate whether additional evidence is required.
A well-substantiated specification prevents disputes during the construction phase. For the antique oak beams we supply, we provide a detailed material description upon request that can serve as the basis for technical documentation.
How Frank Pouwer supports fire class documentation for oak structural timber
Fire safety is a technical subject, but the practical questions that architects and contractors ask are often very specific: what can I demonstrate, what do I need, and what does the supplier provide? We think along with you at every step.
- We supply a written material description for each batch of timber, including wood species, origin, moisture content, and density.
- Our stock of approximately 20,000 m² includes oak structural timber with a natural drying time of more than five years, ensuring stable and measurable properties.
- Through the Woodworldwide platform, more than 15,000 individual pieces are labeled and measured, allowing you to provide your structural engineer with exact dimensions for fire resistance calculations.
- We advise you on selecting the right material for your specific project and occupancy type.
- For projects with special requirements — such as large public buildings or international commissions — we discuss the options with you in person.
Do you have a project in which the fire class properties of oak structural timber are relevant? Request a quote and specify what technical documentation you require. We will make sure you have the right information to put together a complete and watertight specification.
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