Glulam And LVL In Residential Structures

Walk into any house built in the last twenty years and the beam over the kitchen opening is almost certainly engineered timber rather than a sawn hardwood.

The reason is predictability. Engineered products are manufactured to a stated strength with far less variation than a natural piece of timber, which means a smaller member can be used with confidence.

The two products and what they are

LVL, laminated veneer lumber, made from thin veneers glued with the grain all running in one direction. Consistent, strong, available in long lengths and deep sections, and the default for beams and lintels in residential work.

Glulam, glue laminated timber, made from larger laminations stacked and glued. Can be manufactured curved and in very large sections, which is why it turns up in exposed architectural structures rather than hidden in a wall.

Both differ from solid timber in the same way: the defects are distributed through the member rather than concentrated at a knot, so the strength is reliable rather than average.

Cross laminated timber is a different product again, with the laminations crossed for panel action rather than beam action, and it is covered in [cross laminated timber, a viable alternative](/blog/cross-laminated-timber-viable-alternative).

Where it earns its place

Lintels over wide openings, which is the main use. A four metre opening in a load bearing wall is straightforward in LVL and difficult in sawn timber, and the alternative is steel. The comparison is part of [lintels and openings in masonry walls](/blog/lintels-and-openings-in-masonry-walls).

Floor joists at long spans, where depth and stiffness matter and where deflection rather than strength governs.

Rafters and ridge beams on a cathedral ceiling, where there is no ceiling to hide a deeper member.

Posts and columns, where straightness matters.

Any member that has to be straight, because engineered timber does not have the bow, spring and twist that a large sawn section develops.

Deflection almost always governs

Timber beams in houses are rarely limited by strength. They are limited by how much they sag and how much they vibrate.

Which means the design is driven by stiffness, and that has two consequences people find counterintuitive.

A deeper member is dramatically better than a wider one, because stiffness increases with the cube of depth. Two members side by side help far less than one of twice the depth.

Long term deflection is larger than the initial figure, because timber creeps under sustained load. A beam that looks fine on installation continues to sag for years, which is why the design includes a creep factor and why precamber is specified on long spans.

The serviceability thinking behind it is the same as in [deflection limits and serviceability](/blog/deflection-limits-and-serviceability), and the comfort question on a long floor span is the one in [floor vibration and footfall in apartments](/blog/floor-vibration-and-footfall-in-apartments).

The rules that get broken

Do not notch the bottom. A notch in the tension face of a timber beam is the most damaging cut available, and a plumber taking a bite out of the underside of an LVL joist for a waste pipe has removed a large part of its capacity. The parallel is the truss problem in [cutting or modifying a roof truss](/blog/cutting-or-modifying-a-roof-truss).

Holes go through the middle of the depth, small, round, away from the supports, and only where the manufacturer's tables permit them.

No cuts at all in a glulam without checking, because the laminations are arranged with the highest grade material at the extreme fibres, and cutting the top or bottom removes the strongest part.

Bearing length is a design item. An engineered beam concentrates its load, and a long span LVL sitting on a short bearing crushes the timber under it or the plate it sits on. Bearing checks are the most commonly omitted calculation in residential timber design.

Support it in the plane it was designed for, with restraint at the bearings so a deep narrow beam cannot roll.

Moisture, which is the quiet killer

Engineered timber is a dry, internal product. It does not belong in a wet or exposed location unless it is a product specifically manufactured and treated for it.

Standard LVL that gets rained on during construction and stays wet delaminates, and the damage is permanent. Which makes weather protection during the build a real requirement rather than good practice, and it is why a frame left open through a wet month can produce members that need replacing.

The same applies in service. An LVL beam in a bathroom wall with a long term leak, or a member in an unventilated sub floor, will fail eventually. The ventilation argument is the one in [subfloor ventilation and structural timber](/blog/subfloor-ventilation-and-structural-timber).

Termites are a separate question, and engineered timber is not inherently resistant. Treated products exist and the protection strategy is a whole of building matter.

Connections

Timber connections are the weak point and they are rarely as strong as the member.

Which means the bolts, screws, nail plates, hangers and straps are designed rather than selected. Spacing from the end of the member, spacing between fixings, and the direction of load relative to the grain all change the capacity substantially, and a connection detailed by eye is usually the governing element in the structure.

On a job with a steel and timber mix, the interface between the two is where most of the design time goes, and the tolerance question is the same as in [steel fabrication tolerances and site fit](/blog/steel-fabrication-tolerances-and-site-fit).

Specifying it properly

The drawing names the product, the grade, the section, the bearing length, any precamber, the restraint at supports, and whether penetrations are permitted.

A drawing that says a size and nothing else leaves five decisions to the site, and the grades available in LVL differ in strength by a meaningful margin. The general point about timber specification is in [structural timber grades and what they mean](/blog/structural-timber-grades-and-what-they-mean).

ACSES provides structural engineering for residential projects across Sydney. Talk to us about a project.