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The Incredible Lightness of Heavy Timber

May 31, 2017

Seem counterintuitive? Read on.

By Rick Reynolds

Admittedly, the “lightness of heavy timber” sounds like a contradiction in terms. After all, “heavy timber,” by definition, sounds like a “weighty” subject. But buildings built with heavy timber can be “light” in every sense of the word.

Before we get into the lightness of heavy timber construction, let’s start by defining, “heavy timber,” both solid-sawn and engineered laminates.

Not surprisingly, the primary difference between “light” and “heavy timber” is the section size of the construction members. For example, in light-timber construction, small-section stud members form wall and floor assemblies, enclosed with cladding. “Balloon,” or “stick-framed” construction, which became prominent in the beginning of the 20th century and continues today, is an example.

While there is no widely accepted standard for the point where light timber transitions to heavy timber, generally speaking, “heavy timber” has a minimum dimension of solid wood exceeding five inches.  These larger dimensions ensure that timber chars during a fire, developing a protective insulative shell and keeping the core of the timber structurally sound.

Compared with light-timber balloon construction, heavy timber construction requires fewer, large-section wood products for the building superstructure. This may include solid-sawn heavy timbers; however, more and more, modern timber buildings are designed with engineered heavy timber products by virtue of their greater strength and design flexibility.

Post-and-beam laminates (glulams), and panelized cross-laminated timbers (CLTs) are two main examples of this type of engineered heavy timber construction, and there are many different permutations of these.*

Glued laminated timber, called “glulam,” is a structural engineered wood product comprised of a number of layers of dimensional lumber that are bonded together with all the wood fibers parallel, using durable, moisture-resistant structural adhesives. Cross-laminated timber, or CLT, is a large-scale, prefabricated, solid-engineered wood panel that consists of several layers of kiln-dried lumber boards stacked in alternating directions, bonded with structural adhesives, and pressed to form a solid, straight, rectangular product that is dimensionally stable.

As mentioned above, it’s important to emphasize here that heavy timber, whether solid-sawn or an engineered laminate, is extremely fire resistant, protecting itself through the charring effect. Anyone who has ever tried to set a log on fire without kindling knows that it’s nearly impossible to set it aflame. Under high heat, structural steel loses strength rapidly and concrete spalls. Heavy timber, unlike other materials, protects itself in a fire, allowing more time for occupants to evacuate the building and for firefighters to control and extinguish the fire.

In general, engineered heavy-timber construction offers architects and structural engineers longer, unsupported span possibilities, larger open-space plans, and taller buildings. This translates into spacious, airy, lighter-looking spaces. Added to this, flowing, curvilinear shapes and soaring volumes can be achieved that belie any sense of massiveness.

Beyond lighter looking volumes, heavy-timber buildings are actually lighter in weight than comparable steel and concrete structures (which have a higher total carbon offset than single source materials like wood). Lower mass, in turn, allows for simpler materials manufacturing, through off-site fabrication using computer numerical controlled (CNC) machinery, as well as lighter transport mass, and ultimately, lighter foundations.

All this translates into a lighter carbon footprint than other building materials. In addition to sequestering carbon from the atmosphere, wood from managed forests is a renewable resource, with a lifecycle measuring in centuries and practicable downstream recyclability. Wood is also a better insulator than steel and concrete.

Finally, engineered heavy timbers, in particular, are aesthetically lighter, offering a more homogeneous look with less crazing and fewer knots.

So, in sum, the incredible lightness of heavy timbers—both solid-sawn and engineered—is not a misnomer. Collectively they permit:

  • Lighter structures. Fewer support members. Stronger glulam columns and beams. Plus panelized, structural, CLT construction requires no framing.
  • Lighter, longer unsupported spans: airy, light-filled, open plan areas, and taller structures
  • Lighter, soaring, flowing, curvilinear design possibilities
  • Lighter carbon footprint than other building materials; a renewable resource from managed forests
  • Lighter than steel or cement to fabricate, transport, install
  • Lighter building weight allows for lighter foundations, smaller crews
  • Lighter than balloon or light-frame construction
  • Lighter on environment: better insulating, carbon sequestering, longer lifecycle, downstream recyclability
  • Lighter in appearance: Engineered timbers offer a more homogeneous look, with less crazing and fewer knots

*Other examples include NLT ( nail laminated timber), LSL (laminated strand lumber), LVL (laminated veneer lumber), PSL (parallel strand lumber), and SCL (structural composite lumber).