Deck Board Dimensions: How to Choose the Right Size

Are you ready to build or resurface a deck but don’t know what deck board dimensions are right for your plan? Building a deck is a great way to identify your outdoor living space and extend the use and enjoyment of your personal green space. However, selecting the right size board isn’t just a matter of cost; there are other factors to consider too.

Select 5/4×6 inch deck boards to fasten in a perpendicular pattern to joists spaced 12” or 16” on-center (OC), or diagonally on 12” OC joists. Use 2×6 planks for 16” or 24” OC joists, or if running diagonally to 16” OC joists. For high traffic decks, 2×6 planks offer a more solid and safer surface.

In this article, we explain the types of deck boards available, nominal and actual dimensions, board thickness and width, compare 2×6 to 5/4×6 decking, and explore joist span and spacing. When you’re finished perusing this article, you should have a better knowledge of deck board dimensions and how to select what’s best for your project.

Deck Board Dimensions

Types of Decking Boards

The selection of deck boards is greater today than ever before. Cost and maintenance, though, are two factors to consider when choosing deck boards. An inexpensive choice that will require cleaning and staining once or twice a year, or a wood-look synthetic polymer that won’t fade, split or mold for 50-years, but costs 10-times more. The grade of wood – #1 or #2, premium, or standard determines the price and quality of the wood too.

Wood

Untreated pine, spruce, or fir can be used as decking. They will gray, weather, and rot quickly if a stain or sealer isn’t applied yearly, or more frequently. The stain or sealer should be resistant to UV, mold, and insects. (2” x 6”x 12’ = $5 to $9)

Pressure Treated

Wood, commonly pine, protected from fungus and insect damage with chemicals that have been forced into the wood with pressure. The decking should be regularly protected from rot, mold, and insects with a sealer or stain every 2 to 5 years. (5/4” x 6”x 12’ = $8 to $12 and a 2” x 6” x 12’ = $12 to $14)

Red Cedar

Western red cedar, cypress, and redwood have natural oils that resist damage by insects and fungus. Cedar should be protected against UV damage with a clear stain or sealer every 2 to 5-years. (1” x 6”x 12’ = $12 to $20)

Composite Decking

Composite boards are manufactured from recycled plastic and wood to look like wood. They are very resistant to mold, insects, and rot. It is available in more than 24 wood tones and colors, and lengths up to 20-feet. Has a 30-year warranty against staining and fading. (5/4” x 6”x 12’ = $50 to $90 with includes installation clips)

PVC Plastic Boards

PVC boards are synthetic polymer or plastic planks made to imitate wood and carry a 50-year warranty against fading and staining. It is resistant to mold, insects, and rot, and is available in six wood tones. (1” x 6”x 12’ = $60 to $68)

Aluminum Boards

Aluminum planks are weather and waterproof boards up to 36-feet in length. Available in many different anodized or powder coated finishes, as well as silver, sand, or gray tone aluminum. Low maintenance, no warping, cracking or splinters, or any mold, mildew, rot, or UV worries, plus a lifetime guarantee. (1” x 6”x 12’ = $60 to $80)

Deck Board Sizes and Actual Dimensions

When a tree is cut down and then milled into planks, it is commonly 1 or 2-inches thick with varying dimensions in widths. The surfaces are rough to touch and show milling saw marks. The wood is either air or kiln-dried, which decreases the moisture content. As it dries, the wood shrinks, especially in width.

Once the moisture content is stable, the wood goes through a planer with cutting heads that remove saw marks and smooth the plank surfaces to standard dimensions. Kiln drying allows the rough cut lumber to be cut thinner than 2-inches since it better controls the moisture content. The planks are still planed to the same finished or actual dimensions, but more board feet may be produced per log.

The nominal dimensions of lumber are commonly based on the rough cut size before being planed smooth on all four faces. Dimensional lumber is what is produced by planing the rough cut lumber smooth. A nominal or rough cut 2×6 planes down to 1-1/2”x5-1/2” while a 5/4×6 dimensionally is actually 1”x5-1/2”.

Common Deck Board Sizes
Nominal DimensionsActual Dimensions
5/4 x 61 x 5½”
2 x 41½” x 3½”
2 x 61½” x 5½”
2 x 81½” x 7½”

Common Decking Board Thicknesses

Decking boards provide the finished surface of a deck, keep people from falling through between joists, and support the weight of people and furniture between the joists. The plank thickness should provide a solid feel for the unsupported distance between joists. If the board is too thin for the span, it will feel spongy or bouncy.

Deck joists are commonly 12-inch, 16-inch, or 24” on-center (OC). The greater the distance, the more the board must support over that distance, and the greater the potential for flexing. Constant flexing of the plank will weaken it and may cause it to break more easily. It can also create a trip hazard if it dips too far.

Deck board thickness determines the distance it can span between joists in many building codes. A 5/4”x6” deck board is rated for 12” OC and 16” OC, and a 2”x6” plank can span that plus 24” OC. A 2”x6” plank will flex more when supported at 24” OC than the other two distances, and a 5/”x6” will flex more at 16” OC than at 12” OC. The two most common deck board thicknesses are 5/4” and 2”. Greater thicknesses could be used, but increased weight affects the substructure, and greatly increases costs.

What Is the Standard Deck Board Size?

Deck board size

Common deck board widths are 4”, 6” and 8”. The widths are readily available in most lumber or building supply stores, and the number of pieces easily calculated into deck dimensions. Other widths may be used for aesthetics and effect.

Mathematically, one-foot or 12-inches is divisible by 1-2-3-4-6-12, making the number of planks to cover a deck surface easier to calculate. Widths of 1”, 2”, 3” or 4” require more pieces and thus labor to cover the same area. Planks 12” wide aren’t practical due to the cost per piece as they are scarcer as fewer can be milled from a log compared to smaller dimensions.

Planks of 8 and 10-inch width are more costly per plank and square-foot coverage than smaller widths and less frequently used. The most common or standard deck board width is 6-inch wide planks based on square-foot coverage, labor, and material cost.

5/4 Decking vs 2×6

Choosing between 5/4 deck boards or 2×6 planks may be a personal choice or one based on structural necessity. Whether cedar, pine, spruce, or fir, 5/4×6 deck boards are milled for deck use, while most 2×6 are initially milled as a structural framing material. Since 5/4×6 have a specific purpose, they are milled smoother and have a more rounded edge to prevent splintering and scuffing.

Cost may also be a factor and is determined by lumber grade. A premium 5/4×6 deck board is often less expensive than a lower grade 2×6 of equal length. It is important to note, though, that the thicker lumber dries slower and won’t twist, bow, or cup as much as the thinner material.

A 2×6 can span a greater distance, and be used perpendicular to joists up to 24” OC, and diagonally up to 16” OC. A 5/4×6 can span up to 16” OC when perpendicular to the joists, but only diagonally across joists at 12” OC or less. A 2×6 will give a more solid feel to a deck with 16” OC spaced joist than a 5/4×6 beck board will, so if the deck is high traffic or expected to host large numbers of people, the thicker material is often the better choice.

Deck Board Comparison
5/4” x 6”Deck Material2” x 6”
12” OC and 16” OCPerpendicular joists span12” OC, 16” OC & 24” OC
12” OCDiagonal joist span12” OC and 16” OC
$8 to $1212-foot cost$12 to $14
Standard or premiumLumber grade#2 or standard
SmoothFinishLess Smooth
1/2″ roundEdges1/4″ round
8’, 10’, 12’, 16’Standard lengths8’, 10’, 12’, 16’

Deck Joist Spacing for 5/4 Decking

Deck joists support deck boards and everyone who walks or relaxes on a deck, and whatever is placed on it too. Joists are commonly spaced at 12” OC, 16” OC, and 24” OC. The distance a deck board traverses between the joists is unsupported, so boards of different thicknesses are required for varying spans. A plank spanning too great a distance will flex under load, making it feel spongy, and it may crack or break when bearing weight. Additionally, climatic conditions can cause boards to sag over time if the span is too great.

Most building codes limit 5/4 deck boards to maximum joist spacing of 16” OC if laid perpendicular (at right angles) to the joists. Deck boards that are laid at an angle to the joist are limited to 12” OC (or less) joist spacing. Some composite 5/4 deck boards carry recommendations for joists at 12” OC, while others are reinforced with fiberglass and can span joists at 24” OC. However, it is best to check with the local building inspector for allowances in your area.

What Size Boards to Use for Deck Floor

What size boards for deck floor

A deck is commonly a platform raised off the ground by posts or blocks that support beams that carry joists that the deck boards fasten to. Beams are often under the joists but can be part of the joist frame on smaller decks. The load a deck can support is based on the dimensions and spans of the materials from which the frame is constructed. As with deck boards, the greater the span and the thinner a joist, the more it will flex and bounce.

Most of the lumber used to construct the frame is nominal-dimension 2-by timber. The size of the deck determines the length of the joists and unsupported span they must cover, and it influences the joist spacing. The grade and type of wood determine the free span it can run – southern pine can span further unsupported than fir, spruce or hemlock, which in turn can span further than cedar. Check the chart below for a quick comparison.

Common Deck Joist Span and Spacing
Timber Type

(#2 grade or better)

Nominal DimensionsJoist Span & Spacing
12-inch16-inch24-inch
Southern Pine2 x 69’-11”9’-0”7’-7”
2 x 813’-1”11’-10”9’-8”
2 x 1016’-2”14’-0”11’-5”
Spruce-Pine-Fir

(SPF),

Hemlock, Larch

2 x 69’-6”8’-4”6’-10”
2 x 812’-6”11’-1”9’-1”
2 x 1015’-8”13’-7”11’-1”
Cedar, Redwood, Red & Ponderosa Pine2 x 68’-10”8’-0”6’-10”
2 x 811’-8”10’-7”8’-8”
2 x 1014’-11”13’-0”10’-7”

For a 12-foot wide deck, 2×8 southern pine or SPF lumber will span in one length with spacing between the joists of 12” OC or 16” OC, making it a good choice for deck joists. For an 8-foot deck, 2×6 lumber would be ideal. However, if planning to place a hot tub or similar weighty object on the deck, check with the local building department.

Additionally, if you plan to cantilever the end of the deck over a support beam, the non-cantilevered span must be at least twice the length of the cantilevered span. It is recommended you check your local building codes before proceeding.

Conclusion

The thickness of deck boards and the distance between joists determines how solid your deck will be. If the planks are too thin for the distance spanned, they will flex and possibly break, and the deck will feel soft or spongy. Check the distance between joists and select the deck board dimensions best suited for how the deck will be used.

I hope you have a better understanding of the types of deck boards on the market, and how to choose the right dimension lumber for your deck. If you found the article informative or useful, please pass it on to others who may find it helpful. Your comments and suggestions are appreciated.

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