About Durability and Use Grade of Wood
Both the durability and use grade of wood play an important role in the selection process. Here we will discuss it in further detail, which will help you determine if the wood you are considering meets the requirements of your project.
Durability refers to how long the wood will last, usually measured in years. Everything decays, even steel and concrete, so the durability of wood (the number of years it reaches its usefulness) depends on how it performs in a specific environment.
The environment in which the wood is used is called the use grade.
Many hardwood lumbers are naturally durable and can even be used outdoors untreated. On the downside, these naturally durable hardwood species are often very expensive and sometimes in limited supply. Some softwoods are durable to a certain extent, however, most require additional preservative treatment before being used outdoors or in environments prone to condensation or moisture. On the plus side, these softwoods are inexpensive and have a wide range of uses.
Durability Grade Summary: To easily identify the durability of any wood, each species is assigned a durability grade. This shows you how long the wood will last and must be considered relative to where the wood will be used (use category). Standard tests to determine the durability of wood involve putting the heartwood in the ground and monitoring it over time.
Class 5: Not durable. For example, birch and beech or any sapwood. Lifespan 0-5 years.
Class 4: Slightly durable - for example, Scots pine, spruce. Lifespan 5-10 years.
Class 3: Moderately durable - for example, Douglass fir. Lifespan 10-15 years.
Class 2: Durable - for example, oak and cedar. Lifespan 15-25 years.
Class 1: Very durable - for example, teak, greenheart and jarrah. Wood lifespan over 25 years.
If the wood is kept in a protected interior environment, the lifespan can actually be 50+ years, so the lifespan figures in the Class system refer to lifespan in an unprotected exterior environment.
The ultimate end use of preservative treated wood can be classified into one of 5 classes (also called use categories). These are established and defined in the British EN standard BS-EN 355-1 and are based on how dangerous the wood is to insect attack and decay. The Use Class system is used widely throughout the timber industry to help determine the level of treatment required, depending on the environment in which the timber is to be used. The higher the Use Class number, the greater the risk to the timber and, therefore, a stronger treatment will be required.
It is important that you always check with your timber supplier which Use Class the timber you are purchasing is. This is particularly important if you intend for the timber to come into contact with the ground.
The level of the grade achieved during the treatment process depends on the amount of chemicals left in the timber.
It is important to note that durability is based on ground contact, which of course does not always correlate. This is why it is also important to consider the Use Class of the timber, as this will depend on the end use.
Questions to ask before buying timber for your project
• Where will the timber be used during the process?
• How durable is the timber you have chosen?
• Is the timber naturally durable?
• If the timber is not naturally durable, does it have the correct Use Class?
• Do you have a certificate from your supplier that shows which Use Class the timber is considered to be?
The durability rating of a wood reflects its ability to resist decay and insect attack in the heartwood. Durability is a good indication of how long a wood will last, however, it does not take into account all the variables that may affect its lifespan. These variables include humidity, weather, temperature, installation conditions, and the physical stresses that the wood must withstand.