Deck Joist Calculator — Span Tables & Cost Estimator
Span Check
FAIL — Span Exceeded
Your span: 10 ft (120") • Max allowable: 8'2" (98")
Max Span
8'2"
Joist Count
10
Total Lumber
100 LF
Material Cost
$150 – $250
Total w/ Hangers
$165 – $280
Span Exceeded: Your 10 ft span exceeds the maximum 8'2" for 2×8 at 16" OC. Use 2×10 or larger, or reduce spacing.
Recommended: For a 10 ft span at 16" OC, use 2×10 or larger joists with SPF #2 (Spruce-Pine-Fir).
Cantilever Rule: Joists can cantilever (overhang) up to 1/4 of the allowable span beyond the beam. For 2×8 at 16" OC, max cantilever is 24" (2.0 ft).
Blocking Required: For spans over 8 ft, solid blocking or bridging is required at mid-span per IRC R507.6 to prevent joist rotation and improve structural stability.
Code Reference: Span tables are based on IRC Table R507.6 for 40 PSF live load + 10 PSF dead load. Always verify with your local building department — jurisdictions may have stricter requirements.
Disclaimer: Estimates are for budgeting purposes only. Actual costs vary by location, contractor, material availability, and project complexity. Always get professional quotes for exact pricing.
Deck Joist Span Tables & Costs
Maximum Joist Spans — SPF #2 (IRC R507.6)
| Joist Size | 12" OC | 16" OC | 24" OC |
|---|---|---|---|
| 2×6 | 7’2” | 6’2” | 5’4” |
| 2×8 | 9’5” | 8’2” | 7’0” |
| 2×10 | 12’0” | 10’5” | 8’11” |
| 2×12 | 14’7” | 12’8” | 10’10” |
* Based on IRC Table R507.6, 40 PSF live + 10 PSF dead load. Douglas Fir and Southern Pine allow longer spans.
Pressure-Treated Lumber Cost per Linear Foot
| Joist Size | Cost per LF | Hanger Cost (each) |
|---|---|---|
| 2×6 | $1.00 – $1.75 | $1.50 – $3.00 |
| 2×8 | $1.50 – $2.50 | $1.50 – $3.00 |
| 2×10 | $2.00 – $3.50 | $1.50 – $3.00 |
| 2×12 | $2.75 – $4.50 | $1.50 – $3.00 |
How We Calculate
Our deck joist calculator combines IRC span table lookups with material cost estimation. Enter your deck dimensions, joist size, spacing, and wood species to check if your span is within code limits and estimate lumber costs.
How span checking works: We reference IRC Table R507.6 maximum joist spans for 40 PSF live load + 10 PSF dead load (standard residential deck loading). The table provides maximum spans by joist size (2×6 through 2×12), spacing (12"/16"/24" OC), and wood species (SPF #2, Douglas Fir #2, Southern Pine #2). Your span passes if it's within the table value.
Deck Joist Material Calculator
Joist count = (deck length × 12 ÷ spacing) + 1. Total lumber = joist count × joist length (deck width). Material cost = total LF × cost per LF for the selected joist size. Joist hanger cost ($1.50–$3.00 each) is added per joist.
Wood Species Comparison
SPF #2 (Spruce-Pine-Fir) is the most common and affordable pressure-treated lumber. Douglas Fir #2 is stronger (15–20% longer spans) and more common on the West Coast. Southern Pine #2 is the strongest of the three (20–25% longer spans than SPF) and dominates the Southeast market. All are available as pressure-treated lumber for deck use.
Cantilever Rules
Joists may cantilever (extend beyond the beam) up to 1/4 of the allowable back span. This lets you extend the deck without adding another beam, but the back span must still be within table limits. Cantilever length plus back span cannot exceed the maximum allowable span.
Additional Costs
Our estimate covers joist lumber and hangers only. A complete deck also requires: beams ($3–$8/LF), posts and footings ($50–$200 each), decking boards ($2–$8/sq ft), ledger board and flashing ($100–$300), fasteners ($50–$150), and railings ($20–$60/LF). Labor adds $8–$22/sq ft for professional installation.
Data Sources
Span tables are from the International Residential Code (IRC) Table R507.6. Lumber pricing reflects pressure-treated (PT) lumber costs from Home Depot, Lowe's, and lumber yards, verified February 2026. Costs vary by region and availability.
Last updated: 2026-02-04
Frequently Asked Questions
What is the maximum span for deck joists by size? +
Maximum joist spans at 16" OC with SPF #2 lumber: 2×6 spans up to 6'2", 2×8 up to 8'2", 2×10 up to 10'5", and 2×12 up to 12'8". Douglas Fir and Southern Pine allow longer spans. These values are per IRC Table R507.6 at 40 PSF live + 10 PSF dead load.
How far can a 2×8 deck joist span? +
A 2×8 joist can span 7'0" to 11'10" depending on wood species and spacing. SPF #2 at 16" OC: 8'2". Douglas Fir #2 at 16" OC: 9'5". Southern Pine #2 at 16" OC: 10'2". Closer spacing (12" OC) increases the allowable span.
Should I use 12", 16", or 24" joist spacing? +
16" OC is standard for most residential decks and supports all common decking materials. Use 12" OC for composite decking (manufacturer often requires it), longer spans, or heavy-use areas. 24" OC is only suitable for short spans and 5/4 or thicker decking — many composite brands don't allow it.
How does joist size affect span? +
Larger joists span further. Going from 2×6 to 2×8 increases allowable span by about 30%. From 2×8 to 2×10 adds another 25%. The cost per linear foot increases roughly 40–50% per size upgrade, so it's most economical to match joist size to your actual span requirement.
What are the cantilever rules for deck joists? +
Deck joists can cantilever (overhang the beam) up to 1/4 of the allowable back span per IRC. For example, if a 2×10 at 16" OC has a 10'5" max span, the max cantilever is 2'7". The cantilever reduces the effective span between supports but adds usable deck area beyond the beam.
What are deflection limits for deck joists? +
IRC requires that deck joists meet L/360 deflection limits for live load, meaning the joist can deflect no more than 1/360th of its span. For a 12 ft span, max deflection is 0.4 inches. The IRC span tables already account for deflection — if the span passes the table, it meets deflection requirements.
Do I need a permit to build a deck? +
Most jurisdictions require a building permit for decks over 30 inches above grade or attached to the house. Permits typically cost $100–$500 and require a site plan, framing details, and footing specifications. Inspection is usually required for footings, framing, and final approval.
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