What type of treated timber is not allowed where used in external structures?

The type of treated timber that is not permitted for use in external structures is Chromated Copper Arsenate (CCA). CCA is a wood preservative that has historically been used to protect timber from decay and insect damage; however, it contains harmful chemicals, including arsenic, which can leach into the environment, posing health risks to humans and wildlife. Due to these environmental and health concerns, many building codes and guidelines now restrict or prohibit the use of CCA-treated wood for external applications where it may be exposed to the elements.

In contrast, other types of wood such as treated pine, oak, and redwood may be used for external structures. Treated pine, for example, is often treated with safer preservatives suitable for outdoor use. Oak and redwood are naturally resistant to decay, making them suitable choices for outdoor applications without the same risks associated with CCA. Therefore, allowing safer alternatives while banning CCA aligns with environmentally responsible practices in construction.