Suspected faults in EWP surfaces are often just natural variations in the timber.
“There are things known and there are things unknown, and in between are the doors of perception.” Aldous Huxley
In home construction, EWP beams are often chosen for their beauty in exposed applications, e.g. feature beams and alfresco structures. Because these EWP members are so visible in these open designs, normal seasoning changes in the timber surface are very noticeable and can raise concern with the building owner. Unfortunately, these seasonal characteristics are often misunderstood; the word ‘delamination’ is used to describe what most likely is normal timber checking.
As timber is a natural product, ‘checking’ is to be expected. When checking is present, the timber can seem to have the appearance of a surface split or a crack.
Checking is the separation of continuous wood fibres and is a naturally occurring consequence of the seasoning process of wood. The outer fibres lose moisture to the surrounding atmosphere and attempt to shrink, but the inner portion of the timber loses moisture at a much slower rate. The different rates of shrinkage can cause the wood to experience surface cracking (checking) or splitting.
Rapid drying increases the differential moisture content between the inner and outer fibres and thus increases the propensity for checking in the timber. The checking (and shrinkage) process will stabilise as the moisture content of the timber reaches equilibrium with the surrounding environmental conditions.
Checks in glue laminate timber (GLT or glulam) may appear as openings parallel to the grain on the sides of timber.
For GLT, while seasoning checks may occur for exactly the same reasons they do in sawn timber, checking will generally occur to a much lesser degree because of the manufacturing process, which involves careful control of the moisture content in the timbers used as laminates.
In LVL and plywood, a check is a naturally occurring lengthwise separation between wood fibres running parallel to the grain of the outer veneer. Checking is expected on these outer veneers, even when the panels are finished with paint or stain.
In general, checks have little effect on the strength of any glued laminated members, be they GLT, LVL or plywood.
Unlike checking, delamination is not naturally occurring. It is an uncommon manufacturing fault which indicates a manufacturing process failure. In delamination, openings are separations between the laminations at a glueline, not in the wood fibre.
Delamination occurs when the glue bond is not adequate to resist moisture cycling. Openings due to inadequate adhesive bonding may appear as smooth wood surface separations.
With this said, delamination is unlikely in EWP because they are manufactured with durable wet-use adhesives under closely controlled manufacturing procedures.
Myth: Weathering causes Delamination
Mythbusting fact: Weathering doesn’t cause delamination. Delamination is more apparent/obvious when engineered wood products are exposed to regular changes to moisture, but the root cause of delamination is bond failure from an issue in manufacture.
Delamination means a manufacturing fault, while checking just indicates an adverse moisture condition.
Jobsite storage should be such that the members are protected from direct exposure to the elements. Members should be stored to ensure that there is no ground contact and blocked to provide ventilation around the members. Protective wrappings should be maintained intact, but slit on the bottom side to allow for drainage of any entrapped water.
For GLT members, after the building is enclosed it is critical that rapid lowering of the relative humidity be avoided. A gradual seasoning period of moderate temperature should be provided to allow the EWP to slowly reach its equilibrium moisture content level and thus minimise checking.
Direct blowing of heat onto the EWP members using temporary heating units should be avoided. Permanent heating outlets should be designed to deflect heat away from EWP members.
Checking that occurs in members in enclosed buildings is usually completed within the first full cycle of environmental conditioning of the space. However, changes in the end use of the structure may affect future checking.
Can checking be repaired?
Since checking is a natural phenomenon associated with normal moisture cycling during transportation, job site storage and installation – a sequence over which the manufacturer has no control – the EWP manufacturer is not responsible for repairing checks in the product.
If checking does occur and is deemed by the designer, contractor or owner to be visually unacceptable, the checks can be filled with an elastomeric filler. When colour-tinted to match the finished colour of the glulam, these fillers will help to minimise the visual impact of the checks.
Fillers should not be applied until the glulam members have cycled through at least one full heating and cooling season, as further checks are likely.
Rigid epoxy-type fillers should not be used to fill checks since these can cause the checks to worsen during normal moisture cycle changes.
Is my beam structurally OK?
The question will then invariably focus on the point at which the checks are sufficiently large to present a structural problem.
The APA – The Engineered Wood Association in North America – has a long and extensive experience in the use of EWP and has produced a ‘Rule of Thumb’ flow chart for all the industry professionals to analyse the beam and determine whether further engineering advice is needed to determine the structural adequacy of a GLT beam (not applicable to LVL). See Fig. 1.
If, after the use of the flow chart in Fig. 1, it is determined that the checking needs engineering evaluation, there are analysis methods available to the engineer to evaluate whether the reductions in strength caused by the checking is significant enough to warrant replacement or reinforcement.
This information is generic in nature as an initial assessment for uniformly loaded beams only and does not replace expert engineering advice on a case by case basis.
For more information on this topic, contact Craig Kay and the Tilling engineers via email at firstname.lastname@example.org
Image: Natural checking in sawn timber product. Photo Art_of Marble/Shutterstock.com