Timber Fundamentals, Materials and Eurocode Design Basis — Timber Design

StandardEN 1995-1-1:2004+A2:2014 with the applicable National Annex

Source1 source file

Review stateApproved · 2026-08-19

LEARNING OUTCOMES

After this chapter, you should be able to

Describe timber as a natural, orthotropic and moisture-sensitive structural material.
Identify longitudinal, radial and tangential directions and select parallel/perpendicular properties.
Explain how sawing, moisture, defects, durability and grading affect structural performance.
Distinguish solid timber from glulam, LVL, plywood, OSB, CLT and built-up products.
Read an EN 338 strength class as a complete strength, stiffness and density set.
Explain the relationship between EN 1990, EN 1991, EN 1995-1-1, the National Annex and product data.
Select service class, load-duration class, kmod, kdef and γM for a stated teaching case.
Convert characteristic strength to a basic design value without prematurely applying check-specific factors.
Prepare the reusable Chapter 1 timber design-data card.

1.1 Purpose of this chapterSource §1.1

This chapter forms the common foundation for the timber-design module. It combines material fundamentals with the Eurocode design basis so that the learner finishes with one verified timber design-data card for later member, connection, racking and engineered-timber calculations.

1.2 Timber as a structural materialSource §1.2

Timber can act efficiently in tension, compression, bending and shear. Its low density gives a high strength-to-weight ratio, but the material is variable, direction-dependent and moisture-sensitive. Durability depends on exposure and detailing, while connection design is often decisive.

Material features and structural significance
Feature	Structural significance
High strength-to-weight ratio	Reduces dead load and can reduce support and foundation demands.
Cellular / fibrous structure	Strength and stiffness are high parallel to grain but much lower perpendicular to grain.
Hygroscopic behaviour	Moisture exchange affects shrinkage, swelling, creep and durability.
Natural variability	Structural properties require grading and declared values; species name alone is insufficient.
Combustibility	Fire design is required separately, although charring can be treated systematically.
Workability and connections	Members can be cut and connected in many ways; local detailing and fasteners can govern.

Figure 1.R1 — source-derived timber-building gravity-load path from roof and floor members through walls and supports to the foundations.Approved original STRUCTURA academic diagram

1.3 From tree trunk to structural memberSource §1.3

The biological role of the trunk explains many engineering properties. Wood cells are predominantly aligned along the trunk, producing strong longitudinal behaviour. Growth rings, rays, sapwood, heartwood, pith and juvenile wood create different radial, tangential, mechanical and durability characteristics.

Sapwood: Younger outer wood; commonly less naturally durable but often more receptive to preservative treatment.
Heartwood: Older inactive wood; extractives in some species may increase natural durability.
Growth rings and rays: Their orientation contributes to radial/tangential differences, density variation and distortion after sawing.
Pith and juvenile wood: Can be associated with lower stiffness, higher longitudinal shrinkage and less stable sections.

1.4 Orthotropic behaviour and grain directionSource §1.4

Wood is commonly treated as orthotropic, with longitudinal, radial and tangential principal directions. In structural notation, 0 denotes parallel to grain and 90 denotes perpendicular to grain. The distinction is fundamental because the same piece can have very different strength and stiffness in different directions.

Direction-dependent timber properties
Property direction	Symbol	Design use
Bending	f_m,k	Beam design with fibres parallel to the member axis.
Tension parallel / perpendicular	f_t,0,k / f_t,90,k	Ties and tensile zones / splitting-sensitive regions.
Compression parallel / perpendicular	f_c,0,k / f_c,90,k	Columns and studs / support and local bearing.
Shear	f_v,k	Beams, notches and connection-related member checks.
Elastic modulus	E_0,mean / E_0,05	Mean deformation property / stability property.

Figure 1.R2 — regenerated trunk anatomy, longitudinal/radial/tangential axes and member grain direction.Approved original STRUCTURA academic diagram

1.5 Hardwood, softwood and the Sri Lankan contextSource §1.5

Hardwood and softwood are botanical groupings, not guaranteed mechanical hardness. The source records both international and Sri Lankan examples, but Eurocode design still requires a verified structural grade or declared property set, moisture condition and durability/treatment basis.

Hardwood and softwood teaching comparison
Item	Softwoods	Hardwoods
Botanical form	Usually coniferous / cone-bearing trees.	Usually broad-leaved trees.
Typical structural use	Joists, studs, rafters, trusses, glulam and engineered products.	Heavy members, exposed work, durability or appearance applications.
Examples recorded by source	Spruce, pine, Douglas fir and plantation Grandis.	Jak, teak, mahogany, milla, kumbuk, oak and ash.

1.6 Conversion, sawing, section orientation and target sizeSource §1.6

The cut position relative to growth rings influences shrinkage, cupping, appearance and dimensional stability. Through-and-through sawing is efficient but can produce greater distortion; radial or quarter sawing changes ring orientation and can improve stability at higher cost and waste.

Record the installed orientation of rectangular members.
Use target dimensions and tolerances from the relevant product standard rather than nominal merchant size.
Account for planing, machining, notches and holes where they reduce the structural section.
Use EN 336 for solid-timber target sizes/tolerances and EN 14081-1 for strength-graded structural timber.

1.7 Moisture content, fibre saturation and equilibrium moistureSource §1.7

Timber is hygroscopic: it gains or loses moisture until tending toward equilibrium with the surrounding air. Below the fibre-saturation region, bound-water change causes shrinkage or swelling and strongly influences strength and stiffness.

Moisture content · Equation 1.1

MC = (m_w − m₀) / m₀ × 100%

m_w is the mass at the measured moisture condition and m_0 is the oven-dry mass.

Figure 1.R3 — conceptual moisture response below the fibre-saturation region; this is a teaching relationship, not a design curve.Approved original STRUCTURA academic diagram

Equilibrium moisture content: Depends on surrounding relative humidity and temperature; timber moves toward it with time.
Fibre-saturation region: Around the high-20% moisture range for many species; below it, movement and property changes become important.
Wet installation followed by drying: Can increase creep and deformation and may require special kdef treatment.

1.8 Shrinkage, swelling, distortion and structural defectsSource §1.8

Below fibre saturation, tangential movement is generally larger than radial movement and longitudinal movement is usually much smaller. Differential movement explains cup, bow, spring and twist and must be accommodated where timber meets masonry, concrete, steel, glazing, finishes or services.

Features and defects that influence design
Feature	Typical effect
Knots	Interrupt fibre paths and can reduce tension and bending resistance.
Slope of grain	Reduces strength because load is not carried purely along the fibres.
Checks and splits	May reduce effective section, connection capacity and tension-perpendicular resistance.
Wane	Reduces usable cross-section and may affect connection edge distances.
Warp and twist	Affect fit, bearing, restraint assumptions and erection tolerances.

1.9 Durability, biological attack and preservationSource §1.9

Durability is not achieved by strength alone. Good structural detailing keeps timber dry, permits drainage and ventilation, avoids trapped water, separates timber from persistently wet materials where necessary and specifies suitable treatment or naturally durable material when exposure demands it.

Durability design questions
Question	Required response
Can water reach or become trapped at the member?	Provide weather protection, drainage, ventilation and water-shedding details; avoid moisture pockets.
Is the species naturally durable?	Check declared information and distinguish heartwood from sapwood where relevant.
Is treatment required?	Specify treatment compatible with the intended use, adhesives and metal fasteners.
Will inspection be possible?	Detail critical zones so deterioration can be identified and maintained.

1.10 Strength grading and structural property assignmentSource §1.10

Timber strength varies with species, density, moisture, dimensions, defects and loading history, so structural timber is graded. Visual grading limits visible strength-reducing features; machine grading measures properties correlated with strength and assigns a declared grade or class.

A species name is not a substitute for strength grading.
A grading stamp or Declaration of Performance should link the product to declared properties.
Use the full property set belonging to the selected class, not only the class number.

1.11 EN 338 strength classes and characteristic propertiesSource §1.11

EN 338 provides standardized property sets. C classes are commonly used for coniferous timber, D classes for deciduous timber and T classes for tension-based grading. Characteristic and mean values must be used with the EN 1995-1-1 design framework.

Selected EN 338:2016 property values
Property	Symbol	C16	C24	C30
Bending strength (N/mm²)	f_m,k	16	24	30
Tension parallel (N/mm²)	f_t,0,k	8.5	14.5	19
Tension perpendicular (N/mm²)	f_t,90,k	0.4	0.4	0.4
Compression parallel (N/mm²)	f_c,0,k	17	21	24
Compression perpendicular (N/mm²)	f_c,90,k	2.2	2.5	2.7
Shear strength (N/mm²)	f_v,k	3.2	4.0	4.0
E_0,05 (N/mm²)	E_0,05	5,400	7,400	8,000
E_0,mean (N/mm²)	E_0,mean	8,000	11,000	12,000
G_mean (N/mm²)	G_mean	500	690	750
Characteristic density (kg/m³)	ρ_k	310	350	380
Mean density (kg/m³)	ρ_mean	370	420	460

Source table based on EN 338:2016. Confirm the current product declaration and applicable standard for project design.

1.12 Solid timber and engineered wood productsSource §1.12

Engineered wood products redistribute defects, combine smaller pieces and permit larger or more reliable elements. Product type matters because strength classes, size effects, service classes, connection behaviour and deformation factors can differ.

Timber and engineered wood product families
Product	Typical source	Structural note
Solid structural timber	EN 14081-1 + EN 338	Joists, studs, rafters, beams, columns and truss members; grading is essential.
Glulam	EN 14080	Large straight, tapered or curved laminated members.
LVL	EN 14374 / declared product data	High, relatively uniform strength and stiffness; product lay-up matters.
Plywood / OSB	EN 636 or EN 300 / EN 13986	Structural sheathing, diaphragms and composite systems; orientation and service restrictions matter.
CLT	EN 16351 / manufacturer declaration	Layered wall, floor and roof panels; declared system data are required.
I-joists / built-up products	Manufacturer / ETA / product standard	Design depends on proprietary web/flange properties and connection rules.

1.13 The Eurocode 5 design frameworkSource §1.13

Eurocode 5 operates with EN 1990 for the basis of design and combinations, EN 1991 for actions, EN 1995-1-1 for timber-specific rules, the applicable National Annex for nationally determined parameters, and product standards or Declarations of Performance for properties and restrictions.

Documents used by the timber module
Document	Role
EN 1990	Basis of design, ULS/SLS verification and action combinations.
EN 1991	Permanent, imposed, wind, snow and other actions.
EN 1995-1-1	Normal-temperature timber member, system, serviceability and connection rules.
EN 1995-1-2	Structural fire design, developed later in the source module.
National Annex	Nationally determined parameters and accepted complementary information.
Product standards / DoP	Characteristic properties, dimensions, limitations and declared performance.

Figure 1.R4 — source-derived Eurocode workflow from actions and product data to ULS/SLS verification and a recorded design decision.Approved original STRUCTURA academic diagram

1.14 Limit states and the basic verificationSource §1.14

Ultimate limit states concern collapse, loss of equilibrium, material failure and instability. Serviceability limit states concern deflection, vibration, movement and joint slip during normal use.

Basic limit-state verification · Equation 1.2

E_d ≤ R_d

E_d is the design effect of actions and R_d is the corresponding design resistance. Utilisation above 1.0 means the selected resistance check is not satisfied.

Timber limit-state examples
Limit state	Typical examples
ULS	Tie rupture, buckling, bending/shear failure, bearing, connection yield or brittle splitting, racking failure.
SLS	Beam/floor deflection, vibration, joint slip, movement and differential creep.

1.15 Actions and action combinationsSource §1.15

Permanent actions remain essentially constant; variable actions include imposed loading, wind and snow. Characteristic actions come from EN 1991 or other applicable standards and are combined using EN 1990 and the National Annex. When several variable actions exist, vary the leading action where necessary.

Generic fundamental ULS combination · Equation 1.3

E_d = Σ_j γ_G,j G_k,j + γ_Q,1 Q_k,1 + Σ_i>1 γ_Q,i ψ_0,i Q_k,i

The exact partial and combination factors depend on the applicable National Annex.

Quasi-permanent combination · Equation 1.4

E_qp = Σ_j G_k,j + Σ_i ψ_2,i Q_k,i

The quasi-permanent combination is important for time-dependent timber deformation.

1.16 Characteristic values, design values and check-specific factorsSource §1.16

Characteristic strengths are not normally inserted directly into a ULS resistance check. Eurocode 5 modifies characteristic strength for load duration and moisture through kmod and divides by the material factor γM.

Basic timber design value · Equation 1.5

X_d = k_mod X_k / γ_M

X_d is the basic design value before any check-specific factor such as k_h, k_sys, k_c or k_crit is applied.

1.17 Service classesSource §1.17

Service class describes the expected in-service moisture environment, not simply a delivery moisture reading. It affects strength through kmod and deformation through kdef.

Simplified EC5 service-class interpretation
Class	Typical condition	Examples
1	For most softwoods, average moisture content normally not exceeding about 12%.	Heated, protected internal members.
2	For most softwoods, average moisture content normally not exceeding about 20%.	Protected but unheated roofs, garages and ventilated buildings.
3	Moisture condition higher than Service Class 2.	External or exposed timber not protected from precipitation.

Figure 1.R5 — source-derived comparison of Service Classes 1, 2 and 3; project exposure must be confirmed.Approved original STRUCTURA academic diagram

1.18 Load-duration classesSource §1.18

Timber strength is sensitive to load duration. kmod is selected from both the load-duration class and service class. In combinations with different durations, kmod is normally associated with the shortest-duration action present, but the governing case is the one with highest utilisation, not automatically the largest force.

EC5 load-duration teaching classes
Class	Approximate accumulated duration	Typical actions
Permanent	> 10 years	Self-weight and permanent actions.
Long-term	6 months–10 years	Storage or sustained variable actions.
Medium-term	1 week–6 months	Imposed floor load; snow in some applications.
Short-term	< 1 week	Wind, maintenance and short snow events.
Instantaneous	Instantaneous	Wind gust, impact and accidental action.

1.19 Selecting kmod and the material partial factor γMSource §1.19

The source provides teaching values based on first-generation EN 1995-1-1 and supplied references. Confirm the applicable National Annex and product documentation before professional design.

Material factors used in the source teaching examples
Material	γM
Solid structural timber	1.30
Glulam	1.25
LVL, plywood and OSB	1.20
Timber connections unless stated otherwise	1.30
Punched metal plate connections under controlled production	1.25

Typical kmod values for solid timber and glulam
Load duration	SC1	SC2	SC3
Permanent	0.60	0.60	0.50
Long-term	0.70	0.70	0.55
Medium-term	0.80	0.80	0.65
Short-term	0.90	0.90	0.70
Instantaneous	1.10	1.10	0.90

1.20 kdef, creep and long-term stiffnessSource §1.20

kdef accounts for time-dependent deformation associated with moisture and sustained loading. It depends on product and service class. The source gives 0.60, 0.80 and 2.00 for solid timber, glulam and LVL in Service Classes 1, 2 and 3.

Typical first-generation EC5 kdef values
Material	SC1	SC2	SC3
Solid timber	0.60	0.80	2.00
Glulam	0.60	0.80	2.00
LVL	0.60	0.80	2.00
OSB/3 or OSB/4	1.50	2.25	Not generally given

Final-stiffness concept · Equation 1.6

E_mean,fin = E_mean / (1 + k_def)

A conceptual material relationship only. Complete beam/floor deflection uses the relevant SLS combinations and creep components.

1.21 Density and preliminary self-weightSource §1.21

Mean density is commonly used for preliminary self-weight, while characteristic density is important in several resistance and connection equations.

Approximate timber unit weight · Equation 1.7

γ_timber ≈ ρ_mean g / 1000 [kN/m³]

Use ρ_mean in kg/m³ and g ≈ 9.81 m/s².

Rectangular member self-weight · Equation 1.8

w_k = γ_timber b h [kN/m]

Use b and h in metres with γ_timber in kN/m³.

1.22 Timber material and Eurocode design-data cardSource §1.22

Later calculators should call this reusable card rather than asking the user to re-enter the same product and material information.

Required Chapter 1 data-card entries
Data group	Required entry or output	Later use
Product and grade	Product type, standard/DoP, grade or strength class	Select property tables and product restrictions.
Geometry and directions	Target dimensions, panel lay-up, 0°/90° and L-R-T reminder	Section properties, tolerances and correct property direction.
Strength and stiffness	Characteristic strengths, E_0,mean, E_0,05, E_90,mean and G_mean	Member, local, deformation and stability checks.
Density	ρ_k and ρ_mean	Connections and self-weight.
Environment	Durability/treatment, service class and load duration	Detailing, kmod and kdef.
Design conversion	γM and basic X_d values	Starting point for later checks before check-specific factors.

1.23 Interactive design-data calculatorSource §1.23

APPROVED ACADEMIC CALCULATOR

Timber Material and Eurocode Design-Data Calculator

Generate and validate the reusable Chapter 1 material card for source-supported EN 338 C16, C24 and C30 solid timber. It does not check member adequacy.

Inputs: Strength class · Service class · Load-duration class · γM · Target width and depth · Conditioned and oven-dry masses
Outputs: Characteristic properties · kmod · kdef · Basic design strengths · Moisture content · Final-stiffness concept · Unit weight · Self-weight
Status states: COMPLETE DESIGN DATA · MISSING DATA · EXPOSURE REVIEW REQUIRED
Validation: Approved against the supplied worked-example results; project-specific verification remains required

EC5 SAMPLE CALCULATOR · SOURCE WE-01

Timber Material and Eurocode Design-Data Calculator

Source-derived teaching implementation for EN 338 C16, C24 and C30 solid timber. It prepares material data only.

COMPLETE DESIGN DATA

C24, Service Class 1, Medium-term; kmod = 0.80. No member adequacy is checked.

kmod: 0.80
kdef: 0.60
γM: 1.30
Moisture content: 12.00 %
E0,mean,fin: 6875 N/mm²
Unit weight: 4.12 kN/m³
Self-weight: 0.041 kN/m
ρk / ρmean: 350 / 420 kg/m³

Characteristic → basic design strength

Bending fm: 24 N/mm² → 14.77 N/mm²
Tension ∥ grain ft,0: 14.5 N/mm² → 8.92 N/mm²
Tension ⟂ grain ft,90: 0.4 N/mm² → 0.25 N/mm²
Compression ∥ grain fc,0: 21 N/mm² → 12.92 N/mm²
Compression ⟂ grain fc,90: 2.5 N/mm² → 1.54 N/mm²
Shear fv: 4 N/mm² → 2.46 N/mm²

Source calculation trail

MC = (2.24 − 2.00) / 2.00 × 100 = 12.00%
Xd = kmod Xk / γM = 0.80 Xk / 1.30
Emean,fin = 11000 / (1 + 0.60) = 6875 N/mm²
γtimber = 420 × 9.81 / 1000 = 4.12 kN/m³
wk = 4.12 × 0.045 × 0.220 = 0.041 kN/m

1.24 Worked Example WE-01 — Complete C24 design-data cardSource §1.24

WORKED EXAMPLE

WE-01 · TIM-WE-01 · Rev. A

A machine-graded solid softwood joist to EN 14081-1 has strength class C24 and target section 45 × 220 mm. It is used in a protected, normally heated internal floor. A moisture sample has m_w = 2.24 kg and m₀ = 2.00 kg. Prepare the Chapter 1 data card for a medium-term combination using γ_M = 1.30, k_mod = 0.80 and k_def = 0.60.

Eq. 1.1 · moisture
MC = (2.24 − 2.00) / 2.00 × 100
MC = 12.0%
Service-class interpretation
Protected, normally heated internal floor with no persistent wetting
Service Class 1
Load-duration class
Medium-term action governs the selected design combination
k_mod = 0.80
EN 338 C24 · strengths
f_m,k = 24; f_t,0,k = 14.5; f_t,90,k = 0.4; f_c,0,k = 21; f_c,90,k = 2.5; f_v,k = 4.0 N/mm²
Characteristic strength set complete
EN 338 C24 · stiffness/density
E_0,mean = 11,000; E_0,05 = 7,400; E_90,mean = 370; G_mean = 690 N/mm²; ρ_k = 350; ρ_mean = 420 kg/m³
Stiffness and density set complete
Eq. 1.5 · bending
f_m,d = 0.80(24) / 1.30
f_m,d = 14.77 N/mm²
Eq. 1.5 · tension ∥ grain
f_t,0,d = 0.80(14.5) / 1.30
f_t,0,d = 8.92 N/mm²
Eq. 1.5 · compression ∥ grain
f_c,0,d = 0.80(21) / 1.30
f_c,0,d = 12.92 N/mm²
Eq. 1.5 · compression ⟂ grain
f_c,90,d = 0.80(2.5) / 1.30
f_c,90,d = 1.54 N/mm²
Eq. 1.5 · shear
f_v,d = 0.80(4.0) / 1.30
f_v,d = 2.46 N/mm²
kdef
Solid timber in Service Class 1
k_def = 0.60
Eq. 1.6 · concept
E_mean,fin = 11,000 / (1 + 0.60)
E_mean,fin ≈ 6,875 N/mm²
Eq. 1.7 · unit weight
γ_timber = 420(9.81) / 1000
γ_timber ≈ 4.12 kN/m³
Eq. 1.8 · self-weight
w_k = 4.12(0.045)(0.220)
w_k ≈ 0.041 kN/m
Status logic
Product, class, dimensions, exposure, duration, factors and properties are available
COMPLETE DESIGN DATA

Result. The C24 joist has a complete Chapter 1 design-data card: MC = 12.0%, Service Class 1, k_mod = 0.80, k_def = 0.60, γ_timber ≈ 4.12 kN/m³ and w_k ≈ 0.041 kN/m. No member adequacy has been checked; bending, shear, buckling, bearing, deflection, connections and system effects remain later verifications.

1.25 Common mistakes and design decisionsSource §1.25

Using a species or local name as an EN 338 class.
Ignoring grain direction or using nominal merchant dimensions.
Selecting service class from delivery moisture rather than the in-service environment.
Using one kmod for every combination or characteristic strength directly at ULS.
Applying k_h, k_c or k_crit permanently to the material card.
Interchanging E_0,mean, E_0,05 and E_90,mean.
Treating engineered products as solid timber without declared product data.
Ignoring durability, trapped moisture, treatment compatibility or fastener corrosion.
Mixing Eurocode generations or National Annex assumptions.

1.26 Chapter summarySource §1.26

Key points

Timber is natural, variable, orthotropic and hygroscopic.
Fibre direction controls the selected strength and stiffness property.
Moisture, defects, sawing, durability and grading precede resistance checks.
Strength classes provide a full property set, not only a class number.
Engineered products need product-specific data.
EN 1990 supplies the design basis, EN 1991 actions and EN 1995-1-1 timber rules.
Service class and duration determine kmod; kdef governs long-term deformation behaviour.
Basic design strength is kmod Xk / γM before check-specific factors.
The Chapter 1 calculator validates material data but does not declare a member adequate.

References and source mapSource §References

Source references recorded by the supplied chapter

University of Moratuwa, Timber Part 1 — Introduction to Structural Timber.
Porteous & Kermani, Structural Timber Design to Eurocode 5, Chapter 1.
Swedish Wood, Design of Timber Structures, Volume 1, Chapters 1–2.
Swedish Wood, Design of Timber Structures, Volume 2, Sections 2–3 and 9.
IStructE/TRADA, Manual for the Design of Timber Building Structures to Eurocode 5, Sections 2–4.
The Structural Use of Timber — Handbook for Eurocode 5 Part 1-1, Sections 1–5 and Annexes A–D.
EN 338:2016; EN 14081-1; EN 336; EN 350; EN 335.
EN 14080; EN 14374; EN 636; EN 300; EN 13986; EN 16351 and applicable product declarations.
EN 1990 and relevant parts of EN 1991.
EN 1995-1-1:2004+A2:2014 with the applicable National Annex.