Deck Beam and Post Sizing: A Builder's Guide
The Load Path: Decking to Footing
Deck structural design follows a clear load path: the decking transfers load to the joists, the joists transfer to the beam, the beam transfers to the posts, and the posts transfer to the footings. Each element must be sized to carry its portion of the total load.
The IRC provides prescriptive tables for standard residential decks. These cover common configurations without requiring an engineer — as long as you stay within the table parameters.
Design Loads for Decks
Residential decks design for: 40 psf live load (occupants, furniture, snow where applicable) plus 10 psf dead load (framing, decking, railing weight) = 50 psf total design load.
Some jurisdictions require additional live load for areas adjacent to hot tubs (typically 100 psf) or in heavy snow regions.
Beam Sizing
Deck beam size depends on the beam span (distance between posts) and the joist span (distance from the house to the beam, which determines the tributary width the beam carries).
Common configurations with Douglas Fir-Larch No. 2:
For joists spanning 8 feet to the beam with posts at 8-foot spacing: a double 2×8 beam handles the load.
For joists spanning 10 feet with posts at 8-foot spacing: a double 2×10 beam.
For joists spanning 12 feet with posts at 8-foot spacing: a double 2×12 beam.
Wider post spacing requires larger beams. At 10-foot post spacing with 10-foot joist spans, you’d need a double 2×12 or a 3-ply 2×10.
The IRC deck beam span tables (IRC Table R507.5) provide prescriptive sizes for standard configurations. For anything outside the tables — large spans, heavy loads, unusual geometry — an engineer sizes the beam by calculation.
Post Sizing
Deck posts must resist both compression (the load pushing straight down) and lateral forces (wind, occupant movement). For standard residential decks up to 14 feet above grade, 6×6 posts are the IRC default. 4×4 posts are limited to shorter heights (8 feet maximum for most configurations) and lighter loads.
Post height is measured from the top of the footing to the bottom of the beam. Taller posts are more susceptible to buckling under compression and need to be larger or braced.
Notched vs unnotched: If the beam sits on top of the post (post-to-beam connection with a saddle bracket), the full post cross-section is effective. If the post is notched to receive the beam, the notch reduces the effective cross-section. A 6×6 post notched for a double 2×10 beam loses significant section — verify the notched section is adequate for the load.
Footing Sizing
Footings must be sized to spread the post load over enough soil area that the soil bearing capacity isn’t exceeded. Typical soil bearing capacity for residential construction is 1,500 psf (unless a soil report says otherwise).
Quick footing size estimate: Calculate the total load on each post by multiplying the tributary area by the design load. A post carrying a 10-foot beam span × 10-foot joist span has a tributary area of 100 square feet. At 50 psf total load: 100 × 50 = 5,000 pounds per post.
Footing area required: 5,000 / 1,500 = 3.33 square feet. A 22-inch diameter round footing (2.64 sq ft) is tight; a 24-inch footing (3.14 sq ft) is marginal. A 28-inch footing (4.28 sq ft) provides comfortable margin.
Footings must also extend below the frost line — the depth at which soil freezes in winter. This varies by region: 12 inches in the southern US, 36-48 inches in northern states. Your building department specifies the local frost depth.
Common Mistakes
Undersizing beams for the actual post spacing. A beam that works at 6-foot post spacing may fail at 8-foot spacing with the same joist span. Always check both the beam span and the joist span against the tables.
Using 4×4 posts where 6×6 is required. The IRC limits 4×4 posts to specific height and load combinations. Verify before defaulting to 4×4.
Forgetting lateral bracing. Deck posts above 8 feet require cross-bracing or knee bracing to resist lateral forces. This is both a code requirement and a practical one — a tall, unbraced deck sways noticeably.