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Lumber Sizes and Spacing for Deck Building:


How to Calculate Deck Max Joist Span, Max Beam Span, and Max Post Height

© 2008 by All rights reserved; content may not be copied, rewritten, or republished without author’s written permission. Author’s Google profile

A Deck Contractor

When you design your deck, it’s hard to overestimate the importance of your deck understructure. It’s critical to get right the location of your ledger boards (for an attached structure), beam spans, floor joists, the max post height, concrete post settings, and the deck load area.

Your desired deck design and the understructure’s layout for the framing work together closely. During these steps are where the better deck contractors spend a lot of time.

In part 1 of the deck building series, we looked at deck design fundamentals. In part 2 we covered fundamental deck framing concepts. This is part 3 of the series where we cover your choices regarding lumber sizes and spacing.

These lumber and load calculations are equally applicable whether your project is an attached deck or a freestanding deck. Although these are generally accepted guidelines, always consult your local building code.

The First Thing to do: Calculate Your Maximum Joist Span

When you increase the lumber size of your joists it has a direct effect on your distance between adjacent joists along with as the distance which they will span between beams.

Deck Terminology: The distance between beams is known as the joist span.

  • Joists Spaced 12”ve Apart

2” X 4” joist = 10’ 4” between beams
2” X 6” joist = 10’ 4” between beams
2” X 8” joist = 13’ 8” between beams
2” X 10” joist = 17’ 5” between beams
  • Joists Spaced 16” Apart

2” X 4” joist = 9’ 5” between beams
2” X 6” joist = 9’ 5” between beams
2” X 8” joist = 12’ 5” between beams
2” X 10” joist = 15’ 5” between beams


The Second Thing to do: Calculate your Maximum Beam Size

Your beam dimensions are what determines your post spacing since stronger beams makes for fewer posts. There are times when making these dimension selection are important because you might be limited in the post placement department.

Your beams should be doubled. This means that if your beams are 2” X 6” lumber, you should fasten two together. Then just round up the distance between your beams using the chart below.

Deck Term: The distance between posts is called the beam span.


  • 2” X 6” Beams

Any distance between beams = 6’ between posts
  • 2” X 8” beams

  • 10” between beams = 7’ between posts
    Longer distance between beams = 6’ between posts
  • 2” X 10” beams

10’ between beams = 7’ between posts
Longer distance between beams = 6’ between posts
  • 2” X 12” beams

10’ between beams = 8’ between posts
12’ between beams = 7’ between posts
Longer distance between beams = 6’ between posts

The Third Thing to do: Calculate your Max Post Height

Deck Term: Joist span X beam span = load area.
Deck Terminology:The distance from the ground to the bottom of the deck is called the max post height.

It’s time to figure the load area per post. Use these guidelines to find your post size/deck height. First determine the load area and then round up this figure where needed. Err on the side of caution.

  • 4” X 4” post

36’ to 60’ load area = 10’ max post height
72’ to 84’ load area = 9’ max post height
96’ load area = 8’ max post height
  • 6” X 6” post

All load areas = 17’ max post height

If you would like to find a reputable deck contractor and just do the design work yourself, this is the service I recommend Angie’s List - Where thousands of consumers share their ratings on the local contractors and service companies they hire. Learn more.

This is just the third in a series of six articles in the deck design and building series. So stay tuned!

  1. Part 1: Wood Deck Design Fundamentals
  2. Part 2: Deck Design and Framing Concepts
  3. Part 3: Lumber Sizes And Spacing for Deck Building
  4. Part 4: Deck Design and Post Hole Layout
  5. Part 5: Setting Posts in Deck Construction
  6. Part 6: Deck Building: Beams and Joists
  7. Part 7: Trex Composite or Aluminum Decking vs Natural Wood
  8. Part 8: Popular Deck Board Patterns and Decking Installation Instructions

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© 2008 All rights reserved; content may not be copied, rewritten, or republished without author’s written permission.