Cracks in Glulam – what you need to know

Once a glulam project has been designed, manufactured, and installed, you could assume that the glulam part of your build process is complete. 

It can take up to a year for glulam to settle into its conditions and for movement to stop, and over time sometimes cracks can appear. 

We are often asked to look at cracks in glulam, both on our own and from other suppliers as architects, constructors and clients can be concerned there is something wrong. We thought therefore we’d share a little bit about cracks in glulam to help you understand what is normal and what to expect. 

Glulam is made from kiln-dried sections of timber, which are clamped and glued together. The timber is manufactured around 12 ± 2% moisture content in our factory; by using glulam, you get an end product that has lower moisture content and fewer defects than the equivalent solid section of timber and is therefore less likely to crack. 

However, glulam, like solid timber, still responds to changes in temperature and relative humidity in its surroundings so it can still crack, depending on the changes in the atmospheric conditions it is in.  

What can cause cracks in glulam? 

Cracks happen when the outer surface of timber dries out quicker than the inside, which causes the wood fibres to separate. There are several points during your project process where your glulam may be affected in this way by different conditions: 

• During construction – on site, as the glulam is being erected into place, it is likely to be subject to different levels of moisture in the atmosphere and temperatures. This can lead the moisture content of the glulam to rise. 

• On completion – when your structure is finished, generally it will be part of a building that will require some heat for the building to be used. The glulam, along with the rest of the materials in your project: carpet, roof, insulation, flooring etc, will be heated and subject to much drier atmospheric conditions. 
  This will lead the glulam to shrink, and this is when cracks usually occur. It can take about a year for the glulam to settle into its final conditions and for movement to stop. (Timber doesn’t stop moving, it will always adjust to its surroundings) 

• Over time – weather conditions can affect your structure even after the initial settling-in phase. Very hot weather and very dry cold weather can make the glulam dry faster and overall temperature and humidity will always fluctuate with the seasons and building use – particularly if you have air conditioning or underfloor heating for example. 
  In reality, a section of timber or glulam beam will never truly reach an equilibrium moisture content. But gradual or small variations in surrounding conditions are less likely to cause a timber section to crack.  

Do cracks in glulam beams affect structural integrity? 

‘Checking’ (ie cracks appearing) can happen in all timbers but becomes much more likely in larger timber sections. Cracks will usually follow the direction of the wood grain and will often occur along a glueline because of the slight differences in shrinkage in each layer (lamination) of wood. The end-grain is particularly susceptible to reacting to changes in moisture in the surrounding environment and checking in many cases starts at the end of a beam due to this reason.  

Cracks in glulam that are exposed externally can result in a higher risk if left un-maintained over the long term as it allows a point for water to get in which can lead to deterioration of the timber, but most of the time cracks from checking usually don’t need repairing for structural reasons. 

Is checking the same as delamination? 

Because of how checking happens in glulam it is often confused with something called delamination – a failure in the glue bond itself. This is rare, as the glue we use has gone through rigorous testing, and therefore the bond is stronger than the wood itself! Each batch of glulam that is manufactured has a representative sample tested to ensure the required glue bond has been achieved.  

Shrinkage cracks are often confused with delamination or failure of the glue line. In softwoods it is very unusual for full cracks to happen on the glue line – usually, on closer inspection, the crack will have occurred in the timber often right next to the glue line. 

Checking is identified if on closer inspection, wood fibre separation can be seen, whereas delamination is identified if the opening appears clear with a glossy surface.  

Hardwood timber can be an exception to this as it has a higher timber strength that can shear the glue bond, giving the appearance of delamination. Either way, the cracks need to get quite severe before the strength capacity of the glulam is compromised.  

How can I prevent cracks in my glulam? 

There is no fail-safe way to avoid cracks appearing, but you can protect your glulam during transportation, storage and erection from extremes in temperature, humidity & direct moisture.  

Precautions can be taken during storage (before delivery, and on-site) to minimise changes in moisture content due to weather. Rain, damp, and direct sunlight, are all potentially harmful to timber and wood-based components. 

Site-applied finishes, stains and varnishes can help reduce the occurrence of cracks by impeding the movement of moisture from the timber. The thicker the protective layer the greater the protection. We recommend all exposed end grain have an appropriate sealer and, if externally exposed, a protective capping or flashing.  

Once a building is watertight the beams will need time to adjust to the in-service conditions and if you can avoid rapid drying out, do!  

Can I treat cracks in glulam after they have appeared? 

If you are concerned with the visual impact of cracks appearing there are a few things you can do to help. 

Small cracks can be repaired using glue and sawdust, larger cracks can be fixed by using Thixotropic fillers or resins, and for particularly sensitive visual areas they can be repaired by letting in timber pieces. 

While it may be frustrating to wait to do this, we do recommend you wait for the structure to go through at least one full heating and cooling cycle so you know as much as possible about what you are dealing with. (Winter – Summer)

Cracks appearing in glulam is a natural occurrence that happens because of atmospheric conditions surrounding the timber. At Buckland, there are things we can do to help assist you with prevention, minimisation, and repair, but ultimately, it’s a natural part of working with wood.  

If you have any questions, do get in touch.