Saturday, 21 September 2013

Glulam/CLT - what's the difference?

I was recently talking to a colleague about my experience working for a cross-laminated timber (CLT) manufacturer and how I felt that laminated timber was going to be the material of the future. He assumed I was talking about glulam, but actually glulam and CLT are different products. To help understand the difference, here is my explanation of the two.



cross laminated timber floor

CLT stands for Cross Laminated Timber which basically refers to the layers of timber (lamellas) that are glued perpendicular to their adjacent layers - cross lamination. This characteristic of the product gives it strength in two directions rather than the one as is the case with glulam. This is the main difference between the physical nature of CLT and glulam. CLT is engineered timber that is structural and is used for walls, floors and roofs (see my previous related blog posts on high rise timber and pre-fabricated architecture). The thickness of panels varies from minimum 3 layers up to 7 layers of timber and the sizes of panels is limited by transportation. CLT can be manufactured with varying grades of finish with a base grade (suitable for cladding) to a high quality exposed internal finish. However, note that the product cannot be left exposed on the exterior due to its water permeability and thus must be clad on the outside. This is in contrast to concrete, for example, to which the product can be compared in terms of its application, but concrete can be left exposed to the elements.

CLT is a sustainable product (as long as the timber is sustainably sourced) that uses far less amounts of clean water than concrete production and it is also a renewable material. As long as the timber is not burnt it retains the carbon within it. In fact the least sustainable aspect CLT manufacture is its transportation since the majority of CLT is manufactured abroad (mainland Europe and Scandinavia). The greatest benefits of the material are its time saving on site due to off-site manufacture, the dry nature of the construction process and the precision and accuracy due to pre-fabrication. However, this also requires greater time pre-construction for the co-ordination of design. As all openings, including service openings are cut in the factory, these all need to be fully co-ordinated and incorporated prior to cutting. Minor changes can be made on site, however, these are of course time consuming and the finished cut can never be as clean as that machined off-site. CLT is also much lighter than concrete so small elements can be manually handled.



glulam beam cross section

Glulam is also machined timber but it differs from CLT due to the lamination of the layers (lamella). Glulam stands for 'Glued Laminated Timber' whereby layers of timber are cut in the same grain and jointed and glued together. This gives the glulam strength in one direction and for this reason glulam is predominantly used for structural beams. The advantages of glulam are that it is lighter in weight than steel or concrete and has a higher strength to weight ratio than steel or concrete. It provides flexibility with curved as well as straight forms and is often chosen over steel or concrete due to its softer/ warmer appearance. This is particularly important within residential settings, for example. The biggest disadvantage of using glulam would be that often structural members are larger than would be the case with concrete or steel and this can be problematic in restrictive spaces.

Despite many people being wary of specifying timber due to the performance of wood in fire, both CLT and glulam perform well. This is because the laminated elements of the products make it very difficult for the timber to go 'up in flames' as it chars each layer slowly. It does not deform as steel does under fire. Specification of CLT and glulam is already increasing in the UK and as the construction industry gains confidence in the material, I believe it will become a leader for structural applications due to its sustainable nature.

For anyone interested in learning more about CLT should definitely read the blog by Timber consultancy TimberFirst which explains the characteristics and applications of CLT.


  1. I am a student of civil engineering. I'd like to used the picture of CLT connection from your blog. may I know your name so that I can reference it?

  2. hi tommy, thanks for asking --> Aditi Saxena.