How to Install Deck Balusters Correctly
Balusters (also called spindles or pickets) fill the space between your rails to prevent falls and add visual character to your deck railing. The key code requirement is the 4-inch sphere rule—no gap large enough for a 4-inch ball to pass through, which helps prevent small children from slipping through.
Quick Summary
3-5 hours
Easy to Moderate
$100-400 (materials)
Understanding the 4-Inch Sphere Rule
Building codes use the "4-inch sphere" test to determine if a railing is safe. Imagine a 4-inch diameter ball (roughly the size of an adult fist)—it should not be able to pass through any opening in your railing system. This includes:
- The gap between each baluster
- The gap between balusters and posts
- The gap between the bottom rail and deck surface
- Any decorative openings in panels or balusters
This rule exists to prevent small children from slipping through railings. It's not negotiable—inspectors will check, and you should too.
Baluster Options
You have several choices for baluster material, each with different aesthetics, cost, and installation requirements:
| Type | Cost | Pros/Cons |
|---|---|---|
| Wood pickets (2x2) | $1-3 each | Traditional look, easy to cut, needs staining |
| Turned wood balusters | $3-8 each | Decorative profiles, classic style |
| Metal spindles | $3-10 each | Modern look, maintenance-free, various finishes |
| Composite balusters | $4-12 each | Matches composite decking, no maintenance |
| Cable rail | $100-200/section | Open views, modern, check local code |
Materials and Tools
Materials
- Balusters – calculate quantity based on spacing (typically 15-20 per 8-foot section)
- Deck screws – 2-inch length for wood, or specialized fasteners for metal
- Baluster connectors (optional) – hidden fastening systems
- Wood filler – to conceal screw heads in wood balusters
Tools
- Miter saw or circular saw – for cutting wood balusters
- Drill/driver – for pre-drilling and driving screws
- Tape measure – for calculating spacing and measuring
- Spacing jig – a block cut to your gap width for consistent spacing
- Level – to ensure balusters are plumb
- Hacksaw – for cutting metal spindles if needed
Calculating Baluster Spacing
Getting even spacing with gaps under 4 inches requires some math. Here's how to do it:
Example Calculation
Rail section length: 60 inches (between posts)
Baluster width: 1.5 inches (2x2 actual dimension)
Desired gap: 3.5 inches (under 4-inch max)
- 1. Add baluster width + gap: 1.5 + 3.5 = 5 inches
- 2. Divide section length by this: 60 / 5 = 12
- 3. You need 12 balusters
- 4. Calculate actual gaps: (60 - (12 x 1.5)) / 13 = 3.23 inches
Note: You have 13 gaps (one more than balusters because of gaps at each end)
Pro Tip: Most building supply stores sell spacing calculators or have apps. You can also find free baluster spacing calculators online. Just enter your section length and baluster width to get the correct count and spacing.
Step-by-Step Installation
Step 1: Cut Balusters to Length
All balusters in a section should be exactly the same length:
- Measure distance between top and bottom rails
- Subtract 1/4 inch for clearance (prevents binding)
- Set up a stop block on your saw for consistent cuts
- Cut all balusters for each section at once
Step 2: Create a Spacing Jig
Make a simple spacer from scrap wood:
- 1.Cut a block exactly as wide as your calculated gap
- 2.Make it at least 4-6 inches tall so it stays in place
- 3.Test fit between rails to confirm clearance
Step 3: Install First Baluster
Start at one post and work across:
- 1.Place spacer against the post
- 2.Position first baluster against the spacer
- 3.Check plumb with level
- 4.Pre-drill and drive one screw at top, angled into top rail
- 5.Recheck plumb, then pre-drill and secure at bottom rail
Avoid This Mistake: Don't skip pre-drilling! Driving screws directly into 2x2 balusters will split them, especially near the ends. Use a drill bit slightly smaller than your screw diameter.
Step 4: Continue Across Section
Repeat the spacing process for each baluster:
- Place spacer against installed baluster
- Position next baluster against spacer
- Check plumb, secure top and bottom
- Move spacer, repeat
Step 5: Check Final Gap
When you reach the far post:
- The gap between last baluster and post should be close to your standard gap
- Minor variations (up to 1/4") are normal and not noticeable
- If significantly off, you may need to recalculate and adjust
Step 6: Add Second Screws (Optional)
For extra security, add a second screw at each connection:
- Offset second screw from first
- Prevents baluster from rotating over time
- Especially important for square balusters
Alternative Fastening Methods
Baluster Connectors
Proprietary systems like Deckorators or Fortress use specially designed connectors that hide in grooves cut in the rails. They're more expensive but create a clean, screw-free look. Follow manufacturer instructions carefully.
Pre-Routed Rails
Some railing systems include rails with holes or grooves pre-cut for balusters. Balusters simply drop in and are secured with a cap rail that screws over the top. This is the fastest installation method.
Toe-Screwing
The traditional method used in this guide. Drive screws at an angle through the baluster into the rail. Pre-drilling is essential. Screw heads can be filled with wood filler for a cleaner look.
Quality Check
After installing all balusters, verify your work:
- 4-inch test: Try to pass a 4-inch ball through every gap
- Plumb check: Sight down the row—all balusters should be vertical
- Secure: Grab and wiggle each baluster—none should move
- Even spacing: Gaps should look consistent visually
- Bottom gap: Check gap between deck and bottom rail
FAQ
What is the maximum gap allowed between deck balusters?
Building codes require that balusters be spaced so a 4-inch sphere cannot pass through at any point. This means the gap between balusters must be less than 4 inches. The same rule applies to gaps at posts and above the deck.
How do I calculate baluster spacing?
Measure the distance between posts, divide by (baluster width + desired gap), round up for number of balusters, then recalculate the actual gap by distributing remaining space evenly. Online calculators make this easy.
Should balusters be installed inside or outside the rails?
Installing between rails (inside) creates a cleaner look and protects baluster ends from weather. Face-mounting is faster but shows end grain and fasteners. Most residential decks use between-rail installation.
Can I use horizontal rails instead of vertical balusters?
Some jurisdictions allow horizontal rails or cable rails, but many prohibit them because they can be climbed by children. Check your local code before planning a horizontal design.