Prefabricated engineered timber is increasingly being used in mid-rise, large-scale and complex constructions worldwide due to its sustainability, environmental credentials and cost efficiencies, including reduced construction timeline, lesser dead load, and improved safety overall.

Aligned with global trends, Australia has also seen an increase in the use of Cross Laminated Timber (CLT) and Glue Laminated Timber (glulam) products that offer many superior options to traditional building materials such as steel and concrete.

Even though CLT and glulam are both used for structural timber design, there are material and characteristic differences between the two options. Let’s explore.

Cross-laminated timber 

Cross-laminated timber (CLT) is an exclusive engineered wood product prefabricated utilising several layers of kiln-dried timber (lamellas), with each layer glued together at 90-degree points on their broad faces. CLT boards are regularly comprised of three, five, seven, or nine alternating layers of dimension lumber.

The alternating direction of the layers actually provides higher dimensional stability than stacking them in one direction, ensuring CLT has a high strength-to-weight ratio, which has advantages for structural, fire, thermal, and acoustic performance. Like steel, CLT can be designed to different dimensions: Board thicknesses generally range between 100-300mm, however, boards as thick as 500mm can be created. Board sizes range from 1.2m to 3m in width and 5m to 19.5m in length. Most of the CLT panels are prefabricated and made easy for transportation based on regulations and guidelines.

Glue-laminated timber 

Glue-laminated timber (glulam) is made with different layers of solid wood lumber bonded with high-strength glue forming a solitary structural unit; unlike CLT, the layers are not cross-laid. Glulam usually has a high member capacity and can be utilised broadly in all types of structures.

To create a strong, dimensionally steady building material, glulam is machined and built to exact specifications. Sold in several standard widths and lengths, glulam columns can be tailored to meet almost any design requirements. Also similar to steel, the construction design allows it to be ‘shaped’ without losing its structural integrity, making glulam a material that is regularly utilised for large curved or arching members required in the construction of vaulted roofs, domes, and even bridges.

Glulam is robust and provides more strength and stiffness than sole lumbers. Steel dowels and plates bolted together are commonly specified for structural timber connections.

Key differences between CLT and glulam timbers:

The direction of the layers

As mentioned above, the most significant difference lies in the fact that in cross laminated timber, the layers are cross-matched at 90 degrees, whereas glulam timbers are made of separate layers with the grain lining up. However, both are engineered timbers and manufactured based on the design.

So why is this significant?

There are some key contrasts and reasons behind the two different products. Since CLT is made with alternating layers at 90 degrees, it has multi-directional strength, thus offering two-way traversing attributes similar to concrete slabs, creating primary direction and secondary direction.

Since glulam is made with layers all orientated in the same direction, it is utilised most commonly for one-way traversing requirements such as columns, beams and trusses. Regularly, most timber projects use both glulam and CLT, with each employed to take advantage of their unique characteristics. Thus, glulam is generally utilised for columns and beams while CLT is used for structural core, wall panels and floors.

Charring and protection

Timber burns, but before it burns completely, it develops a black outer layer called char. This char acts as insulation in the first instance and can slow down the rate of burn, which we know as fire resistance. Fire resistance is measured by the time elapsed from the conception of the fire up until the time where the material fails to work, commonly expressed in minutes, e.g. FRL 30, 45, 60 or 120.

While char protects the timber from burning as fast as it may otherwise, the timber is still losing strength with the reduced cross-section area. There must be adequate virgin solid wood remaining behind the char layer to support the loads applied to attain the designed fire resistance period. To increase the life of the timbers under fire, each glulam or CLT panel within the building can be treated with an intumescent coating, which works to protect the timber by slowing down the timber burning rate under the fire to a certain amount of time, thus leaving enough effective cross-section in place.

Considerations in using and protecting timber

The way in which the spread of flames is prevented and penetration of fire is resisted relies upon the nature, form and arrangement of the materials involved in the fire and on the property of the igniting fire.

In a fire test where the character of the igniting fire is controlled, the qualification between the two phases of fire performance is somewhat determinable. The words ‘fire retarding’ or ‘fire retardant’ may be used for treatments that limit flaming performance.

There are two general methods available to reduce the flaring attributes of wood through the use of fire-resistant protections.

Impregnation treatment

Impregnation treatment applies water-borne synthetic compounds within the wood. Numerous synthetic compounds display fire-retarding properties, but because of cost or other factors, few are considered generally practical.

The penetration of the treatment into the wood is typically achieved through vacuum-pressure techniques. Significant considerations include the depth of penetration and the quantity of chemicals deposited in the wood.

Intumescent paints

Another technique for controlling the flaming attributes of wood is to use appropriate coatings on the wood surfaces. All work, however, as a thin-film coating, which preserves the natural beauty of the timber, while adding significant fire rating performance.

The advantage of these treatments is the decrease in the immediate flammability of the wood, minimising its fuel contribution to a burning fire. The fundamental purposes of the intumescent coating will be accomplished if the spread of flame from a growing fire can be significantly slowed, if flaming can be diminished and even cease after the removal of the igniting source, and if the progress of the char into the wood can be delayed or kept under control.


Under sustained, severe fire exposure, there is no impregnation treatment or intumescent paint that will provide fire protection.


Structures that have already been erected are obviously unsuitable for pressure impregnation treatments. For best outcomes in new-build constructions, the lumbers should be cut to finished measurements before treatment with little to no mechanical interference after treatment.

As with specialist materials, while it may look easy to apply, ensuring the right thickness, adequate coverage and use of the right product are exceedingly important and will require the use of a qualified applicator.

The Aithon product is a waterborne intumescent and is not designed for external use. Certain maintenance schedules are required if exposed to water damage in interior environments.

Consult with Permax for more information on the use of intumescent coatings for timber fire protection.