How to Size a Beam for a 20-Foot Span
A 20-foot clear span is where beam selection gets serious. Most standard-dimension lumber can't make this span under typical residential loads, you're into engineered wood or steel territory. Here's how to think through it.
Step 1: Define Your Load
Before you pick a product, you need to know what it's carrying. The two numbers that matter are tributary width and total load (dead + live).
For a typical residential floor, you're looking at 40 psf live load + 10-15 psf dead load. For a roof, it depends on snow load and whether it's a flat or pitched assembly. A beam carrying a 12-foot tributary width at 50 psf total load = 600 PLF (pounds per linear foot).
Step 2: Understand Your Span
20 feet is a critical threshold. Below 16 feet, you have plenty of LVL and glulam options in standard depths. At 20 feet, depth becomes the limiting factor, especially if you're constrained by ceiling height or floor-to-floor dimensions.
Rule of thumb: beam depth is roughly span ÷ 20 for floor loads and span ÷ 24 for roof loads. At 20 feet, that's 12" minimum for floors, and you'll likely need 14" to 16" depending on load.
Step 3: Compare Products
At 20 feet, your realistic options are:
- Glulam (24F-V4 or 24F-V8): Available in 3-1/8", 5-1/8", and 6-3/4" widths. At 5-1/8" × 16-1/2", a 24F-V4 DF glulam handles most residential floor loads at 20'. Architectural grade available for exposed applications.
- LVL (1.9E or 2.0E): Multiple plies of 1-3/4" LVL. A (4) 1-3/4" × 16" 2.0E LVL beam works for moderate loads. Economical but deep.
- PSL (Parallam 2.0E): Available in wider sections. A 5-1/4" × 14" PSL can work at 20' under lighter loads. Higher cost than LVL.
- Steel (W-shape): A W10×22 or W12×16 handles most residential 20' spans easily. Shallower depth than wood options. Cost depends on your market and connection details.
Step 4: Check Deflection
Strength gets you the load. Deflection gets you the feel. At 20 feet, deflection often controls the design, especially for floors. The standard is L/360 for live load and L/240 for total load. At 20', L/360 = 0.67" maximum deflection.
This is where a lot of "this beam works" calls fall apart. The beam may carry the load in bending, but it bounces. You need to check both.
Step 5: Account for Bearing
A 20-foot beam at 600 PLF delivers 6,000 lbs per end. That's real weight. Your bearing condition matters, you need adequate post sizing, proper foundation under the post, and a load path that actually gets to the ground.
The beam is never the whole story. It's the load path, from ridge to foundation, that makes or breaks a structure.
Try It Yourself
Our free Beam Compare tool lets you compare Glulam, LVL, PSL, LSL, and Steel side-by-side. Enter your span and load, see which products work and which don't.
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