What is Mass Timber?
Mass timber is a category of framing styles typically characterized by the use of large solid wood panels for wall, floor, and roof construction. It also includes innovative forms of sculptural buildings, and non- building structures formed from solid wood panel or framing systems of six feet or more in width or depth. Mass timber is constructed by stacking small panels of new-growth wood in perpendicular layers and securing them together with glue or nails. This gives the timber product the equivalent size, strength and fire-resistance of old-growth wood without the ecological damage that arises from logging old-growth forests.
Types of Mass Timber products:
Cross-Laminated Timber:
Cross-laminated timber (CLT) is a wood panel product made from glueing layers of solid-sawn lumber together. Each layer of boards is oriented perpendicular to adjacent layers and glued on the wide faces of each board, usually in a symmetric way so that the outer layers have the same orientation. An odd number of layers is most common, but there are configurations with even numbers as well.
Nail-Laminated Timber:
Nail-laminated timber (NLT) is a mass timber panel system constructed by combining individual dimensioned lumber on edge with nails or screws. NLT panels can be up to 10-feet wide and 42-feet long.
Glulam:
Glued laminated timber, also called glulam, is a type of structural engineered wood product comprising a number of layers of dimensioned lumber bonded together with durable, moisture-resistant structural adhesives.
Why Mass Timber?
The production of steel and concrete emits carbon dioxide. Steel is essentially made by smelting iron ore with carbon in a blast furnace in which the carbon is obtained by burning coal. The majority of greenhouse gas emissions from steel production are produced from the blast furnaces. Concrete is made by combining aggregates with cement paste. Half of concrete’s greenhouse gas emissions are a result of the decomposition of these aggregates, and the other half comes from burning fossil fuels during production. When buildings are constructed out of steel or concrete, these greenhouse gas emissions must be taken into account in the building’s carbon footprint.
Conversely, buildings made out of mass timber actually sequester carbon. Trees capture and store atmospheric carbon during photosynthesis by converting it to organic matter, which is released back into the ecosystem when the trees die and decompose. However, wood that is harvested retains its stored carbon instead of recycling it back to the environment. Furthermore, logging forests for mass timber production promotes carbon sequestration when sustainable forest management is practiced and trees are replanted after harvesting. Taken in combination with the wood’s biophilic effects and low construction costs, mass timber is an excellent low-carbon alternative to traditional building materials such as steel or concrete.
Fire-resistance:
Fire is, of course, the first concern that comes to mind with wood construction. And yet, mass timber is actually safer in a fire than steel. A thick plank of wood will char on the outside, sealing the wood inside from damage. Metal, on the other hand, begins to melt. “Steel, when it burns, it’s like spaghetti,” says B.J. Yeh, the technical services director for APA—the Engineered Wood Association.
Resources for Further Research:
Greenhouse Gas Emissions from Major Industrial Processes
Comparative Life Cycle Assessments of Mass Timber and Concrete
An In-Depth Guide to Mass Timber