Modern timber buildings can be cheap, green, and fireproof. It wasn’t the case in Medieval times when density, open fireplaces, candles, and flammable storage cramped dwellings in narrow irregular streets.
Consider the historic fires that destroyed, for example, London and Moscow during the late-17th and early-19th centuries, respectively. Nobody would be proud of being known to history as the source of a catastrophe capable of destroying a city, knocking down the first domino of a series of chain reactions that could turn an entire urban core to ashes.
If the event happens while you’re acting as a hero, working hard to ensure people can be fed in harsh times, the event is even more cruel. Despite later accounts that falsely accused a Frenchman, Robert Hubert, of igniting the blaze, the Great Fire of London started early in the morning on Sunday, 2 September 1666, at Thomas Farriner’s bakery, when one innocent spark jumped from the oven to a pile of fuel.
A few days in September 1666
Farriner was among the King’s bakers in central London, providing bread to the Royal Navy during the Anglo-Dutch war. Any other such event could have ended in nothing less than an anecdote, though it set the house ablaze that morning.
Hours later, the fire had extended to several houses, escaping control. When it finally did, it had gutted the entire City of London inside the old Roman wall, rising to the West. The Great Fire was so traumatic that it transformed building codes —and construction materials. Wooden buildings were substituted by stone and brick and mortar.
The limitations already in place to limit wooden houses with thatch roofs in dense areas had never been reinforced. Still, the blaze transformed public policy —and forbid the reconstruction of rickety wooden tenements.
Fires were common in crowded cities relying on wood siding and techniques such as roof thatching; in cold climates, open fireplaces, candles, ovens, and stored supplies amplified any hazard, but volunteer firefighting could keep blazes under control to avoid general spread. In the cramped central London of 1666, the fire’s scale destroyed elm pipes used to conduct water from the river Thames and strategic reservoirs nearby.
Moscow in flames before Napoleon’s arrival
Samuel Pepys and John Evelyn chronicled the fire, though their accounts of the transformational events were published in the 19th century. In contrast, street ballads also sprouted and found success right away:
Old London that,
Hath stood in State,
above six hundred years,
In six days space
Woe and alas!
is burn’d and drown’d in tears.
Another big city built almost exclusively on wood, Moscow, burned to the ground from 14 to 18 September 1812, just before Napoleon’s invading troops entered the city after the Battle of Borodino. Considered an act of warfare to restrict logistics and military resupply in the already evacuated city, the blaze extended mercilessly over a city built on wood. Tolstoy wrote about the trauma in “War and Peace”:
“They knew that it was for the army to fight, and that if it could not succeed it would not do to take young ladies and house serfs to the Three Hills quarter of Moscow to fight Napoleon, and that they must go away, sorry as they were to abandon their property to destruction. They went away without thinking of the tremendous significance of that immense and wealthy city being given over to destruction, for a great city with wooden buildings was certain when abandoned by its inhabitants to be burned.”
Unlike the spark that ignited medieval London or the acts of sabotage during war that destroyed Moscow before Napoleon’s arrival to the city, other cities saw their pervasive destruction due to a natural event, like Lisbon’s 1755 earthquake (and later tsunami), which impoverished one of the richest European cities at the time, or the more recent Sant Francisco 1906 earthquake.
The birth of modern building codes
Lisbon tried a new reinforced structure for its building, the “pombaline cage” (named after its author, the Marquis of Pombal), built on wood, though covered with masonry. Unlike London, Lisbon, or Moscow, San Francisco’s residential construction relied heavily on wood for framing and cladding.
The burning to the ground of big cities made of wood inspired classical and biblical images to the extent of inspiring rules against wood-clad tenements, thatched roofs, and cramped, irregular neighborhoods. During the Enlightenment, new sanitary standards and rational ideals inspired in the grids of Classical Greek and Roman cities, as well as new materials to build faster, higher, and more affordably, transformed cities forever, relegating wood from its historical structural roles.
Thanks to cross-laminated timber (CLT), structural wood is making a comeback, and not only in the impressive reconstruction of Notre-Dame de Paris’ roof (a restoration that includes traditional woodworking techniques for the lost mammoth framework), which suffered a devastating fire in 2019 and plans to welcome visitors once again in December 2024.
CLT, a reinforced but simple way to build with wood, is structurally suitable for building tall buildings with less foundation and better insulation. A social housing building outside Barcelona just got completed. Mjostarnet, a skyscraper built on the shore of Lake Mjosa, Norway, is an 85-meter, air-tight, passive solar building that reduces the heating bill during freezing winters. It was the tallest CLT building until Ascent MKE (87 meters) was completed in Milwaukee, Wisconsin. Soon, both buildings will be shorter than planned mass timber high-rises in Ontario and Switzerland.
Cross-laminated timber modules
CLT acquires its sturdiness from gluing together at least three layers of solid-sawn lumber, oriented at right angles to one another, whereas glued laminated timber or glulam, an older technique derived from 19th-century plywood, is a version of engineered wood using all laminations in the same orientation.
Experts believe CLT could spark a new generation of wooden buildings and eventually instigate the comeback of cities made mainly of wood —though this time, they will not be likely to burn that easily. CLT adapts to construction using prefabricated panels, already manufactured with preinstalled, fire-retardant insulation, and modern cities lack most of the hazards of medieval cities. Most importantly, cross-laminated buildings avoid using off-gassing materials that represent a fire and health hazard in conventional construction, like oil-derived paints, insulators, and sealants.
Contrary to common belief, if properly maintained, wood-derived structural materials resist better to fire than steel, whose molecular structure collapses and melts at high temperatures. CLT panels are produced (and tested) with different fire resistances, ranging from half an hour to 90 minutes in uncontrolled blazes, and bear a resistance comparable to that of heavy timber. Though, unlike heavy timber, CLT can be produced from wood extracted from forests that need thinning for better management that increases biodiversity and reduces fire hazards.
To some, mass timber promises a future with more efficient and livable neighborhoods —and even entire cities. A Swedish group recently announced their intention of building an entire city from wood, this time according to the last mass timber technological standards, with all the benefits of wood and few of its contingencies (beginning with the fear of medieval towns burning to the ground).
A city out of reinforced wood
Stockholm Wood City will be a whole town made of wood near Sickla, south of the Swedish capital. Construction will begin in 2025, and the urban development company Atrium Ljungberg, the main promoter, expects to complete it ten years later, when it will contain 2,000 homes, abundant office space, restaurants, and shops.
Sweden benefits from direct access to its local agroforestry, cutting both impact and cost. The project will use some concrete and steel in foundations and infrastructures, but the overall amounts are dramatically reduced: wooden buildings are much lighter, and their foundations are smaller, even keeping higher than normal structural performances.
The development will put to the test CLT construction and prefabricated sections of engineered timber (sections built at finer tolerances in a factory, then transported to the building site) at a big scale: wood grains in different components (floors, walls, beams) are aligned to increase strength. The sections are also installed more easily, reducing construction waste and speeding up construction once the foundation is in place.
The biggest concern, also in Stockholm Wood City, is public perception of wood as the main construction material at a big scale and the fire hazard this represents. Developers state that the buildings will have several fire-protection systems, from sprinklers to flame-resistant layers.
According to The Economist:
“At the same time, researchers are coming to believe that engineered timber is, by its nature, extremely fire resistant. To help win approval for the construction of the Ascent building, the us Forest Service carried out tests on the laminated timber columns it would use. After finding them difficult to burn, the columns were awarded an exemplary three-hour fire-resistance rating because they maintained their structural integrity.
“Without a sustained heat source the charring of the outer layer of a big piece of timber protects the structure inside—try lighting a camp fire when you only have logs. Many of the large urban fires of old, like the Great Fire of London in 1666, were mostly fuelled by small sections of timber acting as kindling. So when it comes to building in wood, it is best to think big.”
Cork and other fire-retardant wood derivatives for cladding and insulation could also be included in such constructions to reduce wooden buildings’ overall flammability. There are examples of such use at a smaller scale, like Andrew Linn compostable house in a DC alley.
Linn, a partner at BLDUS, used BamCore as primary wall material, a hybrid of bamboo and wood plywood panels that eliminates studs in walls. They used a cork cladding for the house’s exterior (cork is a natural fire retardant).
What if wooden buildings and entire cities were as fireproof as those built of concrete and steel, and structurally sounder during more time once attacked by a once-in-a-lifetime blaze?