Post and Pole Strength
Post strength is important for fencers and pole contractors installing product, as good strength means minimal breakage, and a good long life for the material once it is installed. Breakages cost time and money.
Post & Pole strength are a function of a number of factors. These can be categorised in three groups:
1. Product size
2. Post and pole grading
3. Wood quality
Product size and consistency of size is very important in ensuring the product is set firmly in the ground, and breakages are minimised. A bigger post or pole will always be able to be set more firmly in the ground than a smaller post or pole.
Product size is also very important in minimising breakage as a small post or pole will always break more easily than a larger post or pole.
Post and Pole Grading
Consistency of sizing is the most important factor with product grading. At PermaPine a post or pole is graded by measuring the length of the smallest face, for example on a Quarter Round, the smallest of the two faces. If comparing product from various merchants, it pays to check on sizing before making your purchase decisions as some manufacturers grade using an average face size rather than a minimum face size. Roundwood graded using the average face size will be smaller.
The smallest face is 95mm therefore the PermaPine product is graded as a No2 quarter round (75-100mm) not as a 100mm quarter round as other manufacturers using average face will grade this piece.
Wood quality is the least well understood factor in determining wood strength. The wood quality
factor which is generally most important for posts and poles is wood stiffness. Wood stiffness is difficult to measure in standing trees in the forest, so most foresters use wood density as a predictor of stiffness, as wood density can be measured by taking core samples from standing trees.
What factors influence density or stiffness?
Wood stiffness or density is determined by the thickness of the cell walls in the tree.
Wood near the outside of the tree has thicker cell walls so it is denser and stronger than wood near the centre.
The higher the mean annual temperature of the site the tree grew on the greater the average
Genetics affects density.
Average density increases with age.
Note: Studies by the Forest Research Institute have found that growth rate (or distance between each growth ring) and wood density are not strongly related.
Talk Tough on Timber
We have decided it’s about time to correct some of the "Misunderstandings and Misconceptions" about density and strength in posts.
PermaPine is committed to supplying our customers with quality roundwood. We aim to
manufacture and supply roundwood which has generous sizing, which is consistent and durable through being well treated with preservatives. Our product will not only perform well while in use, but it will also look good.
Post strength is important for fencers and contractors installing the product, as good strength means minimal breakage, and a good long life for the product once it is installed. Breakages cost time and money.
The most comprehensive study on density of the New Zealand forest resource was carried out by 3 scientists from the Forest Research Institute. They found:
Differences between high and low density areas of forest averaged 15%.
The increase in density from 15 to 45 year old trees ranges from 12 – 17%.
Differences between bottom and top logs within a tree averaged 7 – 11%.
They found wood density within New Zealand falls into three density groups: High, Auckland,
Coromandel, and Northland. Medium, Rotorua and Nelson, and low Wellington, and the rest of the South Island.
PermaPine source their logs from the Auckland and Rotorua areas which the graph above shows are
the regions with the best density in the country.
1. Radiata Pine. Wood properties survey. Forest Research Institute 1991
2. The mechanical properties of New Zealand-grown Radiata Pine for export to Australia. Forest
Research Institute. 1985