Mark Post Hole Locations for Your Deck
Posts are the foundation of your deck. They transfer the entire load—deck structure, furniture, and people—down to the footings in the ground. Getting post locations right means understanding how posts, beams, and joists work together. Space them too far apart, and your beam will sag. Space them wrong, and your posts won't line up with your footings.
Quick Summary
Typical spacing
6-8 feet apart
Time needed
30-60 minutes
Accuracy required
Within 1 inch
Understanding Posts, Beams, and Joists
Before marking post locations, you need to understand how the structural elements work together. This determines where posts need to go.
The Load Path
Decking boards rest on joists
↓
Joists rest on beams (and ledger)
↓
Beams rest on posts
↓
Posts rest on footings
↓
Footings transfer load to the ground
The key insight: post locations are determined by where beams need support. And beam locations are determined by how far joists can span. Work backward from your deck design to calculate post positions.
Calculating Post Spacing
Post spacing depends on beam size and the load it carries. Here are typical maximum spans for common beam configurations:
| Beam Size | Joist Span 6' | Joist Span 8' | Joist Span 10' |
|---|---|---|---|
| Double 2x8 | 7' | 6' | 5' |
| Double 2x10 | 9' | 8' | 7' |
| Double 2x12 | 11' | 9' | 8' |
Note: These are approximate maximums for #2 grade lumber. Always verify with your local building code and the span tables approved in your jurisdiction. Loads, species, and grades affect actual allowable spans.
How to Mark Post Hole Locations
1. Establish Beam Centerlines
Using your string layout from the previous step, run strings to mark where the center of each beam will be.
- •For attached decks, the outer beam is parallel to the house
- •Beam distance from house = joist length minus overhang
- •String should be level and tight
2. Calculate Post Positions Along the Beam
Determine how many posts you need and where they go along the beam line.
Example Calculation
For a 16-foot wide deck with double 2x10 beam:
- • Maximum beam span = 8 feet between posts
- • 16 feet ÷ 8 feet = 2 spans needed
- • You need 3 posts (at 0', 8', and 16')
Pro tip: Corner posts often need to be inset slightly (12-18 inches) from the deck edge to avoid having the beam extend past the post. Check your plans.
3. Mark Post Centerlines
For each post, you need to mark the exact center point where the post will sit. This is where strings intersect.
- •Run perpendicular strings from your deck perimeter to the beam line
- •Where each perpendicular string crosses the beam string = post center
- •Use a plumb bob to transfer this point straight down to the ground
4. Transfer to Ground and Mark
Once you have the string intersection identified, mark it permanently on the ground.
- •Hold plumb bob at string intersection and mark where it touches ground
- •Spray a circle of paint around the mark
- •Drive a stake at the exact center
- •Add a flag or ribbon for visibility
Important: The stake marks the CENTER of the post hole. The actual hole will be dug around this point, typically 12 inches in diameter.
5. Double-Check All Measurements
Before moving on, verify that all post locations are correct:
- •Measure between adjacent posts (should match your calculated spacing)
- •Verify all posts are aligned on the beam centerline
- •Check that corner posts are properly inset if required
- •Confirm posts don't conflict with utility markings from 811
Common Mistakes to Avoid
Spacing posts too far apart
Exceeding beam span limits causes sagging over time. Even if the deck passes inspection initially, an undersized or overspanned beam will develop a visible dip. Always consult span tables.
Forgetting about cantilever limits
If your beam extends past the outer posts (cantilever), there are limits. Generally, a beam can cantilever no more than 1/4 of its span between posts. A beam spanning 8 feet can only cantilever 2 feet.
Not accounting for post thickness
If your plans show post centers, remember that a 6x6 post is actually 5.5 inches wide. The edge of the post extends 2.75 inches from center in each direction. Make sure this doesn't create conflicts with your layout.
Marking for holes you can't dig
Before finalizing post locations, verify nothing is underground at that exact spot. Check against your 811 markings and private utility research. Moving a post 12 inches is easier now than after you've poured concrete.
Frequently Asked Questions
How far apart should deck posts be?
Posts are typically 6-8 feet apart. The exact spacing depends on beam size, load, and code requirements. A doubled 2x10 beam can span about 8 feet between posts. Verify with span tables for your specific lumber grade and species.
How many posts do I need for my deck?
Divide your deck width by the maximum beam span, then add 1. For example, a 16-foot deck with 8-foot maximum span needs 3 posts (16 ÷ 8 = 2 spans = 3 posts). Add more for multiple beam lines or complex designs.
Do posts go inside or outside the beam?
The most common residential approach is beam on top of post, secured with a metal post cap. This provides the strongest connection and is easiest to install. Some designs notch the post or sandwich it between doubled beam members.
How accurate do post locations need to be?
Aim for within 1 inch of your marked center. Post holes are typically 12 inches in diameter, giving you some adjustment room. However, the more accurate your marking, the easier installation will be.
Ready for the Next Step?
With post locations marked, it's time to clear the area of vegetation and debris before you start digging.