I have installed hundreds of joists, and I have replaced just as many that were done wrong the first time.
Quick answer: Deck joists are the horizontal boards that hold up your decking. They run between the ledger board and the beam, and they carry the floor load down to the frame. Most decks space joists either 16 inches or 12 inches apart, measured on-center.
What Are Deck Joists?
Deck joists are the boards that make up the floor of your deck. They run in one direction, side by side, from the house side of the deck to the outer beam. Your decking boards sit right on top of them.
Think of joists like the ribs of the deck. Each board takes a small share of the weight above it, then passes that weight down into the beam or the ledger board. The beam then sends that weight down through the posts and into the footings.
I’ve found that a lot of people focus only on the decking boards on top. But the joists underneath do the real work. If the joists are too small, spaced too wide, or installed wrong, the deck floor will feel soft and bouncy, even if the boards on top look perfect.
Joists do not work alone. They are one piece of a bigger structural system, so make sure to read our complete Deck Building guide to see how joists fit into the whole project from the ground up.
Choosing Your Joist Lumber Size
Most joists are pressure-treated lumber. Common sizes are 2×6, 2×8, and 2×10. The bigger the board, the farther it can span before it needs extra support underneath.
A 2×6 joist can only span a short distance before it starts to sag under load. A 2×10 joist can span much farther. This is why picking the right size matters so much. If you pick a size that is too small for your span, the floor will feel weak no matter how well you build everything else.
Standard Joist Span Limits
| Joist Size | Typical Max Span (16″ On-Center) | Notes |
|---|---|---|
| 2×6 | Around 8-9 feet | Best for small, low decks |
| 2×8 | Around 10-12 feet | Common for mid-size decks |
| 2×10 | Around 13-16 feet | Common for larger decks |
| 2×12 | Around 15-18 feet | Used for long spans |
These numbers change based on wood species, grade, and spacing. Always check a current joist span table for your exact lumber and spacing before you cut anything. The American Wood Council publishes span tables that many builders use as their reference point.
The On-Center Rule: Understanding Joist Spacing
Joist spacing is measured “on-center.” This just means the distance from the center of one joist to the center of the next joist. It is not the gap between the boards. It is measured from middle to middle.
Standard wood decking, run straight across the joists, usually needs 16 inches on-center spacing. This is the most common spacing you will see on residential decks.
If you run your decking boards at an angle, like a 45-degree diagonal pattern, you usually need tighter spacing. Diagonal decking often needs 12 inches on-center, because the boards cross the joists at an angle and need more support points to stay stiff.
Composite decking often needs tighter spacing too. From my experience, composite boards can flex more than solid wood between supports, so many manufacturers require 12 inches on-center instead of 16. Always check your specific decking product’s spec sheet before finalizing your joist layout.
Joist Spacing Guide
| Decking Type | Board Direction | Common On-Center Spacing |
|---|---|---|
| Solid wood decking | Straight (perpendicular) | 16 inches |
| Solid wood decking | Diagonal (45 degrees) | 12 inches |
| Composite decking | Straight (perpendicular) | 12 to 16 inches (check manufacturer spec) |
| Composite decking | Diagonal (45 degrees) | 12 inches |
Why You Must Find the “Crown” of the Wood
Almost every piece of lumber has a slight natural curve running along its length. This curve is called the crown. It happens naturally as the tree grows and the wood dries out.
To find the crown, pick up the board and look down its length like you are looking down a rifle barrel. You will usually see a small curve, even if it is subtle. One edge will bow up slightly compared to the other.
You always want to install your joist with the crown facing up. Here is why. Over time, the weight of the deck will press down on the joist. If the crown faces up, that weight slowly presses the board flatter and straighter. If the crown faces down, the weight pushes the curve further down, and the board will sag in the middle over the years.
One mistake beginners make with joists is skipping this step completely, or installing boards randomly without checking the crown at all. This one small check takes a few extra seconds per board, and it makes a real difference in how flat your deck stays over time.
Joist Safety & Prep Checklist
[ ] Check each board for its natural crown before cutting
[ ] Mark the crown side clearly with a pencil
[ ] Set aside any boards with major splits or knots near the ends
[ ] Wear safety glasses when cutting pressure-treated lumber
[ ] Wear gloves when handling treated boards
[ ] Stack cut joists safely off the ground before installing
[ ] Double check lumber size matches your span table requirement
Mid-Span Blocking: Stopping the Deck Bounce
Blocking, sometimes called bridging, is short pieces of lumber installed between your joists. They run perpendicular to the joists, usually at the midpoint of a long span.
Blocking does two important jobs. First, it stops joists from twisting sideways over time, which can happen especially with taller, narrower lumber like 2×10 or 2×12 boards. Second, it adds stiffness across the whole floor system, which reduces bounce when you walk across the deck.
Many span tables and local structural codes require blocking once a joist span passes a certain length. Always check your local requirement based on your exact joist size and span before skipping this step.
Step-by-Step Guide to Installing Deck Joists
A “deck framing diagram” showing the ledger, rim joist, and evenly spaced interior joists would help readers picture this whole layout.
Step 1: Marking the Layout on the Ledger and Rim Joist
Start by marking your joist locations on both the ledger board and the rim joist. Use your chosen on-center spacing, and mark both sides so every joist lines up straight across the deck.
Step 2: Selecting and Cutting Joists to Length
Pick straight boards with no major defects. Measure and cut each joist to the exact length needed for your frame, double checking your measurement before every cut.
Step 3: Squaring the Ends and Facing the Crowns Up
Make sure each cut end is square, not angled. Then check the crown on each board and mark which edge faces up before you install it.
Step 4: Securing Joists Using Joist Hangers and Approved Structural Fasteners
Set each joist into its joist hanger, then fasten it using approved structural hanger nails or screws, never standard drywall screws. The joists rest on top of the beam at the far end, which is where all that floor weight gets passed along, so check our complete Deck Beams guide to see how sizing and spacing on the beam side connects with your joist layout.
Deck Framing: How to Build a Strong, Square Frame That Lasts
Deck Railings: A Complete Guide
Common Joist Errors Beginners Make
Installing the crown facing down. This causes a slow sag in the middle of the joist as the deck ages.
Using cheap drywall screws in joist hangers. These fasteners are not rated for outdoor structural loads and can shear or rust out over time.
Over-spanning a board beyond its rated limit. This creates a bouncy, unsafe feeling floor even if it seems fine on move-in day.
Skipping blocking on long spans. Without it, joists can twist over time, especially taller boards like 2×10 and 2×12.
Joist Installation Quality Checklist
[ ] All joists installed crown facing up
[ ] Joist spacing matches your decking type requirement
[ ] All joist hangers fastened with approved structural nails or screws
[ ] Blocking installed on any qualifying long spans
[ ] All joist ends cut square and sitting flush in hangers
[ ] Rim joist fastened securely across all joist ends
[ ] Overall frame re-checked for square after joists are set
Frequently Asked Questions
Can deck joists cantilever past the beam? Yes, many joists can extend past the beam a certain distance, but the allowed cantilever span depends on your joist size, so check your span table for the exact limit.
Can I splice two joists together over a beam? Yes, this is common on longer decks, and the splice should land directly over the beam with proper overlap and fastening.
What nails or screws go in joist hangers? Joist hangers need approved structural hanger nails or specific structural screws rated for hangers, not general-purpose screws.
How do I know if my joist spacing is too wide? If the floor feels bouncy or the decking flexes noticeably when you walk on it, the spacing may be too wide for your decking material.
Do all decks need blocking between joists? Not always, but many span tables and local codes require it once the joist span passes a certain length.
Can I use 2×6 joists for a large deck? You can, but 2×6 joists span a shorter distance, so a large deck would need closer post and beam spacing to support them.
What is the rim joist, and is it different from a regular joist? The rim joist runs along the outer edge of the deck and ties the ends of the interior joists together, rather than supporting decking directly along its own length.
How far can a 2×10 joist span? This depends on spacing, species, and grade, so always check a current span table for your exact lumber before finalizing your layout.
Do joists need to touch the ledger board directly? Yes, joists typically sit in hangers attached directly to the ledger board on one end and the beam on the other end.
Why does my deck floor feel bouncy even with the right size joists? This can happen if the crown was installed facing down, the spacing is too wide for your decking, or blocking is missing on a long span.
Final Thoughts
Deck joists are simple boards, but they carry the whole floor. Pick the right size for your span, space them correctly for your decking type, and always face the crown up. These small details add up to a floor that stays flat and solid for years.
If you want to see how joists connect to the rest of the build, check out our complete Deck Building guide. Got a question about your own joist layout? Drop a comment below. I read every one.