Matt Ruckman

Cut a tree down and look at the cross-section. You will see a small dark dot at the center, a pale ring of newer wood near the outside, and a darker, denser zone in between.¹ The denser zone is the part that holds the tree up. It is also, by the time you are looking at it, dead.

This is one of the strangest arrangements in structural biology, and almost no one knows it is happening. Wood does the work of holding the tree (and later, the building) up by retiring the cells in its core: pulling them out of active circulation, locking their walls in place with a polymer the rest of the planet has never quite figured out how to digest, and leaving them in position. The tree grows outward around them. The inert core gets bigger every year. By the time the trunk is felled and milled and shipped to a lumberyard, almost all of what you would call the wood is scaffolding the cambium produced decades before the chainsaw arrived.

This is also the first hint of why wood lasts longer than you think it would. There is nothing in it left to die.

The empirical record is the oddest part. Stone is supposed to be the permanent material; concrete is supposed to last a century; wood is supposed to be the medium of cabins and porches, the thing you build with when you cannot afford to build with anything else. Except that the oldest wooden building on Earth is fourteen centuries old and still in daily use. The Norwegian stave churches have stood for nine hundred winters. The American barn behind your grandparents' house, which has not been maintained in forty years, is probably still standing. Sagging, but standing. Wood, properly understood, may be the most quietly persistent structural material humans have ever used.

The question of why is one of the strangest stories in chemistry. It involves a molecule that took sixty million years to invent, another sixty million years for the planet to figure out how to digest, and a stretch in between during which entire forests piled up in swamps and slowly became coal because nothing alive could eat them. The buildings come later. The chemistry comes first.

A polymer the planet wasn't ready for

For most of the history of life on Earth, plants were small and soft. The first land plants, around 470 million years ago, were close cousins of the modern liverwort: low, mossy, dependent on direct contact with water, and unable to grow more than a few centimeters tall. They had no internal plumbing to lift water against gravity. They had no structural tissue to hold themselves upright. The land was green in patches and the green hugged the ground.

This persisted for tens of millions of years, because growing taller turns out to be a problem that requires solving two things at once. To grow up, a plant needs pipes that move water from roots to leaves, and it needs posts that hold the plant up against gravity. The two requirements are mechanically incompatible: thin tubes leak strength, thick stems leak water. A plant that solves either problem in isolation gets killed by the other one.

Sometime around 425 million years ago, in the late Silurian, a group of early vascular plants stumbled into a single biochemical innovation that solved both problems simultaneously. They evolved lignin: a complex organic polymer built from three closely related alcohols (coniferyl, sinapyl, and p-coumaryl), cross-linked into a three-dimensional structure of extraordinary mechanical strength and chemical stability. Plants impregnated lignin into the walls of their water-conducting cells (the xylem), where it did two contradictory things at once. It stiffened the cell walls enough to resist crushing under load, and it waterproofed them enough that water could flow through the cell's interior without leaking out the sides. The same molecule turned the pipes into posts.

The consequence was immediate, in geological terms. Plants with lignified xylem could grow tall. Within a few tens of millions of years, land plants went from a few centimeters to trees the size of modern oaks. By the middle Devonian, around 385 million years ago, the first true forests appeared, dominated by a tree called Archaeopteris that grew 30 meters tall and had branches and a leafy crown not all that different from a modern conifer. The land, for the first time, had a vertical dimension. Everything that we associate with terrestrial ecology (the canopy, the understory, the leaf litter, the soil it eventually built, the animals that climbed and nested and hunted in all of it, the atmosphere that filled with oxygen as a result, the climate that cooled as the new forests pulled CO₂ out of the air) followed from this one polymer.

Lignin is the chemistry that made the world three-dimensional.

Here is where the story turns into something genuinely strange. When a plant dies, what happens to its body is fundamentally a question of what other organisms can digest the molecules in it. Most plant tissue (cellulose, pectin, sugars) is comparatively edible. Many microbes and fungi and animals can break it down. They have been doing so for hundreds of millions of years.

Lignin is harder. The cross-linked irregular structure that makes it strong also makes it resistant to enzymatic attack. There is no simple key for the lock. Breaking lignin down requires a specific class of enzymes called peroxidases and laccases, which use oxidative chemistry (essentially, controlled burning) rather than the hydrolysis that breaks down most organic compounds. These enzymes are difficult to evolve. The white-rot fungi that have ever truly mastered the trick did not get their full toolkit assembled until somewhere around 290 to 300 million years ago, near the end of the Carboniferous period.

Cellulose fibers (cream) embedded in lignin matrix (ochre). Stacked into cells, into tissues, into the structure of the trunk: this is what wood is made of.

What happened in the meantime is one of the strangest accidents in the history of life. Trees grew, died, and fell. Their lignified bodies, full of indigestible polymer, accumulated in swamps and lowland forests across the equatorial belt of the supercontinent. Layer upon layer of dead trees piled up, slowly compressed by their own descendants and the sediments that buried them, sealed away from the oxygen that might have slowly oxidized them. Over millions of years, the buried plant matter was cooked into peat, then lignite, then bituminous coal, then anthracite.

This is the coal. All of it.² Every coal seam in the world is a Carboniferous forest that died before the fungi were ready and got buried because nothing alive could eat it. The Industrial Revolution ran on stored sunlight from a 60-million-year window during which a single biochemical breakthrough had not yet happened. We have been burning Carboniferous forests for energy for two centuries, and the reason there is a Carboniferous forest to burn is that, for a brief and weird stretch of evolutionary time, the planet's recyclers couldn't keep up with the planet's builders.

When the fungi finally caught up, the coal accumulation slowed dramatically. Forests kept growing, but their dead bodies began to rot in the conventional way rather than being preserved underground. The Permian and Triassic and Jurassic and Cretaceous forests are not, by and large, our coal. The Carboniferous forests are.

The properties of a successful lock

The lignin story explains nearly every property of structural timber that matters, which is convenient, because we are now going to talk about timber as a building material, and it will help to know that everything the material does follows from the chemistry it inherited.

Wood is, mechanically, a composite. A typical structural softwood is roughly 40% cellulose, 30% hemicellulose, and 25% to 30% lignin by weight, with the remainder being various extractives, resins, and water. The cellulose provides tensile strength: long molecular fibers that resist being pulled apart. The lignin provides compressive strength and rigidity: a stiff matrix that resists being crushed. The combination is, in its mechanical principles, identical to reinforced concrete: tensile fibers embedded in a compressive matrix. Wood was the original composite material, perfected by evolution 400 million years before any human engineer thought to combine steel and concrete.

Strength-to-weight, structural timber outperforms steel and dramatically outperforms concrete.³

None of this would surprise anyone who has split firewood. A log set on its end takes the maul with a soft tock and falls open along the grain; the same log laid sideways takes the same maul and the same swing and the maul bounces off. Your shoulders work out the principle in about three minutes.⁴ There's a word for it, and it is the real catch: anisotropy. The lignified cells are aligned vertically in the tree, because the tree needed to resist gravity and move water upward. The grain is the alignment of those cells. A piece of dimensional lumber (2x4, 2x6, 2x8, etc.) is dramatically stronger along its grain than across it. Perpendicular to grain, the same softwood that handles 7,000 psi parallel might fail at 500 psi. This is the single most important thing to understand about wood as a structural material: it has a direction, and the direction matters.

Every traditional timber-building tradition exists to accommodate this. The post-and-beam frame uses vertical posts (loaded parallel to grain, where wood is strong) supporting horizontal beams (also loaded parallel to grain, where wood is strong) connected by joints (which transfer load across the grain at small enough scale that the cross-grain weakness doesn't matter). The Japanese joinery tradition produced wood-on-wood connections of such precision that they outperform metal hardware in earthquakes, but every joint is designed around the grain. The grain is the rule. Everything else is craft.

A cross-laminated timber panel, exploded. Every other layer is rotated ninety degrees, which is what turns a one-directional material into a two-directional one.

The contemporary innovation, the one that has made it possible to build 25-story towers out of wood, is something called cross-laminated timber (CLT). CLT panels are stacks of dimensional lumber where each layer is rotated 90 degrees from the one below it, glued under pressure into a single composite slab. The cross-grain stacking solves the anisotropy problem by giving the panel strength in both directions. A CLT panel behaves, structurally, more like reinforced concrete than like solid wood: predictable in two dimensions, dimensionally stable, able to act as both a structural element and an enclosure surface.

The other modern composite is glued laminated timber, glulam, which has been around since 1906. Glulam is just dimensional lumber stacked with the grain all running the same direction and glued together, producing beams and columns longer and stronger than any single tree could provide. It is what most exposed mass-timber structures use for columns, beams, and trusses. CLT is what they use for floor and wall panels. Together, with a few specialized cousins (dowel-laminated timber, nail-laminated timber, laminated veneer lumber), glulam and CLT make up the family called mass timber, and they are the basis of the current revival.

The forests

Timber is a deeply regional material. The species and the building traditions are local, and the pilgrimage list of great timber buildings turns out, on closer inspection, to be a map of the forests that produced them. A tour, by forest:

The Suzzallo Reading Room⁵, University of Washington, Seattle. Gothic Revival stone overhead, long oak reading tables underfoot, a hush that comes partly from the timber.

The Pacific Northwest. Douglas fir, western hemlock, and western red cedar. The classic American structural softwoods. The forests stretch from northern California to southeast Alaska. The grain is tight, the strength is exceptional, and the trees grow to enormous size. Old-growth Douglas fir routinely reached 70 meters tall and 2 meters in diameter, though most contemporary harvest is from second- or third-growth forests with smaller trees. Douglas fir is the structural backbone of the West Coast mass-timber industry, the species you will smell in any Vancouver or Portland or Seattle mass-timber building. The red cedar, meanwhile, produced the largest pre-industrial wooden buildings ever raised in the Americas: the Indigenous longhouses of the Pacific Northwest coast, plank structures some over 60 meters long, housing entire extended families. Most were destroyed in the colonial period, but the tradition survives in the work of contemporary Indigenous builders, and you have never heard of them.

The American South. Southern yellow pine: loblolly, longleaf, slash, shortleaf. The fastest-growing structural softwood in the world, with rotation cycles as short as 25 years on well-managed plantations. The South now produces more structural timber than any other region in North America. This is the species that will most likely be the substrate of any contemporary timber project anywhere from Virginia to Texas, because the supply is enormous and the price is competitive.

A mature longleaf pine savanna. The widely spaced trunks and grassy understory are the natural state of the species; the dense pine plantations that supply most of today's Southern timber look nothing like this. The Southern building tradition was built from these older, slower-grown trees, and the heart-pine floors of antebellum houses are mostly what's left of them.

The American Northeast and Midwest. White oak, red oak, maple, hickory, ash. The great hardwoods of the Eastern forest. Less common in modern mass timber, which favors the faster-growing softwoods for economic reasons, but historically the substrate of the most underappreciated structural-timber tradition on the continent: the American barn. Pennsylvania bank barns, New England English barns, Midwestern dairy barns, the great tobacco barns of Kentucky and North Carolina. Post-and-beam structures of remarkable engineering, built by farmers and small-town carpenters using local hardwoods (white oak especially, one of the world's great structural hardwoods: rot-resistant, dimensionally stable, beautiful, slow-growing), often standing 150 years and still in use. The barn behind your grandparents' house is probably better engineered than the strip mall in town.⁶

A Pennsylvania bank barn in Adams County. The forebay overhang shelters stalls below; the bank itself lets wagons roll into the haymow at grade on the uphill side. Hand-hewn white oak throughout.

Scandinavia. Norway spruce, Scots pine, larch. The classical European structural timbers, the basis of the great timber farmhouses of Norway and now of the European mass-timber industry; most European glulam runs on these species. They are also the timbers of the Norwegian stave churches, of which twenty-eight survive. All were built between roughly 1130 and 1350. Urnes Stave Church, the oldest, dates to around 1130. They are constructed entirely of timber (Scots pine, mostly) with stone sill plates lifting the wood off the wet Norwegian ground. The carving is among the finest medieval woodwork anywhere. They have stood through 900 winters. Several have burned and been rebuilt; most have not.

Borgund Stave Church in Lærdal, Norway, built around 1180. The best-preserved of the twenty-eight survivors. Pine-tar coating on the exterior, reapplied every few decades, has kept it from rotting for 800 years.

The Alps. Larch, spruce, silver fir. The Tyrolean, Bavarian, and Swiss building traditions. The great timber chalets and Tyrolean barns, centuries old and blackened by sun and weather to a deep silver, many still in everyday use. The Heidi cliché has obscured how seriously these buildings were engineered; many used (and still use) joinery that would impress a modern Japanese temple carpenter. Larch is unusual among conifers in producing a hard, dense, weather-resistant wood that ages to a deep silver-black; entire Alpine villages are clad in larch shingles that have stood for centuries. The wood gets harder as it ages, which is not how most materials work.

Hand-split larch shingles on a contemporary South Tyrolean building, newly installed. Some of the older roofs in the region are pushing 200 years without replacement.

Japan. Hinoki (Japanese cypress), sugi (Japanese cedar), keyaki (zelkova).⁷ The masterpieces of the world tradition cluster around Nara and Kyoto. Hōryū-ji, near Nara, is the canonical site: the Kondō and the five-story pagoda are the oldest wooden buildings on Earth, at roughly 1,400 years; the pagoda's central pillar was felled in 594 CE. This is what wood does when somebody bothers to keep the roof on. Tōdai-ji's Daibutsuden, also in Nara, is the largest historical wooden building in the world, housing a 15-meter bronze Buddha. The current structure dates to 1709, after fire took the original 752 CE building. It remained the world's largest until very recently.

And then there is the Ise Shrine, in Mie Prefecture, ritually rebuilt from new hinoki cypress every twenty years for roughly 1,300 years. Same dimensions, same joinery, same orientation, same species. The current shrine is the 62nd; the 63rd begins in 2033. The carpenters are trained from childhood; the cypress trees are planted in dedicated forests 200 years before they will be needed.

The Ise Grand Shrine, Mie Prefecture.

The Ise model is the opposite of the cathedral model. Stone cathedrals try to make a single building last forever. The Ise Shrine makes the practice of rebuilding last forever, which turns out to be more achievable and arguably more meaningful.

Australia. Eucalyptus species and radiata pine. The Australian mass-timber industry is small but growing. Eucalyptus produces extremely hard, dense structural timber unlike anything in the Northern Hemisphere. Some species (jarrah, ironbark) are so dense they sink in water.

A mature karri forest (Eucalyptus diversicolor) in Western Australia. Karri is among the tallest hardwoods in the world, reaching 90 meters.

Tropical regions. Mass timber is largely a temperate-and-boreal technology. Tropical structural building is the domain of bamboo, which is genuinely a different essay. The tropics also produce a few species that prefer to be left alone.⁸

Tropical rainforest canopy on Barro Colorado Island, Panama, the Smithsonian Tropical Research Institute's long-term research forest.

On price

Mass timber is no longer a premium product. In most North American and European markets it is cost-competitive with concrete and steel for buildings in the 4-to-12 story range, and often cheaper once the schedule is honestly tallied.⁹

The schedule is where the real economics lives. A nine-story CLT building like Stadthaus at Murray Grove was erected and enclosed in eight weeks of timber assembly; the concrete equivalent runs six to eight months. Construction-loan interest on a major project runs into the hundreds of thousands of dollars per week. Cut twenty weeks out of the calendar, and you have saved more money than any per-square-foot difference will ever amount to. Lighter foundations and heavy prefabrication help at the margins, but time is the dominant variable. By the time the concrete project is still pouring, the timber project is generating rent.

The lifecycle math points the same direction. Mass-timber buildings have shorter design lives than stone (typically 75–150 years versus 300-plus), but they are easier to take apart, repair, and reassemble somewhere else. A reclaimed-timber market is already running, on the same logic as the reclaimed-barn-beam market that has existed for a century. The carbon balance is the decisive argument: a mass-timber building disassembled and reused doubles the sequestration benefit, since the carbon stays locked up through the second life.

Insurance is the friction point that has not yet caught up. Many North American underwriters still price mass-timber projects as if they were stick-frame construction, which is dramatically more flammable, and premiums reflect the miscategorization. The actuarial data is starting to correct things, but it remains a cost wedge.

Coming back, almost everywhere

The contemporary mass-timber revival is barely fifteen years old, and the timeline is worth tracing.

The first significant mid-rise mass-timber project was the Stadthaus at Murray Grove in London, completed in 2009. Nine stories, residential, built entirely from CLT panels in eight weeks of timber erection. At the time it was the tallest residential timber building in the world and was widely considered a stunt. A decade later, the same building was conventional.

The European market matured rapidly through the 2010s. Austria, Germany, Switzerland, and Scandinavia developed extensive supply chains, building codes adapted to mass timber, and a generation of architects who designed in the material as their default. Mjøstårnet in Brumunddal, Norway, completed in 2019, was the world's tallest timber building at the time at 85.4 meters (18 stories). It is built almost entirely from glulam, with concrete only in the foundation. The timber came from Norwegian forests within a hundred miles of the site.

Ascent in Milwaukee, Wisconsin, completed in 2022, is currently the world's tallest mass-timber building at 86.6 meters (25 stories). It uses a hybrid concrete podium with mass timber above, a typology that has become standard for high-rise timber in North America. The next record is the Rocket and Tigerli tower in Winterthur, Switzerland, projected to top out at 100 meters in 2026. This is the threshold at which mass timber stops being experimental and starts being a serious competitor to concrete for high-rise construction.

The North American market lags Europe by about a decade but is accelerating quickly. The US International Building Code added formal recognition of mass timber up to 18 stories in the 2021 cycle (the previous limit was 6 stories). Major mass-timber projects are now under construction or completed in Portland, Seattle, Vancouver, Toronto, Atlanta, Boston, and New York.

In Seattle, an eight-story workforce-housing project in Capitol Hill topped out in 2025 as the tallest mass-timber building in the city: a Swinerton build, concrete podium with mass timber above, opening to residents this year. The building is named Heartwood.

Heartwood

The word is finally what the essay is about, so it is worth saying what it is.

When you cut down a tree and section the trunk, you see two zones. The outer ring, lighter in color, is sapwood: living cells, still pumping water from the roots, still part of the tree's active life. The inner zone, darker and denser and more resinous, is heartwood. It is dead. The cells in heartwood stopped conducting water and filled themselves with tannins and resins years before. They are no longer waiting to be cut.

An incomplete inventory of the heartwood currently within four feet of you:

The floor, almost certainly. The desk or table, if it is wood. The chair, in part, even the ones with metal frames, because the seat is probably plywood, and plywood is heartwood sliced thin and laid back together in a hurry. The pencil, if you still own a pencil. The shelves. The frames of the windows and the frame of the door and the frame of the door behind that door. Most of the cabinets in most of the rooms in this building. The studs inside the walls you cannot see, holding up the ceiling you also cannot see, which is also probably heartwood, painted white so you will not think about it. The roof beams above the ceiling. The trusses above the roof beams.

All of it was a tree once. All of it is the dead part of the tree. None of it is doing anything except holding everything else up.

You have been inside it your whole life. You have just been calling it a home.