What distinguishes Type 4 construction from the other types?

Type 4 construction is characterized primarily by its reliance on wood and other combustible materials. This construction type is often referred to as "heavy timber" or "mill construction" and features structural elements made of solid wood or laminated wood panels, which, due to their thickness, provide inherent fire resistance properties when compared to lighter wood structures.

The key aspect that distinguishes Type 4 from the other construction types is its complete reliance on these combustible materials for both structural integrity and overall construction, without the incorporation of non-combustible materials such as masonry or concrete as primary structural elements. Thus, this type offers unique challenges in terms of fire inspection and prevention, as it requires a comprehensive understanding of how combustion can be managed and mitigated in buildings that feature extensive use of timber and similar materials.

While fire-resistant materials, concrete supports, and masonry construction play significant roles in other construction classifications, they are not defining traits of Type 4, which maintains its identity through the predominance of wood-based materials creating a specific fire behavior and risk profile.