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Calculate the exact number of joists needed for your deck project. Verify load capacity, determine spacing layouts, and estimate costs with industry-standard span tables for different wood species.
Maximum recommended: 2 feet
Effective Span: 12.00 ft | Max: 13.10 ft
Based on 12" OC spacing without cantilever
| Joist # | From Starting Edge |
|---|---|
| Joist 1 | 0" |
| Joist 2 | 1'4" |
| Joist 3 | 2'8" |
| Joist 4 | 4'0" |
| Joist 5 | 5'4" |
| Joist 6 | 6'8" |
| Joist 7 | 7'12" |
| Joist 8 | 9'4" |
| Joist 9 | 10'0" |
Spacing shown as center-to-center distances
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Deck joists are the horizontal support beams that form the main structure of a deck. They rest on the beams and support the deck boards (decking) that you walk on.
Joists are typically 2x6, 2x8, 2x10, or 2x12 lumber. The size and spacing depend on:
Maximum span in feet for residential decks (L/240 deflection limit, 12" OC spacing):
Note: Spans may decrease with wider spacing. Always check local building codes.
Spacing between joist centers: 12 inches
Joists per foot: 1
Best for: Thin decking, long spans
Spacing between joist centers: 16 inches
Joists per foot: 0.75
Most Common: Standard residential
Spacing between joist centers: 24 inches
Joists per foot: 0.5
Best for: Thick decking, shorter spans
Number of Joists = (Width / Spacing) + 1
Deck dimensions: 12 ft long x 10 ft wide
Spacing: 16" OC
Joist direction: Perpendicular
Joists = (10 ft / (16"/12")) + 1
Joists = (10 / 1.33) + 1
Joists = 7.5 + 1 = 8-9 joists
Linear Feet = Joists × Joist Length
Using our example:
Linear Feet = 9 joists × 12 ft
Linear Feet = 108 ft
Note: Add 10-15% extra for cutting waste and mistakes.
| Joist Size | Actual Dimensions | Best Use | Max Span (PT) |
|---|---|---|---|
| 2x6 | 1.5" × 5.5" | Small decks, short spans | 9.9 ft |
| 2x8 | 1.5" × 7.25" | Most common residential | 13.1 ft |
| 2x10 | 1.5" × 9.25" | Large decks, wider spans | 16.4 ft |
| 2x12 | 1.5" × 11.25" | Long spans, premium decks | 19.6 ft |
The most common spacing is 16 inches on center (OC). This is the standard for residential decks. 12" OC is used for thinner decking or longer spans, while 24" OC can be used with thicker decking and shorter spans. Always check your local building codes.
Joist size depends on the span (width of the deck), wood species, and spacing. Use the span tables in this calculator or consult the Span Tables section. Longer decks need larger joists. Always check local building codes for specific requirements in your area.
It's generally not recommended. Using the same size throughout the deck ensures uniform strength and appearance. However, you may use larger joists for edge joists (the outermost joists). Always consult with a structural engineer or follow local building codes.
On-center (OC) spacing measures the distance between the centers of two joists. Clear spacing is the space between the edges of joists. On-center is the standard measurement used in construction and building codes. A 16" OC spacing means joist centers are 16 inches apart.
Yes, treating your joists is essential for longevity. Pressure-treated lumber comes pre-treated for rot resistance. For other wood types like cedar or redwood, apply a deck stain or sealer every 1-2 years. This protects against moisture, rot, and UV damage.
Building codes typically require decks to support a live load of 40 PSF (pounds per square foot) plus a dead load of 10 PSF for the structure itself. This allows for people, furniture, and moderate snow load. Always check your local building codes for exact requirements.
Yes, composite joists are available and offer benefits like low maintenance and rot resistance. However, they are more expensive and require proper fastening. Some composites may sag more than wood, so check manufacturer specifications for span tables.
Use pressure-treated lumber or naturally rot-resistant woods. Ensure proper drainage under the deck to prevent water pooling. Install flashing where joists meet the house. Apply a water-resistant stain or sealer. Maintain good ventilation under the deck to allow moisture to evaporate.
Check Local Building Codes: This calculator provides general guidance. Always verify requirements with your local building inspector before starting your project.
Consult a Structural Engineer: For large decks, complex designs, or in areas with high snow/wind loads, consult a professional engineer.
Permit Requirements: Most areas require deck permits. Obtain necessary permits before construction and have inspections completed.
Safety Factors: This calculator uses standard deflection limits (L/240). Results are conservative but always add extra capacity for safety.
Material Quality: Ensure all lumber is properly graded and meets specifications. Inspect for defects, splits, or knots that could weaken the structure.
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