Have you thought of adding some bit of forest-based decorations to your home? In truth, when it comes to decking, cladding, pergolas, and landscaping, using timber is a practical and attractive option. Failure to use the best type of wood in particular outdoor setting might give you undesirable results.

We found that in many cases, the problem is not of the kind of timber material in use but merely the failure of developers to understand the different species of wood, their installation procedure, suitability and also maintenance.


The Different Classes of Timber

Interestingly, timber is classified into different categories. These include four types of durability in in-ground contact and another four in durability while used outside above ground. The rate of strength goes increasing from the first level to the last. Hence, the first class represents the most durable timber, and class number four commonly consists of wood with the least durability[1].


Durability and Strengths Table

Timber Species Strength Durability Rating
Tallowwood F17 – F34 Class 1
Blackbutt F17 – F34 Class 1
Ironbark F22 – F42 Class 1
Spotted Gum F17 – F34 Class 1
Blue Gum F14 – F24 Class 2
Forest Red Gum F14 – F22 Class 1
Mountain Ash F14 – F22 Class 3
Stringybark F14 – F22 Class 3
Jarrah F11 – F22 Class 2
Karri F11 – F22 Class 2
Brushbox F14 – F24 Class 3
Oregon(Douglas Fir) F8 –  F14 Class 3/4

Durability Ratings for Timber[2]

Usually, while choosing the best timber for external use, we recommend the first and second classes. The main reason behind this is because of their natural durability. Besides, they offer a service life of close to fifteen years for in-ground contact. In the case of outside and above ground, they provide easier maintenance and a better service life.

Wood with the durability of classes three and four are used in outside and above ground. All that is needed is maintaining proper care while fixing, nailing, and detailing joints. Also, remember to follow the right instructions while adding the most suitable coating.


Preservative Treatment Timber

Instead of using wood with low durability for outdoor environments, a good alternative is using preservative timber. Treated wood simply means wood that has gone through a chemical treatment process. Why? This is expected to improve the wood’s resistance to attack from algae or fungi. Simply put, the timber’s resistance is improved making it suitable for outdoor home environments!


The Hazard Level Measuring System

Treated wood or timber comes with a service rating which is referred to as the risk level guide. It’s meant to measure the rate of danger caused by increased exposure. H1 is the lowest scale, and H6 is the highest.


Risk Level Guide

Harvard level Exposure Uses
H1 Inside and above ground Flooring, framing, furniture, and interior joinery
H2 Inside and above ground Applied in dry environments for flooring and framing
H3 Outside and above ground Building pergola posts, framing, and decking
H4 Outside and above ground Fence and pergola posts, building greenhouses as well as timber for landscaping
H5 Outside and in ground contact, with presence or absence of fresh water Used for retaining walls, constructing polls and cooling the tower fill
H6 Marine waters For boat hulls and marine piles

Hazard Level Measuring System

Let’s look at a simple example! Preservative timber meant for decking should be that of level H3 while pergola or fence posts should be H4. There are many various types of treatment tips and chemicals used, and hence, it’s necessary to understand the application as well as the condition of the service before selection.

Similar to non-preservative timber, ensure that your treated wood is well protected from weathering. By doing so, you’ll keep your wood from decreasing in value. Remember that preservative timber is safe from fungi or algae, but it’s not protected against damage from weathering.


Key Points to Remember on Treated Timber

  • When using preservative treated lumber a “brush on preservative” should be fully applied to rebates, cut surfaces, and even checkouts.
  • It’s not advisable to buy large pieces of preservative timber only to chop them into smaller pieces. In fact, this is a terrible practice!



We have constructed many home patios and decks using timber. Therefore, it’s critical to have the best type and size of the material. Also, in countries such as Australia, following the right building code regulations is key to ensuring a successful and fruitful process! We hope that with this small bit of knowledge, you can now create some of the best perfect decks or patios using only the best possible timber!




[1] timber-species-strength-durability

[2] timber-species-strength-durability/

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