Deck Joist Spacing & Blocking:
Proper Techniques & How-To

When it comes to DIY-ing your dream deck, having a structurally sound foundation is critical. Before you start the process of deck joist spacing and installation, you'll want to make sure you've also locked down some other important considerations, like deck size, railing, and more.

BEFORE YOU BEGIN
Make sure to consult all appropriate installation guides. Ensure your new or existing framing is structurally sound.

DESIGN CONSIDERATIONS BEFORE YOU BUILD YOUR DECK

Deck Size

Before you know how to execute deck joist spacing and blocking on your DIY project, you'll need to know the size of your deck and the amount of building material you'll need. Make sure you've calculated for all necessary materials.

Railing

Railing installation methods can affect the blocking required for your framing. Choose a TimberTech composite or aluminum railing system to bring a complementary finishing touch to your outdoor living space.

Deck Material

Depending on the deck material and fasteners you select for your deck, you may have to add blocking at butt joints or seams. Refer to the appropriate installation guide for the line of TimberTech decking you've selected.

Deck Board Patterns

Different deck board patterns can require extra blocking / framing (for example, picture frame borders or diagonal inlays). Confirm your design and planned orientation of your deck boards before you begin deck joist work.

Need More Planning Guidance?

Print out this project guide to help you through the design, education, and budgeting tools linked above. Then, use your finished guide to build the deck of your dreams.

deck joist spacing and blocking

Deck Joist Spacing & Blocking Guide

Estimated based on a 12' x 12' deck; time will vary depending on the size of deck and complexity of the build.

ESTIMATED TIME

1 Hour


RECOMMENDED MAN-POWER

1 Person

PREREQUISITES

• Secured framing, properly
attached to the house

• A confirmed deck design

OVERVIEW

Step 1: Check That Your Deck Frame Is Structurally Sound

Step 2: Run String Lines to Ensure All Joists Are Level & In Plane

Step 3: Make Sure All Joists Are Consistent in Size

Step 4: Make Sure Your Joists Are Spaced Correctly

Step 5: Install Your Blocking

Step 6: Prolong the Health of Your Framing With Joist Tape

Deck Joist Spacing & Blocking: Watch the Step-by-Step Overview

Tools You'll Need

Once you've done the prep work, it's time to get your hands dirty. First, you'll want to assemble all the tools needed for proper deck joist spacing and blocking.
  • Safety glasses
  • String line (to check that framing is level and in plane across the tops) 
  • Hammer or mallet
  • Replacement lumber if needed (if required, generally at least 2" x 8")
  • Drill
  • Fasteners (exterior-grade fasteners for wood or wood composite decking)
  • Miter saw / circular saw
  • Tape measure
  • Level (4" minimum)
  • TimberTech® PRO-Tac™ Flashing and Joist Tape

Step 1: Check That Your Deck Frame Is Structurally Sound

Before you begin, make sure you're familiar with the different components of a deck frame.
You will want to check each for any structural damage before you begin building your new outdoor living space.

deck frame and joist spacing

How to Check for Deck Frame Damage

Regardless of whether your deck frame is new or not, it's important to:

  • Check for soft spots, broken pieces, splits, cracks, or other damage to the wood from moisture or the elements.
  • Make sure the frame is securely fastened to the home and/or to a properly secured post and beam frame. Do not build from floating sub-structures.
  • Ensure the framing is held with exterior-grade fasteners; additional exterior-grade fasteners may be required to secure existing framing, as well as fastening reinforcement.
  • Inspect framing for contraction of structural lumber and ensure all fasteners are sound and showing no signs of corrosion.
  • Look for any highs / lows / uneven joists; joists must be level and in plane for a flat surface. Use a tight string line to verify.
  • Be sure to use a water management system such as a drip cap or flashing wherever framing meets the home to keep moisture moving away from the home and deck.
  • Note any screws or fasteners that may be protruding from the surface of the joist. These will need to be removed.

If you are unsure about anything or have questions, contact a professional.

Step 2: Run String Lines to Ensure All Joists Are Level & In Plane

String Lines

Before installing any decking, run string line frequently to ensure that the tops of the joists are in plane and level, as TimberTech decking will conform to the contour of the joists to which it is attached.

The Importance of String Lines

While a particular joist may be level itself, the top of that joist may still not be in plane with surrounding joists. Joists that aren't level can result in high or low spots throughout the deck.

Installed composite decking will follow the contour of the joists beneath; if the joists are uneven, the result can be a wavy-looking deck.

deck joist string lines

Step 3: Make Sure All Joists Are Consistent in Size

MAKE SURE JOISTS ARE LEVEL & IN PLANE
It is important to properly measure your joist widths / heights and adjust the boards as needed for consistency. Make sure that joists are level and in plane, both with the deck frame and with eachother.

notching a joist

Notching a Joist

A new joist may be slightly wider / taller than existing joists. To even the joists, cut a notch in the board to align with the correct height.

shimming a deck joist

Shimming a Joist

A new joist may be slightly narrower / shorter than existing joists, requiring a moisture-resistant shim to raise it to correct height. 

Deck Joist Issues & Solves

deck joist spacing for high deck joist
Solve For

High Joist

  • Remove and replace.
  • Cut fasteners and lower. Reattach.
  • Use a power planer or power sander.
Solve For

Crowned Joist (Crowned Up)

  • Remove and replace.
  • Use a power planer or power sander.
deck joist spacing for crowned deck joist
deck joist spacing for low deck joist
Solve For

Low Joist

  • Remove and replace.
  • Leave the crowned joist and apply sister joists by sandwiching the board with two new joists.
  • Cut fasteners and raise. Reattach.
Solve For

Reverse Crowned Joist

  • Remove and replace.
  • Leave the crowned joist and apply sister joists by sandwiching the board with two new joists.
deck joist spacing for reverse crowned deck joist

DOUBLE CHECK YOUR LEVELING
Always make sure your joist, blocking, and any added framing are level and in plane with each other.

Step 4: Make Sure Your Joists Are Spaced Correctly

joist blocking and spacing

Proper deck joist spacing is required for proper installation. Deck joist spacing should never exceed 16” on center (with the exception of MAX deck boards, which allow for maximum 24" on center). For a more rigid feel, 12” or less may be preferred. If adding additional framing, be sure to keep all boards level and in plane across the tops.

horizontal deck joist spacing

Horizontal Board Placement

Board placement should be 16" on center.

diagonal deck joist spacing

Diagonal Board Placement

Board placement should be 16" on center.

Step 5: Install Your Blocking

space joists

First, Space...

Your solid wood blocking should be equally spaced in rows, with a maximum of 4' to 6' apart.

placing joists

Then, Place...

Blocking should be installed edge to edge to allow for fastening of the interior blocking.

fastened joists

Then, Fasten

Use exterior-grade fasteners to fasten through the outside of the joist.

WHY BLOCKING MATTERS
Solid wood blocking will help reduce up or down movement and the twisting of joists. Limiting joist movement also helps prevent gapping between deck boards for a uniform finish.

Step 6: Prolong the Health of Your Framing With Joist Tape

To extend the life of your decking substructure, apply joist tape. This will help prevent water penetration and will protect the joist lumber from rot.

NOTE: It’s best to apply this tape to clean, dry lumber. Try to avoid wrinkles or air bubbles during application.

Select a tape width that allows for overhang on each side.

For single joists:
Use 1.625” or 2.5” tape.
For double beams:
Use 3.25” tape.
For general flashing:
Use 4” and 12” tape.



Lock Your Design In & Moisture Out

You've laid the foundation — now keep it strong. Banish moisture and protect against rot with all-weather PRO-Tac™ Flashing and Joist Tape.

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