Understanding Checking in Glulam

Checking, the separation of continuous wood fibers, is a naturally occurring consequence of the seasoning process of wood. The outer fibers lose moisture to the surrounding atmosphere and attempt to shrink, but the inner portion of the timber member loses moisture at a much slower rate. The different rates of shrinkage can cause the wood to check or split. Rapid drying increases the differential moisture content between the inner and outer fibers and thus increases the propensity for checking in the timber. The checking (and shrinkage) process will stabilize as the moisture content of the member reaches equilibrium with the surrounding environmental conditions.

Glued laminated timber normally has fewer and smaller checks than solid-sawn timber since the individual laminations used to manufacture glulam have been dried before manufacturing. Glulam has an average moisture content of 12 percent at time of manufacture, which is relatively close to the equilibrium moisture content it will reach in most interior environments. This lower moisture content is one of glulam's inherent advantages over sawn timbers, which are typically supplied at a higher moisture content.

How Do You Identify Checks?

Checks are openings that occur parallel to the natural grain of the laminations. They show the separated wood fiber on each exposed surface at the check openings.

Seasoning checks, as shown at left, are commonly found near a glueline in a glulam member and often appear along the first glueline adjacent to the outmost lamination, where the amount of surface exposed by the outmost lamination is greatest and differential drying stresses are the highest.

Seasoning checks should not be confused with delamination. In delamination, openings are separations between the laminations at a glueline, not the wood fiber. Delamination occurs when the glue bond is not adequate to resist moisture cycling. With delamination, the surfaces of the laminations at the opening are smooth and often reveal the dark color of the phenol-resorcinol adhesive typically used for face bonding of glulam. When lighter-colored adhesives have been used, it may be more difficult to verify the presence or absence of wood fibers on the lamination surfaces. Delamination is highly unlikely in glulam members because they are manufactured with durable wet-use adhesives under closely controlled manufacturing procedures.

Do Checks Significantly Affect the Strength of Glulam?

Building owners commonly ask, "Do checks significantly affect the strength of glulam?" First, the way the member is being used must be considered in evaluating the checks' effect on strength.

For a glulam column or post, the only time a check becomes a structural concern is if it develops into a full-length (the entire height of the column or post, on both sides of the glulam) split. In this very unusual case, the length-to-depth (or L/d) ratio used in the design of columns will change, and the resulting structural capacity of the column should be confirmed by a qualified design professional. A partial check is not a structural concern.

In bending members (beams, rafters, girders, etc.), checks are commonly observed on the face of the bottom lamination, on the side of the members, and at the end of the members.

The picture at right shows a face check on the bottom face of the member. As long as these checks are parallel to the natural grain of the lumber, they are not considered to be of structural significance, even if they are the full depth of the face lamination.

The picture at left illustrates typical side checks. Testing at the APA Research Center reveals side checks that are no greater than one-third the width and one-third the length of the glulam have a negligible effect on the structural performance of the bending member. Side checks can appear at any depth of the member.

Testing also shows end checks or end splits, depicted in the photo at right, with a length less than half the depth of the bending member also pose no structural concern.

How Can Checks Be Prevented?

While checking is not a structural concern for most glulam applications, checks can be unsightly and detract from the visual appearance of the glued laminated member. Glulam members should be protected from extremes in temperature and humidity during transportation, storage and installation. Good storage and installation practices that minimize direct exposure to the elements will minimize the extent of checking. During and after installation, it is important that the glulam members not be exposed to rapid movement of air or to concentrated heating and cooling sources such as furnace or air conditioner, heater air outlets or job-site heating units.

When the building is enclosed, the beams should be allowed to adjust slowly to the ambient temperature and humidity of the building by avoiding rapid lowering of the humidity and/or exposure to high temperatures. Unless there are significant changes in ambient ­temperature and humidity, additional checking generally will not occur after the first full seasonal cycle of environmental conditioning of the building.

Site-applied finishes, such as oil-based semitransparent stains, oil-based solid color stains or latex-based paints, can also help mitigate checking by retarding moisture movement into and out of the wood.

Can Checks 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 manufacturer is not responsible for repairing checks. 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 color-tinted to match the finish color of the glulam, these fillers will help to minimize 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.

Rigid epoxy-type fillers should not be used to fill checks since these can cause the checks to worsen during normal moisture cycle changes.