Type D Construction: What It Is, Where It's Used, and How It Compares

When you hear Type D construction, a rare classification under the International Building Code for buildings with non-combustible structural elements but limited fire resistance. Also known as Type IV Heavy Timber, it combines heavy wood framing with fire-resistant treatments, making it a niche but intentional choice for specific projects. Unlike Type A or B, which rely on steel or concrete, Type D uses thick, solid timber beams and columns that char slowly under fire, buying time for evacuation and firefighting. It’s not common in modern high-rises, but you’ll find it in historic renovations, warehouses, and some rural commercial buildings where aesthetics and sustainability matter as much as safety.

Why does Type D still exist? Because it’s not just about code compliance—it’s about balancing tradition, cost, and environmental impact. Heavy timber construction has been used for centuries, and modern versions use engineered wood like glulam and cross-laminated timber (CLT) to meet today’s standards. Compared to Type C construction, a more common commercial type using steel frames and non-combustible walls, Type D is slower to build but often cheaper in materials and has a lower carbon footprint. It doesn’t offer the same fire rating as Type A (steel and concrete), but its charring behavior makes it predictable under fire, unlike lightweight steel that can buckle suddenly. This is why some architects choose it for barns, workshops, or even boutique hotels where exposed wood adds warmth and character.

There’s a big difference between Type D and Type 5 construction, the most common residential type using light wood framing. Type 5 is everywhere in homes and small offices because it’s fast and cheap. But it’s also the most vulnerable to fire. Type D, by contrast, uses much thicker members—often 6 inches or more—that resist fire longer without needing sprinklers or extra coatings. That’s why some municipalities allow Type D in areas where Type 5 would be restricted, especially in fire-prone zones or near protected historic districts. It’s not a loophole—it’s a code-approved alternative that values material honesty over speed.

What you won’t find in Type D? Thin drywall, plastic insulation, or metal studs. The whole point is the structural wood itself doing the work. That means higher upfront costs for timber, skilled carpenters, and longer build times. But it also means fewer synthetic materials, less waste, and a structure that ages gracefully. If you’ve ever walked into an old factory with exposed beams and felt the weight of its craftsmanship, you’ve experienced Type D in action.

Below, you’ll find real-world examples, code breakdowns, and comparisons with other construction types—everything you need to understand where Type D fits, when to consider it, and why it’s making a quiet comeback in sustainable building circles.