How to Install Deck Floor Joists
Floor joists are the horizontal members that span from your ledger board to the outer rim joist, creating the framework that supports your decking. Proper installation with correct spacing and hardware ensures a solid, safe deck surface.
Quick Summary
3-5 hours
16" on center
Joist hangers
Understanding Joist Sizing and Spacing
The size of your joists and how far apart you space them determines how stiff and bounce-free your deck will feel. This isn't just about meeting code minimums - it's about building a deck that feels solid underfoot.
| Joist Size | 16" OC Span | 12" OC Span | Best For |
|---|---|---|---|
| 2x6 | 6 feet max | 7 feet max | Low decks, lightweight use |
| 2x8 | 10 feet max | 11 feet max | Most standard decks |
| 2x10 | 13 feet max | 15 feet max | Larger decks, heavy loads |
| 2x12 | 16 feet max | 18 feet max | Long spans, minimal bounce |
Pro Tip: These spans are for Southern Yellow Pine #2 grade. Other species like Hem-Fir or SPF have shorter allowable spans. Always check the span tables in your local building code for your specific lumber species and grade.
When to Use 12-Inch vs. 16-Inch Spacing
While 16-inch on-center is standard for most decks, some situations require tighter 12-inch spacing. Knowing when to use each helps you build to appropriate standards.
Use 16" On-Center When:
- - Wood decking runs perpendicular to joists
- - Standard residential traffic only
- - Decking is 5/4x6 or thicker
- - No heavy loads like hot tubs
Use 12" On-Center When:
- - Composite decking at 45-degree angle
- - Hot tub or heavy equipment on deck
- - Decking manufacturer requires it
- - You want extra stiffness underfoot
Tools and Materials You'll Need
Essential Tools
- Circular saw - For cutting joists to length
- Framing nailer or hammer - For driving joist hanger nails
- 25-foot tape measure - For layout and cutting
- Speed square - For marking square cuts
- 4-foot level - For checking joist alignment
- Pencil - For layout marks
Materials
- Pressure-treated joists - 2x8 or 2x10, same size as rim joists
- Joist hangers - LUS28 for 2x8, LUS210 for 2x10 (Simpson Strong-Tie or equivalent)
- Joist hanger nails - 1-1/2" x .148" (10d short) - special short, thick nails
- Hurricane ties - For securing joists to the beam
- 16d galvanized nails - For toe-nailing at beam (alternative to hurricane ties)
Step-by-Step Installation
Step 1: Mark the Joist Layout
Consistent layout marks ensure even joist spacing across the entire deck. Start from the same end on both the ledger and outer rim joist.
- 1.Hook your tape on the outside face of one rim joist where it meets the ledger
- 2.Make a mark at 15-1/4" (this puts the first joist at 16" on center including the rim joist)
- 3.Continue marking every 16" from that first mark
- 4.Draw an X on the side of the line where the joist will sit (same side for all marks)
- 5.Transfer identical marks to the outer rim joist, measuring from the same starting point
Pro Tip: Draw a line across the full width of the ledger at each mark, then put an X on the side where the joist goes. This prevents accidentally installing a joist on the wrong side of the line.
Step 2: Install Joist Hangers at the Ledger
Joist hangers at the ledger are required by code - no toe-nailing allowed at this critical connection. Install them before dropping in joists.
- 1.Position the hanger - Align one side of the hanger with your layout mark/line
- 2.Align the bottom - The hanger bottom should be flush with the ledger bottom
- 3.Drive a nail - Put one nail through the top hole on each flange to hold the hanger in place
- 4.Verify position - Check that it's still aligned with your mark
Important: Don't fill all the hanger holes yet. Leave some open so you can adjust the hanger slightly when you drop in the joist. You'll complete nailing after the joist is in place.
Step 3: Measure and Cut Joists
Even if your frame is perfectly square, measure for each joist individually. Lumber dimensions can vary, and you want tight fits.
- 1.Measure from the face of the ledger to the inner face of the outer rim joist at each joist location
- 2.Subtract 1/8" for clearance (joists should fit without forcing)
- 3.Mark and cut with a square cut at each end
- 4.If most joists are the same length, cut several at once to save time
Step 4: Check and Orient the Crown
Every piece of lumber has a slight curve or "crown." Installing joists with the crown up is critical - it allows the joist to flatten under load rather than sag.
- 1.Sight down the edge - Hold the joist at eye level and look down its length
- 2.Identify the crown - One edge will curve upward slightly (that's the crown)
- 3.Mark the crown - Draw an arrow pointing up on the end of the joist
- 4.Install crown up - Every joist should have its crown facing the sky
Pro Tip: If a joist has more than 1/4" crown over its length, set it aside for blocking or other shorter uses. Excessively crowned joists create high and low spots in your decking.
Step 5: Install Each Joist
Drop each joist into its hanger at the ledger and rest it on the beam, then secure both ends.
- 1.Insert in hanger - Drop the joist into the hanger with crown up, push it tight against the ledger
- 2.Position on beam - The other end rests on top of the beam at your layout mark
- 3.Check level - Place a level on the joist to verify it's reasonably level
- 4.Complete hanger nailing - Fill every nail hole in the joist hanger
- 5.Secure at beam - Toe-nail with 16d nails or install hurricane tie
- 6.Attach to rim joist - Face-nail through the outer rim joist into the end of the field joist
Step 6: Complete All Joist Hanger Nailing
This is non-negotiable: every hole in every joist hanger must have a nail. Inspectors will check this.
- Use only joist hanger nails - 1-1/2" x .148" (short and fat)
- Drive nails through the angled holes on the hanger flanges into the joist
- Drive nails through the face holes into the ledger
- Do not substitute regular nails or screws unless they're specifically approved
Code Requirement: Missing even one nail from a joist hanger can reduce its load capacity by 20-50%. Building inspectors know this and will fail inspections for incomplete hanger nailing.
Common Mistakes to Avoid
Installing Joists Crown-Down
Joists installed with the crown facing down will sag over time, creating a wavy deck surface.
Solution: Sight each joist before installing. Mark the crown direction. Always install crown-up.
Using Wrong Nails in Hangers
Regular 16d nails are too long and too thin for joist hangers. They can split the wood or protrude dangerously.
Solution: Use only 1-1/2" x .148" joist hanger nails or approved structural screws.
Skipping Nail Holes in Hangers
Every empty hole is lost capacity. Inspectors will catch this, and it compromises safety.
Solution: Fill every hole. Walk the deck before inspection and check each hanger.
Toe-Nailing at the Ledger
Toe-nailing (angled nailing) at the ledger connection is not code-compliant. This critical connection requires joist hangers.
Solution: Always use joist hangers at the ledger. Toe-nailing is only acceptable at the beam end.
FAQ
Should deck joists be 12 or 16 inches on center?
Most decks use 16-inch spacing. Use 12-inch spacing for composite decking at angles, hot tub support, or when the manufacturer requires it. Check your decking material's installation guide.
What size joist should I use for a deck?
2x8 joists work for spans up to 8-10 feet at 16" spacing. 2x10 joists handle spans up to 11-13 feet. For longer spans or minimal bounce, go with 2x10 or 2x12. Check local code span tables for your lumber species.
Do all joist hanger holes need nails?
Yes, every hole must be filled with the proper nail. Missing nails can reduce hanger capacity by 20-50%. Use only 1-1/2" joist hanger nails or approved structural screws.
Which way should the crown face on joists?
Always install joists with the crown (the slight curve) facing UP. The joist will flatten under load. Crown-down installation leads to sagging over time.