An introduction to glued laminated timber (glulam)

Glulam Beam Inspections

We have been inspecting glulam timber structures for over ten years, collaborating with contractors and supervising repairs for nearly as long, so thought it a good idea to share with you some of our experiences in the hope that it will help you prolong the life of your engineered timber.

While this article is not intended to be a comparison piece between solid-sawn timber and glu-lam, it will make reference to differences if thought informative.

What is glulam timber?

Glued Laminated Timber, commonly referred to as glulam, is an engineered timber product manufactured by gluing together short pieces of strength graded timber. These laminates, typically 45mm thick, are finger-jointed to make continuous lengths with layers of laminations then bonded to create both straight and curved beams.

Beams can be manufactured to truly impressive proportions: we have assessed the conditions of individual beams that were over 30 metres in length, 400mm wide and 1600mm deep, producing clear spans well in excess of 60M (we had to invest in another microdrill to assess these massive beams –  this super long, 600mm non-destructive assessment tool now helps us to assess the biggest beams).

Why use glulam timber?

This product allows designers to realise spans that create fantastic spaces that would not be possible to achieve with ordinary solid-sawn timber. It is successfully used in bridge construction, forms massive and complex roof structures, and helps create interesting architecture featured in both commercial and residential projects.

Glulam beams offers specifiers performance advantages over traditional structural materials such as concrete and steel: glulam  components are lighter, have a superior strength to weight ratio, and as an engineered product, it is consistent and predicable. Large sections perform well in fire tests, the outer surface charring and preserving the core, with fire resistance further improved by the ease of application of surface retardants.

How durable is glulam timber?

There are examples of ‘glulam’ structural components in buildings and semi-protected external structures that have survived well over 100 years. Its durability is largely dependent on the same things that determine the service life of regular sawn timber, namely the level of natural durability the specified species of timber will provide, the robustness of design detailing (especially fixings), preservative/protective coatings, maintenance and ultimately, its exposure.

Used internally

Used internally, glulam should be very durable, in fact the only issues we see are caused by water ingress via the external envelope, or those caused by challenging environments such as swimming pools where issues can develop if humidity levels are not adequately managed. Problems can occur where cold bridging has facilitated condensation around the fixings between the roof structure and glulam, and where a glulam ring beam or other component is directly exposed to the external elements.

Used outdoors

Many outdoor structures have been successfully constructed utilising glulam timber, with the vast majority of our condition assessments concerning these structures.

Of those issues we encounter, most are either caused by limitations in design, often compounded by the limited inherent durability of timber species and issues concerning maintenance: I think it is fair to say that any chink in its armour will be found out by the great British weather.

Preservative and surface coatings for outdoor structures

With our background in conservation and sustainability, it will be of no surprise to hear that we would always encourage specifying a more durable timber species rather than a reliance on the use of preservative treatments to resist decay – I believe this to be best longterm approach.

However, unless your beams are manufactured from a naturally durable species of timber, it will be imperative that it receives a level of protection appropriate to the exposure it will face. That preservative treatment is usually applied during the manufacture process, and often part of a ‘system’ to ensure compatibility with subsequent protective and decorative finishes.


We really like glulam and the magnificent structures it helps to create, and therefore want to highlight some of the common ‘issues’ we encounter, so to ensure your project lives up to expectations.

Glulam Inspection

Regular inspection will ensure that any problems are identified before they escalate. If budgets do not allow inspection by a ‘specialist’,  we suggest that those tasked with maintenance are encouraged to report any concerns, no matter how trivial they may appear.

Although a visual inspection will be adequate for most routine inspections, more informative testing may be necessary to quantify any significant defects observed.

Rope Access Inspection Timber Footbridge

Rope Access Inspection Timber Footbridge, Ivybridge, Devon

Our methods include moisture content testing, ultrasonic and microdrill non-destructive  decay detection testing and timber species identification; these tests will inform of condition below the timber surface and the level of risk the component is under from decay.

Top tips

  • Designers and specifiers – make your glulam structures inspection and maintenance friendly… your clients will thank you.
  • Consider committing to a periodic ‘Inspection Contract’, usually more cost effective than one-off inspections.

Glulam Maintenance

Keeping up with a good maintenance regime will significantly extend the life of structures.

Ensure that proposed repairs or redecoration specifications are compatible with existing coatings.

Top tips

  • Regularly ensure leafs, litter and other detritus are removed from any joints and gaps in associated decking and from around the feet of glulam structures – this debris could increase moisture retention and accelerate deterioration.
  • Ensure nearby trees and other vegetation are regularly cut-back.


We have found that glulam (and regular sawn timber) structures suffer more when in close proximity to trees, therefore we recommend the annual cleaning of structures in these environments.

Periodic cleaning will help remove algae and bacteria, which are the precursor to more significant fungal decay.

Top tips

  • We would always promote the use of gentle, non-toxic cleaners – a bit more elbow grease is better than the use of aggressive products that could be harmful to our environment.
  • We specify hot water cleaning near watercourses or other sensitive locations – a thorough cleaning can usually be achieved without the use of any chemicals!

Glulam Repairs – prolonging the life of timber

It is important to consider each repair objectively, the repair strategy will be determined by structural requirements, ease of access, proximity of connectors and fixings, and aesthetic considerations.

There are numerous methods to repair significantly decayed sections of timber including, steel reinforcement, timber-resin splices and other composite repairs.

Top tips

  • Consider how your repair will better resist decay – don’t replicate a bad design.
  • Ensure repairs do not create new problems – avoid creating cold bridges and details that could attract condensation formation.

Common defects with glulam

Unfortunately, most of our glulam related enquiries are reactive responses to discovering problems rather than a proactive request to arrange a periodic inspection as part of an existing maintenance programme.

Just like using sawn timber beams, the successful use of glulam beams will be largely dependent on a robust specification, careful transportation, storage, installation and maintenance practices. And as with all timber products, managing its exposure to moisture will largely define its performance and ultimately, its durability.

Rot in beams

Typically, rot attacks the ends of beams and causes localised decay around connections and fixings.The decay is usually the result of limitations associated with the design, specification or maintenance issues.

Although it is likely to be challenging to retrospectively make improvements that successfully ‘manage’ inherent design inadequacies, such design improvements may provide the best longterm solutions.

As already mentioned, when considering how best to repair glulam components, it is vitally important to establish what has caused the defect and consider how best to ensure your repair design isn’t simply going to suffer the same fate. There are several repair methodologies that lend themselves to the repair of glulam beams, these include timber-resin splices, resin repairs, beam reinforcement and component replacement.

‘Checking’ of beams

Glulam beams will absorb moisture, expanding and contracting in reaction to fluctuating moisture content. This moisture loss can result in ‘checking’, longitudinal cracks occurring as fibres shrink. Although not usually said to be of structural concern, these cracks can spoil the aesthetic appeal of glulam beams.

We regularly hear of ‘cracks’ appearing on the surface of glulam beams soon after heating has been introduced into the new building or structure – it is important to ensure the moisture content of the beams is managed before and after installation in order to minimise checking.

In outdoor structures, checking and other fissures will allow moisture ingress – the risk and extent of decay will be dependent on the positioning of the fissures, and how readily moisture can escape from the cracks – decay will develop if moisture becomes entrapped.

Paint defects affecting beams

The deterioration of the decorative/preservative finish will facilitate the development of decay. It is vitally important that subsequent maintenance coatings are compatible with earlier coatings/treatments.

Access problem associated with large structures

One of the main attractions of using glulam beams, namely their potential to create super long spans, high, intricate designs and elaborate structures, does have associated drawbacks… it creates ongoing access issues: in these situations, the inspection and maintenance of glulam components is likely to become more challenging and costly.
Consider the need to inspect or repair beams high above a swimming pool, it will obviously be challenging, usually requiring specialist access equipment and unfortunately, when finances are stretched, maintenance and routine inspection cycles are often neglected.

Although we use rope access techniques to inspect and test structures while climbing up or abseiling down, and/or powered access equipment (cherrypickers etc), this can add significant costs over the planned lifecycle of the structures.


The following conclusions are based on our observations inspecting and testing the condition of numerous swimming pools, footbridges and other glulam structures.

  • Glulamdoes rot…
  • Timber around connections and fixings is often the first area to begin to decay, especially if not robustly maintained.
  • Glulam beams manufactured from durable timber species will consistently outperform glulam components reliant of preservative treatment and coatings.
  • Structures shaded by trees and vegetation often suffer premature decay.
  • Maintenance is essential – many structures are not maintenance-friendly.

Respect our environment

It is important to ensure that potentially harmful products used in either the manufacture, maintenance or repair of glulam are not allowed to enter the environment.

We have developed protocols for working in marine and other sensitive environments to minimise the risk of pollution – call for advice or speak to your local Environmental Officer.

Lee Harper, of Harper Building Diagnostics, has been assessing all sorts of wonderful glulam structures for over a decade, abseiling down footbridges, swimming pools and other ‘difficult to access’ structures. When he’s not inspecting these modern structures, he’s usually up and down the country assessing timbers in heritage buildings.