Timber grades in construction: an essential guide to choosing quality timber

An article based on Radu Vădan's presentation at nZEB Expo Cluj 2024 - video at the end of the article

Radu Vădan, the Wood Hub, on stage at nZEB Expo Cluj 2024

In the world of timber construction, the quality of the material is fundamental to the durability and safety of the structure. However, many projects are compromised from the outset by the wrong choice of wood quality. Radu Vădan, Wood Hub, recently offered nZEB Expo Cluj Cluj 2024 a detailed overview of the standards and criteria for assessing building timber.

Introduction

In construction, the quality of the wood used is crucial for the durability and safety of the structure. Not every wood is suitable for every type of construction, although the common practice in Romania is to purchase materials without careful evaluation. This can lead to problems such as:

Insect emergence
Roof assessment
Curving elements
Creaking floors
Sound transmission through floors instead of soundproofing

Types of defects in wood

Natural defects (grown with the tree)

1. The nodes

They represent branches that have grown over time
They can penetrate through the plank in various ways:
- Full crossing
- Exit through the edge
- Exit around the corner
Impact: Can significantly affect structural strength
May cause beams to break or walls to buckle

2. Insect holes

Indicator that the tree was no longer alive when cut
Common in resinous wood
Pot indicates:
- Wounded tree
- Blown by the wind
- Attacked by fungi

3. Resin bags

Major problem with layered elements
May prevent interlayer adhesion to glulam beams
Risks compromising solder when heating

4. Bark inclusions

Appear when the tree was injured
Heals by raising the bark over the wound
May compromise structural integrity

5. Blue spots

Commonly associated with insect holes
Indicates the presence of fungi
Can lead to wood softening
Turn C24 wood into C14 wood

6. Nonlinear fiber

Rare in resinous
Occurs when trees grow in difficult conditions
Results in uneven resistance

Cutting defects

1. Corners with bark

Very dangerous, especially on beams
Allow insects access
Can lead to major infestations in 3-7 years
Requires expensive treatments afterwards

2. Curb

Can occur both during cutting and drying

3. Dimensional deviation

Thickness unevenness
Caused by the quality of the gater and its maintenance
Can vary significantly (4-6 cm on a 5 cm plank)

Wood strength grades

C14 (weakest class)

Intended for non-structural walls
Allows:
- Knots up to half the thickness
- Firm rot
- Deep cracks (more than half the thickness)
- Dimensional variations up to 8mm/m

C18 (first structural class)

For lightweight structural applications
Features:
- Knots up to 1/4 of the thickness
- Small spots of firm rot
- Superficial cracks
- Requires four-sided tracing

C24 (common construction class)

Usable for any structural load
Restrictions:
- Knots up to 1/4 of the thickness
- Does not allow rot
- Minor superficial cracks
- Minimum dimensional deviations

C30 and C35 (higher grades)

For special applications
Features:
- Requires lamination and tooth bonding
- Compulsory mechanical processing
- C35 requires individual static load testing

Practical tips

For drying

Outdoor drying is acceptable if:
- Stacks are covered
- They are sheltered from direct rain
- They are protected from direct sunlight
The advantage of tumble drying:
- Kills all insects (including eggs)
- Anti-insect cycle at 75°C for 36 hours

For construction

Check the presence of documentation for the resistance class
Visually inspect for defects
Pay special attention to barky corners
Treat cracks to prevent seepage
Protect wood from rain and UV

Conclusion

Choosing the right grade of timber and understanding possible defects are essential for sustainable and safe construction. It is crucial to respect standards and to pay particular attention to the quality of the timber used in construction.

Below is the video of Radu Vădan's presentation on nZEB Expo Cluj 2024