Cross-Laminated Timber (CLT) was initially developed in Europe as an alternative to stone, masonry and concrete construction. It is essentially mass timber plates made from smaller framing lumber laminated crosswise on their wide faces. North American CLT is typically laid up as three-, five-, seven-, and nine-layer panels of 2x6 lumber finger jointed lamstock.
Here in North America, one might first see these mass timber plates and wonder, “Why on earth would I need that big piece of wood to build with?” Well, there are several common misconceptions and myths surrounding the use of wood as a building material, especially these new mass timber CLT panels.
The first myth I always hear is, “It won’t meet the code,” then next, “It's wood, it will just burn down.” My favorite is, “You are cutting down all our big trees.”
One of the best resources for clarifying the topic is the 2013 US CLT Handbook. FPInnovations—in collaboration with the American Wood Council (AWC), the United States (U.S.) Forest Products Laboratory, APA, and U.S. WoodWorks—published the comprehensive guide to provide technical information for building professionals, illustrating CLT applications adapted to current codes and standards. The handbook addresses a number of these common misconceptions about wood and CLT.
MYTH #1 — “CLT IS NOT IN THE BUILDING CODE.”
CLT panels have great potential for providing cost-effective building solutions for residential, commercial, and institutional buildings, as well as large industrial facilities in accordance with the International Building Code.
In 2015, CLT will be incorporated in the International Building Code (IBC). The IBC recently adopted ANSI CLT Standard PRG 320 into the 2015 IBC, (see US CLT Handbook Chapter 1, p. 2) so you can request a design review based on it now and submit it as an alternate material, design and methods (AMM).
MYTH #2 — “CLT IS A WOOD PRODUCT AND, THEREFORE, EASILY CATCHES ON FIRE.”
Like using a few 12-inch-diameter logs to start a camp fire, mass timber does not catch fire easily. In fact, CLT acts more like concrete. Mass timber is not conventional so it is very hard to light, and once it is lit, it wants to put itself out (see US CLT Handbook Chapter 8, p. 2).
A research project recently completed at FPInnovations showed that CLT panels have the potential to provide excellent fire resistance, often comparable to typical heavy construction assemblies of non-combustible construction. CLT panels can maintain significant structural capacity for an extended duration of time when exposed to fire.
MYTH #3 — “YOU HAVE TO BRING IN A SPECIALIZED CREW TO INSTALL CLT.”
Keep in mind, CLT is just another form of glue laminated timber (glulam). It is just wood, so it designs and builds on the earlier technology. CLT panels, like other industry panels (precast concrete or SIP panels), provide easy handling during construction and a high level of prefabrication facilitation and rapid project completion.
A conventional wood installation crew with other panel experience can lift, set, and screw down CLT panels, and with a manufacturer provided installation plan, it goes even faster (see US CLT Handbook Chapter 12, p. 1).
MYTH #4 — “MASS TIMBER IS NOT GOOD FOR THE ENVIRONMENT SINCE MANY TREES NEED TO BE CUT DOWN TO CREATE THE BUILDING MATERIAL.”
CLT is manufactured 2x6 lumber from trees harvested from sustainably managed forests, and mostly Mountain Pine Beetle kill trees. If we don’t use them, they decay and emit carbon back into the atmosphere.
Wood is also the only primary structural material that grows naturally and is renewable. In fact, according to “Sustainable Forestry in North America,” during the last 50 years less than 2% of the standing tree inventory in the U.S. was harvested each year, while net tree growth was three percent.