Defects in Timber : Natural, Fungi, Seasoning & More

Defects in timber mainly signifies any undesirable character -natural or artificial- that decreases its strength, durability or quality of the timber.

The followings are the 5 main types of defects found in timber:

a. Defects due to Natural Forces

b. Defects due to Attack by Insects

c. Defects due to Fungi

d. Defects due to Defective Seasoning

e. Defects due to Defective Conversion

  1. Defects in Timber Due to Natural Forces  

Defects in Timber are generally caused due to natural forces that are listed below:

i. Knots

A knot is a pattern of the position of growth of a branch on the tree. On the cut board, it is most efficiently characterized by its conspicuously darker appearance and harder character.

A knot is a discontinuity in the timber and hence a zone of weakness.

Types of Knots

Types of Knots are as follows:

a. Rounded

b. Knot cluster

c. Elongated

d. Face Knot

e. Edge Knot

f. Pin Knots

g. Live Knots

h. Dead Knots

Knots are available in different variations in their size, shape, and stage of development. Common types of knots are pin knots, small knots, medium knots, and large knots.

This distinction depends upon their diameter, which is, respectively less than 6.50 mm, between 6.50 – 20 mm, between 20 – 40 mm, and larger than 40 mm.

Similarly, based on shape Knots may be represented as rounded, oval, and spike knots.

On the origin of their mode of occurrence, they can be as single knots, knot clusters, or groups of it.

Knots may also be differentiated into a tight knot and a loose knot, and so on.

a. Live Knot

It is that part (root) of the branch that evolves fully enclosed within the growing trunk of a tree.

In such a knot, there is full structural continuity between the fibers of the branch and the main tree.

b. Dead Knot

In such a knot, the continuity of structure is nearly lost or is only up to 25%.

c. An Inter-grown Knot

This is the word employed when nearly three-fourths of the fibers of the knot (perimeter) offer continuation with the main tree.

d. Encased Knot

It is a knot encircled completely by bark.

e. Edge Knot

It is seen only in saw-on timber, on one edge of the cut portion.

f. Face Knot

It is also seen on cut timber on the face of the board.

Knots of any type and shape are forever to be assumed defects in timber. They impact the quality of timber minimum in two modes:

Firstly, they create the workability of timber entirely hard and challenging. They are harder and show more remarkable opposition to planning and polishing.

Secondly, they decrease the tensile strength of the timber. Compressive and shear strength is, however, not concerned poorly.

ii. Shakes

After knots, shakes are more severe and common defects in timber. They are also known as cracks or fissures.

These natural cracks may occur due to:

a. Shrinkage on the aging of the tree

b. due to movement caused by wind action in the growing tree

c. Freezing of sap in the cells at the time of its ascent.

Relying on their place in the tree trunk as caught in a transverse section, the following types of shakes are commonly recognized:

a. Heart Shakes

These are damages or clefts happening in the heartwood, i.e., towards the internal region. They show the starting of the decay of the tree. They evolve thinner as they grow outer (towards sapwood).

b. Star Shakes

This type of shake starts propagating from the bark towards the sapwood and sometimes even towards the heartwood along the lines of medullary rays.

Cracks are wider on the outer edge of the bark and narrower on the inside (generally sapwood, sometimes heartwood).

The major causes behind star shakes are excessive heat or frosting at the period of the growth of the trees and quick or uneven seasoning after cutting off the timber.

Excessive heat or frost generates temperature difference, which generates shrinkage directed to the crack.

c. Cup and/or Ring Shakes

Cup shakes follow the annual growth ring. It is able to diverge the evolution ring partially or fully. When the crack separates the annual ring, it is called a ring shake.

So, all ring shakes are cup shakes although all cup shakes are not a ring shape. Extreme frost action is the primary cause for the formation of this type of crack.

d. Rind Galls

The definition of the rind is bark and gall is strange growth. So weird growth of the bark of the trees is called rind galls. Inappropriate cutting of branches generates this strange growth. Wood from this part of the timber lacks strength and is desirable in structure.

e. Upsets

Upsets in a different wood show that the tree was subjected to crushing or compression. The inappropriate felling of trees and heavy wind blowing at the period of young age of the tree are the primary causes behind this type of defect.

  2. Defects Due To Insects  

i. Beetles

Tiny insects create holes of size about 2 mm in diameter and damage the sapwood of all species of hardwoods.

Small tunnels are made on all sides of sapwood by the larvae of these beetles and changed into a fine flour-like powder. They do not hamper the external shell or cover and look good.

ii. Marine borers

They create holes or bore tunnels in wood for shelter. The wood damaged by marine borers loses the color and strength of the timber.

iii. Termites

White ants are very quick in eating away the wood from the middle of the cross-section. They create holes inside on various sides and do not hamper the external cover

  3. Defects Due to Fungi  

The attack of timber by fungi when the moisture content of timber is higher than 20% and the existence of air and warmth for the evolution of fungi the following defects are induced.

a. Blue stain

The sap of wood is shaded to a bluish color

b. Brown rot

Decay or disease of timber by reduction of cellulose compounds from wood and wood assumes the brown color.

c. Dry rot

Convert the wood into dry powder form.

c. Heart rot

This is comprised when the branch has come out of a tree and the tree evolves weak and provides a hollow sound when hit with a hammer.

d. Sap stain

The sapwood loses its color because of feed on the cell contents of sapwood.

e. Wet rot

Caused chemical decomposition of wood of the timber and timber alters to greyish brown powder known as wet rot.

f. White rot

Attack lignin of wood and wood takes the impression of white mass.

  4. Defects due to Defective Seasoning  

i. Cupping 

a. When you watch at the edge of the board it seems like a curve.

b. Induced by unequal doses of shrinkage along the growth rings.

ii. Bowing 

a.  When you watch at the edge of the wood it seems curved.

b. Induced by poor stacking

c. Not sufficiently stickers

ii. Twisting/Warping 

a. When the edges of the boards are turned in opposite directions.

b. Induced by shrinkage across a spiral or interlocking grain.

iii. Springing 

a. When the face of the board stays flat and the boundary turns inwards to make a curve.

b. Induced by shrinkage longitudinally along the irregular grain.

iv. End splits 

a. Appear at the uncovered ends of the boards.

c. Induced by quick drying out from the sun

d. The cab be controlled by painting the edges of the timber with bituminous paint (waterproof)

v. Surface splits/checks

a. Generally lie across the grain.

b. Induced by quick drying out on the surface of the wood.

vi. Honeycomb splits/checks 

a. Appear inside the board.

b. Decreases strength.

vii. Case hardening

a. Where the outer of the board is dry and hard but moisture is entrapped in the center cells of the wood.

b. Induced by quick drying

  5. Defects Due To Defective Conversion  

While converting timber into commercial form, the following defects may arise are as follows:

a. Chip Mark

Mark or sign positioned by chip on the smooth surface of the timber

b. Diagonal grain

Due to inappropriate sawing of timber, these defects may arise.

c. Torn grain

Due to the falling of the tool small impression is constructed.

d. Wane

Existence of the original round surface on the manufactured piece of timber

This was for the Defects in Timber.

Read Also: Types of Bearings for Bridges