Main Structural Deck Components: JOISTS – Part IV of VI

joist is main structural deck component

Joists are the horizontal substrate, part of the superstructure that supports the walking deck surface of the backyard deck system, particularly for decks installed on the exterior rear of a building. These horizontal framing members serve as the primary load-bearing components, supporting the weight of the deck surface, furniture, and occupants. 

The primary function of joists is to transfer the loads from the deck surface to the supporting structure, such as the building’s exterior wall or freestanding deck posts. Joists are typically made of pressure-treated lumber or engineered wood products, designed to withstand the specific load requirements of the deck. They are spaced at regular intervals, depending on the deck’s span, load capacity, and local building code requirements.

Joists distribute the concentrated loads from the deck surface in a dispersed force across their length, preventing excessive deflection or sagging. They work in tandem with other structural components, such as ledger boards (attached to the building’s exterior wall) or beam supports (for freestanding decks), to create a stable and supported framing system.

In addition to their load-bearing capabilities, joists also provide a secure attachment point for the deck boards or decking material. Deck boards are typically fastened to the top of the joists using appropriate fasteners (nails, screws, or hidden fastening systems), to create a level walking surface.  In decades past, deck installers often used galvanized nails to secure the deck boards to the joist, but in modern times it’s much more common for professionals to use exterior grade screws.  With changes related to moisture and weather and pressure, deck nails will often loosen over time and the heads will become exposed or raised above the walking surface of the deck.  Alternatively, screws have a better shear resistance and will more often stay suppressed or countersunk below the surface of the deck.

why joist is main structural deck component

Cross blocking or solid bridging between joists is often interlaid to increase the overall lateral stability and rigidity of the deck structure. These additional framing members help distribute lateral loads, such as wind forces or heavy furniture, across multiple joists, preventing excessive racking or twisting of the deck frame.  Also, in addition to lateral load distribution, cross blocking also just gives the deck a better overall stiffness and feel of rigidity, it can even eliminate a bit of the typical bounce found in an elevated deck system when walking or jumping on the deck.

Proper joist sizing, spacing, and span calculations are needed to build the deck to meet the required load-bearing capacity and complies with local building codes and safety standards. Factors such as the deck’s intended use (residential or commercial), live and dead loads, and environmental conditions (snow, wind, etc.) need to be taken into account when determining the appropriate joist dimensions and spacing.  In some cases, joists also serve as attachment points for structural reinforcements, such as diagonal bracing or lateral load connectors, further enhancing the deck’s resistance to lateral forces and create a more stable and secure structure.

Deck framing requires familiarity with local building codes and best practices. Proper joist installation, including adequate support, fastening, and bracing, are essential for the long-term safety, durability, and performance of the deck system. The International Code Council, who’s requirements are specifically referenced and adapted in many cases, here in Washington DC says the following:

“Joists exceeding a nominal 2 inches by 12 inches (51 mm by 305 mm) shall be supported laterally by solid blocking, diagonal bridging (wood or metal), or a continuous 1-inch by 3-inch (25 mm by 76 mm) strip nailed across the bottom of joists perpendicular to joists at intervals not exceeding 8 feet (2438 mm).”

In simple terms, this means that cross blocking is required between deck joists longer than 8 foot in length.  The diagram below shows an illustration of how in the span of the joists, from the ledger to the outer rim joist, a row of cross blocking is installed between each joist.  

how joist work in main structural deck component

Often in architectural drawings, and here in our rendering, the cross blocking is set in a linear fashion, almost where each small cross block meets the next or is in line across the plane of the separating joist. In reality, this blocking is often staggered by a few inches.  

Staggering the cross blocking allows for end-nailing of the deck boards. End-nailing is stronger than toenailing, for securing the deck boards and preventing them from separating over time. Segregated across blocking allows each board and, across the perpendicular joist to be accessible, even just a few inches allows enough space to drive a nail through the joist into the end-grain of the cross block.

From an installation perspective, staggering the cross blocking can make the process more efficient and manageable. Working with shorter cross block sections, rather than running continuous blocks across the entire deck frame, can be easier to handle and position during construction.  Also, applied continuous blocking or strapping can be used in some cases but generally applies an additional element outside of the necessary height level of the existing deck, therefore it looks cleaner and more professional just to install the cross blocking between the joist bays.

Use a contractor who understands and cares about doing things right.  Always, feel free to reach out to us here at Dupont Decks and Patios.  You can call us at (202) 774-9128.  You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us 

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