Choosing the correct joist size is fundamental to a deck’s structural integrity and safety. Joists are horizontal framing members that support deck boards and transfer loads to beams and posts. The right joist dimensions ensure the deck remains stable, durable, and performs well over time.
Key Factors for Joist Selection
The distance a joist spans between supporting beams, known as the span, influences its required depth. Longer spans need deeper joists to prevent deflection and maintain strength.
The deck’s anticipated weight, or load, is another factor. This includes dead load (fixed weight of materials) and live load (temporary weights like people, furniture, and snow). Most residential decks support a minimum 40 pounds per square foot (psf) live load, plus 10 psf dead load.
Joist spacing, measured center to center, also affects size. Closer spacing distributes the load over more members, allowing for smaller joists or longer spans. Common spacing is 16 inches on center. The lumber species and grade also matter, as different wood types have varying strengths. Higher grades, like #1 or #2, indicate greater strength.
2×6 Joists Explained
Two-by-six (2×6) joists measure 1.5 inches by 5.5 inches. They suit smaller, low-to-the-ground decks or applications with short spans and lighter loads. A 2×6 joist can span up to 8 feet when spaced 16 inches on center.
2×6 joists have restrictive load-bearing limits. While they handle 40 psf live loads over short spans, exceeding these limits can cause deflection. Spacing is often 12 or 16 inches on center to maximize their span capabilities. They offer cost savings and easier handling for modest projects like small landings.
However, 2×6 joists are generally inappropriate for longer spans, decks supporting heavy items like hot tubs, or high-traffic areas. Their shallower depth makes them prone to bounce or sag under heavier loads. Larger joists are recommended for most deck construction.
2×8 Joists Explained
Two-by-eight (2×8) joists measure 1.5 inches by 7.25 inches, offering increased strength and versatility over 2x6s. This allows them to span greater distances and support heavier loads, making them a common choice for residential decks. A 2×8 joist can span up to 10 feet when spaced 16 inches on center, or up to 12 feet depending on wood species and grade.
Their increased depth provides greater resistance to bending, resulting in a more stable deck. They handle a 40 psf live load over typical spans. This capacity suits decks supporting heavier furniture, multiple people, or moderate snow loads. They maintain 16-inch on-center spacing for many deck board types while achieving good spans.
2×8 joists are recommended for spans approaching 8 to 10 feet, or when a robust feel is desired. Though heavier and more costly than 2x6s, their structural integrity and design flexibility often outweigh these factors for most projects. This size balances material cost, handling, and performance for many deck designs.
Ensuring Compliance and Safety
Adherence to local building codes is the primary determinant for joist sizing. Codes provide definitive span tables and requirements, ensuring structural safety. Local building departments offer specific information on permissible joist sizes, spacing, and load capacities. Obtaining permits and following regulations is essential for safe construction.
Consulting a qualified professional, like a structural engineer or experienced contractor, is advisable for complex designs or code uncertainties. Experts assess project details, including soil conditions and intended use, to provide precise recommendations. Their expertise ensures the deck design meets safety standards.
Beyond code compliance for strength, consider deflection for user comfort. Deflection is how much a joist bends under load. Even if code-compliant, excessive deflection can make a deck feel bouncy. Engineers design for maximum allowable deflection, such as L/360, to ensure a comfortable feel.