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Deck Joist Span Calculator

Calculate maximum deck joist spans based on 2024 IRC building codes. Find the right joist size and spacing for your deck project.

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IRC 2024 Code Reference

Spans based on 40 psf live load and 10 psf dead load per IRC Table R507.6. Always verify with local building codes. Most common: 2×10 at 16" o.c. spans up to 14' with Southern Pine.

🌲 Wood Species

📏 Joist Size

Nominal lumber dimensions (actual: 1.5" × depth)

↔️ Joist Spacing (On-Center)

16" o.c. is most common • Composite decking often requires ≤16"

🎯 Your Desired Span (Optional)

feet

Maximum Joist Span

14'0"

2×10 at 16" o.c.

Max Cantilever

3'6"

(Span ÷ 4)

Total Deck Depth

17'6"

(Span + Cantilever)

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Your span is achievable!

12' span is within the 14'0" maximum

📋 Options for 12' Span (Southern Pine)

2×8 @ 12" o.c.BEST
13'1"+3'3" cant.
2×12 @ 24" o.c.
13'6"+3'4" cant.
2×10 @ 16" o.c.
14'0"+3'6" cant.
2×10 @ 12" o.c.
16'2"+4'0" cant.

📊 Complete Joist Span Chart (IRC 2024)

Based on 40 psf live load, 10 psf dead load, No. 2 grade lumber

🌲 Southern PineSELECTED

Joist Size12" o.c.16" o.c.24" o.c.
2×69'11"9'0"7'7"
2×813'1"11'10"9'8"
2×1016'2"14'0"11'5"
2×1218'0"16'6"13'6"

🌲 Douglas Fir-Larch

Joist Size12" o.c.16" o.c.24" o.c.
2×69'6"8'4"6'10"
2×812'6"10'11"8'11"
2×1015'8"13'7"11'1"
2×1218'0"16'2"13'2"

🌲 Hem-Fir

Joist Size12" o.c.16" o.c.24" o.c.
2×68'10"7'7"6'2"
2×811'8"10'0"8'2"
2×1014'11"12'10"10'6"
2×1217'5"15'0"12'3"

🌲 Redwood / Western Cedar

Joist Size12" o.c.16" o.c.24" o.c.
2×68'4"7'3"5'11"
2×811'0"9'6"7'9"
2×1014'0"12'2"9'11"
2×1216'4"14'2"11'7"

🔧 How to Choose the Right Joist Size

1

Measure Your Span

Determine the distance from ledger board to beam (or beam to beam). This is your required joist span.

2

Check Decking Requirements

Composite decking typically requires 16" o.c. max (12" for diagonal). Wood decking may allow 24" o.c.

3

Select Joist Size from Chart

Find a size where the maximum span exceeds your required span. Consider cost—wider spacing needs fewer joists.

4

Verify with Local Codes

Check local building department requirements. Some jurisdictions have stricter requirements than IRC minimums.

🪵 Decking Material Spacing Requirements

Decking TypePerpendicularDiagonal (45°)
5/4×6 Wood Decking16" o.c. max12" o.c. max
2×6 Wood Decking24" o.c. max16" o.c. max
Composite Decking16" o.c. max12" o.c. max
PVC Decking16" o.c. max12" o.c. max

* Always check manufacturer specifications as requirements may vary by brand and product line.

📐 Quick Reference

Most Common Setup:

2×10 @ 16" o.c. = 14' span

Cantilever Rule:

Max = Span ÷ 4

Pressure Treated:

Required for outdoor decks

⚠️ Important Notes

  • Always obtain a permit before building
  • Verify local code requirements
  • Use pressure-treated lumber outdoors
  • Hire a professional if unsure

Frequently Asked Questions

Deck joist span depends on three factors: joist size (2×6, 2×8, 2×10, 2×12), spacing between joists (12", 16", or 24" on-center), and wood species. The IRC (International Residential Code) provides span tables based on these variables. For example, a Southern Pine 2×10 at 16" o.c. can span up to 14 feet. Always check local building codes as they may have additional requirements.

The choice between 2×8 and 2×10 depends on your span requirements. For spans up to 10-12 feet, 2×8 joists at 16" o.c. are typically sufficient. For spans of 12-14+ feet, 2×10 joists are recommended. If using 24" spacing for cost savings, you may need to upsize to 2×10 even for shorter spans. Consider your decking material too—composite decking often requires 16" o.c. maximum spacing.

A 2×8 deck joist can span from 7'9" to 13'1" depending on wood species and spacing. With Southern Pine at 12" o.c., maximum span is 13'1". At 16" o.c., it's 11'10". At 24" o.c., it's 9'8". Douglas Fir and Hem-Fir have slightly shorter spans. Always use the appropriate span for your specific lumber species and spacing.

A 2×10 deck joist can span from 9'11" to 16'2" depending on wood species and spacing. Southern Pine at 12" o.c. allows the maximum 16'2" span. At the common 16" o.c. spacing, Southern Pine 2×10s can span 14'0", Douglas Fir can span 13'7", and Hem-Fir can span 12'10". These are the most versatile joists for medium to large decks.

"16 on-center" (16" o.c.) means the distance from the center of one joist to the center of the next joist is 16 inches. This is the most common spacing for deck joists and provides a good balance between structural support and material cost. Most composite decking manufacturers require maximum 16" o.c. spacing. For diagonal decking installations, 12" o.c. is often required.

Yes, 2×6 joists can be used for deck framing, but with significant limitations. Maximum spans range from 5'11" to 9'11" depending on species and spacing—suitable only for small decks or short spans. They're commonly used for ground-level decks, stair landings, or areas with additional support. For most residential decks, 2×8 or larger is recommended for better structural performance.

The maximum cantilever (overhang beyond the beam) is typically limited to 1/4 of the joist back-span according to IRC guidelines. For example, if your joist span is 12 feet, the maximum cantilever would be 3 feet. However, specific cantilever limits vary by joist size and back-span length. Always verify with the IRC cantilever tables and local building codes for your specific situation.

🪵 Disclaimer: This calculator provides estimates based on IRC 2024 Table R507.6 for residential deck construction. Actual requirements may vary based on local building codes, lumber grade, and specific site conditions. Always consult with your local building department and/or a licensed contractor before construction.