Lumber Sizes and Spacing for Deck Building:
Calculate Deck Max Joist Span, Max Beam Span, and Max Post Height
© 2008 by Kelly Smith all rights reserved
When you design your deck, give special consideration to the planning of the deck understructure.
Consider the location of the ledger boards, beam spans, floor joists, the max post height, concrete post
settings, and the deck load area.
Deck design and the understructure’s framing layout go hand in hand. These are the steps are where
the best deck contractors spend quite a bit of time. In part 1 of the deck building series,
deck design
fundamentals were looked at. Part 2 focused on basic deck framing concepts. Here you have part 3 of the
series. It explains your options 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
Increasing the lumber size of the joists directly affects your distance between adjacent joists as
well as the distance which they will span between beams.
Deck Term: The distance between beams is called 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
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
The size of your beam will determine your post spacing. Why? Because stronger beams = fewer posts.
Beams should be doubled. In other words, if your beams are 2” X 6” lumber, two beams should be
fastened together. Then round up the distance between beams from the chart below.
Deck Term: The distance between posts is called the beam span.
Any distance between beams = 6’ between posts
- 2” X 8” beams
10” between beams = 7’ between posts
Longer distance between beams = 6’ between posts
10’ between beams = 7’ between posts
Longer distance between beams = 6’ between posts
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 Term: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 this guideline 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.
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
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: Find thousands of unbiased ratings on services for home
improvements, car repairs, and more.
Try Angie’s List .
This is just the second in a series of six articles in the deck design and building series. So stay
tuned!
- Wood Deck Design Fundamentals
- Deck Design and Framing Concepts
- Lumber Sizes And Spacing for Deck Building
- Deck Design and Post Hole Layout
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