The right beam size for a deck depends on three things: how far the beam spans between posts, how much deck area it supports, and what type of lumber you’re using. Most residential decks use doubled or tripled 2×8, 2×10, or 2×12 lumber for beams, with the exact size determined by span and load. A doubled 2×8 can handle short spans of around 6 feet, while a tripled 2×12 can stretch to 14 feet or more under normal residential loading.
How Deck Beams Are Sized
Beam sizing isn’t about the total size of your deck. It’s about two measurements: the beam span (distance between posts) and the tributary width (the area of deck the beam is responsible for holding up). The tributary width is typically half the joist span on each side of the beam. So if your joists span 8 feet from the ledger to the beam, that beam carries a tributary width of 4 feet (half the joist span).
Residential decks are designed for a combined load of 50 to 70 pounds per square foot, which includes a 10 psf dead load (the weight of the materials themselves) plus a 40 to 60 psf live load (people, furniture, snow). Some jurisdictions, including parts of Washington State, have bumped the live load requirement to 60 psf, bringing the total design load to 70 psf. Your local building department will tell you which number applies to your area.
Multiply the beam span by the tributary width, then multiply by the total load per square foot. That gives you the total load the beam must carry. For example, a beam spanning 8 feet with a 6-foot tributary width at 50 psf total load carries 2,400 pounds. The beam needs to be stiff enough to carry that weight without deflecting more than the code allows.
Common Beam Sizes by Span
These are general guidelines for built-up beams (two or three pieces of dimensional lumber bolted together) using No. 2 grade southern pine or Douglas fir, assuming a typical tributary width of 4 to 6 feet and standard 50 psf total loading. Wider tributary widths or heavier loads push you to the next size up.
- Up to 6-foot span: doubled 2×8
- 6 to 8-foot span: doubled 2×10
- 8 to 10-foot span: doubled 2×12
- 10 to 12-foot span: tripled 2×10 or tripled 2×12
- 12 to 14-foot span: tripled 2×12
Solid timber beams are another option. A 6×12 or 8×10 can handle spans that would require a tripled 2×10 in built-up construction. Solid beams look cleaner and eliminate the need to bolt multiple plies together, but they’re heavier, more expensive, and harder to find in pressure-treated lumber.
Lumber species matters more than people expect. Southern pine and Douglas fir are significantly stronger than hem-fir or spruce-pine-fir. If you’re using a weaker species, you’ll need to go up a size. A doubled 2×10 in Douglas fir might handle an 8-foot span comfortably, while the same configuration in hem-fir might not pass code.
Drop Beams vs. Flush Beams
A drop beam (also called a “dropped” or “under-slung” beam) sits below the joists, which rest on top of it. This is the simpler approach and the better choice for most DIY builders. The joists don’t need to be cut precisely, and the connection is straightforward: joists sit on the beam and get toenailed or attached with hurricane ties.
A flush beam sits at the same height as the joists, with each joist hanging from it using metal joist hangers. This creates a cleaner look since the beam doesn’t hang below the deck frame, and it works well when you have limited clearance between the deck surface and the ground. The tradeoff is more hardware, more precise cutting, and a slightly more complex build. With a flush beam, you also gain post height, since the beam height (9.25 inches for a 2×10) gets added to the post length instead of sitting below the joist frame. That extra height makes post-to-beam connections much easier when the deck is close to the ground.
From a structural standpoint, both approaches work. The beam size itself doesn’t change based on mounting style. What changes is the connection hardware and the framing sequence.
Cantilever Rules for Beams
You can extend a beam past the last support post, which is called a cantilever. The building code limits this to one-quarter of the beam’s actual span between posts. So if your beam spans 8 feet between posts, it can cantilever up to 2 feet beyond the last post. This is useful for pushing the deck edge out a bit further without adding another footing.
Individual plies in a built-up beam must be continuous across the cantilever. You can’t splice the beam at or near the post and then extend it. The beam also needs at least 1.5 inches of bearing surface on each post (the full width of the beam sitting on wood or metal) or 3 inches if it bears on concrete or masonry.
Post-to-Beam Connections
How you attach the beam to the post is just as important as the beam size itself. Toenailing a beam to a post is explicitly prohibited by code. You need a metal post cap connector designed for your specific beam configuration (2-ply or 3-ply) and post size. Any post that supports a beam splice, where two beams meet end to end, must be at least a 6×6.
Post caps come in sizes to match standard lumber dimensions. A post cap for a doubled 2×10 on a 6×6 post is a different product than one for a tripled 2×12 on a 4×4. Check the manufacturer’s load ratings and installation instructions, since improper installation can void the capacity of even the right hardware.
Sizing for Heavy Loads Like Hot Tubs
A filled hot tub with people in it can weigh 8,000 pounds or more. That concentrated weight completely changes the beam calculation. Standard deck framing won’t handle it without modification.
For a hot tub deck with an 8-foot beam span and 4-foot tributary width using hem-fir No. 2 lumber, you’d need a tripled 2×10 as a built-up beam, or a solid 6×12 or 8×10 timber. A single 4×12 also works in that scenario. The key difference is that the hot tub area can’t share its structural members with the rest of the deck in the usual way. You’ll likely need closer post spacing (shorter beam spans), closer joist spacing (12 inches on center instead of 16), and possibly a dedicated beam directly under the hot tub’s footprint.
If you’re planning a hot tub, get an engineer involved. The footing sizes, soil bearing capacity, and connection details all change significantly, and the consequences of undersizing are severe.
How to Confirm Your Beam Size
Span tables published in the International Residential Code (IRC Section R507) give you prescriptive beam sizes based on species, span, and tributary load area. These tables are the fastest way to pick a beam size without running calculations. Your local building department likely has a version of these tables available, sometimes with regional amendments for snow load or other factors.
To use a span table, you need three numbers: beam span (post to post), joist span (which determines tributary width), and the lumber species you’re using. Find the row matching your beam span, the column matching your tributary width, and the table tells you the minimum beam size. If you’re between numbers, always round up to the next size.
Online beam calculators from engineering software companies can also verify your sizing. You’ll input the span, load per linear foot (derived from tributary width times the design load), and lumber properties. These calculators check both bending strength and deflection, since a beam that’s strong enough to hold the weight might still bounce or sag more than code allows.