Cross Laminated Timber

Cross Laminated Timber (or CLT for short) is a structural building material that can replace concrete and steel in new building construction.

I wrote about CLT back in April and mentioned that the Gotham Gal and I are in the process of making two CLT buildings right now.

The paper version of the New York Times has an excellent op-ed today that explains why making buildings out of wood is much better for our climate than making them out of concrete and steel. What CLT does is make it possible to make tall and strong buildings out of wood.

This explanation from that NYT op-ed is particularly good:

Trees remove carbon dioxide from the atmosphere and store it in their wood. … This will allow us to pump carbon from the atmosphere and store it both in forests and in cities.

There are challenges to making buildings out of CLT. For example, CLT is not yet an approved building material in the five boroughs of NYC. That is changing however. It looks like the city will add CLT to the NYC building code soon.

I strongly encourage the NYC City Council to act quickly and approve the addition of CLT to the NYC building code.

#climate crisis#NYC#policy

Comments (Archived):

  1. William Mougayar

    Are there weather considerations for hot / cold climates? or ones by the sea for eg?

    1. fredwilson

      Not that I know of

      1. aim4itnow

        Hey Fred, you should check out this approved new wood stud called the tstud at that is insulated to R20-R30 depending on dimensions, is quicker to build with, 4x stronger, and just plain easier to work with than just plain wood, and combined with CLT could make building taller buildings that are just as strong as steel! (P.S.: I’m not an engineer, but this product has been used for years now in the US and Canada for residential buildings, so who knows if commercial applications have been tested yet for taller building, but they say it’s for commercial use)

    2. JLM

      .Of course there are. There are for steel, composite, and concrete buildings. It all falls under the heading of materials science and construction methods.As an example, if you are building in a coastal zone you will use epoxy coated reinforcing steel to ensure that the expansion of the rebar – concrete interface does not create spalling. This is the materials part of the equation.You will be incredibly careful as to cutting and joining rebar. You will power wash the rebar and forms with clean water the day of the concrete placement. This is the construction methods part of the equation.There are a hundred different similar considerations including some chemical reaction – corrosion issues (which is the rebar issue also).The problem with laminated wood will be the connections between the individual pieces — should they be mild steel, galvanized steel, or stainless steel.The obvious answer is stainless steel. Even in an “ordinary” high rise building of any appreciable quality one uses stainless steel fasteners to install any stone on the outside of a building.JLMwww.themusingsofthebigredca…

      1. Richard

        I recently watched the taking down of a 100 year old 5 story brick apartment building – it had all the technology of its time including a telephone on every floor! Almost every brick was meticulously cleaned and stacked for use on another project.

        1. JLM

          .Facade brick is usually not a structural member. It is a cladding of the structure and installed with ties back to the structure. It typically carries only its own weight.If a building has a horizontal relief angle every 2-3 stories, and a vertical caulked joint every fifty feet, a brick facade can last forever, if the exterior clay is hard fired.There are tons of lovely buildings with brick facades in which the exterior clay firing is failing. Once that happens, it is a death sentence. It can be fixed easily.Modern brick should be sealed with a high quality polyurethane sealer every 10-15 years. It is the water, the freeze-thaw cycle that kills them. That and no relief angles or caulked joints.JLMwww.themusingsofthebigredca…

          1. Richard

            No much freezing and thawing in LA!

      2. William Mougayar


  2. zakumanoff

    Very interested to read more about your development. I googled, but couldn’t find anything online. If there is anything available, would appreciate a link. Thanks in advance.

  3. kidmercury

    It is fun now that carbon is a big market, and as such very fashionable, everyone is a climate expert. (Unless you are a contrarian scientist who disagrees with consensus, then you should be burned at the wooden stake :)) but alas there is always something to complain about, doubly so in the climate market. Wood leads to deforestation and ensuing climate problems, but more crucially, is a low density endeavor (ie takes a lot of land mass to get all the wood needed). This impacts scalability, cost, and environmental health…….

    1. awaldstein

      True.I don’t agree that everyone is an expert who participates but personally intrigued by the carbon farming piece of regenerative farming that is bubbling up. Some cool blockchain projects trying to measure and build carbon swapping credits similar to cap and trade type of things.

      1. Richard

        When you replace the word blockchain to database or index card for that matter, you see really cool these projects are.

    2. JLM

      .The “wooden stake” is a nice touch. Well played.I think you should also use “strike anywhere” wooden matches.JLMwww.themusingsofthebigredca…

    3. Richard

      Put another way, we are storing all that carbon in the atmosphere for later use. carbon the new renewable energy ! The odds that we will genetically engineer our trees and forests and seas into carbon capturing systems are increasing with each decade.

  4. Andrew E Solomon

    Thanks for sharing Fred. Was just thinking about this last week. Apart from the ethical motivation to build using CLT, are developers incentivized in terms of cost to utilize CLT? As urbanization increases (and buildings continue to get taller) I wonder if this will provide a long term solution. In general, very cool to see a natural carbon sequestration technique in use. Trying to think up other scalable ways to do this.

  5. pointsnfigures

    The material makes great beams for homes. My friend has been using engineered lumber for years. Doesn’t warp ever.

  6. iggyfanlo

    This is NOT meant to be obsequious but it’s the early adopters that allow a new technology to take hold and become standardized as well as affordable and acceptable for others.. particularly those technologies like CLT and other greent ech that move us to a cleaner planet (fingers crossed)… I say THANKS

    1. Richard

      Go ahead and be as obsequious as you please. I’m a huge believer in the power of obsequiousness. We as a society should encourage the power of being an obsequios ableist. Stand proud!

  7. jason wright

    I wonder what the maximum possible height is for a building made from CLT? Manhattan in particular seems an unlikely place for CLT to take hold.I blame Henry Bessemer 🙂

      1. jason wright

        I’m surprised, and impressed. The design is bland, but as a proof it’s great.As a child i lived in a fifteen storey steel and concrete tower block. It was fun, and safe. Today i would want a parachute under my bed.

  8. awaldstein

    Kudos to the two of you for building a path towards usage and acceptance of environmental materials.Struggling a bit/a lot with my environmental biases as researching a building spot and my aesthetic sense so wants a steel and glass prefab with solar panels on the roof.

  9. JLM

    .CLT has been around for a long time. It has been used to make beams in the residential and light commercial market for more than half a century — maybe longer. When I studied structural engineering back in the late 1960s, we used to study GluLam beam design.It is a fairly new material as a “building system.” A building system is the frame — columns, beams, floors, skin, roof — upon which a building is built.A building is a foundation, a frame, and finishes (interior and exterior — windows, doors, penetrations) plus the surface finishes (floors, walls, ceilings, interior doors, rest rooms, mop closets, central plant) and the HVAC, electrical, plumbing, fire protection, controls, elevators systems.In mid-rise and high-rise construction and building design there are essentially only three systems for the frame — structural steel, reinforced concrete, and composite (structural steel wrapped in concrete).Steel is more flexible than reinforced concrete. Steel is stronger in tension and less strong in compression. Concrete is stronger in compression and less strong in tension.Compression = pushing. Tension = pulling.One puts reinforcing rods — rebar — in concrete to compensate for the tension weakness. This makes reinforced concrete a very useful building material. In addition, reinforced concrete is “job built” meaning the materials come to the site unassembled and are assembled by hand on the site. No single workman can screw up a reinforced concrete building frame. It is subject to technical inspection at each step.You create concrete samples from every truck load and test them while you test the “slump” of every truckload before it goes in the concrete pump. It is very scientific. Concrete is way overdesigned.One can move concrete long distances with modern concrete pumps and one can design the concrete mix to meet differing standards of strength. As an example, the concrete on the ground floor of a building will be 12,000 PSI concrete while the concrete required on the 45th floor may be 3,000 PSI concrete. Different loads at different levels.If I build a 50-story reinforced concrete frame — as I have done — the building ends up as a monolithic structure — meaning it is one big piece of reinforced concrete from foundation to roof. It will behave under load as a single monolith — NO JOINTS TO FAIL.The issue with CLT as a building system is exactly that — its behavior as a whole when subjected to differing forces as statistically magnified by the great number of joints. There are a ton of connections in a CLT building. Thousands of them. One connection failure can create a cascade of failure that may destroy the building. Each connection has the fingerprints of a single worker. One bonehead can wreck a connection.A building is subjected to forces — wind, sunlight — that differ during the course of a day. As an example, I used to go to an investment bank in the World Trade Center and when you sat at a conference table you could see a glass of water lurch to one side when the sun moved around the building at about 2:00 PM. I used to like to schedule meetings at that time for that reason.The sun heats the side of a building, it expands (material co-efficient of expansion becomes very important as well as how the exterior is affixed to the building) and that side of the building becomes longer, making the building tip toward the other — cooler — side. This is of enormous importance when building tall buildings with small footprints.There is also a repetitive nature to the daily movements that is of concern at the wood to metal interface in a CLT building. This is the place where two different materials may behave differently initially, but also where the constant movement through the years may create a failure mechanism that is impossible to detect today. It is like bending a coat hanger back and forth until it breaks.In the wood-metal connector interface, one material is flexible while the other is brittle. That is never a good thing.A reinforced concrete frame takes this kind of stuff in stride, that monolithic frame advantage. A composite structure — like the World Trade Center Towers — is a little less forgiving, but it was impossible to pump concrete that high during those days. They bucketed it to make the composite frame.That myriad of connections in a tall CLT building should give structural engineers, contractors, and developers real pause. In my professional judgment it is driven by a certain “gee whiz, isn’t this cool” environmental state of mind.I also want to sound a note of warning in the arena of what professional you are listening to — architects, architectural engineers, structural engineers, and builders all have a different view of things. In high rise buildings, the structural engineers are the guys you have to listen to. Period.The structural engineers often work FOR the architects, so they are unwilling to speak up because they could be losing the next job.CLT is currently a much more expensive building system, more prone to individual installation failure, harder to inspect with technology, a transportation pain in the ass, and an inherently more difficult material to deal with when it comes time to repair. This repair consideration is currently overlooked because it is such a novel material.I would have no qualms using it as a material for a 5-story building as none of its flaws — except for cost — are potentially deadly.I have had a fire in a mid-rise office building and the fire suppression system, and sprinkler system put it out exactly as designed. It was back in operation in three days. The water damage to a wood frame would not have allowed such a quick rehab. The combination of wood and water, the attendant expansion, the long time necessary to drain and dry it, and the impact at the connections gives me real pause.I fear that wood is a delicate product even when one considers the multi-direction orientation of the individual layers, the quality of the glues — which are superb. The material itself will perform on the job exactly as it does in the lab.The problem will be what happens at all of those connections. I have seen buildings being built using mild steel connections — something perfectly normal in light commercial, but that same product being used in mid-rise or high rise buildings sends a shiver down my spine.I renovated some tallish early 1900s buildings — 6 to 18 stories — and was appalled at the damage to the metal parts behind the facade. I had no idea how much water damage and corrosion was actually there. There was no external evidence of degradation. In time, water gets into everything especially when it is high off the ground and the wind is blowing.CLT is not a new material and its problems are not unique. They are straight out of Structural Engineering I, II, III, IV. It is not the material; it is the connections.As a structural engineer, I would be unwilling to take the thousands of connections risk v a single monolithic frame. I have to ask — why? Make no mistake, this is a building safety issue.CLT for low rise buildings sounds fine if it scratches your particular itch. For mid-rise and high-rise construction, I think it is a real, real, real stretch for the reasons I have enunciated.Can the multiple connections, locus of failure issue be solved? Probably, but why?If somebody approached me to invest in a CLT high rise today, I would pass. The insurance industry is already discussing a different rate approach.Watch what the building code folks, the schools of structural engineering, the insurance industry, the testing labs, and the fire professionals say about it. Pay attention to what contractors and re-hab contractors say about it.Last word about testing labs — when you build a high rise building, you take your wall and window assembly to a testing lab where they stick it in a wind tunnel and run water and hurricane conditions at it. They are testing the strength of the assembly and the seals. The seals almost always fail the initial testing. You work on the seals, use a lubricant to ensure they are seated fully, re-run the test. [You will also field test the assembly by hanging a similar test fan off the building and running the same test in situ.]You cannot get insurance unless you get a clean bill of health from the testing lab.When you are finished, you test to destruction — increasing the water and the wind until the assembly disintegrates. This is always a fun day at the lab. Sometimes, the lab runs out of wind power. Then, you throw a few handfuls of gravel into the chamber — mimicking what happens when a gravel roof is next door to your building.There will have to be a similar regimen developed to test the multi-floor connections in CLT buildings to demonstrate their structural soundness. Right now, that doesn’t exist. Amongst folks who have ever built a high rise, the chatter is already there.As a structural engineer, as a developer, as a guy who has actually built high rise office buildings, I counsel caution. Be careful about taking too much building systems and materials science advice from VCs. <<< Hey, that’s a joke.JLMwww.themusingsofthebigredca…

    1. Richard

      Wow – that is some materials science lecture! The only thing two things I would add are that fibers increase the shear strength as well and the fracture mechanics of the interface between a carbon fiber and a polymer laminate are difficult to model.PS I have a material science engineering degree from Case Western Reserve University and did a research and development work on the many of the carbon fiber stealth fighters flying in the sky and protecting us today.

      1. JLM

        .Isn’t it a classic “flex until failure” model? Meaning physical test model.Test a bunch of different variables.Make a model, and stick it on a motor driven arm and let it crank. Count the number of reps until the thing falls apart.JLMwww.themusingsofthebigredca…

        1. Richard

          Before the failure there is modeling cracks and crack propagation – crack propagation is a multi variable problem with variables both endogenous (Shape of part etc) and exogenous (temp etc.)

          1. JLM

            .Definition of failure issue. We are saying the same thing — failure for a window test is the instant at which blown water can make it inside the building envelope. Same sort of thing here.I would also like to know how much time a connection has between water intrusion and total failure. If the gap is 10,000 reps v 1,000,000 reps my urgency in fixing it would be different.JLMwww.themusingsofthebigredca…

      2. JLM

        .Composites in aviation are one of the biggest breakthroughs in aviation. I had a relationship with a company who just took every airplane — big commercial airplanes — and made a composite replacement for every non-structural part.They didn’t ask permission. They just bought a part and reverse engineered it in composite, tested it, and went to see the company that made the airplane and said, “Here, want to save 500 lbs on every flight for the rest of time?”They got bought by some big aviation company. One plane they worked on, they saved more than 15% of the total weight.All of this translates to less wear and tear on the equipment (landing gear) and less fuel. Lower maintenance costs because of less weight induced stress.You have to redo the weight balance which is no big deal.JLMwww.themusingsofthebigredca…

      3. JLM

        .Buildings are designed with limitations that make the consideration of shear almost irrelevant. A beam is often designed with a maximum deflection at the center point limit of length/360 or length/240.This small deflection negates any necessity for shear analysis.In addition, while buildings are designed for code imposed live and dead loads, there is virtually no live load of any significance. The only exception is elevators, but a building only encapsulates the elevators in a shaft leaving the elevator system itself to deal with the dynamic load.JLMwww.themusingsofthebigredca…

    2. Girish Mehta

      Very interesting and educational…Thanks.

    3. Matt A. Myers

      “Each connection has the fingerprints of a single worker.”Eh, the majority of houses in the future may use automation to up-to-fully be built – so we’ll use computer vision to check the integrity of everything including joints.

      1. JLM

        .The use of video to monitor high rise construction is already a thing. Some builders mount a set of cameras and record everything to ensure they have a building latent defects defense.The issue is the guy with the hangover who puts in the connection, doesn’t torque it properly, over torques it, doesn’t seat it square, sees a crack in the wooden member (which could be epoxied before the connection is made), or doesn’t make sure the connection is perfectly flush.Remember I am only talking mid-rise and high-rise construction, not residential.These are things you can’t see on a two dimensional monitor.These are things that don’t exist in reinforced concrete frame construction.These are not things subsumed in the “factor of safety.”This is why you pressure test systems before you cover them up.When you are 200′ in the air, the hawk is blowing, it is 15F, you will make mistakes.JLMwww.themusingsofthebigredca…

    4. Chimpwithcans

      How to coax a strong JLM response: write something to do with buildings, throw in some climate change. All that’s missing in Fred’s post is some Trump bashing and crypto – THEN we would have had a response!Great post JLM.

      1. JLM

        .I have gotten a couple of calls from people who want me to take a look at a set of plans for a CLT project.OMG, I just realized there was no crypto.Here’s some crypto for you:https://themusingsofthebigrhttps://themusingsofthebigr…Be well.JLMwww.themusingsofthebigredca…

    5. Greg Howes

      Hi JLM, Thanks for the comprehensive and lengthy perspective on CLT and mass timber. Regarding structural engineering of tall timber buildings, I highly recommend looking at the European mass timber community and engineering firms like Blumer-Lehmann https://www.lehmann-gruppe…. and their many complex projects. Also, the 25th International Holzbau (building with wood) Forum will take place in Innsbruck in December 4th-6th… and would provide a very good intro to European companies. I also highly recommend joining the 93,000 plus people who attend the Ligna (Latin for wood) conference every two years in Germany to see how technically-advanced the wood industry already is at the literal cutting edge.The Swiss, Germans and Austrians are famous for their engineering and they too care about safety, fires, sustainability, earthquakes, energy costs, quality of life, etc. I work with an immigrant Swiss wood engineer at our factory and we use German and Swiss CNC machines, robots and processes to fabricate complex building which we “localize” for the U.S. market and our owners and clients. We have experience fabricating hundreds of complex buildings which can then be asembled on site far faster than can be done using conventional construction processes. See this video of our TED stage installation for just one example – The design was by the Rockwell Group in New York https://www.rockwellgroup.c

      1. JLM

        .Hey, Greg, thanks. I am up to speed as it relates to the technical elements of the product. No question as to the strength of the individual materials, meaning the ability to transfer vertical loads.When choosing a building system to build a high rise — something I have done more than a few times — I am skeptical of the idea of a system with many connections v a monolithic frame.I frankly don’t see much in the literature about the connections. I do see some basic gravity joinery that avoids some of the issues, but if I have to choose between a monolithic structure without a single joint v any system with many joints, it is hard as a developer and a structural engineer to invite that problem into the mix. [I am no longer in the development business, but I was quite active once upon a time.]The product can clearly be made into a beautiful finished look.JLMwww.themusingsofthebigredca…

  10. Richard

    Any blog post that relies on a quote from the NYT op-ed section, particularly a quote is on science – that appears to be taken from the children’s book – the giving tree – is some lazy blogging Fred. Respect you readers.

  11. John Francis Charles

    Sorry to be dense, however we would need to plant the trees and manage the forest for approximately 70 years before harvesting them in order for this to be an effective carbon mitigation strategy, correct?

    1. Rob Pennoyer

      I think they can make CLT from spruce, which grows 30-60 inches per year. A 40′ tree could grow in a decade or so. Not easy, but not impossible to manage for this.Also, this article says there’s a surfeit of wood from trees they have to kill to prevent the spread of Mountain Pine Beetles.

      1. JLM

        .In the wood game, the first big question is hardwood v softwood. A lot of the discussion has to be driven by the data derived from density, modulus of rupture, and modulus of elasticity. These taken together indicate the structural health of the wood.PB killed lumber is a real crap shoot as it relates to performance. I wouldn’t hesitate to use it in standard 2×4-6 construction if I knew it had been dead on the stump for no longer than one season, but I would be very cautious using it in structural members like beams and columns because of its quick degradation.PB damaged trees in the South and places like NW Colorado lose up to 30-40% of their structural strength if left on the stump for more than 12 months.If you are a timber guy and someone says they will sell you the stumpage on 1,000 acres of NW Colorado land — you better find out how old the timber is. If it is more than a season, it is what is called Class No. 3 and shouldn’t be used for structural would be great for concrete forms, but not structural.At the granular level the invasion of the PBs has dramatically changed the permeability of the lumber and it will dry quicker. It would be unsuitable for pressure treating and would take a lot of product.PB lumber, which turns blue, is fine for boxing, crating, forms, saw horses, packaging, some furniture, and uses that do not carry a lot of weight — like soldier studs around doors and windows, but it is a pain in the ass to work in the field.A lot of guys bought portable sawyering equipment and worked NW Colorado felling trees and making 2×4 and made a lot of money, but the testing today is not as encouraging.If your CLT is made with 1X material, you might use PB damaged material for a couple of the levels of an 11-member CLT product.PB has been around a long time, so there are 40-year old studies one can peruse.JLMwww.themusingsofthebigredca…

  12. Rob Pennoyer

    Which companies are doing this now? Fred, can you share the names of your suppliers for design, manufacture, and build?Aside from building one’s own apartment building, how can someone invest in CLT?

  13. Rick Bashkoff

    Another solid option worth researching is hempcrete and hempwood. Googling either will provide a wealth of information. Both are pretty fascinating.

  14. sigmaalgebra

    But, but, but wood seems so natural!!Can leave the wooden beams exposed and feel like are in a forest with Thumper and Bambi with little birds tweating, Mickey Mouse scurrying around.The whole building will have that special, 100% all natural aroma that can ONLY be achieved with 100% all natural exposed wooden beams!And we can expect heaping platters of home cooked foods, organic, paleo, artisanal, vegen, GMO-free, gluten-free, from the roof garden, served by the women, including Snow White in person, in floor length, hand sewn, homespun peasant dresses!Lighting ONLY via hand dipped tallow candles on pewter candle holders!Can stain and varnish the wooden beams and be like living inside fine furniture!Can HUG the wood and get affection, even if no parent, child, or spouse ever gave any hugs!!! Concrete, steel — can’t hug them!!!Can gather and sing along with the Carter family recording Will the Circle Be Unbroken!!…And every second of this heaven, are saving Mother Nature’s 100% all-natural, pure, precious, pristine, delicate, virginal, vulnerable, irreplaceable environment!!!Are joined, bonded, with Mother Nature!One by one approach the fireplace and reaffirm to all present total devotion to Mother Nature’s 100% all natural environment!Then the offering plate will be passed.What’s not to love about pure environmental heaven????Okay, I’ll admit: Disney’s Snow White, modeled, IIRC, by Marge Champion, was a DISH!!!! For music, I’d want to take her to a performance of, say,…at, say, Lincoln Center with Levine doing his best with it!

  15. Greg Howes

    Great post Fred. Thanks for supporting mass timber and CLT by actually making two buildings.

  16. Greg Howes

    Thanks Fred for supporting CLT, mass timber and digital fabrication by making two CLT buildings. Here are many more articles supporting the use of wood in construction and managing our forests as a renewable resource.The U.S. is, by far, the world’s largest market for wood for construction and 90% of Americans live in a wood-framed building. We should also lead in smart, green, efficient building with wood as construction evolves into an industrialized technology industry.Trees are the answer. Ask Greta. David Attenborough 3 trillion trees the Economist the BBC Google The Nature Conservancy the World Economic Forum Autodesk Microsoft Outside magazine the forests you do counts. Join us at a Mass Timber Hackathon in Portland, Oregon on January 24th-26th to explore the cutting-edge of mass timber (I’m the main organizer of the hackathon). Fred – can you join us as a presenter and to share more about your two CLT projects?

  17. jason wright

    It’s not CLT construction, but check this out if you’re anywhere near Milan;https://www.stefanoboeriarc…Imagine NYC with one of these towers. It would be a statement of intent. Give Bill de Blasio a call. He might be able to construct an incentive package for developers. Hudson Yards was a crime against the environment and an offensive eyesore to aesthetic sensibilities.