This document discusses defects and diseases that can occur in wood. It describes natural defects like knots, resin pockets, shakes and waney edges that form during tree growth. It also covers artificial defects such as cupping, bowing, twisting and end splitting caused by improper drying. Fungal diseases and insect pests that damage wood are also examined, along with conditions needed for decay. The life cycles of fungi and boring insects are detailed.
3. Defects
Defects in timber can occur
1. naturally (during the growth of a tree) or
2. artificially (as a result of incorrect stacking or seasoning)
Some defects can be decorative and very pleasing to the
eye, so these will be kept
Others may cause problems during the manufacture of a
project, such defects must be overcome
5. Knots:
These are formed from small
branches which are cut off
the tree or fall off.
They reduce timber strength
and make it hard to work
Resin pocket:
• Small cavities in the wood
which are full of resin. Exist
deep in the wood, especially in
softwoods.
•
− Live knot
− Dead knot(loose)
•
Cracks caused by high wind or
extremes in temperature
Reduce the strength of the timber
7. Shakes
Shakes are splits in the end grain of wood
Occur along either:
rings
Ray lines
Annual
Caused by tension forces which build up as
the tree is growing. When it is felled, or
during seasoning weaker points break and
the wood splits causing shakes
8. Waney Edge
Occurs during the conversion process
Occurs when the bark is left on the edge of the
plank
10. Artificial Defects
Occur as a result of incorrect stacking or
inappropriate drying schedule, stresses will be
created in the wood that will cause
– Cupping
– Bowing
– Twisting / Warping
– End Splitting
– Case Hardening
– Honeycomb Checks
14. End Splitting
Caused if the ends of the
boards dry out too quickly
due to exposure to the
sun or heat
Occurs during Air
Seasoning
Can be prevented by
sealing or covering the
end grain of the planks of
wood
15. Artificial defects – seasoning
Case hardening
− Moisture is trapped in the
centre if surfaces dry too
quickly
− Boards bend when
tension is released
17. Types of rot
Wet rot
− Occurs outside
− Doors and window frames
− White residue
Dry rot
− Under wood floors
− Musty smell
− Poor ventilation
− Charred wood appearance
18. Dry rot
Treatment
− All infected wood removed
and 500mm beyond
− All blockwork and wood
treated with fungicide
− The cause of infection must
be found and fixed
− All new timber must be
treated with preservative
20. Insect attack
Insect life cycle
Eggs are laid in wood
The larva (worm) hatches
and begins eating the
wood
When grown, the larva
makes a cocoon (pupa)
and changes into the
adult beetle
Adult emerges at the
surface
22. Name the defects shown in the diagrams
− Cupping
− Warping/twisting
− Heart shake
23. The life cycle of a wood boring insect is shown. At which
stage does the most damage occur?
Most damage occurs
while the insect is at the
larva stage
The larva bores tunnels
under the surface of the
wood
24. List the conditions necessary for a fungal attack to occur
in wood
Conditions
– A moisture content
above 20%
– A supply of food (wood)
– Oxygen
25. Questions
1 What is the difference between natural and artificial
defects in timber.
2 Give an example using a Sketch of both a radial
shake and a tangential shake.
3 Describe using sketches what is meant by bowing,
cupping and twisting in planks.
4 Why does case hardening occur?
