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, check out a service such as Angie’s List or just ask any neighbors who have recently had a deck built or repaired. If you do it yourself, just follow these guidelines for your lumber sizes and spacing for deck building.
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About the Author:
Kelly R. Smith was a commercial carpenter for 20 years before returning to night school at the University of Houston where he earned a Bachelor’s Degree in Computer Science. After working at NASA for a few years, he went on to develop software for the transportation and financial and energy trading industries. He has been writing, in one capacity or another, since he could hold a pencil. As a freelance writer now, he specializes in producing articles and blog content for a variety of clients. His personal blog is at I Can Fix Up My Home Blog where he muses on many different topics.
Kelly R. Smith was a commercial carpenter for 20 years before returning to night school at the University of Houston where he earned a Bachelor’s Degree in Computer Science. After working at NASA for a few years, he went on to develop software for the transportation and financial and energy trading industries. He has been writing, in one capacity or another, since he could hold a pencil. As a freelance writer now, he specializes in producing articles and blog content for a variety of clients. His personal blog is at 