The Load Bearers of Your Deck
Deck posts are the vertical, heavy-duty timber columns (typically 4x4 or 6x6 treated pine) that transfer the immense weight of the entire deck structure down into the concrete footings. If a joist fails, a section of the deck sags. If a post fails, the entire deck collapses.
Because of this critical structural role, knowing exactly how many posts you need—and spacing them correctly beneath the support beams—is the most important safety calculation in deck building.
Understanding Post Spacing
The distance you can safely span between posts is dictated by two main factors:
- The Size of the Beam: A massive triple 2x12 beam can span a much further distance between posts than a single 2x8 beam.
- The "Tributary Area": This is the total square footage of the deck that is directly supported by that specific post.
Most local building codes provide prescriptive "span tables" that dictate maximum post spacing. However, a very common and safe rule of thumb for standard residential decks using double 2x8 or 2x10 beams is spacing the posts 6 feet to 8 feet apart.
How to Estimate the Number of Posts
To estimate the total number of posts required, you must lay out a theoretical grid under your deck.
The Calculation Logic
- Determine how many Rows of Beams you need. If a deck is 12 feet wide, you typically need a beam near the house (or a ledger board) and another beam near the outer edge. If the deck is wider than 14 feet, you often need a third beam running down the middle.
- Determine how many Posts per Beam you need by dividing the total length of the deck by your allowed post spacing, rounding up, and adding 1 (for the starter post).
- Multiply the Rows of Beams by the Posts per Beam.
Note: If your deck is attached to your house with a securely bolted "ledger board," the house itself acts as the first "beam," meaning you need one less row of posts.
Example Calculation
Imagine a freestanding deck (not attached to a house) that is 16 feet long and 12 feet wide, with posts spaced every 6 feet.
- You need two rows of beams (one at the front, one at the back) to span the 12-foot width.
- For a 16-foot long beam with 6-foot post spacing:
16 ÷ 6 = 2.66. Round up to 3 spans. Add 1 for the starter post = 4 posts per beam. - 2 beams × 4 posts per beam = 8 total posts.
4x4 vs. 6x6 Posts
Historically, 4x4 posts were the standard for residential decks. However, modern building codes have become much stricter regarding lateral stability (how much the deck sways side-to-side) and "notching."
It is now highly recommended—and often legally required—to use 6x6 posts for any deck that is more than a few feet off the ground. A 6x6 post is massive enough that you can safely "notch" the top of it, creating a solid wooden shelf for the heavy support beams to rest directly on, rather than relying entirely on metal bolts to hold the weight.