Deck blocking consists of short pieces of lumber installed between deck joists. These blocks tie adjacent joists together, helping prevent joist rotation, improve frame stiffness, support deck-board details, and reinforce concentrated load areas.

Blocking is commonly installed:

• at mid-span on longer joists
• around railing posts
• around stair openings
• under picture-frame borders
• near framing interruptions
• where extra stiffness is desired

Deck blocking improves framing performance in several ways.

Prevents joist rotation

Joists can twist or roll under load, especially over longer spans. Blocking ties adjacent joists together and helps keep them upright.

Improves load sharing

Blocking can help distribute localized loads across neighboring joists instead of allowing one joist to act completely on its own.

Increases perceived stiffness

By reducing joist movement and rotation, blocking can make the deck feel more solid underfoot.

Creates fastening support

Blocking provides backing for picture-frame boards, railing posts, stair openings, access hatches, and other deck details.

Deck joists are designed to span between supports, but they can still move laterally or twist under load.

Blocking connects adjacent joists so the framing behaves more like a unified system instead of a series of isolated members.

This becomes more important when:

• joist spans are long
• deck traffic is heavy
• railings create lateral forces
• composite decking makes framing movement more noticeable
• deck-board borders require extra fastening support

Related: Deck Joist Span Chart and Deck Joist Spacing.

Blocking and bridging are sometimes confused, but they are not the same detail.

Blocking is not universally required everywhere in a deck frame, but it is often required or strongly recommended in specific structural conditions.

Blocking may be required or necessary around:

• guardrail post locations
• stair openings
• framing interruptions
• picture-frame borders
• load-transfer points
• manufacturer-specific decking details

Blocking is often recommended for:

• long joist spans
• high-traffic decks
• decks that feel bouncy
• composite decking installations
• large outdoor living decks

Railing posts are one of the most important places to use reinforcement blocking.

Guardrail posts are exposed to lateral loads when people lean, push, or fall against the railing. Blocking helps transfer those forces into multiple framing members rather than concentrating stress at one joist or rim board.

Blocking around railing posts helps improve:

• post stiffness
• guardrail stability
• load distribution
• connection strength

Blocking placement depends on the deck design and the reason blocking is being installed.

Blocking layout depends on joist span and deck design.

Common patterns include:

• one row of blocking at mid-span for standard decks
• two rows of blocking for longer spans
• additional blocking at railing posts, stairs, and openings
• continuous perimeter blocking for picture-frame borders

Blocking is usually positioned based on framing layout rather than a single universal spacing number.

Picture-frame decking often requires extra perimeter blocking because border boards need solid backing along their length.

Picture-frame boards are commonly installed perpendicular to the main deck boards or around the outside edge of the deck. Without blocking, the border may lack enough support or fastening surface.

Perimeter blocking helps:

• support border boards
• reduce edge flex
• provide secure fastening points
• create a cleaner finished edge

Related: Grooved vs Square Edge Decking.

Blocking is commonly used where the normal joist layout is interrupted.

Examples include:

• stair openings
• access panels
• built-in seating
• deck hatches
• framing around posts

These areas need extra framing support so loads transfer properly around the opening.

Blocking is often confused with other framing members.

Blocking can be especially helpful with composite decking because composite boards may make framing movement more noticeable than wood decking.

Blocking helps improve:

• deck stiffness
• joist alignment
• picture-frame support
• border-board fastening
• overall walking feel

Blocking does not replace correct joist spacing. Composite decking still needs proper framing support based on manufacturer requirements.

Related: Composite Decking Guide and Deck Board Spacing Guide.

Blocking must be fastened securely so it can help tie joists together.

Common fasteners include:

• framing nails
• structural nails
• structural screws

Fasteners are typically driven through the joist into the end of the block. Where access is limited, toe-nailing or angled fastening may be used.

1. Measure the joist bay

Measure the distance between joists where the block will be installed.

2. Cut blocks to fit

Cut blocks from the same size lumber as the joists when possible.

3. Position the blocking

Install blocks at mid-span, perimeter areas, railing posts, or other required locations.

4. Fasten securely

Fasten through the joists into the block ends using framing nails or structural screws.

5. Keep tops flush

The top of each block should be flush with the joist tops so decking boards sit flat.

• skipping blocking on long joist spans
• forgetting blocking at railing posts
• not adding perimeter support for picture framing
• using inconsistent placement
• fastening blocks poorly
• installing blocks above or below joist height
• assuming blocking can fix undersized joists

Skipping blocking may not cause immediate failure, but it can reduce deck performance.

Possible issues include:

• increased deck bounce
• joist twisting over time
• less stable railing posts
• weaker picture-frame edges
• reduced framing stiffness
• more noticeable movement under composite decking

These problems are more likely on longer spans, larger decks, and high-traffic outdoor living spaces.

Deck blocking usually adds only a modest amount of material cost because many blocks can be cut from framing offcuts.

The larger cost is usually labor, especially if blocking is added after framing is already complete.

In most projects, the improvement in deck stiffness, railing support, and perimeter detailing is worth the small additional cost.

Proper blocking installation helps improve deck stiffness, reinforce railing connections, and provide support for picture-frame borders and framing interruptions. These are some of the most useful tools and hardware categories for deck framing projects.

Metabo HPT 30° Framing Nailer

A favorite among professional deck builders. Significantly speeds installation of blocking, framing members, and structural connections on larger deck projects.

View Metabo HPT Framing Nailer →

DEWALT DW088LG Green Cross-Line Laser Level

Helpful for establishing level framing lines, verifying joist alignment, and maintaining consistent deck elevations before decking is installed.

View DEWALT Green Laser Level →

Bosch Blaze GLM165-40 Laser Distance Measure

Useful for measuring joist spans, blocking layouts, railing locations, and overall framing dimensions with excellent accuracy.

View Bosch Blaze Laser Distance Measure →

DEWALT 25-Foot ToughSeries Tape Measure

An essential tool for laying out blocking rows, verifying joist spacing, and checking framing dimensions throughout construction.

View DEWALT ToughSeries Tape Measure →

Do I need blocking for my deck?

Not always, but blocking is often recommended for long spans, composite decking, railing posts, picture-frame borders, and areas where framing is interrupted.

How far apart should deck blocking be?

Many decks use one row of blocking at mid-span, with additional rows or targeted blocking for longer spans, railings, openings, and perimeter details.

Is blocking required for composite decking?

Blocking is not always universally required for composite decking, but it is commonly recommended to improve stiffness and support border details.

Can blocking replace joist hangers?

No. Blocking and joist hangers serve different purposes. Joist hangers support joist ends, while blocking ties joists together and supports specific framing details.

Should deck blocking be staggered?

Staggering blocking can make installation easier because it provides better access for fastening through joists.

Does blocking make a deck stronger?

Blocking can improve stiffness and reduce joist rotation, but it does not replace proper joist sizing, beam design, or footing layout.

Deck blocking is a simple framing detail that can significantly improve deck stability, reduce joist movement, support railings, and provide backing for picture-frame borders and openings.

While blocking is not always required across every deck frame, it is often worth adding in high-value locations where movement, load concentration, or fastening support matter.

Last reviewed: May 2026

• American Wood Council
• International Code Council
• North American Deck and Railing Association