Choosing between 2x6 and 2x8 lumber for deck joists primarily depends on the span of your deck and the spacing of your joists. For residential decks with standard 16-inch joist spacing, a 2x6 can span a shorter distance than a 2x8.
Understanding Joist Span
The "span" refers to the unsupported length a joist covers between two support beams or a beam and a ledger board. The longer the span, the deeper (and sometimes wider) the joist needs to be to prevent excessive deflection (bouncing or sagging) and safely support the deck's weight.
Common Joist Sizes and Their Spans
When joists are spaced 16 inches apart on center, which is standard for most residential decks, the required joist size is determined by the maximum allowable span:
| Joist Size | Maximum Deck Span (at 16" O.C. spacing) |
|---|---|
| 2x6 | Up to 9 1/2 feet (9' 6") |
| 2x8 | Up to 13 feet |
| 2x10 | Up to 16 feet |
Based on this, you would use 2x6 joists if your deck's span is 9 1/2 feet or less. For deck spans greater than 9 1/2 feet but up to 13 feet, 2x8 joists would be the appropriate choice. If your deck needs to span even further, such as up to 16 feet, then 2x10 joists would be necessary.
Factors Beyond Span to Consider
While span is the primary determinant, other critical factors influence joist selection:
- Local Building Codes: Always consult your local building department or an experienced builder. Building codes dictate minimum requirements for lumber size, spacing, and fastening methods, ensuring structural integrity and safety. Codes often specify design values based on wood species and grade. You can often find general guidelines from organizations like the International Code Council (ICC) or local municipal websites.
- Live Load and Dead Load:
- Live Load: The weight of people, furniture, snow, etc., that the deck will support. Most residential decks are designed for a live load of 40 pounds per square foot (psf).
- Dead Load: The weight of the deck structure itself, including the joists, decking, railings, etc.
- Higher loads might necessitate larger joists or closer spacing.
- Joist Spacing: While 16 inches on center (O.C.) is common, some decking materials (like certain composites or thinner boards) might require closer joist spacing (e.g., 12 inches O.C.) to prevent deflection of the decking itself. Closer spacing can sometimes allow for a smaller joist size, but it's typically driven by the decking material.
- Wood Species and Grade: Different types of lumber (e.g., Southern Yellow Pine, Douglas Fir, Redwood) have varying strength properties. The grade of the lumber (e.g., No. 2 grade or better) also impacts its load-bearing capacity. Ensure you use appropriate lumber for structural outdoor use.
- Beam and Post Spacing: The size and spacing of your supporting beams and posts will directly impact the required joist span, as joists run from one beam to another, or from a ledger to a beam.
Practical Insight
When in doubt, it's generally safer and more structurally sound to overbuild slightly rather than underbuild. Using a 2x8 where a 2x6 might just suffice for a very short span (e.g., 9 feet) can provide a stiffer feel to the deck and offer a greater margin of safety. However, for a 6-foot span, a 2x6 is perfectly adequate and more cost-effective.
Ultimately, measure your deck's intended span carefully and consult the relevant span tables and local building codes to make the correct joist size selection.
