Question 1

How do I size a structural beam?

Accepted Answer

To size a beam, you need three things: the clear span between supports, the tributary width (typically half the floor span on each side), and the design loads (live and dead). Multiply load (psf) by tributary width to get pounds per linear foot. Then check beam capacity tables or use this calculator to find a size that handles the load over the span without excessive deflection (typically L/360 for live load).

Question 2

What is tributary width?

Accepted Answer

Tributary width (or tributary area) is the portion of the floor or roof load that a single beam supports. For a beam in the middle of a room, it equals half the joist span on each side. For example, if a beam supports joists that span 12 feet on one side and 14 feet on the other, the tributary width is 6 + 7 = 13 feet. Multiply this by the load (psf) to get the load per foot of beam.

Question 3

When should I use LVL or glulam instead of dimensional lumber?

Accepted Answer

Use engineered wood (LVL, glulam, PSL) when the span exceeds about 14 to 16 feet, when the load is unusually high, or when you need a shallower beam to fit within an existing pocket. Engineered beams are dimensionally stable, do not warp, and offer up to 2 to 3 times the strength of solid lumber. They cost more but often eliminate the need for intermediate columns or bearing walls.

Question 4

What loads do I use for residential beam design?

Accepted Answer

Standard residential loads are 40 psf live load plus 10 psf dead load for floors (50 psf total), or 30 psf live (sleeping) plus 10 psf dead. For roof beams use snow load (varies by region from 20 to 70 psf) plus 15 psf dead load. Decks use 40 psf live plus 10 psf dead. Check your local code as snow loads and seismic adjustments vary considerably by location.

Question 5

How deep does a beam need to be?

Accepted Answer

A useful rule of thumb is depth equals span in inches divided by 12. So a 12 foot span needs about a 12 inch deep beam (like a 2x12 or LVL 9-1/4 to 11-1/4 inch). For longer spans or heavier loads, increase depth proportionally. Deeper beams are stiffer and more efficient than wider ones for the same cross-section area, so depth matters more than width.

Question 6

Can I use 2x10s for a beam to support a load-bearing wall above?

Accepted Answer

Built-up 2x10s can work as a beam, but the right size depends on span and load. A doubled 2x10 (2-ply) handles roughly 1700 plf at short spans, while a tripled 2x10 (3-ply) increases capacity to about 2500 plf. For point loads from above (such as columns or concentrated loads from the roof), always have a structural engineer verify the design. Beam underspecification can cause floor sag and structural failure.

Question 7

Do I need a permit and engineer for installing a structural beam?

Accepted Answer

Yes, almost all structural beam work requires a building permit and most jurisdictions require sealed engineering drawings for any beam that supports a load-bearing wall, removes a bearing wall, or replaces a header in an exterior wall. This calculator provides preliminary sizing only. Always have a structural engineer or architect verify the final beam size before construction.

Span	Built-up Dimensional	LVL (1-3/4")	Steel
8 ft	(3) 2x8	(2) 9-1/4"	W6x9
12 ft	(3) 2x10 or (2) 2x12	(2) 11-7/8"	W8x10
16 ft	(3) 2x12	(2) 14"	W10x15
20 ft	Not feasible	(3) 16"	W12x22

Material	Best For	Max Span	Cost
Built-up 2x Lumber	Short interior spans	~14 ft	$
LVL (laminated veneer)	Residential headers, beams	~30 ft	$$
Glulam (glued laminated)	Exposed architectural beams	~60 ft	$$$
PSL (parallel strand)	Heavy loads, long spans	~40 ft	$$$
Steel (W-shape)	Very long or heavy spans	60+ ft	$$$$

Beam Size Calculator

Span and Load Geometry

Design Loads

Beam Material

All Built-up Lumber Options

How to Size a Structural Beam

Tributary Width

Load Per Linear Foot

Beam Options by Span

Beam Material Comparison

Frequently Asked Questions

Span and Load Geometry

Design Loads

Beam Material

All Built-up Lumber Options