Mass Timber: Thinking Critically About Ingredients

Adohi Hall nestled within old-growth white oaks. Photography by Tim Hursley.

By Ashley Rao

Ashley Rao is a senior associate at Leers Weinzapfel Associates. She was project manager for Adohi Hall at the University of Arkansas, the largest completed mass timber project in the US. A passionate advocate for sustainable design (and good eating), Ashley relishes new challenges and is eager to share what she has learned.

“Eat food. Not too much. Mostly plants.”

When I read Michael Pollan’s Food Rules: An Eater’s Manual a decade ago, it changed my life forever. With three simple edicts, he laid out a critical framework for making choices about food. My consumption of beans has been on the uptick ever since! Although I regularly enjoy a crispy-skinned chicken thigh, Pollan’s deep-seated suspicion about highly-processed foods has colored my thinking far beyond the kitchen.

Perhaps Michael Pollan’s adage for healthy eating applies to healthy building as well.

Thinking critically about ingredients has transformed the way I approach design and construction. Mass timber is an important part of that recipe, with a powerful, expressive materiality that warrants further investigation into the details of sourcing and processing.

Working in mass timber underscores the core design values of Leers Weinzapfel Associates – an approach to materials that embraces both exploration and economy, a commitment to sustainability, and a true partnership with clients. Designing and building with mass timber raises new questions and disrupts conventional practice in ways that continue to surprise and challenge me as an architect.

Mass timber construction, courtesy of Leers Weinzapfel Associates.

More Salad, Please.

Mass timber is both a natural product and a highly engineered one.

The natural par: trees. Living beings grown from a seed, stretched out tall toward the sun, strengthened by carbon pulled from the air. The transformation from tree to timber is purely mechanical – felling the tree, stripping the branches and bark, sawing and planing. The natural structure of the tree – consistently oriented chains of carbon stored as cellulose – remain intact. The size and shape are changed, but the internal structure is not.

In contrast, conventional building materials (such as steel and concrete), are not naturally found in usable form. They require a chemical transformation – a change of state (melting) and a change of internal structure to either separate fused elements or combine disparate ones.

Concrete begins life as limestone, sand, clay, and gypsum. These naturally-occurring materials are ground to powder, then clinkered – heated to a partial melting point that fuses the multiple materials together into a new, single material.

Steel begins life as iron ore, melted in the presence of charcoal, coke, and limestone to purify the ore into iron. This molten iron is injected with oxygen which pulls excess carbon from the molten iron, forming carbon monoxide and a steel alloy with a highly regulated mix of iron and carbon.

Timber is a chopped salad.

Concrete is bread.

Steel is rendered fat, or maybe a sizzling steak. Or string cheese? Pork rinds as slag?

Mass timber in the factory, courtesy of Leers Weinzapfel Associates.

Carbon Sequestration

Beyond the economy of energy required to transform trees into construction materials, wood has another benefit – it is literally is a form of stored carbon, carbon that has been actively removed from the atmosphere during the process of the tree’s growth.

Carbon is stored within the roots, trunk, branches, bark, and leaves of the living tree. Under natural forest conditions, the stored carbon is slowly released back into the atmosphere after the tree dies and decomposes. However, if the wood from the tree is sustainably harvested, this carbon storage can outlast the life span of the tree. The timber – with its stored carbon – can endure for centuries, incorporated into our buildings. A seedling growing in the place of the harvested tree will begin the process of carbon sequestration anew.

For years, the idea of a “carbon footprint” has focused exclusively on energy use. The embodied energy required to manufacture buildings was simply left off the nutrition label. However, once operational energy use has been substantially reduced, then the embodied energy of the building itself becomes a driver of the total carbon footprint.

The Match vs. The Marshmallow

Although wood is already broadly used in North America, it has typically been used in low-rise, light-frame residential construction. Although this system is cheap to produce, fast to construct, and doesn’t rely on skilled labor, light-wood framing has some significant drawbacks. Spans and loads are limited, and the arrangement of minimally-sized pieces of wood encased within long pockets of air creates a distinct fire hazard. Building code recognizes this, placing the most restrictive limitations (number of floors, height, area) on this type of construction, Type V.

Light wood framing construction is akin to corrugated cardboard. Lightweight, strong along one axis, but essentially a paper-thin diaphragm with mostly air inside. In contrast, think of CLT as chipboard, the old studio stand-by. Like chipboard, CLT is a monolithic, solid material, but one that is built up from a series of layers – this is the highly-engineered aspect of mass timber.

Cross-laminated timber (CLT) is made of a series of lamellas – a single layer of wood, typically 2x4s or other commodity dimensional lumber, carefully monitored for moisture content and finger jointed to continuous lengths, that are laid out side-by-side to form a continuous sheet. A layer of glue is spread on top, followed by another sheet of wood members. The dominant grain of each consecutive lamella is oriented perpendicular to that of the previous layer. The total “sandwich” of layers is always an odd number, ensuring that both the bottom and the top layer are oriented the same way. It’s like Jenga, just glued together.

This process aggregates small pieces of wood (typically from fast-growing softwoods) to create a large, solid panel of wood with extremely regulated physical characteristics. The laminated panel performs like traditional heavy timber in fire.

Instead of burning like a match, mass timber scorches like a marshmallow – creating an exterior coating of self-protective char that slows the penetration of the fire deeper into the wood.

These innovations in combustability and structural properties make mass timber a game changer for larger scale, commercial and institutional grade construction, dramatically expanding the scope of buildings that can be built in wood and magnifying the carbon sequestration potential of wood construction.

Shop-drilled penetrations in the CLT slab, courtesy of Leers Weinzapfel Associates.

Species and Terroir

Despite all the testing and engineering that underlies the creation of mass timber element, a tree is not a standardized unit. A W12 is a W12 no matter where it is produced…but a glulam beam or cross-laminated timber panel sourced from Canada’s northeastern black spruce forests will have different structural properties and different member sizes than a Douglas fir member from the west coast, or a European spruce-pine-fir mix from Austria. Species and terroir still matter.

This came as a revelation to me during the mass timber design assist phase on Adohi Hall, when the design team worked directly with the fabricator to finalize construction details. All the mass timber elements on the project had originally been based on Douglas fir, reflecting the dominant North American market. However, Binderholz/Holzpak was selected as the mass timber supplier, sourcing their glulam and CLT from the spruce, pine, and fir forests of the Italian and Austrian Alps. That species change impacted the size of every single structural member. In many cases, the difference was miniscule – the CLT depth increased from 6 7/8” to 7 1/16”. However, the typical glulam beam depth increased from 18” to 20 ½” – a critical difference that was unworkable in student bathroom zones, where ceiling height was already limited and mechanical/plumbing coordination was non-negotiable. A compromise was reached to address the immediate issue, but I learned a much broader lesson about the importance of species and terroir.

Beauty and Biophilia… But What About the “Bark”?

One of the most exciting aspects of mass timber design is the opportunity it creates to explore and expose the beauty of natural wood. At Adohi Hall, even simple, cellular spaces – like student dorm rooms and study spaces – are enlivened by the natural wood ceilings, the exposed underside of the structural CLT slab. “Exposed structure” doesn’t have to recall a pre-cast concrete parking garage, or a W-section covered in cementitious spray-on fireproofing. It can mean – as it does in the main “cabin” social space at Adohi — smooth, round spruce columns, still smelling of the forest; a queen-post timber and steel rod truss system; and CLT roof panels exposed as ceiling planes.

Although exposed wood structure is beautiful, it carries potential risks as well, of water as well as fire. Careful detailing at the column base, especially on the ground floor ensures that vulnerable end grain remains protected. On the exterior, a critical question emerges – what’s the right “bark” for a mass timber project? On a living tree, the exterior bark is visibly distinct from the inner wood – its primary role is to protect the living wood within. The dense outer bark deters insects as well as fire and maintains the water content of the inner wood.

Adohi Hall shies away from using unprotected natural wood on the exterior, opting instead for an enclosure that fills the practical role of bark, protecting the wood inside. It incorporates a lightweight metal rainscreen systems that minimizes additional loading on the mass timber structure. Natural wood is used to express “cuts” into the building form, at protected soffits and signaling points of entry.

Mass timber in Adohi's main cabin interior. Photography by Tim Hursley.

Exposed CLT ceiling in a typical study lounge. Photography by Tim Hursley.

Sometimes Less Means More

Mass timber is often touted as a path to less – less carbon, less construction waste, noise, and time on-site. All of that is true. But it is important to realize that some of that on-site savings doesn’t fully eliminate waste, noise, or time, just displaces it. Those dislocations in place and time can be deeply disruptive to conventional building practice.

Less construction time on-site means more work being done off the construction site (and in 3D models) both before and during construction. It means more people at the table and heightened coordination – not just between design disciplines but also between construction trades – to fully leverage the schedule savings. When the primary structural material is also the exposed finish material, it means more care and protection is required during construction to prevent damage from sun, water, and impact.

Coordinating the plumbing penetrations on the Adohi Hall project highlighted both the promise of accelerated mass timber construction, and the challenges that it poses to conventional construction sequencing.

In order to expedite on-site construction time, the design team and CM sought to locate and factory pre-drill all 4,116 penetrations through the CLT slab. Many of these were modular iterations, tied to the two primary student room types. But any unique conditions in the structural frame, CLT panelization, or MEPFP systems required careful coordination and verification with the architect, MEPFP engineers, structural engineer, and mass timber fabricator. The penetration model was ultimately owned and managed by Nabholz Construction, the CM, aggregating information from both the design team’s BIM model and the mass timber fabrication model. The aggressive schedule for design and construction simply didn’t anticipate the need for this joint coordination phase. On the Adohi Hall project, structural timber was released as an early package, with final detailing to be developed with the mass timber supplier through a design assist process. In retrospect, the savings in on-site construction time involved trade-offs with off-site coordination time, and the design assist effort could have been strengthened by involving more parties earlier in the process.

From Here

Building code literally codifies collective knowledge about safe and responsible building practices, defining a systematic, prescriptive response based on precedent. It protects us from yesterday’s mistakes (like the 1871 Chicago Fire), but today architects need the flexibility to respond to current challenges, most critically climate change. The height and limitations of code have long acted as a brake on the expansion of mass timber construction in the United States, prohibiting it from competing with conventional steel and concrete on larger, taller buildings. The revisions adopted in IBC 2021 change that, expanding the Type IV “heavy timber” construction type to recognize the unique properties of mass timber.

We may soon be at a turning point, where the master recipe for good building uses mass timber as the first ingredient.