How to Size Deck Support Posts
Deck posts transfer the entire weight of the deck down to the footings. Post size depends on the load each post carries and the unsupported height. The 2018 and newer IRC requires 6x6 posts for most residential decks, with 4x4 allowed only for small, low decks.
Post Capacity by Size (IRC R507.4)
| Post | 6 ft | 8 ft | 10 ft |
|---|---|---|---|
| 4x4 | ~10,000 lb | ~7,000 lb | ~5,000 lb |
| 4x6 | ~16,000 lb | ~13,000 lb | ~9,000 lb |
| 6x6 | ~27,000 lb | ~23,000 lb | ~17,000 lb |
| 8x8 | ~52,000 lb | ~46,000 lb | ~38,000 lb |
Post Base vs. Buried Posts
Best practice is to mount posts on top of the concrete footing using a post base bracket (Simpson ABU44, ABU66, or equivalent). This keeps wood above grade where it can dry out and dramatically extends post life. Buried posts are no longer recommended in modern deck construction.
Notched Posts for Beams
A 6x6 post can be notched to support a beam on top - but not arbitrarily. The notch depth cannot exceed one-third the post width (about 1-7/8 inch for a 6x6). Notching more than that creates a failure point where the post can split under load. A safer option is bolting the beam to the side of an un-notched post with heavy lag bolts through-bolts or using a galvanized post cap.
Tributary Area per Post
Use this formula to determine how much deck area each post carries. Then multiply by 50 psf total load (40 psf live + 10 psf dead) to find the load per post:
When to Upgrade to 8x8
- Post height exceeds 12 ft (IRC R507.4)
- Rooftop deck or deck over living space
- Hot tub, spa, or concentrated heavy load on deck
- Any engineered design requires it