What is the recommended dimension for lumber classified as heavy timber?

The recommended dimension for lumber classified as heavy timber is indeed a minimum of 6 inches by 6 inches. This classification is based on building codes and fire safety regulations that define heavy timber as a material that has fire-resistive characteristics suitable for certain construction types.

Heavy timber is typically used in structures where the timber is exposed to significant fire load. The larger dimensions help to ensure that these timber elements have adequate mass and thickness, which allows them to withstand fire exposure for longer periods compared to smaller dimensions. A dimension of 6x6 provides enough structural stability and resistance to heat, making it a preferred choice in applications such as commercial buildings, bridges, and large-span structures.

The other dimensions provided may lack the necessary mass to meet the requirements laid out in building codes for heavy timber, thus making 6x6 the minimum requirement that qualifies as heavy timber for safety and structural integrity during a fire event.