Pressure treated timber is a popular choice for an array of outdoor applications, such as decks, fencing, and garden furniture. This is mainly due to its durability and resistance to insects, fungal decay, and harsh weather conditions. One common concern, however, is the presence of green marks on pressure treated timber. This article will discuss what causes these green marks and provide useful information on whether they should be of concern, how long they last, and removal techniques.

What Causes Green Marks on Pressure Treated Timber?

The green marks typically observed on pressure treated timber are caused by the chemical preservatives used in the treatment process. These preservatives contain copper, which gives the wood a greenish hue. This is a natural and normal occurrence, and it indicates that the wood has been treated effectively to prevent decay and damage from insects and fungi.

Should I Worry About Green Marks?

Green marks on pressure treated timber are completely normal and should not be a cause for worry. They are an indication that the timber has been through the chemical treatment process and, as a result, has enhanced durability and resistance to pests and decay. The green marks are not harmful to you, your pets, or the environment, and they do not affect the integrity or performance of the treated timber.

How Long do Green Marks Last?

The green marks on pressure treated timber will gradually fade over time as the wood weathers and is exposed to sunlight. This process usually takes around six months to one year, depending on environmental factors and the specific preservative treatment used. After the natural weathering process, the treated timber will turn into a more subtle grey or brown shade, blending seamlessly with the natural wood tones.

How do I Remove Green Marks?

While there is no need to remove the green marks from your pressure treated timber, there may be instances where you prefer the wood to have a more natural colour. In this case, you may opt to stain or paint the timber to achieve a different aesthetic. Before doing so, it is recommended to let the wood weather naturally for a few weeks to several months, so the preservatives can set properly. Then, use a high-quality exterior wood stain or paint that is compatible with pressure treated timber. It's crucial to follow the manufacturer's instructions for application and drying times.


In summary, green marks on pressure treated timber are a normal occurrence, resulting from the copper content present in the chemical preservatives used in the treatment process. You should not worry about these marks, as they are an indication of effective protection against decay and pests. Over time, the marks will fade naturally, leaving a subtle grey or brown shade. If desired, the timber can be stained or painted after a proper weathering period. Ultimately, pressure treated timber is an excellent choice for various outdoor applications, and taking the necessary precautions to maintain and care for it will ensure its longevity and structural integrity.