Deck Framing: How to Determine Deck Framing Lumber Sizes - Building a Deck DIY

Determining the correct lumber to frame a deck doesn't have to be hard. First, you have to understand what the different members do, and how the loads they carry are transferred down to the footings that bear the weight. Read this comprehensive guide to choosing deck framing lumber.

How to Determine Deck Framing Lumber Sizes
How to Determine Deck Framing Lumber Sizes

Deck Framing Design

A deck frame is essentially a wooden platform raised off the ground and supported by beams and posts. The posts stand on concrete footings called piers. The platform frame is made primarily from of 2-by lumber called joists. On a small deck, the beams are sometimes incorporated into the joist framing. On larger decks, the beams support the joists from underneath.

Deck Framing
Deck Framing

The rim joists are the joists that make up the outside of the frame. If the deck is fastened to a house or other building, the board that fastens to the house framing is the ledger board. The ledger board is another joist and is usually the same size lumber as the other joists.

It rarely makes sense to use different size joists for the rims joists, ledger and interior frame joists. If you use 2-by-8 joists for the rim joists or ledger, then use 2-by-8 joists for the rest. Learn how to determine the correct size for deck joists and what governs joist sizing.


When you're putting together a deck plan, the joist size depends on the space between the joists and the length of the joists. The minimum joist size is a 2-by-6, which may span up to 11 feet 7 inches without additional support, if they are spaced just 12 inches apart.

It's easy to compare the cost of a 2-by-6 to the cost of a 2-by-8 or 2-by-10 and think that using 2-by-6 joists will save money. Sometimes it will, if the deck is small. If the deck is large, you'll end up buying more joist hangers, more bolts and you'll have to dig and pour more concrete footings. On a small deck or stairway landing or similar structure, 2-by-6 joists make sense. On larger decks, determine the joist size using the charts to find the correct joist size.

Deck Joists and Joist Spacing
Deck Joists and Joist Spacing

Look at the chart to the left and learn how to use it. This chart is for southern yellow pine (SYP) framing lumber and similar strength woods. The table gives the maximum distance between joist supports which include beams and the ledger board. For example, if the joists are spaced 16 inches apart and are 12 feet long, the logical choice is a 2-by-8 joist which can span up to 12 feet 10 inches at this spacing. 

If the joists were 15 feet long, then 2-by-10 joists are required, which can span up to 16 feet 5 inches. Note that with these spans, no additional support is required under the joists. However, using 2-by-6 joists would require you to add an additional beam with support posts and footings underneath it.

Beams and Deck Post Spacing

The beams support the joists and transfer the load from the deck onto the posts and ledger. The minimum beam size is a 4-by-6 made from two, 2-by-6 planks. Looking at the chart, if the joists span 10 feet, you can use a 4-by-8 beam with posts spaced 8 feet apart, or a 4-by-12 beam with posts spaced 12 feet apart. This is significant, especially if you need several beams.

Deck Beams and Post Spacing
Deck Beams and Post Spacing

On a large deck, using the 4-by-12 beam will save you the labor and cost of pouring additional footings and installing additional posts. If the deck is high enough to walk under, it also increases the open space beneath the deck for a patio or storage area.


The posts transfer the load from the beams onto the footings. Two factors are important: the post size and the distance between posts.

The distance required between posts depends on the beam size. A 4-by-10 beam that supports a joist span of 12 feet, requires posts no more than 9 feet apart. If the beam is about 9 feet long, then two posts will support it properly. If the beam is 12 feet long, then three posts are needed.

Post Size from Height and Load Area
Post Size from Height and Load Area

Post size depends on the height above the ground and the load area. The load area is the distance between the beams and the distance between the posts. Height is less than 6 feet or 6 feet or higher.

Using the chart at the left, we can see that if posts are spaced 6 feet apart and the beams are 10 feet apart, we have a load area of (6 x 10 = 60) 60. If the deck is below 6 feet, then we can use a 4-by-4 post, but if the deck is over 6 feet high, then a 6-by-6 post is required.

Putting it All Together

It might seem overwhelming at first, but in practice it is not hard at all. Decide what size deck you want to build. Determine the joist lumber that requires the least amount of support for the size of the deck, then choose beams, post spacing and post sizes. Determining the lumber to use is one of the first steps in estimating deck building costs. Let's look at two examples.

Deck one - 24 x 10 feet: There's no need to use 2-by-10s for joists if the deck extends just 10 feet from the house. According to the Deck Joist and Joist Sizing Chart, 2-by-6 joists on 16 inch centers will span 9 feet 9 inches. With joists just 9 feet 7 1/2 inches long, we add in 1 1/2 inches for the ledger thickness, and another 3 inches for the rim joist which will act as our beam, and the span is exactly 10 feet. Perfect. We use 2-by-6 joists.

Next, determine the beam size that will adequately support the deck with the least number of footings needed. Our joist span is 10 feet, so we can now determine that a 24 foot long 4-by-12 beam with posts spaced 12 feet apart will support our joists. We need 3 posts and 3 footings—one on the left size of the deck, one in the middle 12 feet from each end and one at the right side.

The post size is determined by the load area and the deck height. In this case, the beams are 10 feet apart (ledger to beam = 10 feet) and the distance between the posts is 12 feet. Our load area is 10 x 12 = 120 feet and according to the chart, we need 6-by-6 posts regardless of how high the deck is.

Deck two
- 12 x 12 feet. From the joist chart, we see that a 2-by-8 joist will span 12 feet without additional center support.

We'll use one beam along the front of the deck since we don't need a center support beam. According to the Deck Beam and Post Spacing chart, a 4-by-8 beam supporting a 12 foot joist span requires posts spaced no more than 7 feet apart, so we can use a 4-by-8 beam with three posts and footings spaced 6 feet apart for our 12 foot width.

Our load area is 6 x 12 = 72. We can use 4-by-4 posts for decks lower than 6 feet, or 6-by-6 posts for decks higher than 6 feet.

Deck Building 101: You Can Do It Yourself

You can build a deck yourself. We've got tips, construction drawings, plans, cost estimating and more in this series on building decks. You'll find the information you need to turn your dream of outdoor living space into reality.

  1. Tips for Deck Building
  2. Directions For Deck Stairs Hand Rails
  3. Make an Easy and Inexpensive Deck
  4. Deck Stair Building: Stringer Attachment Methods
  5. How to Determine Deck Framing Lumber Sizes