Here’s the scene: You are standing in an otherwise-empty space, facing a beam to support across a gap of, say, thirty feet. There is no columns permitted. A beam of solid wood would be out of the question over such distance. Steel could also do the job, but it would feel cold to the touch and need fireproofing, at additional cost.
There, wedged between two extremes, is glue-laminated timber. It is warm enough to consider leaving it exposed as a design element and strong enough to hold up the roof.
How to Choose the Right Glulam Beam
How do you figure out how big a beam to order; big enough to do the job, but not too big, and thus more expensive than necessary? Here’s how it all works together on a basic level.
This chart (above) illustrates how much capacity a given size beam has in terms of its span, or length. As you can see, to achieve greater capacity. Longer span, the amount of depth is far more important than its width. For instance, adding one more inch in width adds some bending strength, whereas going up by one inch in depth do so exponentially. This explains why you’ll find beams that are 24 inches deep and only five inches wide handling heavy loads.
They appear as though giant boards stacked on top of each other. And they’re actualy an engineered composite made of kiln-dried laminations bonded with structural adhesive. The visual also underscores that you don’t buy simply a board; rather, you’re purchasing an engineered composite made of individual layers (called lams) of lumber that have been bonded using structural adhesive.
Those layers, which are finger-jointed and then pressed together at very high pressures, are typically made of lower-grade wood toward the interior and higher-grade wood on the outside (the tension face), where it takes the most stress.
Where many DIYers (and even some contractors) fall flat is in understanding the grade stamp. In the infographic, we feature 24F-V4, the go-to grade for straight-forward spans. The first number, 24, represent the maximum allowable bending stress of this lumber; the second, V4, means an unbalanced layup, stronger wood on the bottom.
Why does that matter? Because turning that beam over will immediately compromise its capacity. Simple spans should always have the tension face down. For cantilevers or continuous spans, where the wood quality on top and bottom becomes important, you’d seek out a balance grade such as V8 (same strength wood on both faces). It is a tiny detail, but it is a structural sin to get wrong.
Using the selection tables, you can get an instant sense of what size beam works with what size span for both roof and floor beams (by the way, I like the tables… They were adapted from the F.W. They were adapted from the Dodge Construction Codes Manual. A simple beam of 5-1/8 by 15 inches will support a span roughly eighteen and a half feet across at a tributary width of twelve. If you need to go wider, you step up depth.
You’ll note, too, that since the live load (snow) in most cases is lighter than the live load (people) for floors, and deflection limits are relaxed for roof beams, they can span slightly farther.
These figures, though, are planning guidelines, not engineering stamps! Use them only as such and check with a pro if your conditions is different (for example, heavy snow loads; or special point loads due to heavy machinery or chimney supports).
Another source of confusion for first-timers is camber. Camber is a small upward curve in the beam, designed to allow the structure’s dead weight to settle down into an appearance of a flat beam. According to the guide, spans greater than thirty feet should of have camber. Without it, a long beam may appear to droop despite its structural integrity. It’s all about comfort and perception.
You must also decide between framing or architectural grade. Save money on framing grade if the beam will be covered by drywall; splurge for architectural grade with voids filled and a smoother surface if you’re planning on staining and exposing the beam.
Which Species? This depends on where you live, what’s available, and personal preferences. Here in the west we use a lot of Douglas Fir which tends to be very stiff. In the southeast we often see Southern Pine which is very strong but behaves different than the other species.
The chart cautions against applying all the species data without care. The tables apply to the species stamped on your actual beam. It’s not so much about learning any specific number as it is knowing what goes into the layup of the wood and how the beam will be loaded.
You need the correct span for the depth, the correct grade orientation, and the correct finish for the look. Get those three things right and that thirty-foot span becomes the highlight in the room instead of a problem to solve.
