Deck Lumber Sizes & Spacing
Calculate Deck Max Joist Span, Max Beam Span, and Max Post Height
When making a deck design, plan the deck understructure. Consider ledger boards, floor joists, beam spans, concrete post settings, max post height, and deck load area.
Deck Lumber Sizes & Spacing
Deck design and framing layout go hand in hand. These steps are where good deck builders spend a lot of time. In part 1 of the deck building series, deck design fundamentals were covered. Part 2 covered deck framing concepts. This article is part III of the series and takes a look at lumber sizes and spacing.
These calculations apply whether the project is an attached deck or a freestanding deck.
First – Calculate the Maximum Joist Span
Increasing the lumber size of joists affects the distance between joists and the distance they can span between beams.
Deck Term - The distance between beams is called the joist span.
- Joists Spaced 12” 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
Second – Calculate the Maximum Beam Size
The beam size determines post spacing. (Stronger beams = fewer posts.) Beams are meant to be doubled. In other words, if the beam is a 2” X 6”, two beams are to be fastened together. Round up distance between beams from the chart.
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
Third – Calculate the Maximum 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.
Now, calculate the load area per post. This is a guideline to determine post size/deck height. Now determine the load area. Round up this figure as applicable.
- 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
