What's the Difference Between All These Laminated Timbers?

A pile of laminated lumber blocks

Lloyd Alter / CC BY 2.0

We are in the middle of a mass timber construction revolution. What is everyone talking about here?

We are in the middle of a construction revolution, and after attending Woodrise in Quebec City, it appears that the industry really is reaching critical mass timber. Even the New York Times is on it, recently publishing Let’s Fill Our Cities With Taller, Wooden Buildings.

This opportunity arises from cross-laminated timber, or CLT. First introduced in the 1990s, it enables architects and engineers to design tall, fire-safe and beautiful wood buildings. Recent examples in the United States include the seven-story T3 building in Minneapolis, the eight-story Carbon12 building in Portland, Ore., and a six-story dormitory under construction at Rhode Island School of Design in Providence.

Except there is no Cross-Laminated Timber in the T3 Building in Minneapolis; it's built out of Glulam and Nail-Laminated Timber. So perhaps it's time to explain what these different forms of mass timber are and how they are used. Coincidentally, I took a lot of photos in Quebec City to do this kind of story.


Block of pale striped wood
Lloyd Alter/CC BY 2.0 

Glue Laminated Timber, or Glulam, is not a new technology; it dates back to 1866. It was patented in 1872 in Germany. In 1942 , fully water-resistant phenol-resorcinol adhesives were introduced that made it safe for outdoor use. The wood is all oriented in one direction, so it acts like a solid piece of wood, replacing large beams and columns with wood built up from smaller laminating stock or lamstock. Because all the wood is going in the same direction, it can shrink or expand in length, just like solid wood. It's used for columns and beams, and is holding up the T3 building in Minneapolis.


Block of cross laminated timber
Lloyd Alter/CC BY 2.0

Cross-Laminated Timber, or CLT, differs from Glulam in that the wood is glued up with each layer of boards being perpendicular to each other. Because the lamstock is going in two directions, it gets better structural rigidity and doesn't shrink in length or width. Originally invented in Switzerland, the Austrians developed it further in the 1990s; I was told once (but cannot find the source now) that being a landlocked country with high shipping costs, Austrian lumber wasn't competitive internationally, so they developed CLT to add value to their small pieces of lumber.

The first building to get everybody excited was the Murray Grove tower, designed by Waugh Thistleton; interest in the material immediately exploded, given headlines like Nine Storey Apartment Built Of Wood in Nine Weeks By Four Workers.

CLT being pressed on a metal machine
Lloyd Alter/CC BY 2.0

I first saw the real thing in 2012 on a trip to Italy, where they were using it to build houses in an area where stone houses were destroyed by an earthquake. I wrote back then, when it was just barely creeping in to North America:

Perhaps I am overwhelmed by the shock of the new here, but I can't help thinking that this is the ultimate prefab product. It is not the usual old material assembled in a factory instead of on site, but an entirely new way of building, using a new material that is perfectly adapted to computer controlled design and construction. It is cheap to ship and easy to assemble.


A sample of Nail Laminated Timber
Lloyd Alter/CC BY 2.0

Nail Laminated Timber or NLT is the stuff that the T3 building is made of, because there was not enough CLT capacity in North America to make that big a building, and supplier StructureCraft recommended NLT as an alternative. Lucas Epp explained:

The teams’ decision to go with NLT (nail-laminated timber) was formed on a number of factors including structural advantages, lower cost, and faster procurement times. For a one-way span, NLT and GLT (glue-laminated timber) panels are more structurally efficient than CLT panels, as they have all of the wood fibre going in the direction of the span.

NLT is really just a modern name for what has been done forever in warehouses and factories, and used to be called mill decking; you just nail boards together. Anyone can make it anywhere and it has been in the codes for a hundred years. The famous Butler Building in Minneapolis is made of the same stuff, but with solid wood columns and beams instead of Glulam.

Column and beam made of wood
Lloyd Alter / CC BY 2.0

The aesthetics of NLT are a bit rougher, with that warehouse look that people want these days, without all the trouble of old warehouses.


Sample piece of Dowel Laminated Timber
Lloyd Alter/CC BY 2.0

Dowel Laminated Timber or DLT is a more recent development. NLT is full of nails, so you can't work it once it is all assembled without your saw blade complaining. James Henderson of Brettstapel.org explains:

Dübelholz, German for “dowelled wood” refers to the inclusion of wooden dowels which replaced the nails and glue of earlier systems. This innovation involved inserting hardwood dowels into pre-drilled holes perpendicular to the posts.... This system is designed to utilise a moisture content variation between the posts and dowels. Softwood posts (usually fir or spruce) are dried to a moisture content of 12-15%. Hardwood dowels (mostly beech) are dried to a moisture content of 8%. When the two elements are combined, the differing moisture content results in the dowels expanding to achieve moisture equilibrium which locks the posts together.
DLT machine in a factory warehouse
Lloyd Alter / CC BY 2.0

I think it was StructureCraft that renamed it DLT to fit in with all the other LTs.


Laminated veneer lumber sample piece
Lloyd Alter/CC BY 2.0

Laminated Veneer Lumber or LVL is built up from layers of veneer, but with the grain all running in the same direction. If CLT is known as plywood on steroids, LVL is like plywood on a diet. It is used like Glulam, for columns and beams, but compared to lumber it is stronger, straighter and more uniform, and takes greater stresses than Glulam. Andrew Waugh says, "This high performance engineered hardwood permits beams and columns to have smaller cross sections than softwood glulam, thereby offering greater elegance to the timber structure."

It's also really beautiful, as you can see in the Vitsoe headquarters.


thoma wood block
Lloyd Alter/CC BY 2.0

An interesting new variation is this Holz100, which is like CLT, with the lumber running in layers perpendicular to each other, held together with dowels like DLT, so that there is no glue needed. Patented in 1998 by Dr. Edwin Thoma, it seems to be the best of all worlds. Holz100 is a cross laminated timber held together with dowels

All of these different LTs are used in different conditions; CLT has strength in 2 directions and can sit on columns; NLT and DLT have to sit on beams. NLT is cheaper and anyone with a nailgun and a strong back can make it; CLT requires significant investment, which is why it is still expensive. They all store Carbon Dioxide, and are all part of the mass timber revolution. And while the New York Times writers may not get their LTs right, they do come to the right conclusion about forest management and building with wood:

Incentives that encourage the construction of wood buildings that are drawn from forests with outstanding management are key to our climate future and the future of the forest.
Woman standing with several wood samples
Lloyd Alter / CC BY 2.0

Thanks to Caitlin Ryan of StructureFusion for her help and the great samples of the different wood technologies.