How to Build a Deck

The afternoon sun shines on a back deck overlooking a slope.

Building a deck is a rewarding and worthwhile task. It adds a great area for social use, helps you enjoy your outdoor space, and increases the value of your home. While you may have a design and plan already figured out, take a moment to learn more about how you should go about choosing your lumber.

Which Kind of Wood

Not all woods are suited for building a deck, so you can’t just go to a home improvement store and pick the one with the color or texture you like best. Because of their rot-resistant properties, some of the best choices for decking lumber are pressure-treated lumber, cedar, and redwood. Pressure-treated lumber is generally more cost-effective than the other choices, but the quality of the pieces can vary quite a bit more. Cedar and redwood are both long-lasting options that look nicer than pressure-treated, but you may reconsider this option if you anticipate a lot of abuse to your deck.

There are also a lot of advancements and kits for composite decking, which has its own benefits and drawbacks over traditional wood.

Decking Size

It’s not enough to decide on the wood type; you also need to consider what size pieces you will need. Refer to the chart below for information on purchasing lumber thicknesses. They will come in sizes of either 1.25x6-inch or the standard 2x4-inch, and the number of joists you need will determine the size you choose.

Joist Spacing
5/4x6 lumber 16" maximum
2-inch lumber 24" maximum, 16" preferred

Lumber sized at 5x4x6 should be your minimum thickness as using anything thinner will produce a weak, springy deck. The length of the planks that you buy obviously will vary based on your needs, but you should aim to buy pieces that can span most of your deck length. The fewer cuts you have to make to cover the entire decking surface, the better you will be. More water absorbs in the joints where boards meet, which can lead to quicker deterioration down the line if you aren't vigilant with sealing treatments.

Joist Size

2x6-inch lumber with 2x10-inch lumber is a popular combination to use for joists. You can save a little money here and get a good, solid substructure by choosing pressure-treated lumber over redwood or cedar since the joists will not be visible. Save your budget money for the decking that will be seen.

Plan the space between the joists firsts. A deck about ground level will mean dealing with creating a structure that with require posts and beams to elevate the joists. If the deck is within six feet of the ground, then the support beams should be spaced by no more than one foot.

Beam Spacing
Joist Size (joists 16" o.c.)
Up to 8 feet 2x6 (Southern pine, Douglas fir, Western red cedar, S-P-F, or Hem-Fir)
2x8 (redwood, Northern white cedar)
8 to 10 feet 2x8 (all species listed above)
10 to 12 feet 2x8 (Southern pine, Douglas fir, Western red cedar, S-P-F, or Hem-fir)
2 x 10 (redwood, Northern white cedar)
Beam Spacing
Joist Size (joists 24" o.c.)
Up to 8 feet 2x6 (Southern pine, or Douglas fir)
2x8 (Western red cedar, S-P-F, Hem-Fir, redwood, or Northern white cedar)
8 to 10 feet 2x8 (all species listed above)
10 to 12 feet 2x8 (Southern pine, or Douglas fir)
2x10 (Western red cedar, S-P-F, or Hem-Fir, redwood, or Northern white cedar)

Beam Size

The chart below offers information based on the assumption that the posts are spaced by six feet or less and that the beams are spaced by 12 feet or less. 4x4 lumber is best for such cases.

Beam Spacing (round down to nearest foot)
Minimum Beam Size (doubled 2" material may be used in place of 4" thickness)
Up to 6 feet 4x6 (Southern pine or Douglas fir)
4x8 (Western red cedar, S-P-F, Hem-Fir, redwood, or Northern white cedar)
Up to 7 feet 4x8 (all species listed above)
Up to 9 feet 4x8 (Southern pine, Douglas fir, Western red cedar, S-P-F, or Hem-Fir)
4x10 (redwood, Northern white cedar)
Up to 11 feet 4x8 (Southern pine or Douglas fir)
4x10 (Western red cedar, S-P-F, Hem-Fir, redwood or Northern white cedar)
Up to 12 feet 4x10 (all species listed above)

Deck Railings

Always check your local building codes for specifications on the height and construction of your deck railings, as this will affect the lumber you need to buy. Many places require that the railing shall be between 32 inches and 48 inches high with a maximum space between balusters of 3-1/2 inches. The balusters must also be larger than ¾-inch to prevent small children from squeezing through and getting injured. Other areas have no specifications at all, so it depends where you live.

Generally, 2x4s will suit just fine for the horizontal rails screwed to the posts. Your balusters can either be pre-purchased in a specific design and height, although you will have to be careful to match these to your decking, or you can cut and shape them yourself from any additional 2x4s you might have.

Any more pieces you need to build your deck besides those outlined in this article will vary according to your specific designs. Use your own judgment and utilize the expertise of your local home improvement store professionals to make the best possible choices.