Deck Joist Spacing: A Complete Guide

Find out how to properly space the joists to adequately support the deck

Deck joist spacing is an integral part of deck planning and construction. Before purchasing materials or starting the build, it's necessary to plan the size and shape of the deck, including the deck joist spacing and the joist span. Joist span refers to the length of the joist from one end to the other, while joist spacing is the distance between each joist, which is typically measured from the center of one joist to the center of the next joist. This type of measurement is known as on-center-spacing.

The span and spacing of the deck joists can vary depending on the type of material used in the construction of the deck. For instance, metal joists can typically span several more feet than a standard wooden joist. The deck joist spacing can range from just 12 inches to 24 inches, though it's important to check the local building codes to determine if your deck meets the standards for a safe and structurally sound build. Use this guide to learn how to calculate deck joist spacing.

How to Calculate Deck Joist Spacing

Decks joists are typically spaced 12 inches or 16 inches on center, though this can vary depending on the deck size, deck shape, deck material, joist size, and the load on the deck.

Prepare the Deck Plan

Draw out a deck plan to follow. The size, shape, and layout for the deck boards will determine the joist span and spacing.

Standard rectangular decks with boards running perpendicular to the joists are the most straightforward option. These decks typically require deck joist spacing of about 16 inches on center. However, if the boards are positioned in a diagonal pattern, then they must span a greater distance, increasing the strain on the boards. To rectify this situation, you can decrease the deck joist spacing to 12 inches on center.

The plan should also identify the number of beams, support posts, and joists, and the intended layout for the deck boards. Just keep in mind that the more elaborate the decking plan, the greater the joist structure requirements. Make sure to put together a material list, so you can complete the deck on time and on budget.

Select Decking Material

The deck joist span and spacing can vary depending on the type of decking material. This is because the amount of structure support differs based on whether you are using softwoods, hardwoods, composite, or even steel joists.

Pressure treated lumber is one of the most commonly used products. It's available in a range of sizes, including 2x6, 2x8, 2x10, and 2x12. Similarly, cedar, pine, and fir wood can be found in 2x6, 2x8, 2x10, and 2x12 sizes. If you are set on steel joists for the project, then you are generally limited to 2x6, though this size comes in both 16 gauge and 18 gauge.

Use this helpful deck joist span and spacing table to find out the right deck joist size, spacing, and span for your project.

Choose the Joist Size

The joist size should be a primary consideration when you are calculating the span and spacing. The wider the joist, the more support it can provide for the deck boards and any people or objects that may be sitting on the deck.

For instance, 2x6 joists are less expensive than 2x12 joists, but if the deck will need to support any heavy equipment, like a hot tub, then 2x6 joists may not be able to hold up under the weight. A good way of coming up with a suitable joist width is to divide the total span of the joist by two, then add two:

(Joist Span ÷ 2) + 2 = Joist Width

So, a 12-foot span would require 2x8 joists:

(12 ÷ 2) + 2 = 8

Calculate Deck Joist Span and Spacing

Once you know the material, joist size, and you have a plan with the intended size and shape of the deck, you can calculate the desired deck joist span and spacing. Joists are generally spaced either 12 inches or 16 inches on center, though some builds may opt for eight inches or 24 inches on center.

Typically, going with 12-inch or 16-inch joist spacing is a safe bet for the project, but if you want to ensure the spacing is adequate for the project, use the following formula:

Deck Length ÷ (Number of Joists - 1) = Joist Spacing

For example, a deck that measures 20 feet or 240 inches in length with 16 joists should have 16-inch joist spacing.

240 ÷ (16 - 1) = 16

If the deck will have a diagonal layout, then reduce the spacing to account for the additional distance the boards must cross between joists.

Calculating the deck joist span is significantly more difficult and requires the use of several different formulas. It's recommended to use a joist span calculator or to refer to a joist sizing and span table.

Deck Joist Spacing Considerations

Deck joist spacing can vary depending on six key factors, including the shape of the deck, position of the deck, deck joist material, joist size, load on the deck, and the deck board layout. Also, local building codes can sometimes have different joist spacing requirements, so it's necessary to check with the city's building department to ensure the plans are up to code.

Shape of the Deck

To find the ideal joist spacing, it's necessary to know exactly how big the deck will be, as well as the planned shape of the deck. Square and rectangle decks are relatively straight forward, making it easier to calculate the appropriate spacing to support the deck boards.

If you are building an odd-shaped deck, break the design down into normal shapes before calculating the size and planning the construction. The shape of the deck also determines if the joists will be cantilevered or if they will end at the exact edge of the substructure.

Position of the Deck

The location of the deck is also necessary to consider when you are planning this construction project. Decks that are close to the ground can have greater joist spacing and spans than decks that are built high off the ground, such as a second-story deck. The higher the deck, the greater the need for post and beam support, and the shorter the joist spacing and spans.

Deck Joist Material

When you are planning the construction of a new deck, the deck joist material is a crucial factor to keep in mind while determining the joist span and spacing. Whether you are using steel framing, composite decking, or lumber, there is a difference in the strength and flexibility of each material.

• Steel joists are made for strength and durability. Each 2x6 steel joist provides roughly the same strength as a 2x10 wood joist, allowing the material to span a greater distance without increasing the size. Steel joists can be spaced 12, 16, 20, or even 24 inches apart, depending on the load restrictions for the deck. The greater the expected load, the smaller the joist spacing.

• Composite joists will generally come with detailed instructions for positioning and placement. Composite decking manufacturers typically state that 16-inches on center is the maximum recommended joist spacing for these products.

• Lumber joists should typically be spaced about 12 to 16 inches apart to provide adequate support for the deck. However, if you need the joists to span a greater distance, consider investing in high strength lumber, like Douglas fir or southern pine.
  

Joist Size

The size of the joist will also impact the joist span and spacing. Generally, the wider the joist, the more weight it will be able to hold. However, a wider joist can also be used to increase the span or to use wider spacing between the joists. For instance, doubling the thickness of the joist can increase the span by about 75 or even 100 percent. This means that while a 2x6 joist with 16-inch on center spacing may have a span of nine feet a 2x12 joist with 16-inch on center spacing will have a span of about 18 feet.

Load on the Deck

The weight of the people, pets, furniture, appliances, equipment, and other items collectively make up the load on the deck. This is the amount of weight the deck needs to hold without bending, breaking, or collapsing. The greater the load on the deck, the smaller the deck joist spacing and the shorter the joist span. Heavy items, like hot tubs, may require additional joists and support beams.

Deck Board Layout

If you are laying the boards perpendicular to the joists, then 16-inches on center joist spacing will likely be good enough for the project. However, if you are laying the boards diagonally or creating a custom pattern with the deck boards, then use 12-inches on center joist spacing to provide additional support and make up for the extra distance the boards need to travel between the joists.

Deck Joist Spacing Tips

If you are using an existing frame to build the new deck, then before you start, do a thorough check of the deck for any signs of damage, like broken pieces of wood, cracks, sagging, and moisture damage. You should also inspect the existing screws, nails, and brackets for signs of corrosion. Replace any fasteners that look suspect.

You can run a string across the deck frame during the construction process to ensure the joists are level. If a joist is too high, either replace the joist completely, undo the fasteners to reposition the joist, or use a power sander to trim the wood. If a joist is too low, either replace the joist, reposition the joist, or shim the joist to correct the height.

Additionally, it's important to keep in mind that different deck board patterns may require extra blocking and smaller joist spacing to support the deck boards. For instance, you should limit the joist spacing to just 12 inches for diagonal inlays.

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